U.S. Geological Survey
Hydrologic atlas 730-E
Paul D. Ryder, 1996

REGIONAL SUMMARY
INTRODUCTION
The two States, Oklahoma and Texas, that compose Segment 4 of this Atlas are located in the southcentral part of the Nation. These States are drained by numerous rivers and streams, the largest being the Arkansas, the Canadian, the Red, the Sabine, the Trinity, the Brazos, the Colorado, and the Pecos Rivers and the Rio Grande. Many of these rivers and their tributaries supply large amounts of water for human use, mostly in the eastern parts of the two States. The large perennial streams in the east with their many associated impoundments coincide with areas that have dense populations. Large metropolitan areas such as Oklahoma City and Tulsa, Okla., and Dallas, Fort Worth, Houston, and Austin, Tex., are supplied largely or entirely by surface water. However, in 1985 more than 7.5 million people, or about 42 percent of the population of the two States, depended on ground water as a source of water supply. The metropolitan areas of San Antonio and El Paso, Tex., and numerous smaller communities depend largely or entirely on ground water for their source of supply. The ground water is contained in aquifers that consist of unconsolidated deposits and consolidated sedimentary rocks. This chapter describes the geology and hydrology of each of the principal aquifers throughout the twoState area.
Precipitation is the source of all the water in Oklahoma and Texas. Average annual precipitation ranges from about 8 inches per year in southwestern Texas to about 56 inches per year in southeastern Texas (fig. 1). In general, precipitation increases rather uniformly from west to east in the two States.
Much of the precipitation either flows directly into rivers and streams as overland runoff or indirectly as base flow that discharges from aquifers where the water has been stored for some time. Accordingly, the areal distribution of average annual runoff from 1951 to 1980 (fig. 2) reflects that of average annual precipitation. Average annual runoff in the twoState area ranges from about 0.2 inch in the western part of the Oklahoma panhandle and parts of west Texas to about 20 inches in southeastern Oklahoma.
Comparison of the precipitation and runoff maps shows that runoff is greater where precipitation is greater. However, precipitation is greater than runoff everywhere in the twoState area. Much of the precipitation that falls on the area is returned to the atmosphere by evapotranspiration, which is the combination of evaporation from surfacewater bodies, such as lakes and marshes, and transpiration from plants. Part of the precipitation percolates downward through the soil and permeable rocks and is available for aquifer recharge throughout the area.
Oklahoma and Texas lie within six major physiographic provinces which are differentiated on the basis of differences in landforms and geology (fig. 3). The physiographic features vary greatly and range from the low, flat Coastal Plain Province through the high, gently rolling High Plains Province to mountain ranges in the Ouachita and the Basin and Range Provinces.
MAJOR AQUIFERS
The numerous aquifers in Oklahoma and Texas are in geologic units that range from unconsolidated sand along major streams to consolidated carbonate rocks and sandstones that extend over wide areas. These aquifers are grouped into 16 major aquifers or aquifer systems on the basis of differences in their rock types and groundwater flow systems. An aquifer system is a grouping of two or more aquifers and can be of two types. One type consists of vertically stacked aquifers that are separated by confining units but are hydraulically connected‹that is, their flow systems function in a similar manner, and a change in conditions in one aquifer affects the other aquifer(s). The second type is a set of aquifers that are not physically connected but share common geologic and hydrologic characteristics and can thus be studied and described together. Both types of aquifer systems are in Segment 4.
The approximate areal extent of 15 of the major aquifers or aquifer systems at the land surface is shown in figure 4: the 16th category, alluvial aquifers along major streams, is not shown in the figure. Where they are exposed at the land surface, the aquifers generally contain water that is fresh to slightly saline. The aquifers in this chapter generally are mapped only where they contain fresh to moderately saline water, except where physical boundaries determine the aquifer limits. Salinity in this report refers to the concentration of dissolved solids in water, which commonly is used as an indicator of the general suitability of the water for human use. Recommendations by the U.S. Environmental Protection Agency state that the dissolvedsolids concentration in drinking water should not exceed 500 milligrams per liter. Water that has considerably greater concentrations can be suitable for other uses.
The terms used in this report to describe water with different concentrations of dissolved solids are as follows:

	Dissolvedsolids concentration, 
	in milligrams per liter
Freshwater		Less than 1,000
Slightly saline water		1,000 to 3,000
Moderately saline water	3,000 to 10,000
Very saline water		10,000 to 35,000
Brine		Greater than 35,000

The general term saline is used to describe water that is not fresh.
In some areas, a deep aquifer that contains fresh to slightly saline water underlies the outcrop of a major aquifer mapped here, For example, parts of the Edwards­Trinity aquifer are covered by the Pecos River Basin alluvial aquifer in northern Reeves and Pecos Counties, Tex. and by the High Plains aquifer in northern Ector, Midland, and Glasscock Counties, Tex. In addition, in some areas, alluvial aquifers along large streams cover small areas of underlying major aquifers. The rocks not classified as a major aquifer either yield little water or yield sufficient water for most uses but the areal extent of the wateryielding rocks is small.
Alluvial aquifers, which are discussed later in this chapter, are along major streams in Oklahoma and Texas. These aquifers consist of deposits of alluvium in and along stream channels, alluvial terraces that are remnants of older alluvium, and overlying windblown deposits. The deposits consist of clay, silt, sand, and gravel; only the sand and gravel yield water. The deposits range in age from Tertiary to Quaternary and are a few feet to more than 100 feet thick. The alluvial aquifers generally contain water under unconfined conditions. 
The Rio Grande aquifer system in westernmost Texas is in the Basin and Range Physiographic Province (fig. 3). The aquifer system consists of thick deposits of unconsolidated basinfill material, which is mostly sand but may include a variety of rock types and particle sizes that range from clay to boulders. 
The Pecos River Basin alluvial aquifer is in the Great Plains Physiographic Province. The aquifer consists of unconsolidated sand and gravel, some of which was deposited by streams and some by wind. The deposits locally include clay, silt, and boulders. Small amounts of gypsum and caliche, which are formed by chemical processes, are in the Pecos River Basin alluvial aquifer.  The Seymour aquifer is in the Great Plains and Central Lowland Physiographic Provinces. The aquifer consistsmainly of scattered erosional remnants of the alluvial Seymour Formation of Pleistocene age. Saturated thickness of the scattered alluvial deposits is generally less than 100 feet, but large well yields are locally obtainable. The water is used mainly for agricultural purposes. 
The High Plains aquifer is in the Great Plains Physiographic Province. The aquifer is in northwestern Oklahoma and westcentral and northwestern Texas and consists of unconsolidated clay, silt, and sand, with some gravel and caliche. The aquifer provides large amounts of irrigation water and is the most intensively pumped aquifer in Oklahoma and Texas.
The coastal lowlands aquifer system contains numerous local aquifers in a thick sequence of mostly unconsolidated Coastal Plain sediments of alternating and interfingering beds of clay, silt, sand, and gravel. The sequence is generally wedgeshaped and dips and thickens toward the Gulf of Mexico. The local aquifers consist of sand and gravel and have been grouped into five permeable zones of regional extent. Large amounts of water are withdrawn from the aquifer system for municipal, industrial, and irrigation needs. 
The Texas coastal uplands aquifer system is similar in configuration and composition to the coastal lowlands aquifer system. The two aquifer systems, which are situated in the Coastal Plain Physiographic Province, are hydraulically separated by clays of the Vicksburg and the Jackson Groups of Tertiary age, which compose a thick and effective confining unit. Large amounts of irrigation water are withdrawn from the Texas coastal uplands aquifer system in the agricultural Winter Garden area of Texas.
The Edwards­Trinity aquifer system is in rocks of Cretaceous age that are in a wide, looping band that extends across central Texas and into the southeastern corner of Oklahoma. The aquifer system is divided into three parts. In the western part of the Great Plains Physiographic Province, the Edwards­Trinity aquifer consists mostly of sandstone, sand, dolomite, and clay of Early Cretaceous age. In the south, at the contact of the Great Plains and the Coastal Plain Physiographic Provinces, the Edwards aquifer consists of limestone, dolomite, and marl. The rocks are extensively faulted, fractured, and cavernous, thus allowing the largest individual freshwater well yields in Texas and Oklahoma. The Edwards aquifer is the water supply for the city of San Antonio. The Trinity aquifer extends from the southeastern corner of Oklahoma southwestward into Uvalde County in southern Texas. The aquifer spans three physiographic provinces‹the Central Lowland, the Coastal Plain, and the Great Plains. The rocks that compose the aquifer are mostly of Early Cretaceous age. In order of dominance, they consist of sandstone, sand, clay, conglomerate, caliche, shale, limestone, and dolomite.
Several aquifers and one aquifer system in Oklahoma and northern Texas are in Paleozoic rocks; generally, they yield small amounts of water to wells. The Rush Springs aquifer in westcentral Oklahoma consists of finegrained sandstone and is used primarily for irrigation. The Blaine aquifer in southwestern Oklahoma and northern Texas consists of fractured and cavernous gypsum and associated dolomite, and supplies water for irrigation. The Central Oklahoma aquifer consists of finegrained sandstone, shale, and siltstone; it is an important source of water for suburban communities in the Oklahoma City area. The Ada­Vamoosa aquifer in eastcentral Oklahoma consists of sandstone and provides water for public and industrial use. The Rush Springs, the Blaine, the Central Oklahoma, and the Ada­Vamoosa aquifers are in the Central Lowland Physiographic Province. The Arbuckle­Simpson aquifer in southcentral Oklahoma is in the Central Lowland Physiographic Province and consists of limestone, dolomite, and sandstone. The Ozark Plateaus aquifer system in northeastern Oklahoma is in the Ozark Plateaus Physiographic Province and consists of an upper aquifer in cavernous limestone and a lower aquifer in fractured dolomite with sandy zones. 
GEOLOGY
Two general categories of sedimentary rocks comprise most of the rocks that underlie Oklahoma and Texas‹mostly consolidated rocks of Paleozoic and Mesozoic age, and semi-consolidated to unconsolidated rocks of Cenozoic age. The Paleozoic (Cambrian through Permian) and Mesozoic (Triassic through Cretaceous) sedimentary rocks crop out mostly in Oklahoma and northern, central, and westernmost Texas. Cenozoic (Paleocene and younger) rocks underlie the Great Plains in the northwestern parts of Texas and Oklahoma; they also underlie the Coastal Plain where they form a broad, arcuate, coastparallel band. Both categories of rocks have been divided into numerous formations, as shown on the correlation charts that accompany the discussions of the major aquifers in the following sections of this chapter.
The majority of the wateryielding Paleozoic and Mesozoic rocks are limestone and dolomite; however, sandstone formations are productive aquifers, especially in Oklahoma, and some water also is obtained locally from fractured shale, siltstone, and gypsum. Most Cenozoic aquifers are in clastic rocks. 
The geologic and hydrogeologic nomenclature used in this report differs from State to State because of independent geologic interpretations and varied distribution and lithology of rock units. A fairly consistent set of nomenclature, however, can be derived from the most commonly used rock names. Therefore, the nomenclature used in this report is basically a synthesis of that of the U.S. Geological Survey, the Texas Bureau of Economic Geology, and the Oklahoma Geological Survey. Individual sources for nomenclature are identified on the figures prepared for this report.
The geologic map (fig. 5) shows the distribution of rocks by major age category. Numerous geologic features, such as faults and lineaments, are not shown on the geologic map for the sake of simplicity. Where these features are important hydrologically, they will be depicted and discussed in later sections of this chapter. The geologic sections (figs. 6 through 8) show some of the major subsurface structures in Oklahoma and Texas.
FRESH GROUNDWATER WITHDRAWALS
Ground water is the source of water supply for more than 7.5 million people, or about 42 percent of the population in the twoState area. About 7,300 million gallons per day was withdrawn from all the principal aquifers during 1985; 80 percent of this amount was used in rural areas for agricultural, domestic, and commercial supplies. Withdrawals for public supplies were small and accounted for only about 16 percent of the total water withdrawn.
Total freshwater withdrawals during 1985, by county, are shown in figure 9. Counties with the largest withdrawals are those with large irrigated acreage and large population centers. About 94 percent of the ground water was withdrawn in Texas, and the remainder was withdrawn in Oklahoma. Locally, slightly saline groundwater withdrawals are included in the mapped amounts.
Total withdrawals of freshwater (including slightly saline water) during 1985 from each of the principal aquifers in the twoState area are shown in figure 10. The largest withdrawal, 4,508 million gallons per day, was from the High Plains aquifer; this is about four times as much water as was withdrawn from the second most used aquifer, the coastal lowlands aquifer system (1,090 million gallons per day), and more than 2.5 times as much water as was withdrawn from all the other principal aquifers combined. The Edwards aquifer was the third most used aquifer with a withdrawal rate of 467 million gallons per day; more than onehalf was withdrawn in Bexar County, Tex., where ground water is the source of water supply for the city of San Antonio.
About 397 million gallons per day was withdrawn from the Texas coastal uplands aquifer system, the fourth most used aquifer, during 1985. During the same year, the Trinity aquifer provided 182 million gallons per day, and 145 million gallons per day was withdrawn from the Edwards­Trinity aquifer. Withdrawals from the Rio Grande aquifer system and the Seymour aquifer during 1985 were 126 and 121 million gallons per day, respectively. The Pecos River Basin alluvial aquifer and the alluvial aquifers along major streams accounted for withdrawals of 80 and 71 million gallons per day, respectively, during 1985. Withdrawals from the mostly indurated aquifers in Paleozoic rocks in Oklahoma and northern Texas were small; during 1985, the Rush Springs, the Central Oklahoma and the Ada­Vamoosa, the Blaine, the Ozark Plateaus, and the Arbuckle­Simpson aquifers accounted for 52, 48, 24, 9, and 8 million gallons per day, respectively.

ALLUVIAL AQUIFERS ALONG MAJOR STREAMS
Introduction
Alluvial aquifers of major importance are along many of the larger streams in the twoState area (fig. 11). These streams are the Salt Fork Arkansas and the Arkansas, the Cimarron, the North Canadian, the Canadian, the Washita, the North Fork Red and the Red, the Brazos and the Neosho Rivers. The alluvial deposits, with the exception of those along the Brazos and the lower Arkansas Rivers, are limited to the Central Lowland Physiographic Province. The Brazos River deposits are in the Great Plains and the Coastal Plain Provinces, and the lower Arkansas River deposits are in the Ouachita Province.
The aquifers are generally in deposits of Quaternary age, are unconfined, and consist of sand and gravel with some clay and silt. Locally, they include deposits of Tertiary age. The aquifer materials are commonly segregated by size into lenses and beds, which can affect the movement and availability of water. Beds and lenses of sand, gravel, or mixtures of the two yield most of the water. The deposits may be more than 100 feet thick and several miles wide, much of their total thickness is saturated throughout the year, and, in many places, they yield large amounts of water.
Collectively, withdrawals from the alluvial aquifers in Oklahoma and Texas were 71 million gallons per day during 1985. The aquifers are especially important in Oklahoma, where yields of wells completed in them are generally larger than yields of wells finished in adjacent or underlying bedrock. The water in the alluvial aquifers in many places is less mineralized than water in the adjacent streams.
Deposition and downcutting by the major streams were extensive at the end of the Tertiary Period and during the Quaternary Period. Repeated deposition and erosion left remnants of alluvial deposits at higher elevations as the streams progressively lowered their beds. A series of alluvial terraces was often the result, the youngest of which might be only a few feet higher than the presentday flood plain. Alluvium, as distinguished from alluvial terraces, is the most recent material deposited within the confines of the present flood plain. The alluvial terraces and alluvium usually form a single aquifer, although some outlying alluvial terraces are hydraulically independent. Highly permeable windblown sand derived from the alluvium and alluvial terraces overlies the alluvial deposits in many places and readily stores recharge from precipitation and conducts the recharge downward.
Average annual precipitation in the areas of the alluvial deposits varies from about 22 inches in western Oklahoma to about 44 inches in eastern Oklahoma. Precipitation varies from about 32 to 46 inches in the area of the Brazos River alluvial aquifer in southeastern Texas.
Most natural recharge to the aquifers occurs as precipitation that falls directly on the alluvial deposits, infiltration of runoff from adjacent slopes, and infiltration from the streams that cross the deposits, especially during higher flows. Large, additional recharge may occur from induced stream infiltration when groundwater pumpage lowers the water table below the stream levels (fig. 12). During dry periods, water may discharge from the alluvium into the streams, thus contributing to base flow. Discharge also takes place as transpiration from phreatophytes.
The chemical quality of water in the alluvial deposits may vary between the alluvium and alluvial terraces, thus reflecting the quality of the major source of recharge. The source of recharge for the alluvium may be the river and that for the alluvial terraces may be precipitation and leakage from underlying or adjacent aquifers. 
SALT FORK ARKANSAS RIVER AND ARKANSAS RIVER
The Salt Fork Arkansas River originates in the socalled gypsum hills of southern Kansas and contains water with large concentrations of calcium sulfate as it enters Woods County, Okla. The river then receives large amounts of sodium chloride downstream from natural brine springs and salt plains in Alfalfa County, Okla. The river contains saline water, which is unsuitable for most uses, downstream to its junction with the Arkansas River.
Alluvium and alluvial terrace deposits as much as 10 miles wide and 150 feet thick are located along the entire length of the Salt Fork Arkansas River. Water in alluvium close to the river can reflect the chemical quality of the river water. For example, the water in the alluvium that formerly supplied a small city in Woods County, Okla., is a hard, calcium sulfate type. Downstream from the salt plain in Alfalfa County, water from the alluvium that supplies a small city in Grant County, Okla., had a reported chloride concentration of about 370 milligrams per liter.
The main stem of the Arkansas River also enters Oklahoma from Kansas (fig. 11). The alluvium and alluvial terraces along the Arkansas River between its confluence with the Cimarron River and Tulsa average more than 5 miles in width and 45 feet in thickness. The deposits consist mostly of sand and gravel, and the water table is generally less than 20 feet below land surface. Wells constructed mostly for irrigation use yield as much as 600 gallons per minute and average 350 gallons per minute.
Between about the mouth of the Cimarron River and the mouth of the Canadian River, the alluvial aquifer consists mostly of sand and gravel about 40 feet thick. The water table is generally from 10 to 20 feet below land surface. Direct recharge from precipitation results in ground water with smaller concentrations of dissolved solids than the river water. Yields of wells constructed mostly for irrigation purposes commonly are 300 to 500 gallons per minute.
Between the Canadian River junction and the Arkansas State line, the alluvium and alluvial terraces along the Arkansas River are about 40 feet thick and consist mostly of sand and gravel. The diagrammatic section shown in figure 13 represents conditions about 5 miles upstream from the Arkansas border. The alluvial deposits are about 5.5 miles wide at this location and average about 50 feet thick. The saturated thickness averages about 35 feet. Finer grained material, as shown in the figure, typically overlies medium to very coarse sand and gravel, and the water table slopes toward the Arkansas River from either side.
CIMARRON RIVER
The Cimarron River enters Oklahoma from Kansas (fig. 11) and flows across Permian red beds in Harper, Major, and Woodward Counties. The red beds contain thick layers of salt and gypsum that are easily dissolved and are responsible for highly mineralized surface waters and ground water in the alluvium. The alluvium on the southwestern side of the Cimarron River in these counties is a poor source of ground water. The limited supplies taht can be pumped are highly mineralized (calcium sulfate and sodium chloride); some of the water is suitable for livestock, but not for human consumption. However, the alluvial terraces on the northeastern side of the river compose one of the best aquifers in Oklahoma. The alluvial terraces extend for 110 miles from southern Woods County to western Logan County and range from 3 to 15 miles in width; average width is about 10 miles. The terraces consist of sand and gravel with some clay and sandy clay and have an average thickness of about 60 feet and an average saturated thickness of about 40 feet. The alluvial terraces are overlain by windblown sands that readily transmit recharge from local precipitation downward into the alluvial aquifer.
Water in the Cimarron River alluvial terraces is a calcium-magnesium bicarbonate type with dissolvedsolids concentrations of about 400 milligrams per liter or less. Hardness is generally less than 200 milligrams per liter. The water is suitable for municipal purposes as well as for domestic and irrigation supplies. Wells completed in the terrace deposits yield as much as 600 gallons per minute, and 100 gallons per minute usually can be obtained.
Water in the alluvial terraces generally moves toward the alluvium adjacent to the Cimarron River. Where the alluvium is recharged from the alluvial terraces with water that has low dissolvedsolids concentrations, it becomes a source for municipal supplies. Wells in the alluvium that are intensively pumped or are too near the river are subject to infiltration of highly mineralized river water.
NORTH CANADIAN RIVER
The North Canadian River originates in New Mexico and flows eastward across Oklahoma. Alluvial deposits border the river from western Texas County to eastern Beaver County in the Oklahoma Panhandle but supply little water. Between the western edge of Harper County and the northwestern corner of Blaine County, alluvial deposits are mainly on the north side of the river and consist of sand and basal gravel with some clay and silt. High alluvial terraces of Pleistocene age on the north side of the river are 1.5 to 11 miles wide and average about 70 feet thick. Low alluvial terraces of late Pleistocene age are along both sides of the river and average about 50 feet thick; the thickness of the Holocene alluvium in the flood plain adjacent to the river averages about 30 feet. The combined width of the low alluvial terraces and the alluvium ranges from 0.5 to 2 miles. Dune sands that overlie the alluvium and alluvial terraces in much of the area temporarily store recharge from local precipitation and subsequently release the recharge to the underlying deposits.
The water table in the alluvial deposits between western Harper County and northwestern Blaine County ranges from about 20 to 80 feet below land surface. The general direction of groundwater flow is toward the North Canadian River. Specific yield of the deposits is estimated to average 0.29. Specific yield is the volume of water that will drain by gravity from a given volume of soil or rock. It can be expressed as a percentage; in this example, 29 percent of the water in each volume of saturated alluvial material will drain under the influence of gravity alone. Hydraulic conductivity is a measure of the rate at which water will pass through an aquifer‹the higher the hydraulic conductivity, the more permeable the aquifer. Hydraulic conductivity of the aquifer is as much as 160 feet per day and averages 59 feet per day. Recharge by infiltration from precipitation is on the order of 1 inch per year. Wells completed in the deposits yield as much as 1,000 gallons per minute. The water is a calcium-magnesium bicarbonate type, has small concentrations of dissolved solids, and is suitable for municipal and irrigation uses. The aquifer supplies water for several small towns in the area. An estimated 18 million gallons per day was withdrawn from this segment of the alluvial aquifer during 1977.
The lithologic, hydrologic, and waterquality characteristics of the alluvial aquifer along the North Canadian River between northwestern Blaine County and Oklahoma City are similar to those described, above, but the areal extent and thickness of the aquifer are somewhat less in this reach of the river. Pumpage during 1977 was estimated to be 12 million gallons per day, or about twothirds of the rate for the upstream segment.
Alluvium along the North Canadian River from Oklahoma City to its confluence with the Canadian River in McIntosh County is about 2 to 3 miles wide and about 30 to 40 feet thick. Scattered alluvial terraces on either side of the alluvium reach a maximum width of 8 miles but usually have a width of from 2 to 3 miles. The alluvial terraces have a reported maximum thickness of about 80 feet. The alluvium and alluvial terraces consist of sand and gravel with some clay and silt. Locally, windblown sand covers the alluvium and terraces and acts to promote rapid infiltration from local precipitation. In places, the alluvium and alluvial terraces overlie aquifers in Permian and Pennsylvanian bedrock. In such places, the alluvial deposits and the upper part of the bedrock aquifers are hydraulically continuous, and the water levels are the same in the bedrock and alluvial aquifers. Water in this stretch of the North Canadian River is generally more mineralized than water in the alluvial aquifer. At times, the river level is higher than the water level in the alluvial aquifer, and river water could enter the aquifer and degrade the quality of the ground water.
Annual estimates of recharge from precipitation range from about 1 inch at Oklahoma City to about 4 inches downstream at Eufaula Lake. The water table slopes toward the river, and the aquifer contributes to stream base flow. The withdrawal rate from this segment of the alluvial aquifer was reported to be about 19 million gallons per day during 1982. This rate represents the maximum permitted withdrawal rate and is probably larger than the actual rate.
CANADIAN RIVER
The Canadian River enters Texas from New Mexico and flows eastward across the Texas Panhandle. Alluvium and alluvial terraces are located along the river from Dewey County, Okla., southeastward and eastward to the western part of McIntosh County, Okla.
Wells completed in the alluvium and alluvial terraces yield as much as 500 gallons per minute. Where the ground water is not highly mineralized, the aquifer is a source of supply for various uses. However, the chemical quality of the water is variable, and, although the aquifer can be used locally, it has little potential for widescale development.
Results of a study of the potential of the alluvial aquifer along the Canadian River near Norman, Okla. (fig. 14), indicate that the alluvial terraces contain a large amount of potable water. The alluvial terraces are about 50 feet above the flood plain. The alluvium and alluvial terraces consist of clay, silt, sand, and basal gravel and are as much as 80 feet thick. Dune sands that cover the alluvial terraces and alluvium in many places allow the ready infiltration of precipitation. Under natural conditions, movement of water in the aquifer is from recharge areas where precipitation infiltrates the alluvial terraces downgradient to discharge into the river as base flow.
Groundwater recharge in the Norman area is about 8 inches per year, or about onefourth of normal annual precipitation. The specific yield of the saturated deposits is estimated to be 15 percent, and the average hydraulic conductivity of the aquifer is 134 feet per day. 
Water in the terrace deposits is less mineralized than that in the alluvium. In places, more mineralized river water can infiltrate the alluvium, which causes sulfate, chloride, and dissolvedsolids concentrations in the ground water to exceed the limits recommended for drinking water by the U.S. Environmental Protection Agency.
WASHITA RIVER
The Washita River originates in the Texas Panhandle and flows eastward into Oklahoma and then southeastward to discharge into the Red River. Alluvium and alluvial terraces are along the Washita River mainly in Grady and Garvin Counties, Okla. 
Between the Caddo­Grady County line and southeastern Garvin County, the alluvial valley averages about 2 miles in width and has a maximum width of 3 miles. The alluvium has an average thickness of about 64 feet and a maximum thickness of 120 feet. Depth to water in the alluvium is generally less than 20 feet. Maximum thickness of the alluvial terraces is 50 feet. Wells are commonly between 50 and 100 feet in depth. Yields are about 100 to 300 gallons per minute from wells completed in the alluvium and 20 to 100 gallons per minute from wells completed in the alluvial terraces.
Recharge to the older alluvial terraces is mainly from local precipitation and runoff from adjacent uplands; generally, the older terraces are not hydraulically continuous with the younger terraces and alluvium. Discharge from the alluvium contributes to the base flow of the Washita River. During high river stages, the normal hydraulic gradient can be reversed, and river water can enter the alluvium.
Water from the alluvium and alluvial terraces is used for municipal, industrial, and irrigation supplies. The water is generally a calcium-magnesium bicarbonate type with dissolvedsolids concentrations usually less than 1,000 milligrams per liter.
NORTH FORK RED RIVER AND RED RIVER
The North Fork Red River heads just east of Amarillo in the Texas Panhandle and flows eastward into Oklahoma. Quaternary alluvium and alluvial terraces compose an aquifer of major importance along the North Fork Red River from Beckham County, Okla. at the border of the Texas Panhandle to its junction with the Red River and along the Red River eastward to Jefferson County, Okla. Alluvium and alluvial terraces are covered by dune sands in most of the area. The Quaternary deposits are underlain by poorly permeable Permian bedrock.
In central Beckham County, the extensive alluvial terraces, which consist of varying proportions of clay, silt, sand, and gravel, are mainly south of the river¹s flood plain. The maximum width of the saturated part of the deposits is about 7 miles. The terraces range from 18 to 195 feet in thickness, and average about 70 feet; the saturated part averages about 33 feet in thickness. The water table in the alluvial terraces of central Beckham County slopes toward the North Fork Red River, and water discharges from the aquifer to the river.
Wells completed in the moderately to highly permeable terrace deposits supply water for municipal, industrial, rural domestic, and agricultural uses. The common range of well yields is from 200 to 500 gallons per minute. The water is slightly saline, and concentrations of dissolved solids range from 1,000 to 2,000 milligrams per liter.
Another area of alluvium and extensive terraces is at the junction of the North Fork Red River and the Red River in western Tillman County. Alluvium and alluvial terraces in this area consist of sand and gravel with some clay and sandy clay. The alluvium has an average thickness of about 34 feet, and the alluvial terraces average about 42 feet in thickness. The alluvium along the east side of the North Fork Red River and on the north side of the Red River is generally less than 2 miles wide; the adjoining alluvial terraces are 8 to 10 miles wide. Permian red beds that have low permeability underlie and adjoin the unconsolidated deposits. The alluvial aquifers in Cotton County, Okla., and in Wilbarger, Wichita, and Clay Counties, Tex., apparently supply only small quantities of water.
The water table in the alluvial deposits in western Tillman County, Okla., and northern Wilbarger County, Tex., generally slopes toward the North Fork Red and Red Rivers. Recharge to the terrace deposits from local precipitation is estimated to be about 3 inches per year. Well yields, water quality, and water use are similar to those discussed for the aquifer in Beckham County.
BRAZOS RIVER
The Brazos River heads in New Mexico and flows southeastward across Texas to discharge into the Gulf of Mexico. Large quantities of water are available in the alluvial aquifer along the river between northern McLennan and central Fort Bend Counties, Tex. In this reach, the alluvium and alluvial terraces are as much as 8 miles wide. The alluvial terraces, which are of much less significance as a source of water than the floodplain alluvium, are as much as 75 feet thick and consist of clay, silt, sand, and gravel. The floodplain alluvium consists predominantly of gravel and fine to coarse sand, with lesser amounts of clay and silt. Generally, coarsergrained material is present in the lower part of the alluvium. Maximum thickness of the alluvium is about 100 feet, and average thickness is about 45 feet.
The deposits that compose the alluvial aquifer are of Quaternary age and are underlain by rocks that range in age from Late Cretaceous to Quaternary. The underlying rocks dip toward the Gulf of Mexico and contain several major aquifers that crop out in bands parallel to the coast. Where the Brazos River crosses these aquifers, the alluvial aquifer is hydraulically connected to them. 
Hydraulic conductivity values determined by laboratory tests on samples of the alluvium are as great as 2,400 feet per day for gravel. Estimated transmissivity values average about 5,600 feet squared per day, and the average specific yield is estimated to be about 15 percent. Transmissivity is a measure of the ease with which water will pass through an aquifer; transmissivity is hydraulic conductivity multiplied by aquifer thickness. The higher the transmissivity, the more productive the aquifer.
The water table in the alluvium ranges from less than 10 to nearly 50 feet below land surface. The water table slopes toward the river, and seepage from the alluvium contributes to stream base flow. Recharge to the alluvial aquifer is mainly from precipitation that falls directly on the flood plain and alluvial terraces; estimates of recharge range from 2 to 5 inches per year.
Diagrammatic sections for the area where the Brazos River is the boundary between Burleson and Brazos Counties, Tex. are shown in figure 15. In westcentral Brazos County and eastcentral Burleson County, the saturated part of the alluvial aquifer is about 8 miles wide, and the saturated thickness of the basal sand and gravel is as much as 50 feet (fig. 15A).
Water from most wells completed in the alluvial aquifer is used for irrigation. In addition to irrigation, the chemical quality of the water is generally suitable for domestic and livestock watering purposes, although concentrations of dissolved solids in the water commonly exceed 1,000 milligrams per liter and the water is classified as hard. An estimated 1,000 irrigation wells pump water from the alluvial aquifer; yields of most of the wells range from 250 to 500 gallons per minute. An estimated 30 million gallons per day was pumped from the Brazos River alluvial aquifer during 1985.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of fresh and slightly saline water from the collective alluvial aquifers in Oklahoma and Texas totaled about 71 million gallons per day during 1985 (fig. 16). About 53 million gallons per day was withdrawn for agricultural purposes, the principal water use. Withdrawals for public supply were about 12 million gallons per day. About 3 million gallons per day was withdrawn for domestic and commercial uses; withdrawals for industrial, mining, and thermoelectricpower uses were also about 3 million gallons per day.

RIO GRANDE AQUIFER SYSTEM
INTRODUCTION
The Rio Grande aquifer system in westernmost Texas (fig. 17) corresponds to the eastern part of the Southwest alluvial basins aquifer system, which is a large system of aquifers in alluvial basins in the southwestern United States and Mexico. These aquifers were studied as part of the U.S. Geological Survey¹s Regional AquiferSystem Analysis program and are discussed in detail in Chapter C of this Atlas. A brief description and discussion of the aquifers as they exist in Texas are presented here.
The Rio Grande aquifer system in Texas is in Culberson, El Paso, Hudspeth, Jeff Davis, and Presidio Counties. The alluvial aquifers are found in six major basins: the Mesilla, the Hueco, the Salt, the Eagle, the Red Light, and the Presidio (fig. 17).
The Rio Grande aquifer system is in the Basin and Range Physiographic Province. Vertical movement along block faults has resulted in structurally high mountain ranges that trend south and southeast and are separated by structurally low parallel basins. The basin areas are filled with thick sequences of clastic sediments that have eroded from the adjacent highlands.
The basin deposits are of late Tertiary and Quaternary age and consist mostly of clay, silt, sand, and gravel (fig. 18). They are the principal source of water for the city of El Paso and the surrounding area, where precipitation is sparse (8­12 inches annually). Although a large volume of water is stored in the basin deposits, pumpage easily exceeds natural recharge and leads to longterm depletion of the stored water. Collectively, withdrawals from the Rio Grande aquifer system in Texas were about 126 million gallons per day during 1985.
MESILLA BASIN
The Mesilla Basin lies largely in New Mexico and Mexico. A small part of the basin is in western El Paso County between the Franklin Mountains on the east and the Rio Grande on the west. The western part of El Paso, a city which had a population of 464,000 in 1985, is in the southern end of the basin. The alluvial aquifer in the Mesilla Basin is a source of water for the municipal and industrial needs of El Paso.
The alluvial deposits of the Mesilla Basin are of late Tertiary and Quaternary age and are composed of gravel, sand, silt, and clay. The deposits are predominantly coarse grained around the margins of the basin and fine grained near the basin center. The Rio Grande alluvium is part of the Mesilla Basin alluvial aquifer; it overlies the older basin fill, from which it cannot be easily distinguished. The total thickness of the unconsolidated deposits in the Mesilla Basin is estimated to be at least 2,000 feet, and the thickness of the Rio Grande alluvium is 150 feet or less.
The chemical quality of the water in the shallower part of the aquifer is influenced by the quality of the water in the Rio Grande. The water in the shallower part of the aquifer is generally more mineralized than that in the deeper part. Concentrations of dissolved solids in the shallower ground water locally are as much as several thousand milligrams per liter, whereas water from the deeper part of the aquifer commonly has dissolvedsolids concentrations that are less than 300 milligrams per liter. The depth of freshwater extends to as much as 1,400 feet below land surface. Water in the southern onehalf of the basin deposits is more mineralized than elsewhere. This could be due, in part, to the narrow valley outlet at El Paso that restricts groundwater outflow and prevents flushing of water with greater dissolvedsolids concentrations.
Wells completed in the Mesilla Basin alluvial aquifer yield as much as 3,000 gallons per minute. Transmissivity of the aquifer is several thousand feet squared per day. The aquifer receives recharge by infiltration of runoff around the basin margins, and from seepage from the Rio Grande, ephemeral streams, canals, and excess irrigation water. During 1980, about 21 million gallons per day was pumped from the Mesilla Basin alluvial aquifer, nearly all for municipal and industrial uses. Before development, water levels in wells completed in the deeper parts of the aquifer were at land surface or a few feet above land surface, and ground water moved upward from the deeper to the shallower zones. After development, waterlevel gradients were reversed, and water from the Rio Grande alluvium and shallower zones within the basin deposits now leaks downward. This vertical percolation from the shallower deposits has apparently replenished deeper permeable zones in the aquifer and has caused longterm waterlevel changes to stabilize.
Assuming a specific yield of 10 percent for the unconsolidated deposits in the Texas portion of the Mesilla Basin and the adjacent mesa to the east, about 820,000 acrefeet of freshwater is estimated to be in storage in the deposits. The volume of slightly saline water stored in the Rio Grande alluvium is estimated to be about 300,000 acrefeet. (One acrefoot is the volume of water that will cover 1 acre of land to a depth of 1 foot, or about 43,560 cubic feet of water.) Although these volumes of water may be recoverable in theory, the volume of water that can be recovered in practice may be substantially less.
HUECO BASIN
The Hueco Basin is situated in parts of New Mexico, Texas, and Mexico. In Texas, the northern part of the basin lies between the Franklin Mountains on the west and the Hueco Mountains on the east. The unconsolidated alluvial deposits in the Hueco Basin consist of gravel, sand, silt, and clay. The deposits locally are as much as 9,000 feet thick in a deep trough adjacent and parallel to the Franklin Mountains. The deposits that compose the Hueco Basin alluvial aquifer include the Rio Grande alluvium, which is probably not more than 200 feet thick.
Between the Texas­New Mexico border on the north and the city of El Paso, the deposits of the Hueco Basin contain about 10 million acrefeet of freshwater in an approximately 7milewide area adjacent and parallel to the Franklin Mountains. A map of the saturated thickness of the fresh-waterbearing alluvial deposits is shown in figure 19. In January 1980, these saturated deposits were more than 1,000 feet thick about midway between the Texas­New Mexico border and the Rio Grande at El Paso. An additional large amount of slightly saline water is available in deposits that underlie and adjoin the freshwaterbearing deposits to the east. Relatively rapid recharge to the aquifer by runoff from the Franklin Mountains into alluvialfan deposits makes this a favorable area for groundwater development. During 1980, about 66 million gallons per day were withdrawn from the Hueco Basin alluvial aquifer in the El Paso­Fort Bliss Military Reservation area (fig. 20) for municipal, military, and industrial supplies.
Under natural conditions, groundwater movement is toward the Rio Grande and in a downvalley direction. In developed areas, ground water moves toward centers of pumpage. Natural hydraulic gradients have been reversed in intensively pumped artesian areas, and water in the shallow alluvium moves downward across local confining units to replenish water that is pumped from deeper zones. Waterlevel declines have been large near municipal well fields. Net water levels declined more than 100 feet between 1903 and 1989 in the downtown areas of El Paso and Ciudad Juarez, Mexico, as shown in figure 21.
Where the alluvial deposits contain water under unconfined (watertable) conditions, the specific yield of the aquifer is estimated to be between 16 and 30 percent, and the transmissivity of the aquifer is estimated to range from 1,300 to 37,000 feet squared per day. Where local confining units create artesian conditions, the storage coefficient of the aquifer is about 0.0004 and the estimated transmissivity ranges from 6,700 to 16,000 feet squared per day. The storage coefficient and transmissivity for the deposits in the southeastern part of the Hueco Basin probably are substantially smaller. Wells completed in the aquifer yield as much as 3,000 gallons per minute.
The basin fill in the southeastern part of the Hueco Basin is mostly fine grained and probably consists largely of playa deposits. Field data suggest that the thickness of the deposits in this area ranges from 1,000 to 3,000 feet. Sand and gravel are substantial only in the upper 200 to 400 feet, which includes the Rio Grande alluvium. The ground water generally becomes more mineralized with depth from the northern part of the basin toward the southeast. This is shown by a water-quality profile along a line that approximately follows the course of the Rio Grande from El Paso southeastward to about Fort Hancock in Hudspeth County (fig. 22). Water with less than 1,000 milligrams per liter dissolved solids is contained in deposits that are more than 400 feet thick in the vicinity of El Paso, but the freshwater diminishes rapidly toward the southeast. Although water with dissolvedsolids concentrations of less than 1,000 milligrams per liter is desired for most public and industrial uses, waters with greater concentrations are acceptable for such uses as livestock watering and irrigation, and the southeastern part of the Hueco Basin alluvial aquifer is a valuable source of water for these purposes.
The city of El Paso¹s demands for fresh ground water are currently (1996) resulting in depletion of water in storage in parts of the Hueco Basin alluvial aquifer. Results of intensive pumping include declining water levels, decreased well yields, and deteriorating water quality. City planners anticipate that the demand for water in El Paso will soon exceed supply. To reduce demands and to increase future supplies, El Paso city officials are implementing conservation practices and artificial recharge programs. 
SALT BASIN
The Salt Basin lies mostly in Texas, but a small part of the basin extends northward into New Mexico (fig. 17). From the New Mexico­Texas border into Presidio County, the width of the Salt Basin ranges from 5 to 20 miles, and the length is about 140 miles. The basin is bounded by various mountain ranges.
The deposits in the Salt Basin consist of clay, sand, gravel, caliche, and, in places, volcanic rocks and volcanic/clastic deposits (fig. 18). The Salt Basin is a closed basin; that is, no surface drainage leaves the basin. Recharge to the basin fill is mainly by runoff from the bordering mountains into alluvial fans. The water moves laterally and downward into the basin fill and then upward toward playa areas near the center of the basin where it is discharged mainly by evapotranspiration (fig. 23).
In the northern part of the basin, ground water moves upward toward playas that contain salt deposits. The alluvium in this part of the basin is relatively fine grained and mostly contains highly mineralized water. The water is slightly saline around the basin margin, moderately saline along the axis of the basin, and very saline to briny beneath the playa areas. Salt deposits that formed in the playas as a result of precipitation of minerals from ground water were commercially mined from 1863 until the early 1950¹s. Wells completed in the deposits yield as much as 1,200 gallons per minute. An average of about 4.5 million gallons per day was withdrawn from the Salt Basin alluvial aquifer from 1951 to 1972, primarily for irrigation in the northern part of the basin.
The deposits in the central part of the basin are coarse grained and are composed primarily of volcanic rocks and volcanic/clastic deposits. Fresh to slightly saline ground water is in this part of the basin, where the basinfill deposits are as much as 2,400 feet thick. Estimates of specific yield for the deposits in the central part of the basin range from 5 to 10 percent. Largecapacity irrigation wells completed in the deposits yield from 400 to more than 1,000 gallons per minute. Water levels in the central part of the basin declined as much as 6.5 feet per year from 1951 to 1973.
Little groundwater development has occurred in the southern part of the basin, but sparse data indicate that well yields range from about 250 to 1,400 gallons per minute. The amount of freshwater in storage in the Salt Basin alluvial aquifer is estimated to be 6.5 million acrefeet; the estimated volume of slightly saline water in storage is 1.0 million acrefeet. About 75 percent of the total water is assumed to be recoverable. During 1960, about 32 million gallons per day was pumped from the Salt Basin alluvial aquifer.
EAGLE BASIN 
The Eagle Basin is bounded by various mountain ranges. The southern part of the basin extends to the Rio Grande. The width of the basin ranges from 2 to 10 miles, and the length is about 60 miles. Most of the basin is in Hudspeth County, although the southern end extends into Culberson, Jeff Davis, and Presidio Counties.
The deposits in the basin consist of clay, silt, sand, gravel, volcanic rocks, and volcanic/clastic deposits. The deposits are more than 2,000 feet thick in the central part of the basin and in the southern part of the basin near the Rio Grande. Most wells completed in the basinfill deposits are used for watering livestock and have small yields. Some irrigation wells reportedly yield between 1,000 and 1,500 gallons per minute. Specificcapacity data indicate a transmissivity of as much as 13,000 feet squared per day for the Eagle Basin alluvial aquifer in the Rio Grande Valley. Most of the recharge to the alluvial basin aquifer enters at the margins of the basin as runoff from the surrounding mountains. Ground water moves toward the axis of the basin and then southward to discharge to the Rio Grande.
RED LIGHT BASIN  
The Red Light Basin is located in southeastern Hudspeth County. It is bounded on the north, west, and east by various mountains, and extends southward to the Rio Grande. The basin is filled with alluvium which consists of clay, silt, sand, and gravel, combined with volcanic rocks and volcanic/clastic deposits. The basinfill deposits thicken toward the south and are more than 3,000 feet thick at the Rio Grande. The deposits contain at least some freshwater in most of the basin; however, substantial quantities of freshwater are available only in the central part. Only a few wells which have small yields and are used primarily for livestock watering have been completed in the deposits.
PRESIDIO BASIN
The Presidio Basin is in the western part of Presidio County and contains the southernmost aquifer of the Rio Grande aquifer system in Texas (fig. 17). The Rio Grande forms the western boundary for the basin; it is bounded on the east by mountains. The width of the basin ranges from 4 to 10 miles, and the length is about 70 miles. The basin contains great thicknesses of finegrained alluvial deposits, volcanic rocks, and volcanic/clastic deposits. The basinfill deposits are as much as 5,000 feet thick along the axis of the basin near the Rio Grande.
Ground water has been developed along the flood plain of the Rio Grande, where it is used mostly for irrigation; in other parts of the basin, ground water is pumped only for livestock watering and domestic use. Largediameter irrigation wells in the flood plain of the Rio Grande at the southern end of the basin yield from 300 to 800 gallons per minute. Specif-iccapacity data indicate a transmissivity of about 5,000 to 21,000 feet squared per day for the alluvial aquifer in the Rio Grande Valley. Recharge to the basin fill is mainly along the bordering mountains where small streams enter the basin. Ground water flows from the basin margins to the Rio Grande, where it is discharged either by evapotranspiration or by seepage to the river.
In the Rio Grande Valley in the central part of the basin, an estimated 5 million gallons per day of ground water was withdrawn for irrigation during 1960. An estimated 800,000 acrefeet of freshwater is in storage in the Presidio Basin alluvial aquifer; of this amount, an estimated 75 percent can be recovered.
FRESH GROUNDWATER WITHDRAWALS
An estimated 126 million gallons per day of freshwater was withdrawn from the Rio Grande aquifer system during 1985. About 77 million gallons per day was withdrawn for public supply, the principal use (fig. 24). About 30 million gallons per day was withdrawn for agricultural purposes, and about 10 million gallons per day was pumped for industrial, mining, and thermoelectricpower uses. About 9 million gallons per day was withdrawn for domestic and commercial uses.\

PECOS RIVER BASIN ALLUVIAL AQUIFER
INTRODUCTION
Thick and extensive alluvial deposits of Cenozoic age compose the Pecos River Basin alluvial aquifer in western Texas (fig. 25). The aquifer is in the Great Plains Physiographic Province and underlies approximately 5,000 square miles in parts of Andrews, Crane, Ector, Loving, Pecos, Reeves, Ward, and Winkler Counties. The topography in the area consists mostly of flat to rolling plains that slope gently toward the Pecos River. Ground water in the Cenozoic alluvium is of major importance in this area where average annual rainfall is less than 12 inches.
HYDROGEOLOGY
During late Tertiary and Quaternary time, streams that flowed across the area laid down thick, extensive deposits of alluvium. Prevailing winds subsequently deposited a cover of sand in the eastern part of the area. The alluvial deposits, in order of abundance, consist of gravel, sand, silt, and clay; the deposits contain some caliche (fig. 26). The alluvium generally ranges from 100 to 300 feet in thickness; in places, it is as much as 1,500 feet thick.
The alluvium overlies Permian, Triassic, and Cretaceous rocks as shown by a diagrammatic section that extends from northern Reeves County to northern Pecos County approximately parallel to the Pecos River (fig. 27). In places, these underlying rocks can yield substantial quantities of water and may be in hydraulic connection with the overlying alluvium. The maximum thickness of the alluvium in the section is about 1,500 feet in northeastern Reeves County near the Pecos River. Dissolution of evaporites in underlying Permian rocks has resulted in subsidence and the formation of deep troughs in Reeves County and in Winkler and Ward Counties; thick deposits of Cenozoic alluvium have accumulated in the troughs, which are evident in the baseofaquifer map shown in figure 28. The altitude of the base ranges from less than 1,400 feet to more than 3,000 feet above sea level.
Water in the alluvium is generally unconfined; however, confined conditions prevail in local areas where a clay confining unit is present. Under natural conditions, ground water generally moves from recharge areas near the margins of the alluvium toward the Pecos River. However, pumpage for irrigation in such areas as central Reeves and northern Pecos Counties has caused hydraulic gradients to reverse; consequently, water moves toward these areas from all directions (fig. 29). The saturated thickness of the aquifer, based on the altitude of the 1989 potentiometric surface, ranged from 0 to more than 1,000 feet (fig. 30).
Recharge to the alluvium is by direct precipitation, infiltration from intermittent streamflow, return irrigation water, and subsurface flow from older formations. Recharge by precipitation is especially effective in an area that is covered with sand dunes and extends from southwestern Andrews County through parts of Winkler, Ector, and Ward Counties into central Crane County.
The natural concentration of dissolved solids in water in the alluvial aquifer commonly exceeds 1,000 milligrams per liter. The salinity, which has increased substantially in some areas of intense pumpage, is caused primarily by induced infiltration of highly mineralized water from the Pecos River and return flow of irrigation water that has high mineral content caused by concentration by evapotranspiration and leaching of salts and fertilizers from the soil.
Ground water in the alluvial aquifer is used principally for irrigation. Irrigation wells completed in the aquifer generally yield between 200 and 2,500 gallons per minute and average about 1,000 gallons per minute. Aquifer tests in Reeves, Pecos, Winkler, Ward, and Crane Counties show a large variability in the transmissivity of the alluvial aquifer, with values that range from 2,500 to 12,000 feet squared per day.
Annual pumpage from the alluvial aquifer is much greater than annual recharge. In an intensively irrigated area of central Reeves County, water levels declined more than 190 feet between 1951 and 1960.
Some of the area underlain by the alluvial aquifer is not suitable for irrigation from wells because either the terrain is too rough or the saturated thickness of the aquifer is not great enough to sustain well yields. In the areas that are suitable for groundwater withdrawal, more than 30 million acrefeet of fresh to slightly saline ground water is estimated to be in storage. If substantial waterquality degradation by migration of undesirable water is to be avoided, then only about 9.5 million acrefeet, or 32 percent, of this water can be pumped.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the Pecos River Basin alluvial aquifer totaled about 80 million gallons per day during 1985 (fig. 31). About 67 million gallons per day was withdrawn for agricultural purposes, the principal water use. Approximately 7 million gallons per day was withdrawn for public supply. About 5 million gallons per day was withdrawn for industrial, mining, and thermoelectricpower uses, and about 1 million gallons per day was withdrawn for domestic and commercial uses.

SEYMOUR AQUIFER
INTRODUCTION
The Seymour aquifer consists mainly of the scattered erosional remnants of the Seymour Formation of Pleistocene age. The aquifer has been referred to in the literature as the ³northcentral Texas alluvial aquifers² because it is in 22 separate areas of alluvium in parts of 20 Texas counties in the upper Red and upper Brazos River Basins (fig. 32). The areas are predominantly in the Central Lowland Physiographic Province; only parts of the five westernmost areas are in the Great Plains Province. Average annual precipitation in the area ranges from 19 to 26 inches, and average annual runoff ranges from 0.2 to 1 inch. The aquifer generally has less than 100 feet of saturated thickness, but it is an important source of water for domestic, municipal, and irrigation needs.
HYDROGEOLOGY
During Pleistocene time, the eroded bedrock surface, which was developed mostly on poorly permeable red beds of Permian age, was covered by the Seymour Formation. The Seymour Formation consists of clay, silt, sand, and gravel that were deposited by eastwardflowing streams. Subsequent erosion left scattered remnants of the Seymour Formation mostly in interstream areas, and some of the eroded material was redeposited, thus forming the younger alluvium and alluvial terraces in stream valleys. The younger deposits are similar in composition to the Seymour Formation and compose part of the Seymour aquifer (fig. 33).
Areal extents of the individual alluvial areas range from about 20 square miles for an area in Baylor County to about 430 square miles for an area that spans Haskell and Knox Counties. Saturated thickness locally is as much as 100 feet but usually ranges between 20 and 60 feet. Water in the aquifer generally is unconfined; however, it may be confined locally by beds of clay. The alluvium is recharged mainly by direct infiltration of precipitation that falls on the land surface. Ground water moves toward points of discharge along streams or toward pumping wells. Yields of wells completed in the alluvium range from less than 100 to as much as 1,300 gallons per minute and average about 300 gallons per minute. The chemical quality of water in the alluvial aquifer ranges from fresh to slightly saline. In some areas, the water is hard and contains dissolvedsolids concentrations in excess of 2,500 milligrams per liter; consequently, its suitability for some uses is restricted.
About 4.5 million acrefeet of fresh to slightly saline water was estimated to be in storage in the Seymour aquifer in 1974. About 75 percent of this water, or about 3.4 million acrefeet, was estimated to be recoverable.
An estimated 120 million gallons per day was withdrawn from the Seymour aquifer during 1959. About 94 percent was used for irrigation, and the remainder, for public and industrial supplies. More than 50 percent of the total withdrawal was for irrigation in the area that spans Haskell and Knox Counties.
SEYMOUR AQUIFER IN HASKELL AND KNOX COUNTIES
The part of the Seymour aquifer that is most intensively developed is in Haskell and Knox Counties, and is the largest continuous part of the aquifer. The Seymour aquifer is the only available source of water for moderate to large irrigation supplies in the local area. The aquifer furnished water to more than 2,000 irrigation wells during 1976; it also is a widely used source for domestic and livestock watering supplies. The areal extent of this part of the aquifer is about 430 square miles. Saturated thickness of the aquifer is generally 20 to 40 feet but is as much as 60 feet in northern Haskell County (fig. 34). Buried channels and valleys on the surface of the Permian red beds are areas where the Seymour Formation is thick and consists of coarse grained material. Sand and gravel that form productive aquifers are generally in the lower part of the Seymour Formation.
The hydrogeologic section in figure 35 is located along the maximum length of the aquifer from western Haskell County to eastern Knox County, and shows that the Seymour Formation overlies the Clear Fork Group of Permian age. The younger alluvium and alluvial terraces along the Brazos River are at lower altitudes and are not in hydraulic connection with the Seymour Formation. The slope of the potentiometric surface of the Seymour aquifer generally conforms to the slope of the land surface and to the surface of the underlying Permian rocks. The altitude of the potentiometric surface in January 1977 (fig. 36) indicates that ground water moved generally northward toward the Brazos River from a high area on the potentiometric surface in central Haskell County.
Wells completed in the Seymour aquifer are typically 40 to 60 feet deep. Well yields average about 270 gallons per minute and are as great as 1,300 gallons per minute. Transmissivity of the aquifer ranges from 2,700 to more than 40,000 feet squared per day and averages 13,400 feet squared per day. The chemical quality of the ground water is extremely variable. Concentrations of dissolved solids range from 300 to 3,000 milligrams per liter; most values are between 400 and 1,000 milligrams per liter.
Groundwater contamination is a problem in some areas and is related mainly to pesticides and fertilizers used in agriculture and to human and animal wastes (septic tanks, barnyards, feedlots, and sewagetreatment plants). Contamination from brine disposal and leakage from wells that are or were a part of oilfield activities is expected to remain a localized problem.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the Seymour aquifer totaled about 121 million gallons per day during 1985 (fig. 37). Approximately 110 million gallons per day was withdrawn for agricultural purposes, the principal water use. About 9 million gallons per day was withdrawn for public supply, and about 1 million gallons per day was pumped for domestic and commercial uses. About 1 million gallons per day was withdrawn for industrial, mining, and thermoelectricpower uses.

HIGH PLAINS AQUIFER
INTRODUCTION
The High Plains aquifer in Oklahoma and Texas is part of a regional aquifer that extends into parts of Colorado, Kansas, Nebraska, New Mexico, South Dakota, and Wyoming (fig. 38). Only that part of the aquifer in Oklahoma and Texas is described in this chapter; descriptions for other States are in other chapters of this Atlas. The aquifer consists predominantly of the Ogallala Formation of late Tertiary age; locally, unconsolidated deposits of Quaternary age are included in the aquifer. In places, the High Plains aquifer is in hydraulic connection with permeable parts of the underlying bedrock, which ranges in age from Permian to Cretaceous.
The High Plains geographic area is in the Great Plains Physiographic Province and consists of an elevated plain that is relatively undissected. The population of the High Plains geographic area is sparse, but the combination of level topography, excellent soils, and an abundant supply of ground water for irrigation makes this an important agricultural region.
Average annual precipitation ranges from about 12 inches in the southwest to 24 inches in the northeast. Average annual runoff ranges from about 0.2 inch in the west to 0.5 inch in the east. The High Plains aquifer in Segment 4 underlies an area of about 43,000 square miles mostly in the panhandle parts of Oklahoma and Texas. About 4.5 billion gallons of water per day was withdrawn from the High Plains aquifer in Oklahoma and Texas during 1985. The aquifer is by far the most intensively developed aquifer in the twoState area.
HYDROGEOLOGY
The High Plains aquifer described in this chapter has been called the Ogallala aquifer in many published reports. The age of the Ogallala Formation is considered to be Miocene in this chapter, but is listed as Pliocene or Pliocene and Miocene in many published reports.
At the close of deposition of the Ogallala Formation several million years ago, the Great Plains was a vast, gently sloping plain that extended from the edge of the Rocky Mountains eastward for hundreds of miles. Regional uplift and erosion stripped away the plain in many places, but a large central area was little affected by eroding streams and is preserved. This preserved remnant of the uplifted Ogallala Formation is known as the High Plains. Although the surface of the High Plains has been modified little by streams, it has been pitted by carbonate dissolution and deflation, thus forming many playas, or shallow depressions, that collect and store water during periods of precipitation and runoff.
The Canadian River has cut through much of the Ogallala Formation in the Texas Panhandle. The High Plains south of the Canadian River is referred to locally and regionally as the Southern High Plains. This area also is known as the Llano Estacado (Staked Plain).
HYDROGEOLOGIC FRAMEWORK
During Miocene time, the uplifted and tectonically active Rocky Mountains provided source material for deposition of the Ogallala Formation. Valleys and basins that developed by erosion on the surface of Permian, Triassic, Jurassic, and Cretaceous rocks (fig. 39) became filled with Ogallala sediments. In northern Texas, some collapse structures in Permian rocks are filled with Mesozoic (Triassic, Jurassic, or Cretaceous) rocks, as well as with Ogallala deposits. Where the Mesozoic rocks have secondary permeability, they are considered to be part of the High Plains aquifer; however, they are a very minor component. The Ogallala sediments were deposited by braided streams that spread across a generally level plain. The eastwardflowing streams deposited a heterogeneous mixture of gravel, sand, silt, and clay.
Upwarping and climatic change in Pliocene time caused deposition of alluvium to cease and erosion to begin. Preservation of the remnant of uplifted Ogallala Formation that composes most of the High Plains aquifer is due largely to the presence of resistant caliche cap rock that formed over much of the surface of the Ogallala. The cap rock consists of zones that are cemented with calcium carbonate; these zones are resistant to weathering and cause the formation of ledges and escarpments.
The thickness of the Ogallala Formation is as much as 650 feet. The overlying Quaternary alluvium and windblown sand, which are locally as much as 150 feet thick, are part of the High Plains aquifer in some places (fig. 40). The base of the aquifer generally slopes to the east and southeast. The altitude of the base ranges from about 2,000 to 4,000 feet above sea level (fig. 41). The altitude of the water table before development ranged from about 2,400 to more than 4,000 feet above sea level (fig. 42). The regional movement of ground water is from west to east toward the caprock escarpment that forms the eastern margin of the High Plains geographic area.
GROUNDWATER HYDRAULICS
The High Plains aquifer is recharged by the infiltration of precipitation that falls directly on the aquifer. This recharge is estimated to range from 0.024 inch per year in the Southern High Plains of Texas to 2.2 inches per year in Texas County, Okla. and is about 0.1 percent and 12 percent of average annual precipitation, respectively. Additional recharge may occur when a part of the water that is pumped for irrigation infiltrates the soil and returns to the water table. As much as 54 percent of irrigation pumpage might be reentering the aquifer in Castro and Parmer Counties, Tex., whereas only 20 percent of irrigation water applied in the Oklahoma Panhandle might be returned to the High Plains aquifer.
Ground water discharges naturally through seeps and springs, primarily along the eastern escarpment and the Canadian River. Most ground water is discharged artificially through wells.
Hydraulic conductivity and specific yield of the sediments that compose the High Plains aquifer are important properties that control well yields and resulting waterlevel depths and rates of waterlevel declines. The areal distribution of hydraulic conductivity, as shown in figure 43, was estimated from records collected by water well drillers. Values range from less than 1 to 200 feet per day, and the range is 25 to 100 feet per day for most of the aquifer. The average hydraulic conductivity for the 35,450 square miles of High Plains aquifer in Texas is estimated to be 65 feet per day; the average for the 7,350 square miles of the aquifer in Oklahoma is estimated to be 61 feet per day.
Specific yield also was estimated from lithologic descriptions made by drillers during the construction of water wells. The areal distribution of specific yield is shown in figure 44. Values range from less than 1 to 30 percent; most of the area is in the 10 to 20 percent range. The estimated average specific yield for the High Plains aquifer in Texas and Oklahoma is 15.6 percent and 18.5 percent, respectively.
GROUNDWATER QUALITY
Small concentrations of dissolved solids in ground water in the High Plains aquifer indicate that the water either has had a short residence time in the aquifer or has been in contact with relatively insoluble minerals, or both. Larger concentrations indicate longer residence time, contact with soluble minerals such as gypsum, anhydrite, and halite, or mixing with more mineralized water from bedrock.
Water from the High Plains aquifer is used mostly for crop irrigation. If leaching or drainage is adequate, then concentrations of dissolved solids between 500 and 1,500 milligrams per liter in irrigation water are not likely to be harmful to crops. Concentrations of individual chemical constituents, such as sodium, also are important in determining the suitability of the water for most uses. Excessive sodium concentrations, for example, can cause chemical imbalances and can interfere with normal plant growth.
Most of the water in the High Plains aquifer has a dissolvedsolids concentration of less than 500 milligrams per liter (fig. 45). Concentrations exceed 500 milligrams per liter in water from a large part of the Southern High Plains in Texas. In water from the southernmost part of the aquifer in Texas, concentrations of dissolved solids exceed 1,000 milligrams per liter but are generally less than 3,000 milligrams per liter. In this area, highly mineralized water in underlying Mesozoic rocks of marine origin probably moves into the High Plains aquifer in response to hydraulichead differences. Locally, the more mineralized water seems to be associated with several alkali lake basins in areas underlain by Cretaceous rocks in Lamb, Hockley, Terry, Lynn, eastern Gaines, and Martin Counties. Sodium and increased dissolvedsolids concentrations may increase locally because of industrial activities and irrigation practices.
GROUNDWATER DEVELOPMENT
Pumpage of ground water for irrigation on the High Plains began in the early 1900¹s and increased slowly until the mid1940¹s. In Texas, the acreage irrigated by ground water increased rapidly between the mid1940¹s and 1959 but increased little between 1959 and 1980. The irrigated acreage in 1980 on the High Plains of Texas was 3.9 million acres, which was about the 1959 level. This leveling off after 1959 is primarily the result of declining water availability in the Southern High Plains. Acreage irrigated by ground water in the Oklahoma part of the High Plains in 1980 was about 389,000 acres. During the 1980 growing season, an estimated 5,169,000 acrefeet of water was pumped from the High Plains aquifer for irrigation in Texas, and an estimated 540,000 acrefeet was pumped in Oklahoma.
The density of acreage that was irrigated by ground water from the High Plains aquifer during 1978 is shown in figure 46. Most of the irrigated acreage was in the northern onehalf of the Southern High Plains of Texas. In Texas alone, the High Plains aquifer supplied water to about 75,000 irrigation wells.
Because pumpage to satisfy the large demand for crop irrigation has been considerably in excess of recharge, water levels in the High Plains aquifer have declined substantially. The altitude of the water table in the High Plains aquifer in 1980 is shown in figure 47. When compared with the predevelopment water table (fig. 42), the general westward shift of the contours indicates waterlevel declines.
The change between the predevelopment and the 1980 water tables is shown in figure 48. Waterlevel declines of 50 to more than 100 feet have been measured in a large area in the northern part of the Southern High Plains of Texas where the irrigated acreage is most dense. Water levels in most areas declined between 10 and 50 feet but rose in some areas. Waterlevel rises in Texas probably resulted from the clearing of native vegetation for cultivation, which increased the rate of recharge from precipitation by reducing transpiration. Waterlevel rises in Oklahoma probably represent a recovery from abnormally low water levels during the drought of 1933­40. These low water levels were among the earliest data available in Oklahoma and were used to construct the predevelopment watertable map.
The general decline of the water table has resulted in a considerable loss of water from storage and a decreased saturated thickness of the High Plains aquifer. The total volume of drainable water in storage is a product of specific yield, saturated thickness, and area. In 1980, the estimated total volume of drainable water in storage in the High Plains aquifer was 390 million acrefeet in Texas and 114 million acrefeet in Oklahoma. The saturated thickness of the aquifer ranged from 0 to 600 feet in 1980 (fig. 49). The saturated deposits generally thicken from south to north. Most of the aquifer south of the Canadian River had a saturated thickness of less than 100 feet.
Changes in saturated thickness and in well yields are directly related. The saturated thickness of the High Plains aquifer in Texas reportedly decreased by more than 50 percent in large parts of Castro, Crosby, Floyd, Hale, Lubbock, Parmer, and Swisher Counties, south of the Canadian River. From 1958 to 1980, irrigated land in the seven counties decreased from 2.5 million to 1.9 million acres, while the number of irrigation wells increased from about 21,000 to 30,000. The average number of acres irrigated per well decreased from 118 in 1958 to 62 in 1980. Decreased well yields are one result of water-level declines.
Another result of waterlevel declines and decreased saturated thickness is an increase in the depth to water. The generalized depth to water in the High Plains aquifer in 1980 is shown in figure 50. Depths ranged from 0 to 400 feet and exceeded 100 feet for most of the area. Greatest depths to water are in the vicinity of the Canadian River. Increased depths to water equate to increased pumping lifts which, together with decreased well yields, add substantially to the cost of withdrawing water from the High Plains aquifer.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the High Plains aquifer in Texas and Oklahoma totaled 4,508 million gallons per day during 1985 (fig. 51). Agricultural purposes, the principal water use, required about 4,343 million gallons per day. About 93 million gallons per day was withdrawn for public supply and about 9 million gallons per day was pumped for domestic and commercial uses. Withdrawals for industrial, mining, and thermoelectricpower uses were 63 million gallons per day.
POTENTIAL FOR DEVELOPMENT
The map of potential yields of wells completed in the High Plains aquifer shown in figure 52 is based on hydraulic conductivity and the 1980 saturated thickness. In a large part of the area, especially north of the Canadian River, well yields in excess of 750 gallons per minute can be expected. One well capable of yielding 750 gallons per minute can irrigate 160 acres and effectively operate a quartersection (0.25 square-mile) centerpivot irrigation system. Irrigation development is less favorable in areas, such as a large part of the Southern High Plains of Texas, where well yields are less than 250 gallons per minute. Areas with further declines in water level (therefore declining well yields) may experience a decline in irrigated acreage, as noted in the ³GroundWater Development² section above for the sevencounty area in Texas. In some areas, particularly the southernmost area, irrigation development may be limited because of large sodium or dissolvedsolids concentrations (fig. 45).
Because the High Plains aquifer is being pumped far in excess of recharge, the ground water is a limited resource. Questions of major concern are: How long will the ground-water resource last?; and How can the remaining water be managed and used most efficiently? Among the factors that influence further development of the High Plains aquifer are crop prices, energy and other farm costs, droughts and surplus precipitation, conservation practices, regulatory policies, and wateruse technology improvements.
The Texas Department of Water Resources projects an increasing shortage of water from the High Plains aquifer for future irrigation needs. Unless an effective conservation program is implemented, it is estimated that the irrigated acreage on the High Plains of Texas will be decreased by slightly more than onehalf of the present acreage by 2030. Water conservation methods and secondary recovery of capillary water are among some of the alternatives that are being explored to solve the watersupply problems in the High Plains of Texas. To assist in that exploration, digital computer simulations have been used to predict the possible effects of future groundwater pumpage on the High Plains aquifer under various pumpage estimates and management strategies.

COASTAL LOWLANDS AQUIFER SYSTEM
INTRODUCTION
The coastal lowlands aquifer system consists of mostly Miocene and younger unconsolidated deposits that lie above and coastward of the Vicksburg­Jackson confining unit; the deposits extend to land surface (figs. 53 and 54). The aquifer system is in the Coastal Plain Physiographic Province and is in all or parts of 51 counties in Texas. It extends eastward into parts of the Coastal Plain of Louisiana and Mississippi and is further discussed in Chapter F of this Atlas. A small part of the system extends into southern Alabama and the western part of Panhandle Florida where it is called the sand and gravel aquifer (Chapter G of this Atlas). In Texas, the aquifer system underlies about 35,000 square miles of the level, lowlying coastal plain whose surface rises gradually toward the north and northwest.
The major rivers that flow through the area and empty into the Gulf are, from west to east, the Rio Grande, the Nueces, the Frio, the San Antonio, the Guadalupe, the Colorado, the Brazos, the Navasota, the Trinity, the Neches, the Angelina, and the Sabine. Average annual precipitation ranges from about 22 inches in the Rio Grande Valley in the southwest to about 56 inches at the Louisiana border in the east. The coast-warddipping sediments reach thicknesses of thousands of feet and contain waters that range from freshwater to brine. The coastal lowlands aquifer system yields large amounts of water for public, agricultural, and industrial needs.
HYDROGEOLOGY
The deposits that compose the coastal lowlands aquifer system range in age from Oligocene to Holocene (fig. 55). The lithology is generally sand, silt, and clay and reflects three depositional environments‹continental (alluvial plain), transitional (delta, lagoon, and beach), and marine (continental shelf). The gradual subsidence of the depositional basin and rise of the land surface caused the deposits to thicken Gulf-ward, which resulted in a wedgeshaped configuration of the hydrogeologic units as seen in the cross section shown in figure 54. Coarser grained nonmarine deposits updip grade laterally into finergrained material that was deposited in marine environments. Numerous oscillations of ancient shorelines resulted in a complex, overlapping mixture of sand, silt, and clay. This heterogeneity has made it difficult for investigators to subdivide the thick deposits into individual hydrogeologic units, and various schemes of subdivision are found in the literature.
HYDROGEOLOGIC FRAMEWORK 
Different names have been used for the aquifers and confining units of the coastal lowlands aquifer system. The term ³Gulf Coast aquifer² has been used to refer to and describe the composite sands, silts, and clays of the aquifer system. The ³Chicot aquifer² and ³Evangeline aquifer² are commonly used hydrogeologicunit designations for subdivisions of the upper, mostly sandy part of the deposits. In a recently completed regional study that was part of the U.S. Geological Survey¹s Regional AquiferSystem Analysis (RASA) program, the deposits were subdivided into five permeable zones and two confining units. An informal letter designation has been assigned to each subdivision. The basis of this sevenunit subdivision was primarily differences in permeability, but included an evaluation of depths of waterproducing zones and the resultant vertical differences in hydraulic head at large pumping centers in Houston, Tex., and Baton Rouge, La. Comparison of the subdivisions used in this Atlas with names of hydro-geologic units used in Texas is shown in figure 55.
Some of the boundaries of the aquifer system are geographic and some coincide with permeability contrasts. The landward boundary, or updip limit of the aquifer system, is in outcrop areas where the aquifer system feathers out at point of contact with the underlying Vicksburg­Jackson confining unit (figs. 53 and 54). The Gulfward boundary is near the coastline where the ground water becomes increasingly saline; the upper boundary is the land surface.
The base of the aquifer system is either its contact with the top of the Vicksburg­Jackson confining unit or the approximate depth at which the water in the system has a dissolvedsolids concentration of more than 10,000 milligrams per liter. The altitude of the base of the aquifer system is shown in figure 56. The base ranges from a few hundred feet above sea level near the updip limit, to as much as 6,000 feet below sea level in areas about midway between the updip limit and the coastline.
GROUNDWATER HYDRAULICS
The aquifer system is recharged by the infiltration of precipitation that falls on topographically high aquifer outcrop areas. Natural discharge occurs by evapotranspiration, loss of water to streams as base flow, and upward leakage to shallow aquifers in lowlying coastal areas or in the Gulf of Mexico. Recharge and discharge in areas with little or no pumpage are generally between 0 and 1 inch per year. Additional recharge occurs where water levels are lowered by pumping because the vertical hydraulic head gradient is increased. In places where head gradients might become reversed, water might move from former discharge areas along streams into the aquifers.
With the exception of shallow zones in the outcrop, the water in the coastal lowlands aquifer system is under confined conditions. In the shallow zones, the specific yield for sandy deposits ranges generally between 10 and 30 percent; for confined aquifers, the storage coefficient is estimated to range between 1¥10-4 and 1¥10-3. The storage coefficient is the volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. In an unconfined aquifer, the storage coefficient is virtually equal to the specific yield. The storage coefficient is an important factor that determines the size and rate of development of cones of depression that result from groundwater withdrawals.
The productivity of the aquifer system is directly related to the total thickness of the sands in the aquifer system that contain freshwater. This aggregate sand thickness is shown in figure 57. Values range from zero at the updip limit of the aquifer system to as much as 2,000 feet in the east. The transmissivity of the sands is a measure of the ease with which water will move through them. Transmissivity can be calculated by multiplying the average hydraulic conductivity of the sands times the thickness of the sands that contain freshwater. Transmissivity, storage coefficient, and recharge rate control the rate of well yields and the size and shape of the cones of depression that result on an aquifer¹s potentiometric surface because of pumping.
The average hydraulic conductivity of the sands was estimated from a digital computer model. East of the San Antonio River, the average hydraulic conductivity is about 21 feet per day; west of the river, it is about 17 feet per day. By using these values and the freshwater sand thickness as shown in figure 57, an estimate of the transmissivity can be computed and mapped, as shown in figure 58. Values of transmissivity range from less than 5,000 to nearly 35,000 feet squared per day.
GROUNDWATER DEVELOPMENT
For the coastal lowlands aquifer system in general, groundwater pumpage was relatively small and constant from the early 1900¹s until the late 1930¹s. Pumping rates increased sharply during the 1940¹s and 1950¹s until about 1960, when about 800 million gallons per day was withdrawn. Withdrawal rates increased relatively slowly thereafter, and, during 1985, 1,090 million gallons per day was withdrawn.
Withdrawals during 1985 were largely from the east-central area; the largest pumpage was in the Houston area of Harris County. Harris County accounted for 35 percent of the total withdrawals, and the combined withdrawals from Harris and Wharton Counties were 50 percent of the total (table1). Ten counties in the eastcentral area accounted for 82 percent of total withdrawals; the largest usage was divided about equally between public supply and agriculture (table 1).
During 1982, some of the greatest pumpage from the aquifer system was in the coastal area of rice irrigation centered in Jackson and Wharton Counties and including parts of Colorado, Lavaca, Victoria, and Matagorda Counties. About 322 million gallons per day was withdrawn from permeable zone A, the uppermost permeable zone of the aquifer system in this area. Because the permeable zone crops out near this area, recharge in the outcrop area provided a source to quickly balance the large withdrawals. Thus, drawdowns were not large (generally less than 50 feet), but the increase in recharge rates over predevelopment rates was large. Recharge rates were increased by as much as 4 to 6 inches per year in the rice irrigation area, as indicated by the model simulation results shown in figure 59.
Another area that was pumped intensively during 1982 is centered in the city of Houston and includes Harris and all or parts of Chambers, Galveston, Brazoria, Fort Bend, Waller, Montgomery, and Liberty Counties. Withdrawals from permeable zones A, B, and C, the three uppermost wateryielding zones in the aquifer system, were mostly for public and industrial supplies and were about 260, 260, and 165 million gallons per day, respectively. As a result of the intense pumping, the potentiometric surface was lowered in all three zones. The lowering was least severe in zone A, the shallowest zone, where water levels declined to a maximum of 150 feet below sea level. The effect on the potentiometric surface was more severe in the deeper zones because their outcrop recharge areas were far updip from the pumping centers and a substantial amount of water was removed from aquifer storage. In Houston, the 1982 potentiometric surface declined to more than 250 feet below sea level in zone B (fig. 60) and more than 350 feet below sea level in zone C (fig. 61). Maps of the distribution of the change in the predevelopment to 1982 potentiometric surfaces show a decline of more than 300 feet in zone B (fig. 62), and more than 400 feet in zone C (fig. 63).
The large groundwater withdrawals in Harris County and adjacent areas have reduced the artesian pressure sufficiently to cause water from clay beds in the permeable zones to flow into the sands. As the water flows out of the compressible clays, they become irreversibly compacted, which causes permanent subsidence of the land surface. The land has subsided several feet in parts of the area (fig. 64); more than 9 feet of subsidence has been recorded in areas east of the Houston city limits. The subsidence has increased the risk of flood damage to residential and commercial properties (fig. 65) and has activated faults which have caused structural damage.
With the creation of the Harris­Galveston Coastal Subsid-ence District in 1975, the reduction of groundwater pumpage and increased reliance on surfacewater supplies have been emphasized. Pumping rates have been substantially reduced in much of southeastern Harris County and in Galveston County; this has caused a recovery of water levels and a cessation or sharp decrease in the rate of landsurface subsidence in that area. The subsidence that has already occurred, however, is virtually irreversible.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the coastal lowlands aquifer system in Texas totaled about 1,090 million gallons per day-during 1985 (fig. 66). About 476 million gallons per day was withdrawn for public supply, and about 447 million gallons per day was withdrawn for agricultural purposes. Withdrawals for industrial, mining, and thermoelectricpower uses were about 114 million gallons per day. About 53 million gallons per day was withdrawn for domestic and commercial uses. 
POTENTIAL FOR DEVELOPMENT
Problems associated with groundwater pumpage, such as land subsidence and saltwater encroachment, have caused pumping to be curtailed in some areas. The Texas Water Development Board has made projections of groundwater use to 2030. The tentative projections undergo revision and updating as technical and socioeconomic factors change. For the 10 counties that withdrew the largest amounts of water from the coastal lowlands aquifer system during 1985, State officials project a large decline in pumpage for 6 counties and an increase in 4 counties by 2030 (table 2). For the 10 counties, the total projected pumpage in 2030 is 39 percent less than that of 1985.
Although overall use of ground water might decline, some areas can sustain additional development. Pumping from wateryielding zones in geologically older rocks that are farther inland will minimize land subsidence and saltwater encroachment. Pumping in areas that have more abundant precipitation, and thus greater recharge potential, is less likely to cause continuous, steep waterlevel declines and such problems as stream baseflow depletion and greater pumping lifts.

TEXAS COASTAL UPLANDS AQUIFER SYSTEM
INTRODUCTION
The Texas coastal uplands aquifer system consists of Eocene deposits of the Claiborne Group and Eocene and Paleocene deposits of the Wilcox Group. Both groups are below the Vicksburg­Jackson confining unit and above the Midway confining unit (figs. 67 and 68). East of the Texas­Arkansas and Texas­Louisiana State lines, stratigraphically equivalent beds are called the Mississippi Embayment aquifer system. The sediments that compose the Texas coastal uplands aquifer system dip coastward beneath the coastal lowlands aquifer system. The Texas coastal uplands aquifer system underlies an area of about 48,000 square miles in the Coastal Plain Physiographic Province and is in all or parts of 70 counties in Texas. The topography of the coastal uplands is more dissected and rolling than that of the coastal lowlands. Average annual precipitation in the uplands ranges from about 21 inches in the Rio Grande Valley to about 50 inches at the Louisiana border.
The Texas coastal uplands aquifer system furnishes large quantities of water for agricultural, public, and industrial needs. Water withdrawn for public supply generally contains dissolvedsolids concentrations of less than 1,000 milligrams per liter. Slightly saline water with dissolvedsolids concentrations that range from 1,000 to 3,000 milligrams per liter can be used for many agricultural and industrial purposes. Nearly onehalf of all freshwater withdrawn from the Texas coastal uplands aquifer system during 1985 was pumped for agricultural use from Zavala, Frio, Atascosa, and Dimmit Counties in the west.
HYDROGEOLOGY
Deposits of the early Tertiary Claiborne and Wilcox Groups compose the Texas coastal uplands aquifer system (fig. 69). The sediments, in order of dominance, consist mostly of sand, silt, and clay and are distributed as relatively uniform sequences of predominantly fine or coarsegrained material.
The Texas coastal uplands aquifer system is subdivided into four aquifers and two confining units. These are, from shallowest to deepest, the upper Claiborne aquifer; the middle Claiborne confining unit; the middle Claiborne aquifer; the lower Claiborne confining unit; the lower Claiborne­upper Wilcox aquifer; and the middle Wilcox aquifer. The widespread, intensively pumped lower Claiborne­upper Wilcox aquifer has been chosen to illustrate the aquifer system. Other aquifers in the system, though of lesser importance, show similar geometry, hydraulic characteristics, and waterquality trends.
The landward boundary of the aquifer system is at the updip limit of the outcrop of the Wilcox Group. The Gulfward boundary is generally the farthest downdip extent of water in the aquifer system that has a dissolvedsolids concentration of less than 10,000 milligrams per liter (fig. 68). The top of the aquifer system is either land surface or the base of the Vicksburg­Jackson confining unit. The base of the aquifer system is either its contact with the top of the Midway confining unit or the approximate depth at which the water in the system has a dissolvedsolids concentration that exceeds 10,000 milligrams per liter. The altitude of the base of the aquifer system is shown in figure 70. The base ranges from less than 1,000 feet above sea level to nearly 8,000 feet below sea level. The thickness of the freshwater sands of the aquifer system ranges from 0 to nearly 3,000 feet (fig. 71).
LOWER CLAIBORNE­UPPER WILCOX AQUIFER
GroundWater Hydraulics
Highly permeable sands that contain large volumes of freshwater over an extensive area make the lower Claiborne­upper Wilcox aquifer the most important aquifer in the Texas coastal uplands aquifer system. The lower Claiborne­upper Wilcox aquifer is recharged by the infiltration of precipitation that falls on topographically high aquifer outcrop areas. Natural discharge occurs as evapotranspiration, loss of water to streams in outcrop areas, and as upward leakage in downdip areas. Recharge and discharge are generally less than 1 inch per year in areas that have little or no pumpage. Water in the aquifer is generally unconfined in aquifer outcrop areas where the specific yield for the sandy deposits might range between 10 and 30 percent. Water is confined in downdip areas by the overlying lower Claiborne confining unit. In these areas, the storage coefficient of the aquifer is estimated to range between 1.0x10-4 and 1.5x10-3.
The thickness of the sands of the lower Claiborne­upper Wilcox aquifer that contain freshwater is shown in figure 72. Maximum sand thickness is nearly 1,000 feet in some western areas. Transmissivity for the aquifer, as estimated from a digital groundwater flow model, is shown in figure 73. Although the transmissivity is generally less than 5,000 feet squared per day, maximum values are nearly 15,000 feet squared per day in the west.
GroundWater Quality
In extensive areas, the concentration of dissolved solids in water from the lower Claiborne­upper Wilcox aquifer is less than 500 milligrams per liter (fig. 74). The water is fresh (dissolvedsolids concentrations less than 1,000 milligrams per liter) in nearly the entire eastern onehalf of the aquifer and in most of the western onehalf. Concentrations exceed 1,000 milligrams per liter in the central and western downdip areas.
GroundWater Development
Withdrawals from the lower Claiborne­upper Wilcox aquifer during 1985 totaled 296 million gallons per day (table 3). This was nearly threefourths of the total water withdrawn from the Texas coastal uplands aquifer system. Much of the water pumped from the lower Claiborne­upper Wilcox aquifer is used for irrigation in the agricultural Winter Garden area (fig. 75). This area is defined as all or major parts of Atascosa, Dimmit, Frio, La Salle, and Zavala Counties, and minor parts of Bexar, McMullen, and Wilson Counties. Combined withdrawals from Atascosa, Frio, Dimmit, and Zavala Counties accounted for nearly onehalf of the water withdrawn from the Texas coastal uplands aquifer system during 1985 (table 4). The combination of infrequent killing frosts and fertile soils make the Winter Garden area ideal for growing garden vegetables and other food crops. Intense pumpage for irrigation in the Winter Garden area has created a large cone of depression on the potentiometric surface of the lower Claiborne­upper Wilcox aquifer (fig. 75). The lowering of the potentiometric surface from predevelop-ment conditions to 1982 was more than 250 feet in parts of Zavala, Dimmit, and Frio Counties (fig. 76). To sustain the large pumpage, recharge rates in parts of the outcrop are estimated to have increased by about 1 to 3 inches per year, and large amounts of water have been obtained from aquifer storage.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater, including some slightly saline water used predominantly for irrigation, from the Texas coastal uplands aquifer system totaled 397 million gallons per day during 1985 (fig. 77). Approximately 210 million gallons per day was withdrawn for agricultural purposes, the principal water use. About 127 million gallons per day was withdrawn for public supply and about 14 million gallons per day was withdrawn for domestic and commercial uses. About 46 million gallons per day was withdrawn for industrial, mining, and thermoelectricpower uses.
POTENTIAL FOR DEVELOPMENT
For the aquifers of the Texas coastal uplands aquifer system, the potential for development is greater in the east than the west, because precipitation and, thus, recharge potential is higher, and the extent of freshwater in the aquifers is greater. In some areas, particularly the Winter Garden area, the aquifers already are overdeveloped. In this area, the aquifers are being pumped in excess of recharge, and declining water levels are creating problems of excessive pumping lifts and migration of highly mineralized water into the pumped wells.
The Texas Water Development Board has made projections of groundwater use to 2030. For the eight counties that withdrew the largest amounts of water from the Texas coastal uplands aquifer system during 1985, the State projects a large decline in pumpage for seven counties and an increase in one county (table 5). Pumpage is predicted to decline from 36 to 83 percent below 1985 rates. For the combined eight counties, the total projected pumpage in 2030 is 59 percent less than the 1985 pumpage.

EDWARDS­TRINITY AQUIFER SYSTEM
INTRODUCTION
The Edwards­Trinity aquifer system is in carbonate and clastic rocks of Cretaceous age in a 77,000squaremile area that extends from southeastern Oklahoma to western Texas (fig. 78). The aquifer system consists of three complexly interrelated aquifers‹the Edwards­Trinity, the Edwards, and the Trinity aquifers (figs. 78 and 79). The Edwards­Trinity and the Trinity aquifers are stratigraphically equivalent in part and are hydraulically connected in some places. The Edwards aquifer overlies the Trinity aquifer (fig. 79) and the two aquifers are hydraulically connected where no confining unit separates them. The groundwater flow systems and permeability of the three aquifers are sufficiently different, however, to allow them to be separately mapped and described. 
In the Trans-Pecos area (the area west of the Pecos River) and Edwards Plateau area of western and westcentral Texas, the Edwards­Trinity aquifer consists of rocks of the Washita, the Fredericksburg, and the Trinity Stages, and the Coahuilan Series (fig. 80). In the Balcones Fault Zone area of southcentral Texas, the rocks of the Washita and the Fred-ericksburg Stages are far more permeable than those of the overlying confining unit or the underlying Trinity aquifer and constitute the nearly separate flow system of the Edwards aquifer. Rocks of the Trinity Stage and the Coahuilan Series constitute the Trinity aquifer, which crops out on its updip edge from the Hill Country of southcentral Texas into southeastern Oklahoma. In eastcentral Texas and into Oklahoma, rocks of the Washita and the Fredericksburg Stages that overlie the Trinity aquifer constitute a confining unit.
The rocks that compose the Edwards­Trinity aquifer are relatively flatlying and are generally exposed at the land surface in the Trans-Pecos and the Edwards Plateau areas (fig.78). The geologic formations that compose the Trinity and the Edwards aquifers generally are exposed in updip areas, but they dip eastward and southward beneath younger units and lie deep in the subsurface. The downdip boundary of each aquifer approximately coincides with the farthest updip extent of water that contains 10,000 milligrams per liter dissolved solids. 
The base of the Edwards­Trinity aquifer system, which is an erosional unconformity developed on the surface of preCretaceous rocks, is shown in figure 81. Generally, the base slopes toward the southsoutheast; the gradient steepens in a downdip direction. The altitude of the base ranges from more than 5,000 feet below sea level in the northeast to more than 3,000 feet above sea level in the west.
Withdrawals of freshwater from the Edwards­Trinity aquifer system totaled about 794 million gallons per day during 1985 (fig. 82). About 370 million gallons per day was withdrawn for agricultural purposes, slightly more than the 343 million gallons per day withdrawn for public supply. About 43 million gallons per day was pumped for industrial, mining, and thermoelectricpower uses, and the remaining 38 million gallons per day was withdrawn for domestic and commercial uses. 
Areas with the largest rates of withdrawal are shown in figure 83. Withdrawals for municipal and industrial uses during 1980 exceeded 10 million gallons per day in the seven Metropolitan Statistical Areas shown in the figure, as defined by the Texas Department of Water Resources. These areas are Austin, Dallas, Fort Worth­Arlington, Odessa, San Antonio, Sherman­Denison, and Waco. Pumpage of 225 million gallons per day in the San Antonio area was far greater than that of the other Metropolitan Statistical Areas. Withdrawals for irrigation during 1985 exceeded 10 million gallons per day in 10 counties; pumpage was more than 100 million gallons per day in Uvalde County (fig. 83).
Edwards­Trinity Aquifer
The Edwards­Trinity aquifer consists of rocks of Cretaceous age that are present in an area of about 35,500 square miles in westcentral Texas (fig. 84). The aquifer is referred to in much of the literature as the ³Edwards­Trinity (Plateau) aquifer.² The area underlain by the aquifer is mostly on the Edwards Plateau, but it also extends into the Trans-Pecos area. The aquifer is located in the Great Plains Physiographic Province except for a small part that is in the Basin and Range Physiographic Province.
The topography of much of the area is characterized by flat to rolling, largely rocky plains that are dissected in places to form steepwalled canyons (fig. 85). The area is bounded on the west by mountain ranges. The altitude of the land surface ranges from about 1,000 feet at the Rio Grande in Val Verde County to more than 4,500 feet in the Davis Mountains in Jeff Davis County. Average annual precipitation ranges from about 12 inches in the west to about 30 inches in the east. Average annual runoff ranges from about 0.2 inch in the west to about 5 inches in the east. The Edwards­Trinity aquifer supplies large amounts of water for irrigation, particularly in the northwestern area; it also provides water to many small towns and cities.
HYDROGEOLOGY
During Jurassic and very early Cretaceous time, the rocks in the area were subjected to erosion, and a flat to undulating plain was formed. This erosional surface, which underlies the Edwards­Trinity aquifer, was developed on rocks that range in age from Cambrian to Triassic (fig. 86). The early Cretaceous sea then advanced northward from the Gulf of Mexico across Texas. Deposition of clastic and carbonate rocks accompanied repeated advances and retreats of the ancient sea. The rocks that compose the Edwards­Trinity aquifer are generally limestone in the upper part and sand and sandstone in the lower part. The rocks dip and thicken to the southeast. Thickness of the aquifer ranges from a few tens of feet to more than 1,000 feet (fig. 87).
The complex nomenclature and lithologic character of the many geologic formations that compose the Edwards­Trinity aquifer are discussed in detail in several of the reports listed in the ³References² section. The nomenclature is generalized for use in this Atlas (see fig. 80) and follows the local practice in which Early Cretaceous rocks are discussed in terms of provincial stratigraphic stages or series. The rocks of the Washita Stage and the Fredericksburg Stage consist generally of thinbedded to massive limestone; the rocks of the Trinity Stage and the Coahuilan Series consist mostly of sand and sandstone, with some limestone and shale.
The base of the aquifer slopes generally to the south and southeast. The altitude of the base ranges from about 2,000 feet below sea level in the south to more than 3,000 feet above sea level in the west (fig. 88). Most of the rocks that underlie the Edwards­Trinity aquifer are much less permeable than those that compose the aquifer and, thus, serve as a barrier to groundwater flow. Locally, however, the underlying rocks are permeable and are hydraulically connected to the Edwards­Trinity aquifer, thus extending the thickness of the flow system.
The top of the aquifer is at land surface with the exception of areas capped with small, scattered remnants of Del Rio Clay or Buda Limestone and about 1,500 square miles in the northwest where the aquifer is covered by thick deposits of Pecos River alluvium. 
 The aquifer is generally recharged by direct precipitation on the land surface. Water is mostly unconfined in the shallow parts of the aquifer and is confined in the deeper zones.
The altitude of the potentiometric surface of the Edwards­Trinity aquifer during the winter of 1974­75 ranged from about 1,000 feet to more than 3,500 feet above sea level (fig. 89). Ground water in the extreme west moves generally toward the Pecos River; elsewhere, the regional movement of water is toward the southeast. Much of the natural discharge from the aquifer is as spring flows along the southeastern edge of the Edwards Plateau where erosion has cut the rocks of the Edwards Group down to the water table. Springs that are present at the headwaters of many streams in these areas contribute substantially to stream base flow.
Depths of producing wells completed in the Edwards­Trinity aquifer typically range from 150 to 300 feet. Wells commonly yield from 50 to 200 gallons per minute. Well yields can vary greatly depending on the amount of development of secondary permeability in the limestone; yields from jointed and cavernous limestone can be as much as 3,000 gallons per minute. The water is generally a hard, calcium bicarbonate type and typically has concentrations of dissolved solids that range from 400 to 1,000 milligrams per liter.
GROUNDWATER DEVELOPMENT
Irrigation constitutes the most important use of water withdrawn from the Edwards­Trinity aquifer. Irrigation is concentrated in the northwestern part of the area where soil conditions are particularly favorable for farming. For much of the area, the lack of soil cover and the generally rocky terrain are the factors that limit the use of ground water for irrigation, rather than lack of water in the aquifer.
In the Trans-Pecos area, major irrigation areas are in southeastern Reeves County and northern Pecos County. In these areas, pumpage from the aquifer for irrigation in the 1960¹s and 1970¹s was about 89 million gallons per day. Groundwater levels declined nearly 150 feet from the late 1950¹s to the early 1970¹s in an irrigated area about 15 miles westnorthwest of Fort Stockton. Withdrawals in this area were about 8 million gallons per day during 1974. Intense pumpage around Fort Stockton, particularly southwest of the city, has caused the cessation of flow from Comanche Springs at Fort Stockton. The springs once flowed at about 29 million gallons per day.
During 1972, withdrawal from the Edwards­Trinity aquifer for irrigation in the Edwards Plateau area was about 55 million gallons per day, or about 70 percent of the total water withdrawn in that area. Glasscock, Midland, and Reagan Counties are the principal users of irrigation water in the Edwards Plateau. Declining water levels and decreasing well yields have accompanied development. In southern Glasscock County, the groundwater level declined more than 100 feet from 1937 to 1966. In northern Reagan County, the water level declined 95 feet from 1954 to 1969 and another 50 feet from 1970 to 1987.
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the Edwards­Trinity aquifer totaled about 145 million gallons per day during 1985 (fig. 90). About 115 million gallons per day was withdrawn for agricultural purposes, the principal water use. About 14 million gallons per day was withdrawn for each of two use categories: public supply and industrial, mining, and thermoelectricpower use. About 2 million gallons per day was withdrawn for domestic and commercial uses.
Edwards Aquifer
The Edwards aquifer consists of highly faulted and fractured carbonate rocks of Cretaceous age in an area of about 4,000 square miles in southcentral Texas (fig. 91). This aquifer is referred to in some reports as the ³Edwards (Balcones Fault Zone) aquifer.² Most of the aquifer is within the Coastal Plain Physiographic Province, although some updip areas are in the Great Plains Physiographic Province.
The area underlain by the Edwards aquifer is a combination of agricultural and ranch land and areas of dense population, including the cities of Austin in Travis County and San Antonio in Bexar County. The topography consists of a gently rolling plain to the east and moderately hilly country to the west. The altitude of the land surface ranges from about 500 feet above sea level at the Colorado River at Austin to about 1,500 feet above sea level in Uvalde County.
Average annual precipitation ranges from about 22 inches in the west to about 34 inches in the east. Average annual runoff ranges from about 1 inch in the west to about 6 inches in the northeast. Major streams that cross the area flow southward and southeastward and include the Nueces, the Frio, the Medina, the Guadalupe, the Blanco, the Colorado, and the San Gabriel Rivers.
The aquifer underlies parts of 10 counties and is separated into northern and southern parts by a groundwater divide in about the middle of Hays County. The northern part is called the Austin area and consists of Bell, Travis, and Williamson Counties. The southern part, called the San Antonio area, consists of Bexar, Comal, Hays, Kinney, Medina, and Uvalde Counties. The aquifer also underlies an extremely small part of northwestern Guadalupe County; because pumpage in this county is negligible, it is excluded from further discussion.
The 1985 population in the threecounty Austin area was about 810,000. During 1985, withdrawals from the Edwards aquifer in the Austin area were about 17 million gallons per day. The sixcounty San Antonio area had a 1985 population of about 1.3 million. Withdrawals from the Edwards aquifer in the San Antonio area during 1985 were about 450 million gallons per day. The city of San Antonio, which has a population of nearly 1 million, derives its entire water supply from the Edwards aquifer.
HYDROGEOLOGIC FRAMEWORK
The Edwards aquifer consists of limestone and dolomite of the Washita and the Fredericksburg Stages. The complex nomenclature and lithologic character of the formations that compose the aquifer are generalized for purposes of this Atlas, but are discussed in detail in several of the reports listed in the ³References² section. The aquifer generally consists of the Kainer and the Person Formations of the Edwards Group and the Georgetown Formation (fig. 80).
After deposition of Cretaceous rocks in westcentral Texas, tectonic movement caused the relative uplift of the Edwards Plateau and subsidence of the Gulf of Mexico. Structural forces caused deformation and fracturing of the rocks, and a number of en echelon, northeastwardtrending faults formed in the region known as the Balcones Fault Zone. The numerous faults have formed wedges or blocks of rock that are generally downthrown to the south and southeast in the form of stairsteps. The Edwards aquifer is generally coincident with the fault zone. The length of the arcshaped aquifer is about 240 miles. The northern boundary is in central Bell County where the thickness of the aquifer and its importance as a source of ground water are diminished.
The width of the aquifer ranges from about 4 miles at the Colorado River at Austin to about 30 miles in Medina and Williamson Counties. The updip boundary of the aquifer in most places is the farthest updip extent of the Edwards Group, except in the westernmost area where the updip boundary is determined by a decreased incidence of faulting. From Kinney County eastward and northward to the Colorado River at Austin, the updip boundary generally coincides with the Balcones Escarpment. 
The downdip boundary of the aquifer is largely fault controlled. As a result of the faulting, the chemical quality of the water in the Edwards aquifer can change abruptly in a very short distance across a zone often referred to as the ³badwater line.² Along this line, the water is fresh on the upthrown side of a fault and very saline (usually a sodium or calciumsulfate type water) on the downthrown side (fig. 92). The downdip boundary of the aquifer in the San Antonio area is the downdip extent of water that contains less than 1,000 milligrams per liter of dissolved solids, whereas in the Austin area, it is the downdip extent of water that contains less than 3,000 milligrams per liter of dissolved solids.
The Edwards aquifer is underlain by the much less permeable Walnut Formation or Glen Rose Limestone of the Trinity aquifer. Where the Edwards aquifer does not crop out, it is confined above by the Del Rio Clay (fig. 80). The aquifer dips to the south and southeast, and is offset by numerous faults (figs. 93 through 95). The aquifer thickens from northeast to southwest and ranges in thickness from a few feet in outcrop areas to about 800 feet in Medina and Uvalde Counties (fig. 96).
The base of the aquifer slopes generally to the south and southeast. The altitude of the base ranges from more than 2,000 feet below sea level in the south to more than 500 feet above sea level in updip areas (fig. 97). The top of the aquifer, where it is confined by the Del Rio Clay, ranges from 1,500 feet below sea level in the south to more than 500 feet above sea level (fig. 98).
GROUNDWATER MOVEMENT, RECHARGE, AND DISCHARGE
The generalized altitude of the potentiometric surface in 1974 (San Antonio area) and 1981 (Austin area) ranged from less than 500 feet above sea level at the Colorado River to more than 1,100 feet above sea level in Uvalde and Kinney Counties (fig. 99). Groundwater movement is generally downdip, but in the San Antonio area, flow in the confined zone is toward the east and northeast where numerous northeast-wardtrending faults have a substantial influence on the direction of groundwater flow (fig. 100). Vertical displacement of the aquifer along faults may place rocks of high and low permeability opposite each other (figs. 94 and 95) and, thus, may create a partial or total barrier to the normal downdip flow of ground water. In places, flow is diverted to a direction that approximately parallels the faults. The effect of barrier faults on the potentiometric surface and the direction of groundwater movement is shown in more detail for Medina and Uvalde Counties in figure 101.
The faults and fractures also serve as points of entry for recharge, as illustrated in figure 102. Runoff that originates on the Edwards Plateau is augmented by groundwater discharge along the eroded edge of the Plateau. As streams cross the Balcones Fault Zone, water percolates downward along the faults where permeability might be greatly enhanced by partial dissolution of limestone. Secondary sources of recharge are direct infiltration of precipitation that falls on aquifer outcrop areas, internal flow of ground water from the Trinity aquifer where the Edwards and the Trinity aquifers are juxtaposed, and upward leakage from the underlying Trinity aquifer where an upward vertical head gradient exists. Direct recharge to the aquifer can be quite rapid through sinkholes (fig. 103).
Water levels in wells completed in the Edwards aquifer rise immediately and springflows increase quickly after major recharge events, thus attesting to a dynamic flow system and the rapid movement of large volumes of water. Because of the great depth of the water table below land surface in most of the area, groundwater losses to evapotranspiration are assumed to be minor. Diffuse leakage into or out of the aquifer also is assumed to be minor. Recharge from streams and precipitation, and discharge from springs and wells can be measured. Thus, an estimated water budget can be computed for the Edwards aquifer for any period for which records are available.
An analysis of longterm (between 1917 and 1987) re-cords provides an estimate of a longterm average water budget for the Edwards aquifer. Total recharge to the aquifer averaged 686 million gallons per day; average discharge to springs was 425 million gallons per day, and well withdrawals averaged 251 million gallons per day for a total average discharge of 676 million gallons per day. Thus, the small net increase in storage of 10 million gallons per day was small. Withdrawals from the aquifer have been increasing over the years and are now substantially greater than the longterm average presented here. This is clearly shown for the San Antonio area by the graph in figure 104.
The water budget varies considerably from the average during any given month or year, depending largely on the amount and distribution of precipitation. During 1956, which was the final year of a long drought, recharge to the aquifer was only about 7 percent of the longterm average. In contrast, during 1987, which was an exceptionally wet year, recharge was more than 3 times the longterm average. In years of belownormal precipitation and recharge, the ratio of well discharge to spring discharge tends to increase, and water that is stored in the aquifer may be substantially depleted. For example, during 1980, annual precipitation was about 3 or 4 inches less than average. Recharge was only about 380 million gallons per day, or about 55 percent of the longterm average. Discharge to springs was about 300 million gallons per day and about 460 million gallons per day was withdrawn from wells during 1980. Depletion of water from storage in the aquifer was about 380 million gallons per day.
The amount of recharge and discharge varies substantially from county to county within the area because of such factors as topography, streamflow characteristics, soil type, geology, faulting, solution openings, distribution of precipitation, landuse patterns, and so forth. During 1980, about 70 percent of the total recharge to the Edwards aquifer was in Kinney, Uvalde, and Medina Counties. About 84 percent of the total withdrawal from wells was from Bexar (58 percent) and Uvalde (26 percent) Counties. Nearly 70 percent of the total spring discharge was from two springs: 48 percent from Comal Springs in Comal County (fig. 105), and 22 percent from San Marcos Springs in Hays County. Spring locations are shown in figure 106.
AQUIFER HYDRAULIC PROPERTIES AND WATER QUALITY
The Edwards aquifer is the most transmissive of all the aquifers in Texas and Oklahoma. Estimates of transmissivity values for the Edwards aquifer in most of the San Antonio area range from about 200,000 to 2,000,000 feet squared per day. Variations in transmissivity are considerable over relatively short distances and depend upon the amount of development of solution openings along fractures and faults. Large discharges from springs and from flowing and pumped wells attest to the highly permeable nature of the aquifer.
For 61 years of record, Comal Springs, which is the largest spring that issues from the Edwards aquifer, had an average discharge of 185 million gallons per day; a maximum daily discharge of 434 million gallons per day was recorded on November 25, 1985. Some individual wells operated by the city of San Antonio yield more than 16,000 gallons per minute, which ranks them among the largestyielding wells in the world. Many species of subterranean aquatic organisms exist in the large solution openings deep within the aquifer; for example, toothless, blind catfish live more than 1,900 feet beneath the land surface and are occasionally discharged from flowing or pumped wells.
The aquifer becomes less transmissive toward the Austin area, particularly north of the Colorado River where the aquifer is thinner and less permeable. Estimated transmissivity values in this area range from less than 2 to about 40,000 feet squared per day.
The average specific yield in the unconfined zone of the Edwards aquifer in the San Antonio area is estimated to be 3 to 4 percent. The storage coefficient in the confined zone is estimated to range from about 1x105 to 1x104. The estimated volume of water in storage in the confined freshwater zone of the aquifer in the San Antonio area is 19.5 million acrefeet (6.4x10 12 gallons).
The concentration of dissolved solids in water from the Edwards aquifer typically ranges from 300 to 1,200 milligrams per liter. The dissolvedsolids concentration increases from a few hundred milligrams per liter in the recharge zone to more than 1,000 milligrams per liter at varying distances downdip. The transition from water with a dissolvedsolids concentration of 1,000 milligrams per liter to water with a concentration of 3,000 milligrams per liter is generally sharp. The width of this transition zone ranges from less than 1 or 2 miles in most of the area to about 11 miles in Williamson County.
GROUNDWATER DEVELOPMENT
To prehistoric man, Indian tribes, Spanish explorers, cattle drivers, immigrant pioneers, and the present population, the springs and springfed rivers and underground water from the Edwards aquifer have been, and continue to be, an attractive and vital resource in this region. Today, many uses compete for water from the aquifer, including public and industrial supplies, agriculture, tourism, and ecosystems associated with the springs and springriver systems.
Development of water from the Edwards aquifer has been unequal in a geographical sense. Development in the Austin area has been minor, primarily because of reliance on surfacewater supplies for most needs, particularly for the city of Austin. During 1985, groundwater withdrawals in Bell, Travis, and Williamson Counties totaled 17 million gallons per day for mostly smaller public supplies (table 6).
Development in the San Antonio area has been far greater, with a total withdrawal of about 450 million gallons per day during 1985. The largest user of ground water is Bexar County, where about 232 million gallons per day was withdrawn during 1985 (table 6). The city of San Antonio in Bexar County has a population of nearly 1 million and derives its total water supply of about 157 million gallons per day from the Edwards aquifer. Another important use of water from the aquifer in the San Antonio area is for agricultural purposes. During 1985 in Uvalde and Medina Counties, withdrawals, which were mostly for irrigation, totaled about 140 million and 54 million gallons per day, respectively (table 6). Withdrawals in the San Antonio area of about 450 million gallons per day during 1985 were more than four times the rate of withdrawal in the 1930¹s. The increase in withdrawals was relatively steady from 1934 to 1987 (fig. 104).
 A prolonged drought can severely stress the aquifer because of increased groundwater withdrawals. After a prolonged drought that culminated in 1956, regional groundwater levels and springflows reached record lows. Most springs ceased to flow, including Comal Springs where zero flow was recorded from June 13 to November 4, 1956. San Marcos and Barton Springs continued to flow, but at greatly reduced rates. Greaterthannormal precipitation that began in 1957 led to a recovery of water levels and springflow to predrought conditions in less than 2 years (fig. 104).
FRESH GROUNDWATER WITHDRAWALS
Withdrawals of freshwater from the Edwards aquifer totaled about 467 million gallons per day during 1985 (fig. 107). About 229 million gallons per day was withdrawn for public supply, and about 201 million gallons per day was withdrawn for agricultural purposes. About 25 million gallons per day was withdrawn for domestic and commercial uses, and withdrawals for industrial, mining, and thermo-electricpower uses were about 12 million gallons per day.
POTENTIAL FOR DEVELOPMENT
Although well withdrawals have been increasing over the years, the Edwards aquifer has the capacity to sustain the withdrawals during times of normal or abovenormal precipitation. During times of belownormal precipitation, water from storage in the aquifer is temporarily depleted; this water is replenished with the onset of increased precipitation. During times of severe, prolonged drought, concern for declining groundwater levels, decrease