Aquifers of the Edwards Plateau


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Aquifers of the Edwards Plateau

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Aquifers of the Edwards Plateau
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Aquifers of the Edwards Plateau
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Mace, Robert E.
Angle, Edward S.
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Aquifers of the Edwards Plateau, Vol. 360 (2004).

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1Chapter 1 Aquifers of the Edwards Plateau Robert E. Mace1 and Edward S. Angle1 Introduction The Edwards Plateau occupies the west-central part of Texas, extending from the Hill Country near Austin and San Antonio up to the mountains of West Texas and extending into the High Plains. Because of low rainfa lls, the frequency of drought, and few major rivers, groundwater is an im portant source of water to th e people and environmental resources of the Edwards Plateau area. The hydrogeologic centerpiece of the Edwards Plateau is the Edwards–Trinity (Plateau) aquife r, one of the major aquifers of the state. Around and in hydraulic connection with this centerpiece are a number of major and minor aquifers including the Capitan Reef, Cenozoic Pecos Alluvium, Dockum, Edwards (Balcones Fault Zone), Ellenburger-San Saba , Hickory, Lipan, Marble Falls, Ogallala, Rustler, and Trinity aquifers. Many towns a nd rural areas in the Edwards Plateau area rely on groundwater. Total groundwater usag e in the area has ranged from about 500,000 to over 700,000 acre-ft per year over the past 20 years. A better understanding of how these aquifers behave is important for better understanding how to best manage the scarce water resources that do exist in the Edward s Plateau. The purpose of this paper is to present a general overvi ew of the aquifers of the Edward s Plateau and recent scientific and planning activities c oncerning these aquifers. Location, Physiography, and Climate Our focus is on the Edwards Plateau area of Texas (Figure 1-1). This area comprises a large part of state and includes the follo wing 51 counties: Andrews, Bandera, Blanco, Brewster, Brown, Burnet, Coke, Coleman, C oncho, Crane, Crockett, Culberson, Ector, Edwards, Gillespie, Glasscock, Howard, Ir ion, Jeff Davis, Kendall, Kerr, Kimble, Kinney, Lampasas, Llano, Loving, Martin, Ma son, McCulloch, Menard, Midland, Mills, Mitchell, Nolan, Pecos, Reagon, Real, Reeves , Runnels, San Saba, Sc hleicher, Sterling, Sutton, Taylor, Terrell, Tom Green, Upton, Uvalde, Val Ve rde, Ward, and Winkler. A total of 39 counties have popul ations of less than 20,000 pe ople as of 2000 with four counties (Ector, Midland, Taylor, and To m Green) with populations of more than 100,000 people (Table 1-1). The population in the area has grown by almost 80 percent since 1950, increasing by more than 440,000 people (Table 1-1). However, 19 counties 1 Texas Water Development Board

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2 N0 50 miles Texastwdb2003-019 Figure 1-1: Location of the study area in the Edwards Plateau area. Bexar, Comal, Hays, Medina, Presidio, and Travis counties are shown for orientation purposes and are not considered in this paper. have decreased in population since 1950. To tal groundwater use has ranged from 443,000 to 954,000 acre-feet per year ove r the past twenty years (F igure 1-2). In 2000, 26 counties got more than 75 percent of their water from aquifers of the Edwards Plateau (Table 1-1). This part of Texas is primarily located in the southern portion of Great Plains Province (Fenneman, 1931), which is characterized by as ymmetric ridges or mountains and broad intervening basins (Bates and Jackson, 1984). Elevations range from 5,000 feet above sea level in the western portion of the region to 500 feet above sea level on the eastern side. The Rio Grande and the Colorado and Pecos rive rs are the major rivers than cut through the Edwards Plateau area (Figure 1-3). Th e headwaters of the Guadalupe, Nueces, and San Antonio rivers are also located in the Ed wards Plateau area (Figure 1-3). Flow in the Rio Grande in this part of Texas is primar ily controlled by inflows from the Rio Conchos near Presidio. The Pecos River is a major tri butary to the Rio Grande that originates in New Mexico. The river is impounded in Red Bl uff Lake in Loving County and is used for irrigation in Pecos, Reeves, and Ward counties. Most of the study area ranges from subhumid in the eastern portion to semiarid in the western areas (Walker 1979). Average annua l precipitation ranges from less than 10

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3Table 1-1: Population and groundwater us e for counties in the Edwards Plateau area for selected years. _________ Population __________ _ Groundwater use (acre-feet) _ County 1950 1980 1990 2000 1980 1990 2000 %GW Andrews 5,002 13,323 14 ,338 13,004 21,443 15,132 24,123 99.7 Bandera 4,410 7,084 10,562 17,645 1,320 1,848 2,653 85.7 Blanco 3,780 4,681 5,972 8,418 886 1,514 1,288 74.1 Brewster 7,309 7,573 8,681 8,866 3,126 2,551 3,967 92.5 Brown 28,607 33,057 34,371 37,674 1,049 1,611 2,788 13.0 Burnet 10,356 17,803 22,677 34,147 2,123 1,946 2,957 33.2 Coke 4,045 3,196 3,424 3,864 451 678 1,070 37.6 Coleman 15,503 10,439 9,710 9,235 257 113 115 4.0 Concho 5,078 2,915 3,044 3,966 1,595 3,287 3,473 91.1 Crane 3,965 4,600 4,652 3,996 2,780 2,676 2,081 59.1 Crockett 3,981 4,608 4,078 4,099 6,606 4,561 3,376 87.4 Culberson 1,825 3,315 3,407 2,975 76,119 12,580 27,030 99.9 Ector 42,102 115,374 118,934 121,123 25,144 20,551 17,546 28.4 Edwards 2,908 2,033 2,266 2,162 1,310 854 1,041 90.3 Gillespie 10,520 13,532 17,204 20,814 4,242 5,729 6,325 89.2 Glasscock 1,089 1,304 1,447 1,406 40,443 27,491 35,788 99.9 Howard 26,722 33,142 32,343 33,627 2,682 4,141 6,103 38.8 Irion 1,590 1,386 1,629 1,771 1,030 1,458 1,542 56.6 Jeff Davis 2,090 1,647 1,946 2,207 26,872 3,767 1,084 96.4 Kendall 5,423 10,635 14,589 23,743 1,748 2,322 3,499 79.6 Kerr 14,022 28,780 36,304 43,653 5,716 3,176 3,818 43.0 Kimble 4,619 4,063 4,122 4,468 1,103 845 707 25.7 Kinney 2,668 2,279 3,119 3,379 10,834 8,394 15,833 99.4 Lampasas 9,929 12,005 13,521 17,762 1,192 993 1,872 8.0 Llano 5,377 10,144 11,631 17,044 1,958 2,122 1,824 27.4 Loving 227 91 107 67 64 44 46 11.2 Martin 5,541 4,684 4,956 4,746 21,118 13,919 15,693 97.4 Mason 4,945 3,683 3,423 3,738 16,861 18,077 11,602 97.1 Mcculloch 11,701 8,735 8,778 8,205 7,515 6,060 7,137 96.2 Menard 4,175 2,346 2,252 2,360 709 767 1,132 28.4 Midland 25,785 82,636 106,611 116,009 31,975 34,173 32,945 52.3 Mills 5,999 4,477 4,531 5,151 1,340 1,245 952 19.4 Mitchell 14,357 9,088 8,016 9,698 3,611 2,249 7,103 39.1 Nolan 19,808 17,359 16,594 15,802 3,710 3,611 6,079 59.8 Pecos 9,939 14,618 14,675 16,809 111,250 67,552 78,563 97.7 Reagan 3,127 4,135 4,514 3,326 24,378 39,919 18,724 99.8 Real 2,479 2,469 2,412 3,047 632 770 480 51.1 Reeves 11,745 15,801 15 ,852 13,137 120,524 40,117 68,285 85.9 Runnels 16,771 11,872 11,294 11,495 2,027 1,866 973 27.8 San Saba 8,666 6,204 5,401 6,186 3,705 1,919 2,763 45.9 Schleicher 2,852 2,820 2,990 2,935 2,350 2,113 3,364 96.9 Sterling 1,282 1,206 1,438 1,393 2,245 1,814 1,813 96.1 Sutton 3,746 5,130 4,135 4,077 3,799 2,574 3,373 96.8 Taylor 63,370 110,932 119,655 126,555 2,891 914 872 2.0 Terrell 3,189 1,595 1,410 1,081 1,379 1,139 546 85.2 Tom Green 58,929 84,784 98,458 104,010 15,268 28,246 22,609 42.3 Upton 5,307 4,619 4,447 3,404 19,516 16,310 16,098 99.7

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4Table 1-1 (cont): Popula tion and groundwater use for counties in the Edwards Plateau area for selected years. _________ Population __________ _ Groundwater use (acre-feet) _ County 1950 1980 1990 2000 1980 1990 2000 %GW Uvalde 16,015 22,441 23,340 25,926 81,196 144,522 66,083 97.4 Val Verde 16,635 35,910 38,721 44,856 1,673 4,211 16,217 91.7 Ward 13,346 13,976 13,115 10,909 33,311 10,670 12,164 52.5 Winkler 10,064 9,944 8,626 7,173 8,356 3,171 5,516 99.9 Total: 255,811 549,592 660,934 754,099 621,194 310,308 447,935 63.5 % GW = percent of total water use in 1997 that was met with groundwater. Groundwater use includes use from all aquifers, including those not discussed in this paper. 0400 200 600 800 1,000198019841985198619871988198919901991199219931994199519961997Year 199819992000759 713 632 579 493 564 639 573 541 443 954 697 727 674 670 743 593 567twdb2004-001 Figure 1-2: Total groundwater use for the Edwards Plateau area of Texas.

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5 N0 50 miles Texastwdb2003-021 Figure 1-3: Location of major rive rs in the Edwards Plateau area. 10 10 10 10 12 12 14 14 16 16 18 18 20 2022 22 24 24 26 26 28 28 30 30 14 16 12 Average annual precipitation (inches) N0 50 miles Texastwdb2003-022 Figure 1-4: Amount of average annual pr ecipitation in the Edwards Plateau area (after Larkin and Bomar, 1983).

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6 N0 50 miles Average gross lake-surface evaporation rates (inches) Texas 95 91 87 83 81 80 79 80 83 87 91 93 81 79 7775 7975 73 71 69 67 65 63 63 65 67 69 71twdb2003-023 Figure 1-5: Amount of average gross lake-s urface evaporation rates in the Edwards Plateau area (after La rkin and Bomar, 1983). inches in the west to more than 30 inches in east (Figure 1-4). Late spring and early summer bring the greatest rainfalls to the eastern portion of the plateau while late summer results in the heaviest rainfall in the west ern areas. Average annual gross lake-surface evaporation rates range from less than 65 inches in east to more than 90 inches in west (Figure 1-5). Aquifers of the Edwards Plateau The Edwards Plateau area includes all or part of 12 aquifers recognized by the Texas Water Development Board (Figure 1-6). Five major aquifers, the Edwards (Balcones Fault Zone), the Edwards–Trinity (Plateau), the Cenozoic Pecos Alluvium, the Ogallala, and the Trinity are found in the area. Seven mi nor aquifers are also located in the area including the Capitan Reef, Dockum, Elle nburger-San Saba, Hickory, Lipan, Marble Falls, and Rustler aquifers. The Texas Wate r Development Board (TWDB) has assigned a major and minor status to the State's aquife rs based on the quantity of water supplied by each aquifer (Ashworth and Hopkins, 1995). In addition to the aquifer recognized by the TWDB, there are several other geologic formations that locally produce water.

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7 Figure 1-6: Location of recognized major and minor aquifers in Far West Texas (delineations from TWDB, map doe s not include the Igneous, West Texas Bolsons, and Marathon aquifers). The major aquifers have had a number of scie ntific studies done on them. However, with a few exceptions in some local areas, the mi nor aquifers have had little to almost no groundwater studies done on them. The general information presented below is from "Aquifers of Texas" by Ashworth and Hopkins (1995); "Aquifers of West Texas" by Mace and others (2001) ; the regional water plans of the Region F, Lower Colorado, Sout h Central Texas, Plateau, and Far West Texas regions; and water-use information fr om TWDB surveys and estimates. Bradley and Malstaff (2004, Chapter 10 of this volume) discuss drought in the area. Paine (2004, Chapter 12 of this volume) discusses natural an d oil-field contaminatio n in an area of the Edwards Plateau, and Standen and Opdyke (2004 ; Chapter 11 of this volume) discuss aquifer susceptibility to contaminants. Ed wards and others (2004, Chapter 13 of this volume) discuss the aquifer-dependant fishes of the Edwards Plateau. Keese and others (2004, Chapter 14 of this volume) evaluate climate, vegetation, and soil controls on groundwater recharge in Texas, includi ng the Edwards Plateau. Arroyo and Mullican

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8(2004, Chapter 15 of this volume) discuss desalination, and Hart (2004, Chapter 16 of this volume) discuss brush control. Chow dhury and others (2004, Chapter 17 of this volume) discuss the origin of flow to the San Solomon Spring system. Capitan Reef aquifer The Capitan Reef aquifer consists of tw o strips located in Brewster, Culberson, Hudspeth, Jeff Davis, Pecos, Reeves, War d, and Winkler counties (Figure 1-6) and extends northward into New Mexico. The aquife r is an ancient reef consisting of 2,360 ft of dolomite and limestone, and, in Texas, ge nerally has poor water quality except in the exposed areas of the aquifer. Most of the wa ter pumped from the aquifer is in Ward and Winkler counties for water-flooding operations in oil-producing areas. A small amount water is used for irrigation in Pecos and Culberson counties. Carlsbad, New Mexico, relies on the aquifer for municipal use. Pump ing from the aquifer in the Edwards Plateau area over the past twenty years has ranged fr om less than 30 to a bout 12,500 acre-ft per year (Table 1-2). Recent pumping has been less than 50 acre-ft per year. The Capitan Reef aquifer is discussed in more detail by Uliana (2001) in "Aquife rs of West Texas" (Mace and others, 2001). Cenozoic Pecos Alluvium aquifer The Cenozoic Pecos Alluvium aquifer is locate d in Andrews, Crane, Crockett, Culberson, Ector, Jeff Davis, Loving, Pecos, Reeves, Upton, Ward, and Winkler counties (Figure 16) and extends to the north into New Mexico. The aquifer consists of sands, gravels, and clays of ancient river deposits of up to 1,500 ft thick. The aquifer is connected to the Dockum and Edwards–Trinity (Plateau) aq uifers where they exist underneath the alluvium. Water quality is naturally highly vari able and has also been locally impacted by past activities of the petroleu m industry. Water levels have declined more than 200 ft in south-central Reeves and northwest Pecos c ounties but have remained somewhat steady since the 1970s with a decrea se in irrigation. Lowered wa ter levels have decreased baseflow to the Pecos River and, in some cases , now cause the river to lose water to the aquifer. Pumping from the aquifer in the Ed wards Plateau area over the past twenty years has ranged from about 64,000 to about 200,000 ac re-ft per year (Tab le 1-2). Reeves County has been the largest user of groundwater from the aquifer, using 67 percent of the total water pumped in 1997. The Cenozoic Pecos Alluvium aquifer is discussed in more detail by Jones (2004; Chapter 6 of this volume). Dockum aquifer The Dockum aquifer is located in Andrews, Coke, Crane, Crockett, Ector, Glasscock, Howard, Irion, Loving, Martin, Midland, Mi tchell, Nolan, Pecos, Reagon, Reeves, Sterling, Tom Green, Upton, War d, and Winkler counties in the Edwards Plateau area (Figure 1-6) and extends to th e north beneath the Ogallala aquifer and to the northwest

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9Table 1-2: Groundwater use for the different aquifers in the Edwards Plateau area (acre-feet). This table only incl udes groundwater use in the counties listed in the Location section of this paper. Year 1980 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 Aquifer 1997 1998 1999 2000 Capitan Reef Complex 12,450 826 645 95 62 62 582 181 583 156 724 642 510 398 41 26 26 28 Cenozoic Pecos Alluvium 199,327 127,460 102,719 93,947 75,260 79,109 106,541 71,377 70,348 64,220 388,502 152,290 159,427 150,621 151,371 165,084 143,806 132,456 Dockum 12,715 12,894 14,813 10,860 10,004 10,210 9,808 8,301 8,841 8,734 11,024 10,638 7,981 9,277 8,432 9,196 10,019 8,260 Edward (BFZ) 81,265 157,365 156,567 126,656 105,155 139,328 159,878 145,346 119,805 47,592 113,493 99,221 71,447 91,970 66,464 72,485 84,861 72,369 Edwards–Trinity (Plateau) 184,129 205,845 163,347 171,741 149,924 153,225 168,308 153,441 164,327 151,280 212,185 181,165 201,393 184,180 176,708 192,717 156,151 153,371 Ellenburger-San Saba 4,948 6,293 6,293 5,447 6,238 5,222 4,638 6,659 6,413 6,473 6,737 7,498 5,518 5,854 6,172 5,919 6,455 5,853 Hickory 28,348 16,728 28,250 26,764 24,595 27,616 28,130 26,402 26,638 20,857 24, 000 23,523 22,094 20,658 19,603 21,379 19,099 17,634 Igneous 5,135 2,465 2,613 2,677 2,185 2,623 2,118 2,217 2,727 2,646 2,594 2,762 2,697 2,629 2,968 3,237 3,239 2,635 Lipan 10,121 22,794 19,653 17 ,333 14,534 22,672 24,383 24,588 20,512 13,850 63, 867 60,581 76,180 35,230 66,292 72,298 25,781 37,560 Marble Falls 1,350 1,278 1,141 987 718 772 814 790 749 693 702 1,524 1,601 1,647 1,644 1,793 1,646 1,468 Ogallala 73,283 58,124 57,240 49,075 36,434 45,606 51,249 60,217 50,770 62,013 61, 609 88,476 92,930 86,018 76,134 83,031 83,625 73,097 Rustler 371 539 327 297 301 251 268 246 302 310 681 1,486 1,605 1,515 1,584 1,728 1,542 1,380 Trinity 14,693 12,385 13,640 13,326 12,181 13,074 14,465 13,505 13,234 14,250 16, 296 17,093 18,120 19,427 18,765 20,465 19,940 17,296 West Texas Bolsons 75,582 21,548 24,098 19,890 20,238 21,697 15,584 12,752 11,788 13,987 7,936 8,818 8,968 9,387 9,860 10,753 15,240 10,338 Other aquifers 25,308 24,208 23 ,509 20,697 19,002 24,226 23,813 15,989 18,581 16,203 23, 943 18,678 20,351 16,899 20,014 21,827 15,534 15,560 Total 729,025 664,606 614,855 559,792 476,831 545,693 610,579 542,011 515,618 423,264 934,293 674,395 690,822 635,710 626,052 681,938 586,965 549,305

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10into New Mexico. The Dockum aquifer consis ts of up to 700 ft of sand and conglomerate with layers of silt and shale of the Dockum Group. Water quality is variable and is used for water-flooding operations in oil-produci ng areas of the southern High Plains. Pumping from the aquifer in the counties in the study area over the past twenty years has ranged from about 8,000 to about 15,000 acreft per year (Table 1-2). The Dockum aquifer is discussed in more detail by Kala swad and Bradley (2004; Chapter 7 of this volume). Edwards (Balcones Fault Zone) aquifer The Edwards (Balcones Fault Zone) aquifer is located in Bexar, Comal, Hays, Kinney, Medina, Travis, and Uvalde counties in th e Edwards Plateau area (Figure 1-6) and extends north into Bell and Williamson coun ties. The Edwards (Balcones Fault Zone) aquifer consists of the Georgetown Limest one, formations of the Edwards Group and its equivalents, and the Comanche Peak limest one where it exists. The thickness of the aquifer ranges from 200 to 400 feet. Water qua lity and quantity are excellent. Much of the water is used for agricultural and munici pal purposes. Pumping from the aquifer in Kinney and Uvalde counties in the study area over the past twenty years has ranged from about 50,000 to about 160,000 acre-ft per year (Table 1-2). Mace and Anaya (Chapter 18 of this volume) discuss the Edwards (Balc ones Fault Zone) aquifer in Kinney County in more detail. The reader is directed to Maclay and Land (1988) and Klemt and others (1979) for more detailed overviews of the aquifer. The Edwards (Balcones Fault Zone) aquifer is expected to be the focus of an upcoming Aquifers of Texas conference. Edwards–Trinity (Plateau) aquifer The Edwards–Trinity (Plateau) aquifer is the hydrogeologic centerpiece of the Edwards Plateau and is hydraulically c onnected, in one way or the ot her, to every other aquifer discussed in this paper. The Edwards–Tr inity (Plateau) aquifer underlies Bandera, Blanco, Brewster, Coke, Concho, Crane, Crockett, Culberson, Ector, Edwards, Gillespie, Glasscock, Irion, Jeff Davis, Kerr, Kenda ll, Kimble, Kinney, Mason, McCulloch, Menard, Midland, Nolan, Pecos, Reagon, R eal, Reeves, Schleicher, Sterling, Sutton, Taylor, Terrell, Tom Green, Upton, Uvalde, Val Verde, Ward, and Winkler counties (Figure 1-6) and extends s outhward into Mexico. Equivalent rocks of the EdwardsTrinity (Plateau) aquifer exte nd north of the Plateau area under the Ogallala aquifer are recognized as the Edwards–Trinity (High Pl ains) aquifer (Ashworth and Hopkins, 1995). The Edwards–Trinity (Plateau) aquifer consists of rocks of the Comanche Peak, Edwards, and Georgetown Formations and the Trinity Group. The Trinity Group consists primarily of sands (Antlers and Maxim sands) and limes tones. The Comanche Peak, Edwards, and Georgetown Formations consist primarily of limestones and dolomites. Pumping from the aquifer over the past twenty year s has ranged from about 150,000 to about 200,000 acre-ft per yr (Table 1-2). The Edwards–Trin ity (Plateau) aquifer is discussed in more detail by Anaya (2004) for the aquifer in Te xas (Chapter 2 of this volume), by Boghici (2004) for the aquifer in Mexico (Chapter 4 of this volume), and by Nance (2004) on the

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11groundwater chemistry (Chapter 3 of this vol ume). Mace and Anaya (2004; Chapter 18 of this volume) discuss recharge in Kinney County in greater detail. Ellenburger-San Saba aquifer The Ellenburger-San Saba aquifer is lo cated in Blanco, Brown, Burnet, Coleman, Concho, Gillespie, Kendall, Kerr, Kimble , Lampasas, Llano, Mason, McCulloch, Menard, Mills, and San Saba c ounties (Figure 1-6). The aquife r consists of limestone and dolomite facies of Cambrian and early Ordovician age (Ashworth and Hopkins, 1995). The outcrop of the aquifer encircles the core of the Llano Uplift. The down-dip portions of the aquifer are as deep as 3,000 feet. The ma jority of water pumped from this aquifer is use for municipal supplies (Ashworth a nd Hopkins, 1995). Pumping from the aquifer over the past twenty years has ranged from about 5,000 to about 7,500 acre-ft per yr (Table 1-2). The Ellenburger-San Saba aqui fer is discussed in more detail by Smith (2004; Chapter 9 of this volume). Hickory aquifer This Hickory aquifer is located in Blan co, Brown, Burnet, Coleman, Concho, Gillespie, Hays, Kendall, Kerr, Kimble, Lampasas, Ll ano, Mason, McCulloch, Menard, Mills, San Saba, Travis, and Williamson and counties (Figur e 1-6). The aquifer consists primarily of sands and occurs in some of the oldest Ca mbrian sedimentary rocks in Texas (Ashworth and Hopkins, 1995). The outcrop areas encircle and overlie directly on the Precambrian metamorphic rocks that make up the Llano up lift. The down dip portions are as deep as 4,500 feet below land surface. Most of the wate r pumped from this aquifer is used for irrigation, although some high capacity wells are used for municipal supplies as well (Ashworth and Hopkins, 1995). Pumping from the aquifer over the past twenty years has ranged from about 17,000 to about 28,000 acre-ft per yr (Table 1-2). The Hickory aquifer is discussed in more detail by Smith (2004; Chapter 9 of this volume). Lipan aquifer The Lipan aquifer is located in Coke, C oncho, Runnels, and Tom Green counties (Figure 1-6). It consists of 125 feet of alluvial sediments of the Quaternary Leona Formation (Ashworth and Hopkins, 1995). The groundwater from the Lipan usually does not meet drinking water standards but is suitable for irrigation. Pumping from the aquifer over the past twenty years has range d from about 10,000 to about 76,000 acre-ft per yr (Table 12). The Lipan aquifer is discussed in more detail by Beach and Burton (2004; Chapter 8 of this volume). Marble Falls aquifer The Marble Falls aquifer is located in Bl anco, Burnet, Kimble, Lampasas, Llano, Mason, McCulloch, Menard, and San Saba counties (Figure 1-6). The a quifer consists of Pennsylvanian-age limestones and occurs as a series of disconti nuous outcrops that

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12surround the Llano Uplift area (Ashworth and Hopkins, 1995). Water o ccurs in fractures and solution cavities in the formation. Pump ing from the aquifer over the past twenty years has ranged from about 700 to about 1,800 acre-ft per yr (Table 1-2). The Marble Falls aquifer is discuss in detail by Smith (2004; Chapter 8 of this volume). Ogallala aquifer The Ogallala aquifer is located in Andrew s, Ector, Glasscock, Howard, Martin, and Midland counties in the Edwards Plateau ar ea (Figure 1-6). The Ogallala aquifer is composed primarily of sand, gravel, clay, and silt and generally has a saturated thickness of less than 100 ft in the Edwards Plateau area. The quality of water tends to be mixed in this part of the aquifer. The Ogallala a quifer partially overlies the Edwards–Trinity (Plateau) aquifer in the Edwards Plateau area. Pumping from the aquifer in the counties in the study area over the past twenty years has ranged fr om about 36,000 to about 93,000 acre-ft per year (Table 1-2). The Ogallala aquifer and its interaction with the Edwards– Trinity (Plateau) aquifer is discussed in more detail by Blandford and Blazer (2004) (Chapter 18 of this volume). Rustler aquifer The Rustler aquifer is located in Brewster, Culberson, Jeff Davis, Loving, Pecos, Reeves, and Ward counties (Figure 1-6). Groundwater occurs in the partially dissolved dolomite, limestone, and gypsum beds of the Rustler Form ation. The water is of poor quality and is used primarily for irrigation, livestock, and for water-flooding opera tions in oil-producing areas. Pumping from the aquifer in the count ies in the study area ove r the past twenty years has ranged from less than 300 to about 1,700 acre-ft per year (Table 1-2). The Rustler aquifer is discussed in more de tail by Boghici and van Broekhoven (2001) in "Aquifers of West Texas" (Mace and others, 2001). Other aquifers There are areas along the fringes of the Ed ward Plateau that do not have a TWDB recognized major or minor aquifer beneath th em (see white areas in Figure 1-6). This does not mean that there are no groundwater resources in these areas. These areas may have small, local aquifers that can supply water for limited purposes. According to the TWDB information, about 700 to as much as 11,000 acre-ft per year has been pumped from other aquifers in the area (Table 1-2) . Further study and evalua tion will increase our knowledge of water resources in these areas. Groundwater Conservation Districts Groundwater in Texas is governed by the rule of capture. Rule of capture allows a landowner to produce as much groundwater as the landowner chooses, absent malice or willful waste, without liabil ity to neighbors who may claim that pumping has depleted their wells. The Legislature enab led the regulation of groundwat er through the creation of

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13groundwater conservation distri cts the first of which, the High Plains Groundwater Conservation District No. 1, was created in 1949. Groundwater conser vation districts are recognized by the Legislature as the state's preferred me thod of managing groundwater resources. Depending on the rules and regulations of each district, absolute rule of capture does not necessarily apply within the boundary of a groundwater conservation district. The Edwards Plateau area is home to 35 c onfirmed groundwater conservation districts (Figure 1-7): 1. Bandera County River Aut hority/Ground Water District 2. Barton Springs/Edwards Aqui fer Conservation District 3. Blanco-Pedernales Groundwat er Conservation District 4. Brewster County Groundwater Conservation District 5. Coke County Underground Wa ter Conservation District 6. Cow Creek Groundwater Conservation District 7. Culberson County Groundwater Conservation District 8. Edwards Aquifer Authority 9. Emerald Underground Water Conservation District 10. Fox Crossing Water District 11. Glasscock Groundwater C onservation District 12. Hays Trinity Groundwater Conservation District 13. Headwaters Groundwater Conservation District 14. Hickory Underground Water C onservation District No. 1 15. Hill Country Groundwater Conservation District 16. Irion County Water Conservation District 17. Jeff Davis County Underground Water Conservation District 18. Kimble County Groundwater Conservation District 19. Kinney County Groundwater Conservation District 20. Lipan Kickapoo Groundwater Conservation District 21. Lone Wolf Groundwater Conservation District 22. Medina County Groundwater Conservation District 23. Menard County Underground Wa ter Conservation District 24. Middle Pecos Groundwater Conservation District 25. Permian Basin Underground Wa ter Conservation District 26. Plateau Underground Water Cons ervation and Supply District 27. Presidio County Underground Water Conservation District 28. Real and Edwards Conservati on and Reclamation District 29. Santa Rita Underground Wa ter Conservation District 30. Saratosa Underground Wate r Conservation District 31. Sterling County Underground Wa ter Conservation District 32. Sutton County Underground Wa ter Conservation District 33. Trinity Glen Rose Groundwat er Conservation District 34. Uvalde Underground Wate r Conservation District 35. Wes-Tex Groundwater C onservation District

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14 N0 50 miles Texas Presidio County UWCD Brewster County GCD Middle Pecos GCD Emerald UWCDSutton County UWCDPlateau UWC and Supply DistrictReal and Edwards C and R District Kinney County GCDBandera Co. RA/GWD Cow Creek GCD Headwaters GCDBlancoPedernales GCD Hill Country GCDKimble County GCD Menard County UWCDHickory UWCD No. 1 Saratoga UWCDFox Crossing Water District Medina County GCD Uvalde County UWCDTrinity Glen Rose GCD Hays Trinity GCD Barton Spring/ Edwards Aquifer CD Edwards Aquifer Authority WesTex GCD Lone Wolf GCD Lipan Kickapoo GCDSterling County UWCDCoke County UWCD Irion County WCDPermian Basin UWCDSanta Rita UWCD Glasscock GCD C and R= conservation and reclamation CD= conservation district GCD= groundwater conservation district RA/GWD= river authority/ground water district UWC= underground water conservation UWCD= underground water conservation district WCD= water conservation district WD= water districttwdb2003-025 Figure 1-7: Location of confirmed groundw ater conservation districts in the Edwards Plateau area. Regional Water Planning Through Senate Bill 1, the 1997 Legislature enacted comprehensive water management to plan for drought and meet increasing demands as population grows (Hubert, 1999). Senate Bill 1 is a “bottom up” water planning process that al lows individuals representing different interest groups to serve as members of Regiona l Water Planning Groups. The interest groups include the public, counties, municipalities , industries, agriculture, environmental, small business, steam-electric generating utilities, rive r authorities, water districts, water utilities, and others se lected by the Planning Groups. A total of 16 Regional Planning Areas cover the State. The Planning Groups are charged with preparing regional water plans for their resp ective planning areas. Th ese plans will show, for each planning area, how to conserve water, meet future water needs, and respond to future droughts. Each Planning Group submitted their first regi onal water plans in January of 2001. The TWDB integrated their indivi dual plans into a comprehensive State Water Plan which was released on January 5, 2002. The TWDB w ill only provide financial assistance to those projects that are consistent with the regional water plans, and the Texas Commission on Environmental Quality will only issue water right permits for

PAGE 15

15 N0 50 miles Texas Region F Far West TexasPlateau South Central Texas Lower Colorado Brazos G twdb2003-024 Figure 1-8: Location of regional water plan ning areas in the Edwards Plateau area. municipal purposes consistent with the pl an. These water plans are updated every five years. The Edwards Plateau area includes all of Re gion F and parts of the Brazos G, Far West Texas, Lower Colorado, Plateau, and South Cent ral Texas regions (Figure 1-8). All of the regions except the Far West Texas region have cities with needs by 2050. The Plateau region showed that projected water supplie s exceeds projected demands, although there are specific water user groups with needs. They also noted that they need more groundwater information and that endangered and threatened species could limit future water development options. Region F showed that projected water supplies do not meet projected water demands. Over the regions , the Brazos G, Lower Colorado, and South Central Texas regions showed that projected water supplies do not meet projected water demands. The regional water planning groups recommended a number of strategies to meet future needs for water, including: € new groundwater, € expanded use of existing groundwater, € new surface water, € expanded use of surface water, € conservation, and € water reuse.

PAGE 16

16The water plans for these regions can be found on the TWDB Web page (www.twdb.state.tx.us ). A statewide summary of the regi onal water plans is available in the 2002 State Water Plan (TWDB, 2002). Groundwater Availability Modeling Texas is developing new, state-of-the-art co mputer models of groundwater resources. In 1999, the Legislature provided initial funding for development of groundwater availability models (GAMs) for the major a quifers and in Senate Bill 2, enacted by the 77th Texas Legislature (2001), directed the TW DB to develop groundwater availability models for the minor aquifers. There are several completed and ongoing modeli ng projects in the Edward Plateau area. A GAM for the Hill Country part of the Trinit y aquifer that includes part of the Edwards– Trinity (Plateau) aquifer was completed in 2000 (Mace and others, 2000). A GAM for the southern part of the Ogallala aquifer was completed in early 2003 (Blandford and others, 2003). GAMs for the Cenozoic Pecos Alluvium, Edwards (Balcones Fault Zone), Edwards–Trinity (Plateau), Lipan, and northern part of the Trinity aquifers are expected to be completed in 2004. TWDB plans to develop GAMs for the Ca pitan Reef, Dockum, Ellenburger-San Saba, Hickory, Marble Falls, and Rustler aquifers, but development of these GAMs has not yet been scheduled. Planning Groups and groundwater conservation districts will use the models to assess availability of groundwater in the areas or regions. These assessments will be based on the socio-economic needs of their areas a nd may be guided by groundwater management standards that describe the desired future c ondition of the aquifer, such as the quantity and quality of groundwater and the amount of springflow, baseflow, and subsidence (Mace and others, 2001, 2002). Final reports, models, and aquifer information will be posted on the TWDB GAM Web page (www.twdb.state.tx.us/gam) . Summary The Edwards Plateau of Texas is blessed with many aquifers but faces many challenges to meet current and future water needs. The Edwards Plateau is dry and is susceptible to drought. Because of limited surface-water res ources, groundwater is often the only choice of water supply. Because of its size and owners hip of the rocks that form the plateau, the centerpiece of the Edwards Plateau is the Ed wards–Trinity (Plateau) aquifer. However, bordering aquifers are no less im portant to those that rely on them for water. In addition, the Edwards-Trinity (Plateau) aquifer is in hydraulic connection with many of its bordering aquifers. Groundwater conservation dist ricts, regional water plan ning groups, and groundwater availability models are helping to furthe r the understanding of the aquifers and the options for meeting future water needs. Howe ver, additional study is needed, particularly

PAGE 17

17on the less studied minor aquifers in the ar ea and on the less studied areas of the major aquifers. References Anaya, R., 2004, Conceptual model for the Ed wards–Trinity (Plateau) aquifer system, Texas: in Mace, R. E., Angle, E. S., and Mulli can, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 21-62. Arroyo, J. A., and Mullican, III, W. F., 2004, Desalination: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 293-302. Ashworth, J. B., and Hopkins, Janie, 1995, A quifers of Texas: Texas Water Development Board Report 345, 69 p. Bates, R. L., and Jackson, J. A., 1984, Di ctionary of geological terms: Anchor Press/Doubleday, Garden City, New York, 571 p. Beach, J. A., and Burton, S. T., 2004, The Lipan aquifer: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 165-180. Blandford, T. N., and Blazer, D. J., 2004, Hydrologic relationships and numerical simulations of the exchange of water be tween the southern Ogallala and Edwards– Trinity aquifers in southwest Texas: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 115-132. Blandford, T. N., Blazer, D. J., Calhoun, K. C., Dutton, A. R., Naing, T., Reedy, R. C., and Scanlon, B. R., 2003, Groundwater availabi lity of the southern Ogallala aquifer in Texas and New Mexico – Numerical si mulations through 2050: contract report by Daniel B. Stephens and Associates, Inc ., and the Bureau of Economic Geology, The University of Texas at Austin to the Texas Water Development Board, variably paginated. Boghici, R., 2004, Hydrogeology of Edwards–Trin ity aquifer of Texas and Coahuila in the border region: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 91114. Boghici, R., and van Broekhoven, N. G., 2001, Hydrogeology of the Rustler aquifer, Trans-Pecos Texas: in Aquifers of West Texas, Mace, R. E., Mullican, W. F., III, and Angle, E. S., eds., Texas Water Development Board Report 356, p. 207-225. Bradley, R. G., and Kalaswad, S., 2004, The Dockum aquifer in the Edwards Plateau: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 149-164.

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18Bradley, R. G., and Malstaff, G., 2004, Dry pe riods and drought events of the Edwards Plateau, Texas: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Wate r Development Board Report 360, p. 201-210. Chowdhury, A. H., Ridgeway, C., and Mace, R. E., 2004, Origin of the waters in the San Solomon Spring system, Trans-Pecos Texas: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 315-344. Edwards, R. J., Garrett, G. P., and Allan, N. L., 2004, Aquifer-dependent fishes of the Edwards Plateau region: : in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 253-268. Fenneman, N.M., 1931, Physiography of Wester n United States (1st ed.): New York, McGraw-Hill, 534 p. Hart, C., 2004, Brush management for water conservation: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 303-314. Hubert, Martin, 1999, Senate Bill 1: The first big bold step to ward meeting Texas's future water needs: Texas Tech Law Review, v. 30, no. 1, p. 53-70. Jones, I. C., 2004, Cenozoic Pecos Alluvium aquifer: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 133-148. Keese, K. E., Scanlon, B. R., and Reedy, R. C., 2004, Evaluating climate, vegetation, and soil controls on groundwater recharge using unsaturated flow modeling: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 269-292. Klemt, W. B., Knowles, T. R., Elder, G. R., and Sieh, T. W., 1979, Ground-water resources and model applications for the Ed wards (Balcones fault zone) aquifer in the San Antonio region, Texas: Texas Depart ment of Water Resources Report 239, 88 p. Mace, R. E., and Anaya, R., 2004, Estimate of recharge to the Edwards (Balcones Fault Zone)and Edwards-Trinity (Plateau) aquifers in Kinney County, Texas: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 345-366. Mace, R. E., Chowdhury, A. H., Anaya, R., and Way, S.-C., 2000, A numerical groundwater flow model of the Upper and Mi ddle Trinity aquifer, Hill Country area: Texas Water Development Board Open File Report 00-02, 62 p. Mace, R. E., Mullican, W. F. III, and Angle, E. S., eds., 2001, Aquifers of West Texas: Texas Water Development Board Report 356, 272 p. Mace, R. E., Mullican, W. F., III, and Wa y, T. (S.-C.), 2001, Estimating groundwater availability in Texas: in the proceedings of the 1st annual Texas Rural Water Association and Texas Water Conservation Association Water Law Seminar: Water

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19Allocation in Texas: The Legal Issues." Austin, Texas, January 25-26, 2001. Section 1, 16 p. Mace, R. E., Mullican, W. F., III and Williams, C., 2002, Groundwater supplies and availability in Texas: Results from regional water planning: in Dutton, S. P., Ruppel, S. C., and Hentz, T. F., Transactions of the Gulf Coast Asso ciation of Geological Societies and Gulf Coast Section of th e Society of Economic Paleontologists and Mineralogists, Volume LII, p. 681-694. Maclay, R. W., and Land, L. F., 1988, Simulati on of flow in the Edwards aquifer, San Antonio Region, Texas, and refinements of storage and flow concepts: U. S. Geological Survey Report Water-Supply Paper 2336, 48 p. Nance, H. S., 2004, Hydrochemical variability in the Edwards-Trinity aquifer system, Edwards Plateau, Texas: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 6390. Paine, J. G., 2004, Oil-field salinization scr eening on the Edwards Plateau using airborne geophysics: in Mace, R. E., Angle, E. S., and Mu llican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 235-252. Smith, R., 2004, Paleozoic aquifers of the Llano Uplift: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 181-200. Standen, A. R., and Opdyke, D. R., 2004, C ontamination migration, characteristics, and responses for the Edwards–Trinity (Plateau) aquifer: in Mace, R. E., Angle, E. S., and Mullican, W. F., III, eds., Aquifers of the Edwards Plateau: Texas Water Development Board Report 360, p. 211-234. TWDB, 2002, Water for Texas—2002: Texas Wa ter Development Board Document No. GP-7-1, 156 p. Uliana, M., 2001, The geology and hydrogeolog y of the Capitan aquiferA breif overview: in Aquifers of West Texas, Mace, R. E., Mullican, W. F., III, and Angle, E. S., eds., Texas Water Development Board Report 356, p. 153-166. Walker, L. E., 1979, Occurrence, availability, and chemical quality of ground water in the Edwards Plateau Region of Texas: Texas Department of Water Resources Report 235, 337 p.

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