GROUND WATER ATLAS of the UNITED STATES Alaska, Hawaii, Puerto Rico and the U. S. Virgin IslandsHA 730-N

ALASKA

INTRODUCTION
Alaska is the largest State in the Nation and has an area of about 586,400 square miles, or about one-fifth the area of the conterminous United States. The State is geologically and topographically diverse and is characterized by wild, scenic beauty. Alaska contains abundant natural resources, including ground water and surface water of chemical quality that is generally suitable for most uses.
The central part of Alaska is drained by the Yukon River and its tributaries, the largest of which are the Porcupine, the Tanana, and the Koyukuk Rivers. The Yukon River originates in northwestern Canada and, like the Kuskokwim River, which drains a large part of southwestern Alaska, discharges into the Bering Sea. The Noatak River in northwestern Alaska discharges into the Chukchi Sea. Major rivers in southern Alaska include the Susitna and the Matanuska Rivers, which discharge into Cook Inlet, and the Copper River, which discharges into the Gulf of Alaska. North of the Brooks Range, the Colville and the Sagavanirktok Rivers and numerous smaller streams discharge into the Arctic Ocean.
In 1990, Alaska had a population of about 552,000 and, thus, is one of the least populated States in the Nation. Most of the population is concentrated in the cities of Anchorage, Fairbanks, and Juneau, all of which are located in lowland areas. The mountains, the frozen Arctic desert, the interior plateaus, and the areas covered with glaciers lack major population centers. Large parts of Alaska are uninhabited and much of the State is public land. Ground-water development has not occurred over most of these remote areas.
PHYSIOGRAPHYMost of Alaska is on a large peninsula that forms the northwestern corner of the North American continent and separates the Arctic Ocean from the Pacific Ocean (fig. 1). The southeastern part of the State is on the main body of the continental land mass. Alaska is separated from Russia by the narrow Bering Strait and from the conterminous United States by part of western Canada.
Large areas of high, rugged mountains in northern and southern Alaska are extensions of mountain systems in Canada. The Brooks Range in northern Alaska is the western terminus of the Rocky Mountain System. In southern Alaska, the Alaska and the Boundary Ranges, and the Talkeetna, the Wrangell, the Kenai-Chugach, and the St. Elias Mountains are extensions of the Pacific Mountain System. The south peak of Mount McKinley in the Alaska Range is the highest point in the United States and has an altitude of 20,320 feet above sea level. The Aleutian Range that extends as a long peninsula southwestward from the Alaska mainland is an extension of the Alaska Range. Parts of the summits and upper slopes of the southern mountain ranges and chains are covered with glaciers. In contrast, only small valley glaciers are present in the eastern parts of the Brooks Range.
Low mountains, plateaus, and highlands bound the high mountains and are, in turn, bounded by lowland areas (fig. 1). The lowlands are primarily along the courses of major streams and in coastal areas. Most of the cities, towns, and villages in Alaska are in the lowland areas.
CLIMATEAlaska has a variable climate because of its large size, position between two oceans, high latitude, and range in altitude of the land surface. The State is divided into four climatic zones, based on variations in precipitation and temperature (fig. 2). The Arctic zone is characterized by average annual precipitation of less than 20 inches and an average annual temperature of 20 degrees Fahrenheit or less; seasonal variation in temperature is small in this zone. The Continental zone extends over about two-thirds of the State and is characterized by about 20 inches of average annual precipitation and an average temperature of about 22 degrees Fahrenheit. Temperature extremes are greater in the Continental zone than in the other climatic zones. Average annual precipitation in the narrow Transitional zone is about 30 inches, and temperatures average about 27 degrees Fahrenheit annually. The Maritime zone is extremely wet compared to the other three climatic zones; it averages about 70 inches of precipitation annually. Average annual temperature in the Maritime zone is about 42 degrees Fahrenheit. This zone lacks prolonged periods of freezing weather at low altitudes and is characterized by cloudiness and frequent fog. The combination of heavy precipitation and low temperatures at high altitudes in the coastal mountains of southern Alaska accounts for the numerous mountain glaciers.
PRECIPITATION AND RUNOFFAverage annual precipitation (1951-80) varies greatly in Alaska (fig. 3). Areas along the northern part of the North Slope receive less than 10 inches of average annual precipitation, whereas more than 160 inches has been recorded in several places in the southern part of the State, adjacent to the Gulf of Alaska and the Pacific Ocean. Locally, Sitka in the southeastern part of the Maritime climatic zone receives more than 300 inches of precipitation. The amount of precipitation is directly related to topography; high, rugged mountains receive the greatest amounts of precipitation and lowland areas receive least (compare figs. 1 and 3).
Much of the precipitation falls as snow from November through March. Snow might fall year-round in the high mountains, where much of it is stored for long periods in glaciers and icefields. The glaciers of Alaska extend over about 17,000 square miles, which is about the size of the combined areas of Massachusetts and New Hampshire; perennial snowfields extend over an additional 13,000 square miles. The water content of the glacial ice is not released until the ice has moved to altitudes sufficiently low to allow the ice to melt. Likewise, water that is stored as snow on the land surface is not released until the spring melt. Several months (in the case of snow) or many years (in the case of glacial ice) may elapse between the time that precipitation falls and the time that it runs off.
Average annual runoff (1951-80) in Alaska ranges from less than 10 inches in large areas of the northern and central parts of the State, and small areas near Cook Inlet and along part of the Copper River, to more than 160 inches in local areas in the southern and southeastern parts of the State (fig. 4). The pattern of distribution of runoff is similar to that of precipitation: both are least in lowland areas and greatest in mountainous areas. Average annual runoff over about two-thirds of Alaska is less than 20 inches.
Average annual precipitation minus the total of average annual direct runoff plus evapotranspiration (the combination of evaporation and transpiration by plants) is the amount of water potentially available to infiltrate downward to aquifers in their recharge areas. Although about one-half of Alaska receives less than 20 inches of average annual precipitation (fig. 3), conditions of low temperature, high humidity, and cloudy skies that prevail over most of the State minimize the rate of evaporation. Short summers minimize the time during which vegetation actively grows and, thus, negligible amounts of water are returned to the atmosphere by evapotranspiration. The small amount of evapotranspiration that does occur, however, may be a large percentage of the precipitation that falls in northern and western Alaska. Perennially frozen ground, or permafrost, inhibits infiltration of precipitation to underlying aquifers and promotes rapid runoff to streams.
PERMAFROSTPermafrost is soil, unconsolidated deposits, or bedrock that has been continuously at a temperature of 32 degrees Fahrenheit or less for two or more years. The term is synonymous with ³perennially frozen ground² but is defined solely on the basis of temperature; locally, permafrost might contain very little water or ice, or might contain highly mineralized water that remains liquid at temperatures less than 32 degrees Fahrenheit. Most permafrost, however, is consolidated by ice.
Permafrost is widespread in Alaska (fig. 5), but occurs only in small areas at high altitudes elsewhere in the United States. The thickness and areal continuity of permafrost are greatest in the continuous permafrost zone of northern Alaska and diminish southward. Locally, permafrost extends to depths of 2,000 feet below land surface in parts of the continuous permafrost zone. The wide zone of discontinuous permafrost shown in figure 5 contains isolated or interconnected unfrozen zones within the permafrost. In this zone, the thickness and lateral continuity of the permafrost decrease southward until only scattered, isolated areas of frozen ground are found near he southern limit of the zone. Areas near the southern and southeastern coasts of Alaska generally contain no permafrost.
The occurrence of permafrost is controlled by the heat balance at the surface of the earth. This balance is affected regionally by the average annual air temperature, which usually must be several degrees below 32 degrees Fahrenheit in order for permafrost to form and persist. The southward increase in average annual air temperature in Alaska is directly reflected by a southward decrease in the thickness and areal continuity of permafrost. The thickness of permafrost is determined by the average annual temperature of the ground surface, thermal properties of the soil and rock, and the geothermal gradient (the natural increase in the temperature of the earth with depth). Where the average annual ground-surface temperature is below freezing, permafrost may form and extend downward until the heat gained from the earth raises the local temperature above the freezing point. Much of Alaskaıs permafrost is thought to have formed partly during the Pleistocene Epoch, when temperatures were much lower than at present.
Factors that locally affect the surface heat balance, and thus, the presence and thickness of permafrost, include soil and rock type, relief, slope aspect (steepness and the direction which the slope faces), vegetation, snow cover, and the presence of surface-water bodies or flowing ground water. Different types of soils and rocks conduct heat at different rates and, thus, affect the depth and thickness of permafrost. Unvegetated soils may be warmer than vegetated soils because they lack the shading effect of the vegetation. In areas underlain by intrusive igneous rocks, the geothermal gradient may be higher than normal, thus limiting the thickness of permafrost, and where thermal springs issue from such rocks, they provide enough heat to locally thaw the permafrost. In the zone of discontinuous permafrost, silt in the alluvial and glacial deposits in lowlands is more likely to contain permafrost than sand and gravel interbedded with the silt. The sand and gravel beds might contain moving ground water that conducts sufficient heat to melt the permafrost. The insulating effect of thick snow tends to prevent the formation of permafrost locally; thick snow accumulations are more likely to form on gentle slopes than on steep slopes. Southward-facing slopes receive more solar radiation and, thus, are less likely to be underlain by permafrost than are northward-facing slopes. The warming effect of streams, rivers, lakes, and the ocean may extend to a depth of several hundred feet and result in local areas where permafrost is thin or absent.
The relation of surface-water bodies to the depth and thickness of permafrost in the zone of continuous permafrost is shown in figure 6. Beneath the ocean, large, deep lakes, or large streams, permafrost is thin or might be absent. Beneath small, deep lakes or streams that are perennially ice-free, an unfrozen zone overlies the permafrost but is bounded by it on the sides. The water in such an unfrozen zone commonly is highly mineralized. Beneath small, shallow lakes or creeks that completely freeze during the winter, permafrost is present only a few feet below the bottom of the surface-water body. In the discontinuous permafrost zone, the permafrost is thinner, and most lakes and large rivers are underlain by unfrozen zones that perforate the permafrost.
Human activities can affect the local thickness of permafrost because changes in ground surface temperature of only a few degrees can change permafrost thickness. Removing natural vegetation and its insulating effect in the process of clearing land causes increased solar absorption, a rise in surface temperature, and thinning of permafrost. Conversely, adding fill during road building or other construction projects increases the thickness of insulating material above the permafrost and under these insulated roadways the permafrost is less likely to melt. Heat radiating from the floors of buildings constructed directly on the land surface can cause thinning of permafrost, whereas the shading effect of buildings constructed on pilings creates a less likely chance of the permafrost melting.
Permafrost is reflected by distinctive geomorphic features at the land surface. Small lakes are common to abundant in permafrost areas. In the zone of discontinuous permafrost, many of these lakes are ³thaw lakes² that occupy shallow depressions and are unfrozen only in summer. Beaded drainage, or streams along which small pools appear to be connected like beads on a string, is an indicator of permafrost areas. Patterned (polygonal) ground such as that shown in figure 7 forms in permafrost areas where the ground freezes and then contracts on further cooling. Cracks that open during the contraction propagate downward well into the permafrost, where the temperature remains below freezing even during the melt season. When spring melt occurs at the surface, water runs down into the crack and refreezes at depth. This cycle of freezing, contraction, melting, and freezing again occurs each year, adding annual growth margins to the wedges, which intersect to form polygons. Pingos, or isolated, steep-sided, circular to oval hills (fig. 8) that range from 10 to more than 100 feet in height, form in permafrost areas. One way in which pingos form is by freezing of thawed and saturated sediments of drained lake basins. The sediments freeze inward from the sides of the basin and downward from the surface, and expand as they freeze. Water may be expelled from the sediments due to the volume expansion created by the freezing. The water pressure pushes upward an area of thin frozen sediment to form the core of a pingo. A few feet of silt, sand, and peat overlie the ice core.
RELATION OF PERMAFROST AND GROUND WATERThe principles of ground-water recharge, movement, and discharge are, in general, as valid in permafrost areas as in more temperate regions. However, ground-water flow systems in permafrost areas are affected by cold climate and the presence of perennially frozen ground. The generalized diagram of permafrost conditions shown in figure 9 is representative of the northern and middle parts of the zone of discontinuous permafrost. The top of perennially frozen ground is called the permafrost table. Above the permafrost table is the active layer, a zone that freezes in winter and thaws in summer; permeable, saturated parts of the active layer constitute suprapermafrost aquifers. These aquifers are seasonal and are primarily useful as a summer water supply where they contain water of usable chemical quality. Suprapermafrost aquifers are a source of freshwater for some villages near the Arctic Ocean. In recent years, however, water pumped from freshwater lakes in summer and stored in heated tanks for winter use is a more likely source of supply. The permafrost table forms a basal confining unit for the suprapermafrost aquifers.
Permeable material below the base of permafrost constitutes subpermafrost aquifers. In the zone of continuous permafrost, these aquifers consist mostly of consolidated rock; in the discontinuous permafrost zone, they commonly consist of unconsolidated deposits. Subpermafrost aquifers are used as sources of water supply in parts of the basins of the Yukon and Tanana Rivers where the aquifers contain freshwater. However, subpermafrost aquifers in parts of northern and western Alaska and in the Copper River Lowland contain highly mineralized water.
Permafrost affects ground-water recharge, movement, and discharge. The frozen ground blocks the downward percolation of rainfall or meltwater, and thus restricts recharge to subpermafrost aquifers. Where the permafrost table is shallow, it can perch water near the land and surface. Permafrost also blocks the lateral movement of ground water, and acts as a confining unit for water in subpermafrost aquifers. Discharge of water confined beneath the permafrost is possible only through unfrozen zones, or taliks, that perforate the permafrost layer. Although a huge quantity of water is stored in the permafrost, the water cannot be obtained and the presence of thick, continuous permafrost greatly limits the usefulness of most shallow aquifers.
GEOLOGYThe rocks and unconsolidated deposits exposed in Alaska range in age from Precambrian to Holocene. The distribution of major categories of rock types is shown by a generalized geologic map (fig. 10). Within each major mapped category, rocks younger or older than those represented by the category might crop out, but are not shown because of the map scale. Glacial drift deposited during the Pleistocene Epoch by large valley glaciers mantles mountain flanks and adjacent lowland areas in most of the mountainous areas of Alaska. These drift deposits and large areas of windblown silt (called loess) derived from glacial deposits are likewise not shown on the map. Although consolidated bedrock locally yields water to wells, especially where the bedrock is fractured or contains large solution openings, most ground water is withdrawn from permeable Quaternary deposits.
Modern interpretations of the complex geology of Alaska are based on the concept that the State is a mosaic of geologic terranes. A terrane is a body of rock of regional extent that is bounded by faults, and whose geologic history is different than that of adjacent terranes. All the terranes in Alaska represent blocks of the Earthıs crust that have moved large or small distances relative to each other. The movement might have been translational (lateral movement without rotation) or rotational or both. Some of the terranes may have moved only a short distance, whereas others may have moved laterally for several hundred miles or rotated as much as 135 degrees. The pattern of terranes in Alaska reflects the interactions of oceanic crustal plates with the North American plate; large-scale lateral and rotational movements, rifting, and volcanic activity result from these interactions. A detailed discussion of Alaskan terrane theory and evidence is beyond the scope of this Atlas; an excellent summary of the topic can be found in Plafker and Berg (1994), listed in the References section of this atlas. Some of the large faults shown in figure 10 separate terranes.
Lower Paleozoic and Precambrian metamorphic rocks underlie most of central Alaska and much of the Seward Peninsula, the Yukon-Tanana Upland, the Kokrine-Hodzana Highlands, and the southern flank of the Brooks Range (compare fig. 10 with fig. 1). These rocks are primarily gneiss, schist, phyllite, and quartzite, but locally include argillite, marble, and several kinds of metasedimentary rocks. Local areas of Lower Paleozoic and/or Precambrian sandstone, limestone, shale, and chert in the northeast Brooks Range are mapped in this category. In the eastern part of the Yukon-Tanana Upland, Paleozoic intrusive and volcanic rocks of various kinds intrude, overlie, or are faulted against lower Paleozoic and Precambrian metamorphic rocks in an area of complex geology.
Cambrian through Devonian sedimentary rocks are widespread in the Brooks Range, the northwestern part of the Yukon-Tanana Upland, the northwestern and eastern parts of the Kuskokwim Mountains, the northeastern part of the Nushagak-Big River Hills, and southeastern Alaska. Smaller areas of these rocks are exposed in the easternmost part of the Alaska Range and in the Northern Foothills that border that range. These rocks consist mostly of sandstone, shale, and siltstone, but also include beds of limestone, dolomite, and chert. Cambrian through Devonian sedimentary rocks are complexly folded and faulted in the Brooks Range and are less deformed elsewhere.
Mississippian through Permian sedimentary rocks crop out mostly along the northern flanks of the Brooks Range, in the eastern part of the Porcupine Plateau (compare fig. 10 and fig. 1), in the northern and eastern parts of the Alaska Range, in the Wrangell Mountains and the northeastern part of the Kenai-Chugach Mountains, and locally in southeastern Alaska.These rocks are mostly limestone, shale, siltstone, and sandstone, but include beds of conglomerate, dolomite, and chert. Locally, marble, argillite, and metasedimentary and metavolcanic rocks are mapped in this category.
Upper Paleozoic metamorphic, sedimentary, and igneous rocks are exposed mostly in southeastern Alaska, the western parts of the Ahklun Mountains, and the southern part of the Nulato Hills. Metamorphic rock types mapped in this category include schist, gneiss, phyllite, and slate; sedimentary rocks include limestone, dolomite, chert, tuff, and volcaniclastic rocks; and igneous rocks include gabbro and basaltic to andesitic lava flows.
Mesozoic sedimentary rocks underlie large parts of the Arctic Foothills and the Arctic Coastal Plain, the Baird Mountains and the Indian River Upland south of the Brooks Range, most of the Nulato Hills, the Nushagak-Big River Hills, the Kuskokwim and the Ahklun Mountains, and the southern part of the Alaska Range. Smaller exposures of these rocks are in the Wrangell and the Talkeetna Mountains, the northern part of the Aleutian Range, the Kodiak Mountains, and the Chilkat-Baranof Mountains (compare fig. 10 with fig. 1). These rocks are mostly shale, siltstone, and sandstone, but locally include limestone and large deposits of coal.
Mesozoic volcanic rocks crop out in large areas of central Alaska, from the eastern side of the Seward Peninsula to the westernmost part of the Yukon-Tanana Upland to the northern part of the Kuskokwim Mountains. Smaller areas of these rocks are exposed in the Nulato Hills, the western part of the Ahklun Mountains, the Talkeetna and the Wrangell Mountains, and southeastern Alaska. These rocks range in composition from andesite to basalt.
Mesozoic intrusive rocks crop out in smaller areas than the Mesozoic volcanic rocks, but are widespread in central, southern, and southeastern Alaska. These rocks are mostly in upland and mountainous areas and range in composition from granite to gabbro. The widespread occurrence of these rocks, along with that of the Mesozoic volcanic rocks, shows that igneous activity was greatest in Alaska during Mesozoic time.
Mesozoic metamorphic, volcanic, and igneous intrusive rocks underlie large parts of the Kenai-Chugach Mountains and a small part of the Kodiak Mountains. These rocks consist of greenstone, limestone, chert, granodiorite, schist, and layered gabbro. Their contacts and extent are incompletely known because of glacial cover in many places.
Tertiary sedimentary rocks crop out mostly in the northeastern part of the Arctic Coastal Plain, near Cook Inlet and the northern part of the Gulf of Alaska, and in the northern part of the Aleutian Range. Smaller exposures of these rocks are along part of the Northern Foothills that flank the Alaska Range, and on the south-central flank of the Alaska Range. These rocks are primarily sandstone, siltstone, and shale, but also contain beds of coal, mudstone, and conglomerate.
Tertiary intrusive igneous rocks are prominent in the southern part of the Alaska Range and in the Talkeetna Mountains. Smaller exposures of these rocks are found in the eastern part of the Yukon-Tanana Upland, the Kodiak Mountains, and on some of the Aleutian Islands. These rocks range in composition from gabbro to granite.Tertiary volcanic and sedimentary rocks are exposed mostly in the area around Prince William Sound, the southeastern side of Kodiak Island, and in the Aleutian Islands. These rocks consist of complexly interbedded sedimentary and volcanic rocks of early Tertiary (Paleocene through Oligocene) age.
Quaternary and Tertiary volcanic rocks ranging in composition from rhyolite to basalt are prominent in large parts of central and southern Alaska and in the Aleutian Islands. Volcanic eruptions continue along Cook Inlet, on the Alaska Peninsula, and in the Aleutian Islands at the present time.
Quaternary unconsolidated deposits are present in lowland areas throughout Alaska. The deposits mapped in figure 10 represent only thick accumulations of these deposits; small areas of thin deposits are not shown. These deposits consist primarily of alluvium but also include glacial deposits and locally include eolian and beach deposits. In coastal areas, deltaic and marine deposits of Quaternary age are included in this map category.
AREAL DISTRIBUTION OF AQUIFERSInformation on subsurface geology, ground water, and permafrost is sparse in Alaska, and for many places no data are available. In large parts of the State, the surface geology is not well known. It is difficult to extrapolate hydrologic conditions from the few areas where they are known to different localities that have similar geologic settings because local variations in geologic and permafrost conditions significantly affect the occurrence and movement of ground water.
The aquifers of Alaska have never been mapped, except in the immediate vicinity of some of the towns and cities such as Kenai, Anchorage, Juneau, and Fairbanks. In other places, data from widely scattered drill holes, combined with maps of the surficial geology, allow some inference about the availability of ground water. The distribution of coarse-grained, unconsolidated alluvial and glacial-outwash deposits of Quaternary age is shown in figure 11. In many areas, such as the Tanana River basin, these deposits comprise thick aquifers that yield large quantities of water to wells. In other areas, such as the Copper River basin, widespread Quaternary deposits consist mostly of lacustrine silt and clay that are underlain by saline water and do not comprise aquifers. In the coastal area between Norton Sound and Bristol Bay, Quaternary deposits extend over large areas but are generally too fine grained to yield significant amounts of water. However, sand and gravel deposits such as those that provide the water supply for Bethel locally form productive aquifers. From the Brooks Range northward to the Arctic Ocean, Quaternary deposits contain continuous permafrost and, therefore, are not aquifers. In the northern part of the zone of discontinuous permafrost, the alluvial and outwash deposits are frozen during much of the year and exploration for local sources of ground water has generally not been conducted. In this region, however, scattered occurrences of large surface accumulations of ice during the winter indicate the presence of local aquifers.
Unconsolidated Quaternary deposits may locally be as thick as 1,000 feet in large basins such as the Yukon, the Kuskokwim, the Tanana, and the Copper River. The entire thickness, however, does not yield water. At depth, the deposits are likely to consist of fine grained marine or lacustrine sediments. A test hole drilled near Fort Yukon in 1994, for example, penetrated lacustrine sediments from a depth of 600 feet to the bottom of the hole at 2,000 feet.
Igneous, metamorphic, and sedimentary rocks underlie about 70 percent of Alaska. Although these rocks generally yield smaller amounts of water to wells than coarse-grained alluvial and outwash deposits, they are important aquifers in some parts of the State. In the Fairbanks area, approximately half the residents obtain water from wells completed in bedrock. Large springs that issue from carbonate rocks in the eastern part of the Brooks Range are reported to discharge as much as 16,000 gallons per minute. Carbonate bedrock on Admiralty Island in southeastern Alaska also yields large quantities of water from well-developed cave systems. In general, the water-yielding capability of bedrock in Alaska is not well known, however, and bedrock aquifers are not mapped in figure 11.
GROUND-WATER QUALITYThe concentration of dissolved solids in ground water provides a basis for categorizing the general chemical quality of the water. Dissolved solids in ground water primarily result from chemical interaction between the water and the rocks or unconsolidated deposits through which the water moves. Rocks or deposits composed of minerals that are readily dissolved will usually contain water that has large concentrations of dissolved solids. The rate of movement of water through an aquifer also affects dissolved-solids concentrations; the longer the water is in contact with the minerals that compose an aquifer, the more mineralized the water becomes. Thus, larger concentrations of dissolved solids commonly are in water at or near the ends of long ground-water flow paths. Aquifers that are in hydraulic connection with bays, sounds, or the ocean commonly contain saline water, and mixing of fresh ground water with this saline water can result in a large increase in the dissolved-solids concentration of the freshwater. Contamination from human activities can increase the concentration of dissolved solids in ground water; such contamination usually is local but can render the water unfit for human consumption or for many other uses.
The terms used in this report to describe water with different concentrations of dissolved solids are as follows:#The chemical quality of water from aquifers in unconsolidated deposits in Alaska generally is suitable for most uses. The water, classified by the dominant dissolved ions it contains, is a calcium bicarbonate or calcium magnesium bicarbonate type in inland areas. Locally, in areas near the coast, these aquifers contain moderately saline to very saline water in their downgradient parts, where the aquifer is hydraulically connected to seawater of a sodium chloride type. Water in the mixing zone between fresh and saline water in these coastal aquifers commonly is a sodium bicarbonate type.
Dissolved-solids concentrations in water from unconsolidated-deposit aquifers are less than 400 milligrams per liter in most places (table 1). An exception is the Copper River Lowland, where dissolved-solids concentrations in water from shallow and deep wells, and from some springs, exceed the 500 milligrams per liter recommended for drinking water by the U.S. Environmental Protection Agency. The large concentrations of dissolved solids in water from the Copper River Lowland reflect the upward movement of saline water from marine sediments that underlie the unconsolidated deposits.
Water from the aquifers in unconsolidated deposits is hard to moderately hard and, thus, may require treatment for some uses. Concentrations of iron in water from these aquifers are objectionable in many places, but the iron is easily removed from the water by inexpensive treatment. Iron concentrations in excess of 1,000 micrograms per liter are common; concentrations greater than 300 micrograms per liter can cause staining of laundry and porcelain plumbing fixtures, and impart a taste to the water. Locally, excessive concentrations of dissolved manganese and arsenic are reported in water from these aquifers.
Ground-water contamination from human activities can take place rapidly, and shallow aquifers such as those in unconsolidated deposits are particularly susceptible to contamination. Contamination related to human activities is categorized as being from a point source or a nonpoint source. A point source is a specific local site such as an underground storage tank that contains wastes, petroleum products, or chemicals; a landfill; a storage pond, pit, or lagoon; a spill of hazardous chemicals or petroleum products; or a disposal or injection well that receives municipal or industrial wastes. Nonpoint contamination sources are large-scale and can extend over hundreds of acres. Examples of nonpoint sources are: agricultural activities, such as applying fertilizer or chemicals to fields; urban areas with concentrations of septic tanks and cesspools; encroachment of saltwater or highly mineralized geothermal water; mining operations; oilfields and associated tank farms; and salt from highway deicing. Nitrate, a common contaminant from septic tanks, has been reported in ground water near Fairbanks in concentrations greater than the recommended Federal drinking-water standard of 10 milligrams per liter. These large nitrate concentrations, however, were present in places before significant development occurred, and are thought to result in part from the addition of nitrogen to the soil by plants such as alders.
The chemical quality of water from bedrock aquifers in Alaska is known from a few areas where dispersed residential wells have been drilled away from centralized water-distribution systems. In the vicinity of Fairbanks, water from wells completed in bedrock is generally a calcium bicarbonate type and usually is hard, especially on the lower slopes. Locally, concentrations of arsenic and nitrate in excess of the recommended Federal drinking-water standards are reported. Water of chemical quality suitable for most uses is reported from wells completed in bedrock aquifers in the Anchorage-Eagle River area and in coastal communities bordering the Kenai and the Kodiak Mountains. However, water from wells completed in coal-bearing Tertiary strata in the Cook Inlet Basin commonly contains objectionable concentrations of iron and hydrogen sulfide.FRESH GROUND-WATER WITHDRAWALSAlthough surface water is abundant in Alaska, many of the streams, rivers, and lakes are covered with ice for much of the year. In addition, streams that are fed by glaciers transport glacial silt which gives the water a milky appearance and renders it unsuitable for many uses unless the silt is removed by flocculation. Accordingly, ground water is an important source of supply, especially in the zones where permafrost is discontinuous or absent.
During 1990, ground water provided 23 percent of the total freshwater withdrawn in Alaska, but supplied 37 percent of the water withdrawn for public supply and 90 percent of that withdrawn for rural domestic use. Fairbanks, Juneau, and about 50 smaller communities depend almost entirely on ground water for supply. About 50 percent of the Stateıs population is supplied by ground water.
The Municipality of Anchorage withdrew ground water at a rate of 11 million gallons per day during 1969, and withdrawals increased to more than 20 million gallons per day during 1985. Since 1985, however, Anchorage has constructed a new pipeline to Eklutna Lake north of the city. This lake and Ship Creek, which flows through the city, now supply most of the water needed by the municipality.
Most of the ground water in the Anchorage and Juneau areas is withdrawn from aquifers in unconsolidated deposits. Wells completed in unconsolidated deposits provide about one-half of the water withdrawn in the Fairbanks area; the remaining one-half is obtained from wells completed in bedrock. Bedrock supplies water to a significant number of wells in the hillside areas of Anchorage, on Kodiak Island, and on the Kenai Peninsula.
Total fresh ground-water withdrawals in Alaska during 1990 were about 63 million gallons per day (fig. 12). About 54 percent of this amount, or about 34 million gallons per day, was withdrawn for public supply. About 15 million gallons per day, or about 24 percent of the total withdrawals, were pumped for domestic and commercial use. Withdrawals for industrial, mining, and thermoelectric power use accounted for almost all the remainder of the water pumped. Only about 0.2 million gallons per day, or less than one-half of one percent, of the water withdrawn was used for agricultural purposes. About 48 million gallons per day of saline ground water was withdrawn for mining use during 1990.

INTRODUCTION
Unconsolidated deposits of sand and gravel that were deposited as alluvium or glacial outwash or both form the most productive aquifers in Alaska. These aquifers are present in lowland areas, primarily in the flood plains of major rivers, but in some places they also underlie low, rolling hills developed on alluvial-fan deposits that separate the flood plains from nearby mountains. In some areas, such as near Anchorage and Fairbanks, the unconsolidated-deposit aquifers are thick and widespread; in other places, they are present as narrow bands of alluvium in, and adjacent to, river channels. Because the Stateıs major population centers and most of the agricultural development are in lowland areas near the rivers, the unconsolidated-deposit aquifers are an important source of water for public supply, domestic and commercial uses, and growing of crops. The geologic and hydrologic characteristics of these aquifers are described in the example areas shown in figure 13 and discussed in the following sections of this report.
COOK INLET AQUIFER SYSTEMThe Cook Inlet aquifer system underlies the lowland areas along both sides of the northern part of Cook Inlet and the lower reaches of the Susitna and the Matanuska Rivers which discharge into the inlet (fig. 13). The aquifer system provides part of the water supply for Anchorage and for smaller cities and towns including Palmer, Kenai, and Soldotna. A large number of domestic wells also obtain water from the aquifer system.
The unconsolidated sediments that make up the aquifer system consist of clay, silt, sand, gravel and boulders that were deposited primarily by glaciers but also by alluvial and colluvial processes. The sediments are complexly interbedded, with lenses and thin beds of sand and gravel interfingering with beds of clay, silt, and till. The stratigraphic complexity and great variability in grain size of the sediments causes discontinuity and variability in their hydraulic characteristics. Low-permeability sedimentary and metamorphic rocks underlie the aquifer system; locally, small volumes of water might move through these rocks and discharge upward to the unconsolidated-deposit aquifers.
The sand and gravel beds that compose the water-yielding parts of the Cook Inlet aquifer system were deposited mostly as glacial outwash. Locally, alluvial deposits of sand and gravel are present in the upper parts of the aquifer system. Sand and gravel of colluvial origin flank the bedrock hills bordering the sedimentary basin that contains the aquifer system. Poorly sorted, unstratified till (fig. 14) or beds of clay and silt that represent glacial-lake or estuarine deposits are commonly interbedded with the sand and gravel. The till, clay, and silt have minimal permeability and commonly confine water in the unconsolidated-deposit aquifers. The relations of the aquifers and confining units at several places in the Cook Inlet aquifer system are shown in figure 15.
Water in the unconsolidated-deposit aquifers moves from recharge areas near the mountains, down the hydraulic gradient to discharge areas beneath major streams, Cook Inlet, or Knik Arm, the northern fork of the inlet. Where the aquifers are exposed at the land surface, such as the colluvial deposits on the flanks of the mountains or alluvial deposits near streams, they can receive recharge directly from precipitation on outcrop areas. Also, streams that flow from the low-permeability bedrock of the mountains onto sand and gravel deposits (fig. 16) lose water to the unconsolidated-deposit aquifers by leakage through the stream beds. The principal recharge areas for the aquifers are, thus, near the flanks of the mountain ranges. Small amounts of water might leak upward into the aquifers from local permeable zones in the underlying bedrock. Water moves laterally in the unconsolidated-deposit aquifers toward discharge areas, where it moves upward. Some water discharges by evapotranspiration from unconfined aquifers and withdrawals from wells.
The aquifers near Anchorage are the best known part of the Cook Inlet aquifer system. Two principal water-yielding zones contain most of the ground water (fig. 15B); a deep third zone is present in some places but is not well known. The upper zone contains water under unconfined (water table) conditions, whereas a fine-grained unit that underlies it creates confined (artesian) conditions in the lower zone. Hydraulic heads in both zones are sufficiently high to prevent intrusion of saline water from Cook Inlet or Knik Arm. Tidal fluctuations of as much as 37 feet in the inlet produce water-level fluctuations of as much as 4 feet in some wells, as a result of changes in pressure created by the rising and falling tide.
A map of the water table in the shallow, unconfined aquifer in the Anchorage area (fig.17) shows that water in this aquifer moves generally westward and northwestward from recharge areas at the eastern limit of the aquifer system. The configuration of the water table generally corresponds to the configuration of the land surface, but the water table contours are irregular where they cross streams. These irregularities reflect the hydrologic relations between the stream and the aquifer. Ship Creek, north of Anchorage, is a good example of these relations. Eastward from the airstrip at Elmendorf Air Force Base, the water-table contours bend downstream where they cross Ship Creek, indicating that the creek is losing water to the aquifer. Farther westward, the contours point upstream where they cross Ship Creek, indicating that the creek is gaining water from the aquifer in this area. Discharge from the aquifer to other streams is indicated by the shape of the water-table contours where they cross the downstream reaches of Chester Creek and Campbell Creek: the contours bend upstream in both areas.
Before large ground-water withdrawals began, water in the confined aquifer near Anchorage moved regionally from recharge areas near the Chugach Mountains toward discharge areas at Cook Inlet and Knik Arm (fig. 18). The regional direction of movement was similar to that of water in the unconfined aquifer (compare figs. 17 and 18). However, the potentiometric surface of the confined aquifer indicates little or no hydraulic connection between the aquifer and streams that cross it. The fine-grained sediments that overlie this aquifer form an effective confining unit that hydraulically separates the aquifer not only from the streams, but also from the overlying unconfined aquifer. The effectiveness of the confining unit is also shown by the difference in altitude between the water table and the potentiometric surface of the confined aquifer. Water levels in the unconfined aquifer are 20 to 30 feet higher than the potentiometric levels of the confined aquifer everywhere the confining unit is present (compare figs. 17 and 18).
Withdrawal of water from high-capacity wells completed in the confined aquifer causes a decline in artesian pressure in the aquifer that is reflected by depressions on the aquiferıs potentiometric surface (fig. 19). Large withdrawals from pumping centers near Elmendorf Air Force Base and the Glenn Highway caused the potentiometric surface of the aquifer to decline more than 50 feet by March 1969. Most of the ground water withdrawn in the Anchorage area is pumped from the confined aquifer. Withdrawals were sufficient to cause declines of 10 feet or more over an area of more than 40 square miles in March 1969 (fig. 20).
RIVER-VALLEY ALLUVIAL AQUIFERSAlluvial deposits of sand and gravel that are present in the flood plains and terraces of the major river valleys in Alaska, and that are not connected to large alluvial fans, are called river-valley alluvial aquifers in this report. The permeable sand and gravel contain lenses and beds of silt and clay, rich in organic material in some places, that retard the movement of ground water. The alluvial deposits are present mostly in the zone of discontinuous permafrost but also occur in the parts of the State where permafrost is absent. Where present, the permafrost acts as an impermeable barrier to ground-water flow, and creates confined conditions for water that might be in unfrozen permeable beds beneath it. Permafrost is absent beneath the beds of the major streams and in the alluvium adjacent to the streams; thus, the alluvial aquifers are in hydraulic connection with the streams and the movement and level of ground water are directly influenced by the direction of streamflow and the stage of the stream.
The aquifer in deposits of thick alluvium in the flood plain of the Yukon River at Tanana (fig. 13) is an example of a river-valley alluvial aquifer. The alluvial deposits at Tanana consist of sandy gravel and sandy silt, bordered at the sides of the river valley by silty deposits of colluvium that locally contain poorly sorted sand and gravel. Discontinuous permafrost is present in the alluvium except beneath the bed of the river and the flood plain immediately adjacent to it, where the warming effect of the river prevents permanent freezing of the ground. Adjacent to the river, therefore, ground water can move into and out of the riverbanks and stream bed, depending on the elevation of water in the river relative to the water level in the aquifer. Water levels in the aquifer at Tanana rise and fall in response to rises and falls of river level.
Field observations and computer simulation indicate that the movement of ground water near Tanana is toward the Yukon River from the valley walls of the river (fig. 21). Water recharges the aquifer by seepage through the beds of Bear and NC Creeks, and moves locally southward and southwestward to discharge to the Yukon River. Regionally, ground water moves westward, in the same direction as the flow of the river. The local movement of ground water is probably more complex than that shown in the figure because permafrost was assumed to be absent in the computer simulations; the general movement of the water, however, is thought to be correct.
TANANA BASIN AQUIFERThe water-yielding unconsolidated deposits along the Tanana River and the flanks of the hills that surround the river basin (fig. 13) are called the Tanana Basin aquifer in this report. The deposits consist of flood-plain alluvium near the Tanana River and its tributaries, and alluvial-fan deposits on the north flanks of the Alaska Range that borders the river basin to the south. Locally, moraines deposited by glaciers in valleys of the Alaska Range interfinger with the alluvium and are considered to be slightly less permeable parts of the aquifer. Although the alluvial deposits locally comprise several aquifers separated by leaky confining units of silt and clay or by layers of permafrost, they are usually treated as a single aquifer whose permeability varies widely. The bedrock that underlies and surrounds the alluvial deposits consists primarily of folded, faulted metamorphic rocks, locally intruded by igneous rocks. The bedrock is generally dense, compact, and yields little water; locally, however, where it is fractured it will yield significant quantities of water to wells. For example, numerous wells completed in bedrock in the uplands north and northeast of Fairbanks yield sufficient water for domestic supplies.
The alluvial deposits consist of well-stratified layers and lenses of silt, sand, and gravel. Broad alluvial fans of the large rivers that enter the Tanana drainage basin from the Alaska Range coalesce to form a continuous alluvial apron of coarse, permeable sediments at the base of the range. Permeable flood-plain alluvium is also present as narrow to wide bands along the Tanana River and its larger tributaries. The alluvial deposits are very thick in some places: wells have penetrated more than 600 feet of alluvium near Fairbanks and about 550 feet near the junction of the Delta and Tanana Rivers. Where the alluvium is thick and permeable, it is reported to yield as much as 3,000 gallons per minute to large-capacity wells. Water in these widespread alluvial deposits is mostly unconfined.
By contrast, water in the alluvial deposits north and east of the Tanana River occurs under unconfined and confined conditions. The sediments that compose the aquifer here are poorly sorted and, because the aquifer is in the zone of discontinuous permafrost, the permanently frozen ground, as well as beds and lenses of silt and clay, create confined conditions (fig. 22). The silt and clay deposits are more likely to be permanently frozen than beds of sand and gravel. Unfrozen alluvium is present beneath the permafrost, however, in most parts of the aquifer. The water is generally unconfined in the higher parts of the alluvial fans and in the alluvial plains near major streams. Artesian conditions are common on the lower slopes of the alluvial fans, and some wells completed in confined parts of the aquifer in these areas flow at the land surface.
The occurrence and movement of ground water in the Tanana Basin aquifer are directly related to stream levels and streamflow. Most recharge to the aquifer is from seepage through streambeds, rather than from precipitation that falls directly on the aquifer. Water levels in streams that emerge from the bedrock of the Alaska Range onto permeable parts of the alluvial fans at the base of the mountains are much higher than the water table. Much of the flow of the larger streams, and all the flow of some smaller ones, is lost as the water seeps downward to recharge the aquifer. Water in the alluvial fans moves regionally toward the Tanana River and then downstream, in the same direction of flow as the river (fig. 23). Water discharges locally from the aquifer to springs and the lower reaches of Tanana River tributaries and regionally to the Tanana River; a small amount of water discharges to wells.
Water levels in wells located near streams fluctuate in direct response to rises and falls in stream water levels. A hydrograph comparing the water level in the Chena River at Fairbanks with that in a nearby well (fig. 24) shows that rises in river level are soon followed by rises in ground-water levels, indicating that the river and the aquifer are hydraulically connected. Both stream and aquifer water levels rise in response to precipitation events and snow melt.
During most of the year, the water level in a reach of the Tanana River near Fairbanks is higher than that of a nearby reach of the Chena River, a tributary of the Tanana. When this condition occurs, the shallower parts of the Tanana Basin aquifer between the two rivers receive recharge from the Tanana River and discharge to the Chena River (fig. 25). Thus, the local direction of shallow ground-water movement varies from the regional direction, which closely corresponds to the direction of stream flow. Deep ground-water flow, however, is thought to move under the Tanana River and toward the Chena River at all times of the year.
The chemical quality of water in the Tanana Basin aquifer is generally suitable for most uses. The water is a calcium bicarbonate or calcium magnesium bicarbonate type, and locally contains concentrations of iron and manganese that are higher than those recommended for drinking water by the U.S. Environmental Protection Agency.
COASTAL VALLEY AQUIFERSUnconsolidated deposits of silt, sand, and gravel are common along the lower reaches of streams in coastal valleys. The coastal valley deposits near Juneau and in the Mendenhall River Valley (fig. 13) comprise a water-yielding unit that is an example of this type of aquifer. The aquifer consists of alluvial and glacial deposits of Quaternary age and contains water under unconfined conditions in its upper parts. Beds of silt and clay that interfinger with the permeable sand and gravel beds create confined conditions in the lower parts of the aquifer. The fine-grained confining units are mostly discontinuous but are effective enough in some places to create artesian pressures sufficiently high so that early wells completed in permeable strata beneath them would flow at the land surface.
Water enters the aquifer primarily as seepage through the beds of streams such as the Mendenhall River where they flow across deposits of sand and gravel (fig. 26); a smaller amount of recharge to the aquifer is by precipitation that falls directly on permeable strata. Where permeable beds of the aquifer are exposed at the land surface, water levels in wells completed in these beds respond quickly to variations in precipitation (fig. 27). Some of the water stored in glaciers that cap low-permeability bedrock mountains to the north and east of Juneau and the Mendenhall Valley is released as meltwater during the summer months. The meltwater is channeled through bedrock valleys until it emerges onto the alluvial and glacial deposits that comprise the aquifer, where much of the streamflow seeps downward to enter the aquifer. The water subsequently moves coastward, where most of it discharges either to the lower reaches of the streams as base flow or directly into saltwater bodies; however, some of the water discharges by evapotranspiration and some discharges to wells.
Freshwater in some of the sand and gravel beds of the coastal valley aquifer near Juneau and in the Mendenhall Valley is hydraulically connected to saltwater bodies (fig. 26). Under natural conditions, saltwater in the parts of the aquifer beneath Fritz Cove and Gastineau Channel is in balance with freshwater in the inland parts of the aquifer, a condition known as hydraulic equilibrium. If the freshwater column inland is lowered as a result of withdrawal by wells, however, the saltwater can migrate inland, contaminate some of the freshwater in the aquifer, and render it unfit for use.
The hydraulic conditions that are likely to result in saltwater intrustion are summarized in figure 28. Under natural conditions, saltwater in the offshore parts of an unconsolidated-deposit aquifer is balanced by a thicker column of freshwater onshore (fig. 28A). Large withdrawals from wells completed in the freshwater parts of the aquifer cause the water table to decline and the thickness of the freshwater column to decrease (fig. 28B). The equilibrium between the saltwater and freshwater is, thus, imbalanced, and saltwater moves inland. Eventually, the saltwater might enter the pumping wells and contaminate the water to the extent that it is unsuitable for most uses.

BEDROCK AQUIFERS
Consolidated rocks are exposed at the land surface over about 70 percent of Alaska, but are generally less permeable than the unconsolidated deposits discussed in the preceding section of this report. Accordingly, the consolidated rocks are used as a source of water supply only where the unconsolidated deposits are absent, thin, or poorly permeable. Information about the water-yielding characteristics of the consolidated rocks, which are called bedrock aquifers in this report, is scarce. The locations of known bedrock aquifers and the aquifer rock type are shown in figure 29.
Sedimentary bedrock that underlies the southwestern part of the Kenai Peninsula (fig. 29) provides water to numerous domestic wells and supplies some small communities. Poorly consolidated sandstone that is part of the Kenai Group of Tertiary age yields most of the water, but some water is also obtained from coal beds and seams within the group. Interbedded siltstone and claystone yield no water. Although most wells completed in strata of the Kenai Group yield less than 20 gallons per minute, yields of as much as 80 gallons per minute are reported locally. Several springs and seeps near Homer on the Kenai Peninsula discharge from beds of the Kenai Group. Sparse data from the Capps coal field, north of Cook Inlet across from the Kenai Peninsula, indicate that Tertiary beds of coal and poorly consolidated sandstone and conglomerate yield as much as 60 gallons per minute to wells. Water withdrawn from the Kenai Group and the strata in the Capps coal field locally contains objectionable concentrations of iron and hydrogen sulfide, both probably derived from the coal beds. Locally, methane gas that has been reported in water from some wells near Homer is also probably derived from the coal units.
Carbonate rocks comprise known or potential aquifers in some parts of Alaska. In the eastern part of the Brooks Range, at least 25 springs are known to discharge from carbonate rocks; the discharge of one of these springs is as much as 16,000 gallons per minute. The springs are thought to discharge from a network of solution cavities and conduits that have developed by partial dissolution of the carbonate rocks. Carbonate rocks on Admiralty Island in southeast Alaska locally yield large quantities of ground water, probably from a system of caves and solution conduits.
Metamorphic rocks yield water in substantial quantities only where they have been fractured. Perhaps the most important area underlain by a metamorphic-rock aquifer is north and northeast of Fairbanks, where wells completed in fractured schist supply approximately one-half of the population. Fractured slate and metagreywacke in the upland areas near Anchorage supply water to numerous domestic wells. Fractured slate and metamorphosed volcanic rocks on Kodiak Island generally yield less than 15 gallons per minute to wells, but locally yield as much as 100 gallons per minute.
Little is known about the water-yielding potential of the widespread volcanic rocks in Alaska, but they are permeable at least locally, where hot springs issue from them. The permeability in these rocks may be a combination of fractures produced when the rocks cooled and weathered and vesicular layers that developed on the tops of individual basalt flows. Basaltic rocks in the Pribilof Islands yield sufficient water to supply small communities.

GEOTHERMAL WATER
Numerous springs in Alaska discharge geothermal water, or water with a temperature appreciably warmer than the local average annual air temperature. The location of known geothermal springs is shown in figure 30. The temperatures for the mapped springs are those where the water emerges at the land surface; temperatures within the geothermal reservoir that houses the water at depth are greater than those mapped.
Most of the geothermal springs in Alaska issue from consolidated bedrock. The springs are most common in two areas - a belt across central Alaska that is underlain largely by intrusive igneous rocks and an arcuate area in the mountain ranges in the southern part of the State which is underlain largely by volcanic rocks. Many of the springs issue from faults and fractures at the contacts of granitic plutons.
One theory of the origin of geothermal water is that precipitation falling in upland areas circulates to great depths in the consolidated rocks, mainly along faults. At some depth, the water is warmed by the natural increase in temperature with depth in the Earthıs crust (the average increase is about 1 degree Fahrenheit for each 60 to 100 feet of depth) until it becomes lighter than the overlying water. The warm water then moves upward along faults and fractures and discharges to springs.
Geothermal water is a potential source of energy but has been developed only locally in Alaska. Some of the geothermal springs on Baranof Island in southeast Alaska are used to heat buildings and to supply water for a bathhouse. Other springs in scattered mainland areas are used for similar purposes.

HAWAII

INTRODUCTION

The Hawaiian islands are the exposed parts of the Hawaiian Ridge, which is a large volcanic mountain range on the sea
floor. Most of the Hawaiian Ridge is below sea level (fig. 31). The State of Hawaii consists of a group of 132 islands, reefs,
and shoals that extend for more than 1,500 miles from southeast to northwest across the central Pacific Ocean between
about 155 and 179 degrees west longitude and about 19 to 28 degrees north latitude. The main inhabited islands are at
the southeastern end of the group (fig. 31); not all the small islands, reefs, and shoals included in the State are shown.

The Hawaiian islands are geologically youngest in the southeast and oldest in the northwest. This report discusses only
the eight largest islands near the southeastern end of the group; these eight main islands account for practically all of
the 6,426-square-mile land area of the State. The eight islands and their approximate size, in square miles, from
southeast to northwest are Hawaii, 4,021; Maui, 728; Kahoolawe, 45; Lanai, 141; Molokai, 259; Oahu, 603; Kauai, 553; and
Niihau, 71. The total resident population in 1995 was 1,179,198, of which about 75 percent were on the island of Oahu.
Honolulu, which is on Oahu, is the largest and most developed city and had a population of 369,485 in 1995. In addition to
the resident population, a visitor population of about 150,000 has typically been present at any given time during the
1990's. Many of these visitors stay in Honolulu. 
The State Land Use Commission is responsible for classifying the lands of the State into one of four categories called
districts: conservation, agricultural, urban, or rural (fig. 32). In 1995, conservation, agricultural, urban, and rural districts
accounted for about 48, 47, 5, and 0.2 percent of the land area in the State, respectively. Conservation districts include
areas necessary for protecting the State's watersheds and water resources and are typically located in high-altitude,
high-rainfall areas. Much of the urban development in Hawaii is in the lowland coastal areas of each island. Agricultural
irrigation can place large demands on the water resources; prior to the 1990's, one of the largest uses of water was for
sugarcane irrigation. The five largest islands (Hawaii, Maui, Molokai, Oahu, and Kauai) have extensive areas of mountainous
land where urbanization and large-scale agricultural operations are not feasible.

The island of Hawaii is the largest island of the State (fig. 33) and has the highest altitude at 13,796 feet. Maui is about
10,000 feet above sea level in its eastern part and about 5,800 feet above sea level in its western part; a broad lowland
area separates the two parts. Kahoolawe is the smallest of the eight major islands and is only about 1,500 feet above sea
level in its eastern, highest part. Lanai is about 3,400 feet above sea level in its highest part, but much of the island is
less than 1,000 feet above sea level. Molokai is mountainous in its eastern half, where it rises to about 5,000 feet above
sea level, but most of the island is less than 1,000 feet above sea level. Oahu has a mountainous ridge along its eastern
side and another mountainous area along the western side, where it rises to about 4,000 feet above sea level; however,
most of Oahu is less than 1,000 feet above sea level. Kauai is about 5,200 feet above sea level in its central part, but
from the base of the mountains shoreward, large areas of the island are less than 1,000 feet above sea level in the
southern, eastern, and northern parts. Niihau is mostly less than 1,000 feet above sea level, except for a narrow ridge
about 1,300 feet above sea level along its northeastern side. The topography of each island has a profound effect on
development and climate.

Climatic Effects
The Hawaiian islands are near the northern margin of the tropics, and because of the prevailing northeast tradewinds and
the buffering effect of the surrounding ocean, air temperature at a given location in Hawaii is generally equable. At the
Honolulu International Airport, for example, the warmest month of the year is August, which has a mean temperature of
80.5 degrees Fahrenheit, and the coolest month is February, which has a mean temperature of 72.0 degrees Fahrenheit.
Air temperature can vary greatly from one location to another in Hawaii. The air temperature in the eight-island group can
range from about 95 degrees Fahrenheit at sea level to below freezing at the top of some peaks on the island of Hawaii.
In the geologic past, these peaks have been glaciated. 

Northeasterly tradewinds are present about 85 to 95 percent of the time during the summer months (May through
September), and 50 to 80 percent of the time during the winter months (October through April). The tradewinds are
occasionally interrupted by large-scale storm systems which pass near the islands. The southwestern parts of some
islands receive most of their rainfall from these severe storms, which produce a relatively uniform spatial distribution of
precipitation. In general, the northeastern, or windward sides of the islands are wettest (fig. 34). This pattern is
controlled by the orographic lifting of moisture-laden northeasterly tradewinds along the windward slopes of the islands.
The winds blow across open ocean before arriving at the islands; when the moisture-laden air mass rises over the
mountains, the moisture condenses as precipitation. Maximum rainfall occurs between altitudes of 2,000 and 6,000 feet
above sea level, but exact amounts vary depending on the form, location, and topography of each island. Above 6,000 feet,
precipitation decreases and the highest altitudes are semiarid. High mountain areas are dry because the upslope flow of
moist air is prevented from penetrating above altitudes of about 6,000 to 8,000 feet by a temperature inversion. Areas
that are leeward (southwest) of mountain barriers are generally dry because air is desiccated during its ascent over an
upwind orographic barrier. This is known as the rain-shadow effect.

On Kauai, the island summit receives more than 435 inches of average annual rainfall (1916?83). West Maui has a small
area where average annual rainfall is greater than 355 inches. Average annual rainfall is greater than 275 inches on the
northeastern parts of Maui and Oahu, and greater than 235 inches on the northeastern part of the island of Hawaii.
Because the island of Lanai is in the rain shadow of Maui and Molokai, it receives much less rain than the larger islands.
Most of the southwestern coastal areas of all islands receive less than 40 inches of rain annually; the island of Hawaii has
areas at high altitudes that receive less than 20 inches.

Two rainfall seasons are typical-a wet season during the winter months from October through April and a dry season
during the summer months from May through September. An exception is the western side of the island of Hawaii, where
summer months are wettest because of a thermally driven sea breeze.

Evapotranspiration, which is the loss of water to the atmosphere by the combination of transpiration of plants and direct
evaporation from land and water surfaces, is a major component of the hydrologic budget of the islands. In the Honolulu
area of Oahu, for example, actual evapotranspiration was estimated to be about 40 percent of the total water (rainfall
plus irrigation) falling on or applied to the ground surface during 1946?75. Pan evaporation is the main measurement used
in Hawaii to assess the amount of water loss by evapotranspiration. Over the open ocean, the estimated annual
pan-evaporation rate is 65 inches. As with precipitation, pan-evaporation rates in Hawaii are related to topography. At
altitudes between 2,000 and 4,000 feet, where humidity is high and sunlight intensity is reduced because of clouds,
pan-evaporation rates are reduced to as low as 25 percent of the open-ocean rate. In the leeward coastal areas, wind
carrying dry, warm air increases annual pan-evaporation rates to as much as 100 inches. At the summits of Mauna Kea and
Mauna Loa on the island of Hawaii, annual pan-evaporation rates exceed 70 inches because of clear skies and dry air.

The amount of recharge available to enter the aquifers on an annual basis is about equal to average annual precipitation
minus water losses (average annual runoff and evapotranspiration). Runoff is directly related to rainfall, topography, soil
type, and land use, and ranges from less than 5 to as much as 200 inches per year. Runoff typically averages about 10 to
40 percent of the average annual precipitation, but is greater than average where precipitation is high and slopes are
steep and where precipitation falls on less-permeable land surfaces. Runoff is less than average where low amounts of
precipitation fall on gentle slopes or where precipitation falls on highly permeable soils or rocks. Streams generally are
small and have steep gradients, and many flow only immediately after periods of rainfall. Some streams, however, receive
water from aquifers and have perennial flow.

Areal Distribution of Aquifers
The rocks of the Hawaiian islands can be grouped into two general hydrogeologic categories. The principal aquifers occur in
volcanic rocks ranging in age from Miocene to Holocene. Less-important aquifers occur in Quaternary-age sedimentary
deposits of alluvium, coralline limestone, and cemented beach or dune sand. Volcanic-rock aquifers are found throughout
the eight major islands (fig. 35) and are locally overlain by sedimentary deposits. The areas where sedimentary deposits
are at the land surface on the eight major islands are shown in figure 35.

Volcanic-rock aquifers are by far the most extensive and productive aquifers in the Hawaiian islands. These aquifers are
formed by layered sequences of permeable basalt. Less-productive volcanic-rock aquifers are formed by sequences of
less-permeable, thick-bedded basalt. The basalt found in some areas, such as much of Kahoolawe, Niihau, and the western
third of Molokai, may be permeable, but yields little potable water mainly because these areas receive little recharge.

Consolidated sedimentary deposits are found mostly in the coastal areas. The limestone is highly permeable in many places
and usually yields brackish water or saltwater because of good hydraulic connection with the ocean and because of low
recharge to the limestone. The brackish water is used for cooling and industrial purposes, particularly in southern Oahu.
In addition, treated wastewater is injected into the limestone where it contains brackish water or saltwater. Coralline
limestone overlies much of the isthmus area of Maui, but these rocks are not a significant source of potable water.

The unconsolidated sedimentary deposits consist of alluvium, beach and dune sand, and lagoonal mud and clayey sand. In
some places, these deposits are interbedded with consolidated rocks. Sedimentary deposits, as well as weathered
volcanic rocks are important to the ground-water hydrology of the islands in some areas. The combination of weathered
volcanic rocks and overlying sedimentary material forms a low-permeability material called caprock in areas overlying
high-permeability volcanic rocks. The caprock confines water in the volcanic rocks so that, in places such as the coastal
plain of Oahu, freshwater exists in the volcanic rocks beneath brackish water or saltwater in the caprock.

The climate of the Hawaiian islands has a profound effect on weathering processes that affect the hydraulic properties
of sedimentary deposits and volcanic rocks (especially ash and tuff). The permeability of the sediments and volcanic rocks
can be greatly reduced by chemical weathering. During the weathering process, original pore spaces are closed by swelling
of mineral particles as chemical changes cause the deposits and rocks to disintegrate. Weathering processes consist
chiefly of oxidation, hydration, and carbonation (reaction with carbon dioxide) of various minerals in the rocks.

Geology
A long chain of volcanoes known as the Hawaiian Ridge extends northwestward across the central Pacific Ocean. The
volcanoes are youngest in the southeast and become progressively older to the northwest. The volcanoes of the Hawaiian
Ridge have formed as a plate of the Earth's crust beneath the Pacific Ocean moves northward and westward relative to
an area of anomalously high temperature, called a hot spot, in the Earth's mantle. As a volcano moves northwestward away
from the hot spot, eruptions become less frequent, and a new volcano begins to form above the hot spot. Many of the
younger volcanoes have grown above sea level, forming islands. As islands age, they erode and subside, eventually becoming
atolls and then seamounts.

Some of the eight major Hawaiian islands, such as Kahoolawe, are composed of a single volcano, whereas Hawaii is formed
by five volcanoes. Some of the older volcanoes have not erupted for millions of years, but as many as eight of the younger
volcanoes may have erupted in the last 10,000 years. Historic eruptions have been recorded on five volcanoes: East Maui
Volcano-on the island of Maui; Hualalai, Mauna Loa, and Kilauea-on the island of Hawaii; and Loihi-a submarine volcano
currently (1998) forming to the southeast of Hawaii. Kilauea also is currently erupting. The volcanoes are called shield
volcanoes because they are shaped like broad, flattened domes.

The evolution of Hawaiian volcanoes generally progresses through four distinct stages-preshield, shield, postshield, and
rejuvenated. However, not all Hawaiian volcanoes have a postshield stage or a rejuvenated stage. The preshield stage is
the earliest, submarine phase of activity, and is known primarily from studies of Loihi. Lava from the preshield stage
consists predominantly of alkalic basalt (basalt that is low in silica and high in sodium and potassium). Lava from the
principal stage of volcano building, called the shield stage, consists of fluid tholeiitic basalts (silica-saturated basalt) that
characteristically form thin flows. This basalt forms during submarine, as well as subaerial, eruptions. A large central
caldera, or craterlike depression, can form during the preshield or shield stages and might later be partly or completely
filled during subsequent eruptions. Thousands of flows erupt from the central caldera and from two or three rift zones
that radiate out from the caldera. Intrusive dikes fed by rising magma extend down the rift zones and may erupt if they
reach the surface. The shield stage is the most voluminous phase of eruptive activity during which 95 to 98 percent of
the volcano is formed. The postshield stage is marked by a change in lava chemistry and character. Postshield-stage lava
includes alkalic basalt, and more viscous hawaiite, ankaramite, mugearite, and trachyte. Lava from the postshield stage
may erupt from locations outside of the rift zones formed during the shield stage. Postshield-stage lava forms a veneer
atop the shield-stage basalt. Eruptions of more viscous lava generally are explosive and may produce pyroclastic material
(ash, cinder, spatter, and larger blocks), as well as thick, massive lava flows. After a period of quiescence, lava such as
alkalic basalt, nephelinite, and basanite, might issue from isolated vents on the volcano during the rejuvenated stage.
Pyroclastic material can be deposited during all of the subaerial stages of eruption.


Clastic sedimentary deposits, which primarily are alluvium derived from erosion of the volcanic rocks, have accumulated on
the flanks of the islands. In some places, the clastic sediments are interbedded with coralline limestone that formed as
reef deposits in shallow marine waters.

The island of Hawaii consists of five volcanoes, discussed here from oldest to youngest (fig. 36). All of the volcanic rocks
range in age from Pleistocene to Holocene. Kohala Volcano, which forms the island's northwestern tip, consists mostly of
the shield-stage, mainly tholeiitic Pololu Volcanics and is capped by flows of the postshield-stage Hawi Volcanics. Hualalai
Volcano, which forms part of the island's west coast, is covered by the postshield-stage Hualalai Volcanics. Mauna Kea
Volcano, which is southeast of Kohala Volcano, primarily consists of the shield- and postshield-stage Hamakua Volcanics,
which is overlain by the postshield-stage Laupahoehoe Volcanics. In the central part of the island, the bottom unit of
Mauna Loa Volcano is the Ninole Basalt; which is overlain by the Kahuku Basalt; which is in turn overlain by the Kau Basalt,
the most widespread geologic unit on the island. All three units of Mauna Loa Volcano consist of shield-stage tholeiitic
basalt. Kilauea Volcano, which forms the southeastern part of the island, contains shield-stage tholeiitic basalts, the
Hilina Basalt and the younger Puna Basalt. Rift zones, marked by cones and fissures, contain numerous volcanic dikes, and
are found on all the volcanoes. Small beaches composed of thin, unconsolidated sand, some created as lava enters the
ocean (Hawaii's famous black sand beaches), fringe parts of the island's coastline.

Maui consists of two volcanoes-the older West Maui Volcano and the larger East Maui Volcano (Haleakala). The two
volcanoes are separated by an isthmus that is covered with deposits of alluvium and coralline limestone that are as much
as 5 miles wide. The Pleistocene-age rocks of West Maui Volcano consist of the mostly shield-stage Wailuku Basalt, which
is overlain by the postshield-stage Honolua Volcanics and rejuvenated-stage Lahaina Volcanics. The Pleistocene- to
Holocene-age rocks of East Maui Volcano consist of the tholeiitic, shield-stage Honomanu Basalt, which is overlain by the
postshield-stage Kula Volcanics and the younger rejuvenated-stage Hana Volcanics. The Kula Volcanics and the Hana
Volcanics are the most widespread geologic units exposed at the land surface on Maui. The East Maui Volcano has three
rift zones and the West Maui Volcano has two.

Kahoolawe is a single, relatively small volcano. The Pleistocene-age Kanapou Volcanics forms most of the island and includes
both shield-and postshield-stage lava. Rejuvenated-stage vents and small areas of alluvium are present at scattered
places near the shoreline.

The island of Lanai is a single volcano, and all of the island's shield-stage tholeiitic rocks are mapped as the
Pleistocene-age Lanai Basalt. Sedimentary deposits, predominantly alluvium but with small areas of consolidated
conglomerate, fill many of the basins in the island's interior. Dikes are present in three rift zones radiating from the
volcano's summit.

Molokai consists of two volcanoes, the older West Molokai Volcano and the larger East Molokai Volcano, joined by a plain.
Both the West and East Molokai Volcanics consist of shield- and postshield-stage volcanic rocks ranging in age from
Pliocene to Pleistocene. The widespread East Molokai Volcanics is separated into two informal members-the thick lower
member that is mostly tholeiitic basalt and the thinner upper member that contains more alkalic basalt. The
rejuvenated-stage Pleistocene-age Kalaupapa Volcanics forms a peninsula on Molokai's northern coast. Volcanic dikes are
found in the rift zones of both volcanoes.

Oahu consists of two volcanoes-the older Waianae Volcano in the west and the larger Koolau Volcano in the east. The
Pliocene-age Waianae Volcanics is divided into four members. The lower two members, the Lualualei (shield-stage) and the
Kamaileunu (shield- and postshield-stage) Members, are combined on the map in this report, and the upper Palehua and
Kolekole Members, which consist largely of alkalic basalt, are shown separately. The shield-stage tholeiitic rocks of the
younger Koolau Volcano are named the Koolau Basalt. The Pliocene-age Koolau Basalt is the most widespread geologic unit
exposed on Oahu. Rejuvenated-stage eruptions from about 50 vents scattered on the southeastern part of Koolau Volcano
form the Honolulu Volcanics, which ranges in age from Pleistocene to Holocene. The largest rift zones in the Koolau and
Waianae Volcanoes are on a nearly parallel northwest-southeast trend: other rift zones trend north and northeast. Oahu
has larger areas of sedimentary deposits than any other island, and these deposits contain coralline limestone in coastal
areas.

The geology of Kauai is complex and the island may consist of more than one volcano. Two geologic formations-the mainly
shield-stage Waimea Canyon Basalt and the rejuvenated-stage Koloa Volcanics, have been identified. The Waimea Canyon
Basalt, which ranges in age from Miocene to Pliocene, is divided into the Napali, the Haupu, the Olokele, and the Makaweli
Members primarily on the basis of the bedding characteristics and structural relations of the rocks. The Pliocene- to
Pleistocene-age Koloa Volcanics overlies the Waimea Canyon Basalt and is exposed at the land surface over most of the
eastern half of Kauai. Dikes are exposed throughout much of the island. Clastic sedimentary deposits, which include
lithified sand dunes, are scattered primarily around the periphery of the island.

Niihau consists of the deeply eroded remnants of a single volcano. The core of the island is the mainly shield-stage
Miocene- to Pliocene-age Paniau Basalt, which is intruded by dikes and overlain by the rejuvenated-stage Pliocene- to
Pleistocene-age Kiekie Basalt. Alluvial deposits are extensive on Niihau, and some of the alluvium is consolidated.

Ground-Water Occurrence and Movement
Certain geologic and hydrologic characteristics of the Hawaiian islands favor the occurrence and retention of freshwater.
The larger islands have extremely productive freshwater aquifers. However, the geologic and hydrologic characteristics
of the aquifers vary widely. The modes of emplacement of the volcanic rocks and sedimentary deposits and the
subsequent weathering processes to which they have been subjected have resulted in a wide range of the hydraulic
properties that control the storage and flow of water. Sedimentary deposits and some types of volcanic rocks (chiefly
pyroclastic material) that typically are considered to be productive aquifers in much of the conterminous United States
are commonly confining units or relatively poor aquifers in the Hawaiian islands. Basalt with thick lava-flow units,
weathered ash and tuff beds, and unconsolidated coastal-plain and valley-fill sedimentary deposits generally are of low
permeability, and impede the seaward and lateral movement of freshwater (defined in this report as water that contains
less than 1,000 milligrams per liter dissolved solids).

The largest bodies of fresh ground water float on saltwater within the aquifers. This occurrence is known as a
freshwater lens because of the lenticular shape of such bodies of water. The Ghyben?Herzberg principle is named after
two scientists who independently described a freshwater?saltwater relation for conditions in which the two fluids do not
mix and the freshwater is in static equilibrium with the ocean. In a static freshwater or Ghyben-Herzberg lens, the
thickness of the freshwater lens below sea level is directly proportional to the height of the top of freshwater above
sea level. In principle, at a place where the water table stands 1 foot above sea level, for example, 40 feet of freshwater
will be below sea level, and the freshwater lens will thus be 41 feet thick. This relation exists because seawater is about
one-fortieth more dense than freshwater. In most field situations, the lower limit of the freshwater is not a sharp
boundary because mixing creates a zone of transition that separates freshwater from the saltwater body. The transition
zone contains brackish water (water that contains between 1,000 and 35,000 milligrams per liter dissolved solids) and
can be quite thick (several tens to hundreds of feet) depending on the extent of mixing. In many cases, the depth
predicted by the Ghyben-Herzberg principle is about the depth where the brackish water in the transition zone has a
dissolved-solids concentration about 50 percent of seawater.

On a small island that receives little precipitation or is made up of rocks that are highly permeable, the water level is
just above sea level and the thickness of freshwater below sea level may be very thin. In some places where freshwater
is significantly mixed with saltwater, brackish water may exist immediately below the water table (fig. 37A). Where an
aquifer receives more rainfall or is less permeable, the freshwater lens is thicker (fig. 37B). The regional movement of
fresh ground water is from interior areas toward the ocean, and all of the water discharges diffusely to the ocean or at
springs near sea level.

In some coastal areas, such as southern Oahu, highly permeable volcanic-rock aquifers are overlain by a confining unit,
called caprock, that consists of unconsolidated and consolidated sediments and weathered volcanic rock (fig. 37C). The low
overall permeability of the caprock impedes ground-water discharge to the ocean and results in a freshwater wedge
inland that is thicker than it would be in the absence of the caprock. In places with a caprock, inland ground-water levels
are at higher altitudes and the freshwater lens is significantly thicker than in places without a caprock.

For a given recharge rate, freshwater hydraulic heads will be lower in high-permeability rocks than in low-permeability
rocks. In the most permeable volcanic rocks, the water table is generally no more than several tens of feet above sea
level, indicative of a freshwater lens. In low-permeability volcanic rocks, ground-water flow is impeded to a greater
extent, and higher water levels result. Water levels in these rocks are commonly greater than several tens of feet above
sea level, and the rocks are fully saturated below the water table.

In some low-permeability volcanic-rock aquifers, such as in eastern Kauai (fig. 38), a vertically extensive freshwater-lens
system develops with freshwater standing several hundreds or even thousands of feet above sea level. In such vertically
extensive freshwater-lens systems, substantial vertical, freshwater hydraulic-head gradients exist in the aquifer and
the Ghyben-Herzberg principle is not valid for predicting the depth at which saltwater lies beneath freshwater. Much of
the fresh ground water in a vertically extensive freshwater-lens system discharges directly to stream valleys above sea
level where the ground surface intersects the water table.

Dike-impounded water is an important source of low-salinity water on some of the islands. Sub-vertical dike systems tend
to compartmentalize areas of permeable volcanic rocks, chiefly in the rift zones or caldera of a volcano. The dikes and
rocks they intrude are known in Hawaii as dike complexes. Dikes impound water to great heights, as much as 3,300 feet
above sea level on the islands of Maui and Hawaii and as much as 1,600 feet above sea level on Oahu. The depth to which
freshwater extends below sea level within a dike complex is not known. Where dikes have been eroded or fractured,
springs might issue from openings in the dikes. Shafts in dike complexes are particularly important sources of freshwater
on the eastern side of Oahu, where much of the island's precipitation falls along the dike complex in the Koolau Range.

Perched water can occur in areas where low-permeability rocks impede the downward movement of ground water
sufficiently to allow a saturated water body to develop over unsaturated rocks. These low-permeability rocks include
massive, thick-bedded lava flows and extensive soil and weathered ash layers. Some perched water bodies supply usable
quantities of water to wells.

The occurrence of ground water in the volcanic-rock aquifers of Oahu is summarized in figure 39. A freshwater lens
underlies much of Oahu. Well A, which is nearest the coast, produces saltwater from below the transition zone, and well B
produces brackish water from the transition zone. Well C is the inland-most well and produces freshwater. Horizontal
shaft D (sometimes called a Maui shaft) has been dug into the volcanic rocks along and just below the water table and
produces large volumes of freshwater by skimming water from near the top of the freshwater lens (fig. 39). Shafts E and
F (sometimes called Lanai shafts) are dug horizontally into one or more of the dike-bounded compartments. Location G
(fig. 39) indicates a perched water body containing minor amounts of water.

Fresh ground water generally moves from topographically high areas towards the ocean (fig. 39). Fresh ground-water flow
is predominantly downward in the inland areas, upward in the coastal areas, and horizontal in between. Ground water from
the dike compartments recharges downgradient freshwater lenses. A saltwater circulation system exists beneath the
freshwater lens (fig. 39). Saltwater flows landward in the deeper parts of the aquifer, rises, then mixes with fresher
water and discharges to the ocean.

The occurrence of fresh ground water in each of the Hawaiian islands can be depicted using water levels measured in
wells, shafts, and springs as shown in figure 40. Water levels less than 50 feet above sea level were arbitrarily chosen to
show occurrences of freshwater lenses for figure 40. Water levels greater than 50 feet above sea level were chosen to
show areas where vertically extensive freshwater-lens systems or dike-impounded water exist. Non-dike-intruded areas
containing wells that penetrate below sea level and that have high water levels are considered to have vertically
extensive freshwater-lens systems. Where high water levels are found in wells that do not penetrate below sea level,
the possibility of a perched-water system cannot be ruled out. Although many of the ground-water systems of the islands
are well understood, exploration in others is
The Hawaiian islands are the exposed parts of the Hawaiian Ridge, which is a large volcanic mountain range on the sea
floor. Most of the Hawaiian Ridge is below sea level (fig. 31). The State of Hawaii consists of a group of 132 islands, reefs,
and shoals that extend for more than 1,500 miles from southeast to northwest across the central Pacific Ocean between
about 155 and 179 degrees west longitude and about 19 to 28 degrees north latitude. The main inhabited islands are at
the southeastern end of the group (fig. 31); not all the small islands, reefs, and shoals included in the State are shown.

The Hawaiian islands are geologically youngest in the southeast and oldest in the northwest. This report discusses only
the eight largest islands near the southeastern end of the group; these eight main islands account for practically all of
the 6,426-square-mile land area of the State. The eight islands and their approximate size, in square miles, from
southeast to northwest are Hawaii, 4,021; Maui, 728; Kahoolawe, 45; Lanai, 141; Molokai, 259; Oahu, 603; Kauai, 553; and
Niihau, 71. The total resident population in 1995 was 1,179,198, of which about 75 percent were on the island of Oahu.
Honolulu, which is on Oahu, is the largest and most developed city and had a population of 369,485 in 1995. In addition to
the resident population, a visitor population of about 150,000 has typically been present at any given time during the
1990's. Many of these visitors stay in Honolulu. 
The State Land Use Commission is responsible for classifying the lands of the State into one of four categories called
districts: conservation, agricultural, urban, or rural (fig. 32). In 1995, conservation, agricultural, urban, and rural districts
accounted for about 48, 47, 5, and 0.2 percent of the land area in the State, respectively. Conservation districts include
areas necessary for protecting the State's watersheds and water resources and are typically located in high-altitude,
high-rainfall areas. Much of the urban development in Hawaii is in the lowland coastal areas of each island. Agricultural
irrigation can place large demands on the water resources; prior to the 1990's, one of the largest uses of water was for
sugarcane irrigation. The five largest islands (Hawaii, Maui, Molokai, Oahu, and Kauai) have extensive areas of mountainous
land where urbanization and large-scale agricultural operations are not feasible.

The island of Hawaii is the largest island of the State (fig. 33) and has the highest altitude at 13,796 feet. Maui is about
10,000 feet above sea level in its eastern part and about 5,800 feet above sea level in its western part; a broad lowland
area separates the two parts. Kahoolawe is the smallest of the eight major islands and is only about 1,500 feet above sea
level in its eastern, highest part. Lanai is about 3,400 feet above sea level in its highest part, but much of the island is
less than 1,000 feet above sea level. Molokai is mountainous in its eastern half, where it rises to about 5,000 feet above
sea level, but most of the island is less than 1,000 feet above sea level. Oahu has a mountainous ridge along its eastern
side and another mountainous area along the western side, where it rises to about 4,000 feet above sea level; however,
most of Oahu is less than 1,000 feet above sea level. Kauai is about 5,200 feet above sea level in its central part, but
from the base of the mountains shoreward, large areas of the island are less than 1,000 feet above sea level in the
southern, eastern, and northern parts. Niihau is mostly less than 1,000 feet above sea level, except for a narrow ridge
about 1,300 feet above sea level along its northeastern side. The topography of each island has a profound effect on
development and climate.

Climatic Effects
The Hawaiian islands are near the northern margin of the tropics, and because of the prevailing northeast tradewinds and
the buffering effect of the surrounding ocean, air temperature at a given location in Hawaii is generally equable. At the
Honolulu International Airport, for example, the warmest month of the year is August, which has a mean temperature of
80.5 degrees Fahrenheit, and the coolest month is February, which has a mean temperature of 72.0 degrees Fahrenheit.
Air temperature can vary greatly from one location to another in Hawaii. The air temperature in the eight-island group can
range from about 95 degrees Fahrenheit at sea level to below freezing at the top of some peaks on the island of Hawaii.
In the geologic past, these peaks have been glaciated. 

Northeasterly tradewinds are present about 85 to 95 percent of the time during the summer months (May through
September), and 50 to 80 percent of the time during the winter months (October through April). The tradewinds are
occasionally interrupted by large-scale storm systems which pass near the islands. The southwestern parts of some
islands receive most of their rainfall from these severe storms, which produce a relatively uniform spatial distribution of
precipitation. In general, the northeastern, or windward sides of the islands are wettest (fig. 34). This pattern is
controlled by the orographic lifting of moisture-laden northeasterly tradewinds along the windward slopes of the islands.
The winds blow across open ocean before arriving at the islands; when the moisture-laden air mass rises over the
mountains, the moisture condenses as precipitation. Maximum rainfall occurs between altitudes of 2,000 and 6,000 feet
above sea level, but exact amounts vary depending on the form, location, and topography of each island. Above 6,000 feet,
precipitation decreases and the highest altitudes are semiarid. High mountain areas are dry because the upslope flow of
moist air is prevented from penetrating above altitudes of about 6,000 to 8,000 feet by a temperature inversion. Areas
that are leeward (southwest) of mountain barriers are generally dry because air is desiccated during its ascent over an
upwind orographic barrier. This is known as the rain-shadow effect.

On Kauai, the island summit receives more than 435 inches of average annual rainfall (1916?83). West Maui has a small
area where average annual rainfall is greater than 355 inches. Average annual rainfall is greater than 275 inches on the
northeastern parts of Maui and Oahu, and greater than 235 inches on the northeastern part of the island of Hawaii.
Because the island of Lanai is in the rain shadow of Maui and Molokai, it receives much less rain than the larger islands.
Most of the southwestern coastal areas of all islands receive less than 40 inches of rain annually; the island of Hawaii has
areas at high altitudes that receive less than 20 inches.

Two rainfall seasons are typical-a wet season during the winter months from October through April and a dry season
during the summer months from May through September. An exception is the western side of the island of Hawaii, where
summer months are wettest because of a thermally driven sea breeze.

Evapotranspiration, which is the loss of water to the atmosphere by the combination of transpiration of plants and direct
evaporation from land and water surfaces, is a major component of the hydrologic budget of the islands. In the Honolulu
area of Oahu, for example, actual evapotranspiration was estimated to be about 40 percent of the total water (rainfall
plus irrigation) falling on or applied to the ground surface during 1946?75. Pan evaporation is the main measurement used
in Hawaii to assess the amount of water loss by evapotranspiration. Over the open ocean, the estimated annual
pan-evaporation rate is 65 inches. As with precipitation, pan-evaporation rates in Hawaii are related to topography. At
altitudes between 2,000 and 4,000 feet, where humidity is high and sunlight intensity is reduced because of clouds,
pan-evaporation rates are reduced to as low as 25 percent of the open-ocean rate. In the leeward coastal areas, wind
carrying dry, warm air increases annual pan-evaporation rates to as much as 100 inches. At the summits of Mauna Kea and
Mauna Loa on the island of Hawaii, annual pan-evaporation rates exceed 70 inches because of clear skies and dry air.

The amount of recharge available to enter the aquifers on an annual basis is about equal to average annual precipitation
minus water losses (average annual runoff and evapotranspiration). Runoff is directly related to rainfall, topography, soil
type, and land use, and ranges from less than 5 to as much as 200 inches per year. Runoff typically averages about 10 to
40 percent of the average annual precipitation, but is greater than average where precipitation is high and slopes are
steep and where precipitation falls on less-permeable land surfaces. Runoff is less than average where low amounts of
precipitation fall on gentle slopes or where precipitation falls on highly permeable soils or rocks. Streams generally are
small and have steep gradients, and many flow only immediately after periods of rainfall. Some streams, however, receive
water from aquifers and have perennial flow.

Areal Distribution of Aquifers
The rocks of the Hawaiian islands can be grouped into two general hydrogeologic categories. The principal aquifers occur in
volcanic rocks ranging in age from Miocene to Holocene. Less-important aquifers occur in Quaternary-age sedimentary
deposits of alluvium, coralline limestone, and cemented beach or dune sand. Volcanic-rock aquifers are found throughout
the eight major islands (fig. 35) and are locally overlain by sedimentary deposits. The areas where sedimentary deposits
are at the land surface on the eight major islands are shown in figure 35.

Volcanic-rock aquifers are by far the most extensive and productive aquifers in the Hawaiian islands. These aquifers are
formed by layered sequences of permeable basalt. Less-productive volcanic-rock aquifers are formed by sequences of
less-permeable, thick-bedded basalt. The basalt found in some areas, such as much of Kahoolawe, Niihau, and the western
third of Molokai, may be permeable, but yields little potable water mainly because these areas receive little recharge.

Consolidated sedimentary deposits are found mostly in the coastal areas. The limestone is highly permeable in many places
and usually yields brackish water or saltwater because of good hydraulic connection with the ocean and because of low
recharge to the limestone. The brackish water is used for cooling and industrial purposes, particularly in southern Oahu.
In addition, treated wastewater is injected into the limestone where it contains brackish water or saltwater. Coralline
limestone overlies much of the isthmus area of Maui, but these rocks are not a significant source of potable water.

The unconsolidated sedimentary deposits consist of alluvium, beach and dune sand, and lagoonal mud and clayey sand. In
some places, these deposits are interbedded with consolidated rocks. Sedimentary deposits, as well as weathered
volcanic rocks are important to the ground-water hydrology of the islands in some areas. The combination of weathered
volcanic rocks and overlying sedimentary material forms a low-permeability material called caprock in areas overlying
high-permeability volcanic rocks. The caprock confines water in the volcanic rocks so that, in places such as the coastal
plain of Oahu, freshwater exists in the volcanic rocks beneath brackish water or saltwater in the caprock.

The climate of the Hawaiian islands has a profound effect on weathering processes that affect the hydraulic properties
of sedimentary deposits and volcanic rocks (especially ash and tuff). The permeability of the sediments and volcanic rocks
can be greatly reduced by chemical weathering. During the weathering process, original pore spaces are closed by swelling
of mineral particles as chemical changes cause the deposits and rocks to disintegrate. Weathering processes consist
chiefly of oxidation, hydration, and carbonation (reaction with carbon dioxide) of various minerals in the rocks.

Geology
A long chain of volcanoes known as the Hawaiian Ridge extends northwestward across the central Pacific Ocean. The
volcanoes are youngest in the southeast and become progressively older to the northwest. The volcanoes of the Hawaiian
Ridge have formed as a plate of the Earth's crust beneath the Pacific Ocean moves northward and westward relative to
an area of anomalously high temperature, called a hot spot, in the Earth's mantle. As a volcano moves northwestward away
from the hot spot, eruptions become less frequent, and a new volcano begins to form above the hot spot. Many of the
younger volcanoes have grown above sea level, forming islands. As islands age, they erode and subside, eventually becoming
atolls and then seamounts.

Some of the eight major Hawaiian islands, such as Kahoolawe, are composed of a single volcano, whereas Hawaii is formed
by five volcanoes. Some of the older volcanoes have not erupted for millions of years, but as many as eight of the younger
volcanoes may have erupted in the last 10,000 years. Historic eruptions have been recorded on five volcanoes: East Maui
Volcano-on the island of Maui; Hualalai, Mauna Loa, and Kilauea-on the island of Hawaii; and Loihi-a submarine volcano
currently (1998) forming to the southeast of Hawaii. Kilauea also is currently erupting. The volcanoes are called shield
volcanoes because they are shaped like broad, flattened domes.

The evolution of Hawaiian volcanoes generally progresses through four distinct stages-preshield, shield, postshield, and
rejuvenated. However, not all Hawaiian volcanoes have a postshield stage or a rejuvenated stage. The preshield stage is
the earliest, submarine phase of activity, and is known primarily from studies of Loihi. Lava from the preshield stage
consists predominantly of alkalic basalt (basalt that is low in silica and high in sodium and potassium). Lava from the
principal stage of volcano building, called the shield stage, consists of fluid tholeiitic basalts (silica-saturated basalt) that
characteristically form thin flows. This basalt forms during submarine, as well as subaerial, eruptions. A large central
caldera, or craterlike depression, can form during the preshield or shield stages and might later be partly or completely
filled during subsequent eruptions. Thousands of flows erupt from the central caldera and from two or three rift zones
that radiate out from the caldera. Intrusive dikes fed by rising magma extend down the rift zones and may erupt if they
reach the surface. The shield stage is the most voluminous phase of eruptive activity during which 95 to 98 percent of
the volcano is formed. The postshield stage is marked by a change in lava chemistry and character. Postshield-stage lava
includes alkalic basalt, and more viscous hawaiite, ankaramite, mugearite, and trachyte. Lava from the postshield stage
may erupt from locations outside of the rift zones formed during the shield stage. Postshield-stage lava forms a veneer
atop the shield-stage basalt. Eruptions of more viscous lava generally are explosive and may produce pyroclastic material
(ash, cinder, spatter, and larger blocks), as well as thick, massive lava flows. After a period of quiescence, lava such as
alkalic basalt, nephelinite, and basanite, might issue from isolated vents on the volcano during the rejuvenated stage.
Pyroclastic material can be deposited during all of the subaerial stages of eruption.


Clastic sedimentary deposits, which primarily are alluvium derived from erosion of the volcanic rocks, have accumulated on
the flanks of the islands. In some places, the clastic sediments are interbedded with coralline limestone that formed as
reef deposits in shallow marine waters.

The island of Hawaii consists of five volcanoes, discussed here from oldest to youngest (fig. 36). All of the volcanic rocks
range in age from Pleistocene to Holocene. Kohala Volcano, which forms the island's northwestern tip, consists mostly of
the shield-stage, mainly tholeiitic Pololu Volcanics and is capped by flows of the postshield-stage Hawi Volcanics. Hualalai
Volcano, which forms part of the island's west coast, is covered by the postshield-stage Hualalai Volcanics. Mauna Kea
Volcano, which is southeast of Kohala Volcano, primarily consists of the shield- and postshield-stage Hamakua Volcanics,
which is overlain by the postshield-stage Laupahoehoe Volcanics. In the central part of the island, the bottom unit of
Mauna Loa Volcano is the Ninole Basalt; which is overlain by the Kahuku Basalt; which is in turn overlain by the Kau Basalt,
the most widespread geologic unit on the island. All three units of Mauna Loa Volcano consist of shield-stage tholeiitic
basalt. Kilauea Volcano, which forms the southeastern part of the island, contains shield-stage tholeiitic basalts, the
Hilina Basalt and the younger Puna Basalt. Rift zones, marked by cones and fissures, contain numerous volcanic dikes, and
are found on all the volcanoes. Small beaches composed of thin, unconsolidated sand, some created as lava enters the
ocean (Hawaii's famous black sand beaches), fringe parts of the island's coastline.

Maui consists of two volcanoes-the older West Maui Volcano and the larger East Maui Volcano (Haleakala). The two
volcanoes are separated by an isthmus that is covered with deposits of alluvium and coralline limestone that are as much
as 5 miles wide. The Pleistocene-age rocks of West Maui Volcano consist of the mostly shield-stage Wailuku Basalt, which
is overlain by the postshield-stage Honolua Volcanics and rejuvenated-stage Lahaina Volcanics. The Pleistocene- to
Holocene-age rocks of East Maui Volcano consist of the tholeiitic, shield-stage Honomanu Basalt, which is overlain by the
postshield-stage Kula Volcanics and the younger rejuvenated-stage Hana Volcanics. The Kula Volcanics and the Hana
Volcanics are the most widespread geologic units exposed at the land surface on Maui. The East Maui Volcano has three
rift zones and the West Maui Volcano has two.

Kahoolawe is a single, relatively small volcano. The Pleistocene-age Kanapou Volcanics forms most of the island and includes
both shield-and postshield-stage lava. Rejuvenated-stage vents and small areas of alluvium are present at scattered
places near the shoreline.

The island of Lanai is a single volcano, and all of the island's shield-stage tholeiitic rocks are mapped as the
Pleistocene-age Lanai Basalt. Sedimentary deposits, predominantly alluvium but with small areas of consolidated
conglomerate, fill many of the basins in the island's interior. Dikes are present in three rift zones radiating from the
volcano's summit.

Molokai consists of two volcanoes, the older West Molokai Volcano and the larger East Molokai Volcano, joined by a plain.
Both the West and East Molokai Volcanics consist of shield- and postshield-stage volcanic rocks ranging in age from
Pliocene to Pleistocene. The widespread East Molokai Volcanics is separated into two informal members-the thick lower
member that is mostly tholeiitic basalt and the thinner upper member that contains more alkalic basalt. The
rejuvenated-stage Pleistocene-age Kalaupapa Volcanics forms a peninsula on Molokai's northern coast. Volcanic dikes are
found in the rift zones of both volcanoes.

Oahu consists of two volcanoes-the older Waianae Volcano in the west and the larger Koolau Volcano in the east. The
Pliocene-age Waianae Volcanics is divided into four members. The lower two members, the Lualualei (shield-stage) and the
Kamaileunu (shield- and postshield-stage) Members, are combined on the map in this report, and the upper Palehua and
Kolekole Members, which consist largely of alkalic basalt, are shown separately. The shield-stage tholeiitic rocks of the
younger Koolau Volcano are named the Koolau Basalt. The Pliocene-age Koolau Basalt is the most widespread geologic unit
exposed on Oahu. Rejuvenated-stage eruptions from about 50 vents scattered on the southeastern part of Koolau Volcano
form the Honolulu Volcanics, which ranges in age from Pleistocene to Holocene. The largest rift zones in the Koolau and
Waianae Volcanoes are on a nearly parallel northwest-southeast trend: other rift zones trend north and northeast. Oahu
has larger areas of sedimentary deposits than any other island, and these deposits contain coralline limestone in coastal
areas.

The geology of Kauai is complex and the island may consist of more than one volcano. Two geologic formations-the mainly
shield-stage Waimea Canyon Basalt and the rejuvenated-stage Koloa Volcanics, have been identified. The Waimea Canyon
Basalt, which ranges in age from Miocene to Pliocene, is divided into the Napali, the Haupu, the Olokele, and the Makaweli
Members primarily on the basis of the bedding characteristics and structural relations of the rocks. The Pliocene- to
Pleistocene-age Koloa Volcanics overlies the Waimea Canyon Basalt and is exposed at the land surface over most of the
eastern half of Kauai. Dikes are exposed throughout much of the island. Clastic sedimentary deposits, which include
lithified sand dunes, are scattered primarily around the periphery of the island.

Niihau consists of the deeply eroded remnants of a single volcano. The core of the island is the mainly shield-stage
Miocene- to Pliocene-age Paniau Basalt, which is intruded by dikes and overlain by the rejuvenated-stage Pliocene- to
Pleistocene-age Kiekie Basalt. Alluvial deposits are extensive on Niihau, and some of the alluvium is consolidated.

Ground-Water Occurrence and Movement
Certain geologic and hydrologic characteristics of the Hawaiian islands favor the occurrence and retention of freshwater.
The larger islands have extremely productive freshwater aquifers. However, the geologic and hydrologic characteristics
of the aquifers vary widely. The modes of emplacement of the volcanic rocks and sedimentary deposits and the
subsequent weathering processes to which they have been subjected have resulted in a wide range of the hydraulic
properties that control the storage and flow of water. Sedimentary deposits and some types of volcanic rocks (chiefly
pyroclastic material) that typically are considered to be productive aquifers in much of the conterminous United States
are commonly confining units or relatively poor aquifers in the Hawaiian islands. Basalt with thick lava-flow units,
weathered ash and tuff beds, and unconsolidated coastal-plain and valley-fill sedimentary deposits generally are of low
permeability, and impede the seaward and lateral movement of freshwater (defined in this report as water that contains
less than 1,000 milligrams per liter dissolved solids).

The largest bodies of fresh ground water float on saltwater within the aquifers. This occurrence is known as a
freshwater lens because of the lenticular shape of such bodies of water. The Ghyben?Herzberg principle is named after
two scientists who independently described a freshwater?saltwater relation for conditions in which the two fluids do not
mix and the freshwater is in static equilibrium with the ocean. In a static freshwater or Ghyben-Herzberg lens, the
thickness of the freshwater lens below sea level is directly proportional to the height of the top of freshwater above
sea level. In principle, at a place where the water table stands 1 foot above sea level, for example, 40 feet of freshwater
will be below sea level, and the freshwater lens will thus be 41 feet thick. This relation exists because seawater is about
one-fortieth more dense than freshwater. In most field situations, the lower limit of the freshwater is not a sharp
boundary because mixing creates a zone of transition that separates freshwater from the saltwater body. The transition
zone contains brackish water (water that contains between 1,000 and 35,000 milligrams per liter dissolved solids) and
can be quite thick (several tens to hundreds of feet) depending on the extent of mixing. In many cases, the depth
predicted by the Ghyben-Herzberg principle is about the depth where the brackish water in the transition zone has a
dissolved-solids concentration about 50 percent of seawater.

On a small island that receives little precipitation or is made up of rocks that are highly permeable, the water level is
just above sea level and the thickness of freshwater below sea level may be very thin. In some places where freshwater
is significantly mixed with saltwater, brackish water may exist immediately below the water table (fig. 37A). Where an
aquifer receives more rainfall or is less permeable, the freshwater lens is thicker (fig. 37B). The regional movement of
fresh ground water is from interior areas toward the ocean, and all of the water discharges diffusely to the ocean or at
springs near sea level.

In some coastal areas, such as southern Oahu, highly permeable volcanic-rock aquifers are overlain by a confining unit,
called caprock, that consists of unconsolidated and consolidated sediments and weathered volcanic rock (fig. 37C). The low
overall permeability of the caprock impedes ground-water discharge to the ocean and results in a freshwater wedge
inland that is thicker than it would be in the absence of the caprock. In places with a caprock, inland ground-water levels
are at higher altitudes and the freshwater lens is significantly thicker than in places without a caprock.

For a given recharge rate, freshwater hydraulic heads will be lower in high-permeability rocks than in low-permeability
rocks. In the most permeable volcanic rocks, the water table is generally no more than several tens of feet above sea
level, indicative of a freshwater lens. In low-permeability volcanic rocks, ground-water flow is impeded to a greater
extent, and higher water levels result. Water levels in these rocks are commonly greater than several tens of feet above
sea level, and the rocks are fully saturated below the water table.

In some low-permeability volcanic-rock aquifers, such as in eastern Kauai (fig. 38), a vertically extensive freshwater-lens
system develops with freshwater standing several hundreds or even thousands of feet above sea level. In such vertically
extensive freshwater-lens systems, substantial vertical, freshwater hydraulic-head gradients exist in the aquifer and
the Ghyben-Herzberg principle is not valid for predicting the depth at which saltwater lies beneath freshwater. Much of
the fresh ground water in a vertically extensive freshwater-lens system discharges directly to stream valleys above sea
level where the ground surface intersects the water table.

Dike-impounded water is an important source of low-salinity water on some of the islands. Sub-vertical dike systems tend
to compartmentalize areas of permeable volcanic rocks, chiefly in the rift zones or caldera of a volcano. The dikes and
rocks they intrude are known in Hawaii as dike complexes. Dikes impound water to great heights, as much as 3,300 feet
above sea level on the islands of Maui and Hawaii and as much as 1,600 feet above sea level on Oahu. The depth to which
freshwater extends below sea level within a dike complex is not known. Where dikes have been eroded or fractured,
springs might issue from openings in the dikes. Shafts in dike complexes are particularly important sources of freshwater
on the eastern side of Oahu, where much of the island's precipitation falls along the dike complex in the Koolau Range.

Perched water can occur in areas where low-permeability rocks impede the downward movement of ground water
sufficiently to allow a saturated water body to develop over unsaturated rocks. These low-permeability rocks include
massive, thick-bedded lava flows and extensive soil and weathered ash layers. Some perched water bodies supply usable
quantities of water to wells.

The occurrence of ground water in the volcanic-rock aquifers of Oahu is summarized in figure 39. A freshwater lens
underlies much of Oahu. Well A, which is nearest the coast, produces saltwater from below the transition zone, and well B
produces brackish water from the transition zone. Well C is the inland-most well and produces freshwater. Horizontal
shaft D (sometimes called a Maui shaft) has been dug into the volcanic rocks along and just below the water table and
produces large volumes of freshwater by skimming water from near the top of the freshwater lens (fig. 39). Shafts E and
F (sometimes called Lanai shafts) are dug horizontally into one or more of the dike-bounded compartments. Location G
(fig. 39) indicates a perched water body containing minor amounts of water.

Fresh ground water generally moves from topographically high areas towards the ocean (fig. 39). Fresh ground-water flow
is predominantly downward in the inland areas, upward in the coastal areas, and horizontal in between. Ground water from
the dike compartments recharges downgradient freshwater lenses. A saltwater circulation system exists beneath the
freshwater lens (fig. 39). Saltwater flows landward in the deeper parts of the aquifer, rises, then mixes with fresher
water and discharges to the ocean.

The occurrence of fresh ground water in each of the Hawaiian islands can be depicted using water levels measured in
wells, shafts, and springs as shown in figure 40. Water levels less than 50 feet above sea level were arbitrarily chosen to
show occurrences of freshwater lenses for figure 40. Water levels greater than 50 feet above sea level were chosen to
show areas where vertically extensive freshwater-lens systems or dike-impounded water exist. Non-dike-intruded areas
containing wells that penetrate below sea level and that have high water levels are considered to have vertically
extensive freshwater-lens systems. Where high water levels are found in wells that do not penetrate below sea level,
the possibility of a perched-water system cannot be ruled out. Although many of the ground-water systems of the islands
are well understood, exploration in others is only just beginning, and these areas are not fully understood.

The island of Hawaii contains high water levels (greater than 50 feet above sea level) in the rift zones of Kilauea and
Kohala Volcanoes. High water levels, possibly associated with a buried rift zone of Hualalai Volcano or fault scarps draped
with lava flows, also are present along the western coast. Areas of high water levels also are found along the northern
flank and eastern flanks of Mauna Kea near Hilo and on the southeastern flank of Mauna Loa.

The central isthmus and most of the coastal areas of Maui (fig. 40) have low water levels (less than 50 feet above sea
level) indicative of a freshwater lens. High water levels are found in the interior of West Maui Volcano where rocks are
intruded by dikes. On East Maui Volcano, high water levels are found along the northern flanks of the volcano in the high
rainfall areas. Both high and low water levels occur along the northern rift zone of the volcano, indicating that a
perched-water system exists above a freshwater lens. Further to the east outside of any known rift zone, high water
levels occur in wells drilled below sea level indicating that a vertically extensive freshwater-lens system is present.

Few wells exist on Kahoolawe but because rainfall is low, the freshwater lens is probably thin. Lanai has high water levels
in the interior of the island within the rift zone and caldera complex. In the northern part of Molokai, areas of high water
levels are found in association with the northwest rift zone of East Molokai Volcano.

A large number of wells on Oahu (fig. 40) in nearshore areas around most of the periphery of the island have low water
levels. High water levels are found in rift zones near the eastern and western sides of the island and low-permeability
features create high water levels in the central part. Some small areas of perched water in the southern part of Oahu
are in alluvial deposits, but the perched water is not a significant source of supply.

Kauai has a large area with high water levels along the eastern side of the island (fig. 40). High water levels in wells that
penetrate below sea level outside of any known rift zone indicate that a vertically extensive freshwater-lens system is
present. Niihau receives little rain and data from existing wells indicate that a thin freshwater lens is present
throughout much of the island.

Water-Level Fluctuations
Water levels in wells fluctuate in response to both short- and long-term natural factors and human-induced stresses.
Short-term (diurnal time scale or shorter) water-level fluctuations are caused by ocean tides, barometric pressure
changes, evapotranspiration by phreatophytes, or earthquakes, and also by human-induced stress. Long-term fluctuations
can be caused by pumping and changes in recharge.

Changes in ground-water storage caused by withdrawals of water from distant or nearby wells are reflected by
water-level changes in wells. Data from an observation well in northern Oahu (fig. 41) show declines and recoveries of the
water table caused by intermittent pumping from nearby wells. The magnitude of the water-level decline caused by
pumping is dependent on the distance between the pumped well and the observation well, the rate at which water is
withdrawn, and the hydraulic characteristics of the rocks. In addition, ocean tides can cause water-level variations that
are superimposed on the water-level declines caused by pumping (fig. 41); ocean tides account for about 0.1 foot of
water-level fluctuation at this well. The magnitude of the water-level fluctuation caused by ocean tides is dependent on
the distance of the observation well from the coast and the hydraulic characteristics of the rocks. In addition to diurnal
and semidiurnal ocean tides, longer term variations in ocean level also affect ground-water levels.

Ground-water levels generally are highest in the winter months because of greater rainfall and reduced demand for
ground water, and decline during the summer months when demand for ground water is greatest. In the Pearl Harbor
area, where ground-water demand for agriculture historically has been high, seasonal fluctuations in water level range
from a few feet to as much as 10 feet (fig. 42). In the Honolulu area, seasonal fluctuations in water level are less
pronounced. Long-term records indicate that water levels in parts of southern Oahu reflect an overall downward trend
since the early 1900's because of increased ground-water withdrawals. In the extensively developed Honolulu area, water
levels have declined from about 43 feet above sea level in 1880 to about 20 to 25 feet above sea level during the early
1990's. In the Pearl Harbor area, water levels have declined from about 20 to 25 feet above sea level in 1910 to about 15
to 20 feet above sea level during the early 1990's.

In the Schofield area of central Oahu, where ground-water withdrawals have generally been small relative to recharge,
ground-water levels fluctuate mainly in response to changes in rainfall (fig. 43). The water-level response generally lags
the averaged rainfall by several months to a year.

Spring discharge is related to aquifer water levels. The discharge of springs that issue from the volcanic-rock aquifer in
the Pearl Harbor area of Oahu (fig. 44) varies directly with changes in the water level (hydraulic head) in the aquifer.
When the water level in the aquifer is highest, spring discharge is greatest, and, conversely, when the water level is
lowered, spring discharge decreases.

Ground-Water Quality
The source of fresh ground water in Hawaii is precipitation that originates as water evaporated from the surrounding
ocean. The water vapor condenses on salt nuclei in the atmosphere, which are also commonly of oceanic origin. Accordingly,
the rain that falls on the islands contains diluted concentrations of the same ions as those in seawater; the rainfall is
particularly enriched with sodium and chloride, the major components of seawater. Even on days when no rainfall occurs,
small concentrations of ocean salts accumulate on the land surface because the salts are transported as aerosols carried
ashore by prevailing winds. These conditions account for the fact that most ground water in the Hawaiian islands, even in
high-altitude recharge areas, contains sodium and chloride as the dominant ions.

The rainfall is altered naturally when it partly dissolves volcanic rocks and sedimentary deposits as the water moves in
the subsurface from interior areas to discharge areas near the ocean. The water acquires calcium, magnesium, sodium,
silica, and iron from volcanic rocks and alluvium. The water acquires bicarbonate and calcium as it infiltrates the
consolidated sedimentary deposits especially where these deposits are calcareous.

In general, salinity of ground water in the Hawaiian islands decreases with distance inland from the coast and increases
with depth in the aquifer. Elevated concentrations of sodium and chloride in ground water in nearshore rocks generally are
the result of mixing of fresh ground water with saltwater derived from the ocean. These mixing effects are most
pronounced in aquifers with highly permeable rocks exposed at the ocean floor because saltwater can readily flow into
such aquifers. In addition, elevated concentrations of sodium and chloride in ground water may reflect a low recharge
rate. In some of the western parts of the island of Hawaii, for example, a freshwater lens does not exist: only brackish
water overlies saltwater in the highly permeable volcanic-rock aquifer because of both low recharge and lack of a coastal
caprock. Water in coralline limestone along the southern coast of Oahu is also generally brackish because recharge is low
and because highly permeable limestone crops out at the ocean floor, allowing easy inflow of saltwater.

When water is withdrawn from a freshwater lens, the freshwater lens shrinks and saltwater will encroach or intrude
into parts of the aquifer that formerly contained freshwater. The degree of saltwater intrusion depends on several
factors, which include the hydraulic properties of the rocks, recharge rate, and pumping rate. The effect of intrusion on a
particular well depends on the vertical and lateral distance between the well and the transition zone.

In the Honolulu area of Oahu, some free-flowing artesian wells that originally produced fresh ground water were later
abandoned because of increased salinity associated with saltwater intrusion. Pumping from a well can cause the
freshwater-saltwater transition zone to rise into the pumped well. Many wells in Hawaii that are pumped at high rates or
drilled too deeply are affected by this process, resulting in increased concentrations of sodium and chloride in pumped
water.

Ground water is chemically altered as a result of human activities in developed areas. Shallow, unconfined aquifers are
most susceptible to contamination through the land surface, especially where infiltration of water from the surface
rapidly recharges the aquifers. Even deeply buried aquifers are not immune to contamination. In general, areas that
receive large amounts of rainfall or irrigation water and that have highly per- meable soils are susceptible to
ground-water contamination.

Sources of ground-water contamination that result from human activities are classified as point or nonpoint. Point sources
are specific local sites from which pollutants are discharged. Common types of point sources are cesspools, disposal wells,
landfills, industrial sites, and underground storage tanks. Nonpoint sources extend over broad areas and include
agricultural fields treated with pesticides or fertilizers and residential areas where chemicals are used near homes and
on lawns. Human activities associated with agricultural, industrial, and residential areas can have profound effects on the
quality of water in affected aquifers.

Since the early 1980's, organic-chemical contaminants associated with agricultural, industrial, and urban activities have
been detected in water samples from wells in the State (fig. 45; table 2). The chemicals 1,2-dibromo-3-chloropropane
(DBCP), 1,2-dibromoethane or ethylene dibromide (EDB), and 1,2,3-trichloropropane (TCP), which are associated with
nematicides previously used in pineapple cultivation in Hawaii, have been detected in ground-water samples from wells on
the islands of Oahu and Maui. Locations of contaminated well sites are in or downgradient from areas of past and present
pineapple cultivation. EDB contamination on Oahu also may be associated with fuel pipeline leaks. Concentrations of DBCP in
water samples ar