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Texas Water Development BoardReport 316Evaluation ofGround-Water ResourcesIn theLower Rio Grande Valley, TexasbyT. Wesley McCoy, GeologistJanuary 1990
Evaluation of Ground-Water Resources in the Lower Rio Grande Valley, TexasJanuary 1990Texas Water Development BoardG. E. (Sonny) Kretzschmar, Executive AdministratorTexas Water Development BoardWalter W. Cardwell, III, ChairmanWesley E. PittmanThomas M. DunningStuart S. Coleman, Vice ChairmanGlen E. RoneyCharles W. JennessAuthorization for use or reproduction of any origianl material contained in thispublication, i.e., not obtained from other sources, is freely granted. The Board wouldappreciate acknowledgement.Published and Distributedby theTexas Water Development BoardP.O. Box 13231, Capitol StationAustin, Texas 78711·3231111
Evaluation of Ground-Water Reaourcetl in the Lower Rio Grande Valley. TexasJanuary 1990ABSTRACTIn 1985, the Sixty-ninth Texas Legislature enacted House Bill 2, whichdirected the State water agencies to identify critical ground-waterareas within the State of Texas, conduct studies on those areas, andsubmi t the findings along with recom mendations on whether a groundwater conservation district should be established in the study area.One area so identified for study is in Cameron, Hidalgo, Starr, andWillacy Counties, collectively known as the Lower Rio Grande Valley.The study area is located at the southern tip of the Texas Gulf CoastalPlain and has a semi-arid climate with low to moderate rainfall and ahigh rate of evaporation. Agriculture dominates the region's economy.Water needs for the Lower Rio Grande Valley are provided almostentirely from the Rio Grande via storage in Amistad and FalconReservoirs. Surface water accounts for over 97 percent of the totalwater used in the Valley, with irrigation being the predominant use.Ground water in the area is produced in small amounts from Eoceneage strata and the Miocene-age Oakville Sandstone in Starr County,and in moderate to large amounts from the Evangeline and Chicotaquifers in Cameron, Hidalgo, and Willacy Counties. Water levels inthe study area declined dramatically in the 1950's due to heavyirrigation pumpage and a severe drought. Since the construction ofreservoirs on the Rio Grande and the shift to surface-water use, areawater levels have been rising steadily.The chemical quality of ground water over most of the study area ispoor and does not meet Texas Department of Health recommendeddrinking water standards. Dissolved solids usually range from 1,000to 5,000 milligrams per liter, with sodium, chloride, and sulfatedominating the hydrochemistry. Additionally, high boron and nitrateconcentrations appear to be widespread throughout the area. Ingeneral, the ground wa ter is un sui table for irriga tion wi thout practicingspecial agricultural techniques.In 1985, the total pumpage of ground water in the Lower Rio GrandeValley was 17,268 acre-feet. Total surface-water use was 824,250 acrefeet. Surface water has been and will continue to be the most importantsource of water supply for the area. Projected total annual water usethrough the year 2010 does not exceed total water use in the area in1980 Adequate ground water should be available to meet projectedneeds through the year 2010.v
Evaluation of Ground- Water Resources in the Lower Rio Grande Valley, TexaaJanuary 1990TABLE OF CONTENTSPageABS'fRACT . vINTRODUCTION . 1Purpose . 1Geography. 1Topography and Drainage . 1Climate . 3Economy . 3Previous Investigations . 3Acknowledgements . 7GEOHYDROLOGY. 8Geology. 8Structure . 8Stratigraphy . 8Source and Occurrence of Ground Water. 10Hydraulics. 21Water Levels . 21Water Quality . 27GROUND-WATER PROBLEMS . 37Ground-Water Quality. 37PROJECTED WATER DEMANI) . 38Population . 38Water Use . 38AVAILABILITY OF WATER. 43Current and Projected Availability of Ground Water. 43Current and Projected Availability of Surface Water. 43Potential for Conjunctive Use of Ground and Surface Water. 44Potential for Additional Ground-Water Development. .44Projected Availability Through the Year 2010. .44SUMMARY. 45SELECTED REFERENCES . 46TABLES1. Stratigraphic and Hydrologic Section of the LowerRio Grande Valley Area . 142. Current and Projected Population of Study Area . 393. Current and Projected Water Use by Demand in Study Area . .41Vll
Evaluation or Ground-Water Reaourcee in the Lower Rio Gnlnde Valley, TexalJanuary 1990TABLE OF CONTENTS -(continued)FIGURESPage1. Location of Study Area . 22. Average Annual Precipitation, and Average Monthly Precipitation versusGross Lake Evaporation for Period of Record at Selected Stations . 53. Structural and Physiographic Regional Setting of thePost-Eocene Texas Gulf Coastal Plain . 94. Geologic Map of Study Area . 115. Approximate Productive Areas of the Major Sources of Ground Waterin the Lower Rio Grande Valley . . 156. Stratigraphic Cross-Section A - A', Lower Rio Grande Valley . 177. Stratigraphic Cross-Section B - B', Lower Rio Grande Valley . 198. Altitude of Water Level in Evangeline and Chi cot Aquifers in theLower Rio Grande Valley, 1988 . 239. Water-Level Rise and Decline in Selected Wells in theLower Rio Grande Valley, 1970 through 1988 . 2510. Chemical-Quality Diagrams Showing Concentration and Ratio of Constituents inWater from Selected Wells and from the Rio Grande . 2911. Chemical-Quality Diagrams Showing the Change in the Concentration andRatio of Constituents in Water from Selected Wells . . 3112. Chemical Quality of Water in the Evangeline and Chicot Aquifers . 3313. Classification oflrrigation Waters in the Lower Rio Grande Valley . 3614. Current and Projected Population, Lower Rio Grande Valley . 4015. Current and Projected Water Use, Lower Rio Grande Valley . 42IX
Evaluation oCGround-Water Reaources in the Lower Rio Grande Valley, TexasJanuary 1990INTRODUCTIONRecognizing that certain areas of the State were experiencing, or wereexpected to experience within the next 20 years, critical ground-waterproblems, the Sixty-ninth Texas Legislature enacted House Bill 2 in1985. House Bill 2 directed the State water agencies to identify criticalground-water areas, conduct studies of those areas, and submit thefindings and recommendations on whether a groun d-water conservationdistrict should be established in the respective areas in order toaddress ground-water problems (Subchapter C, Chapter 52, TexasWater Code).PurposeThe study area covered by this report includes Cameron, Hidalgo,Starr, and Willacy Counties, an ar !a collectively referred to as theLower Rio Grande Valley (Figure 1). In this report, major emphasis isplaced on eastern Starr County and all of Cameron, Hidalgo, andWillacy Counties.GeographyThe Lower Rio Grande Valley is the southernmost portion of the TexasGulf Coast Plain. The most prominent feature of the region is the RioGrande, which forms the western and southernmost boundaries of theValley, as well as Texas' border with Mexico.Topography andDrainageMost ofthe Lower Rio Grande Valley forms a broad, flat plain whichrises gently from sea level at the Gulf of Mexico in the east to anelevation of approximately 500 feet in western Starr County. Thewestern edge ofthis plain culminates in a westward-facing escarpmentkno\\-n as the Bordas escarptment. The upland plain consists primarilyof unconsolidated sand and clay, while the escarpment is made up ofmore resistant sandstone and clay.Drainage in the region is into either the Rio Grande or the LagunaMadre through small coastal streams. The Rio Grande has no largetributaries in the area, but several small intermittent streams draininto the river in Starr County and western Hidalgo County. TheArroyo Colorado floodway is a prominent drainage feature whichheads near Mission in southern Hid.algo County, flows east-northeastthrough western Cameron County to form part of the county linebetween Cameron and Willacy Counties, and eventually empties intothe Laguna Madre.Much of the drainage in Cameron County empties into the LagunaMadre through former distributary channels ofthe Rio Grande calledresacas. Drainage in northeastern Starr County, northern HidalgoCounty, and much ofWillacy County is into shallow depressions whichform small lakes or ponds; the water dissipates by percolation into thesubsurface and by evaporation.1
Evaluation of Ground-Water Reoourc . in the Lower Rio Grande Valley. Texa . January 1990Figure 1LOCATION OF STUDY AREA2
Evaluation of Ground-Water Resources in the Lower Rio Grande Valley, TexasJanuary 1990The climate of the Lower Rio Grande Valley is subtropical and semiarid. Temperature extremes, either high or low, are very uncommon.The annual average daily low temperature is about 63 F and the dailyhigh temperature averages about 84 l". Summer temperatures usuallyreach into the mid to upper 90's and winter temperatures can drop intothe 20's, though the latter is rare.ClimateThe average annual precipitation ranges from about 22 to 26 inches,increasing from west to east across the study area (Figure 2). Mostprecipitation falls during the spring from April through June, andduring the late summer and early fall, from August through October.Spring precipitation is the result of seasonal transition as inflowingwarm, moist air from the Gulf of Mexico and the Pacific Oceangenerates thunderstorms. The period oflate summer to early fall is thehurricane season, during which Atlantic and Gulf storms may moveashore along the Texas or upper Mexican Gulf Coast. These storms cangenerate tremendous amounts of rainfall over a very short period oftime causing extensive flooding due to the flat nature of the terrain.The average annual gross lake evaporation varies from 81 inches inStarr County to 61 inches in Cameron County. Lake-surface evaporationrates are highest in the summer months (Figure 2).The economy ofthe Lower Rio Grande Valley is primarily agricultural,including both intensive irrigation farming and ranching. In 1985,agriculture generated some 462.5 million for the entire region (TexasAgricultural Statistics Service, 1985). In that same year, approximately570,000 acres was irrigated, mainly with surface water from the RioGrande. The area is a leader in the State in the production of citrusfruit, vegetables, and cotton.EconomyOil and gas production is a secondary, but locally important industryin Hidalgo, Starr, and WHlacy Counties. Oil was first discovered in theregion in Starr County in 1929. In 1H86, 5.9 million barrels of crude oilwas produced in the counties ofthe Lower Rio Grande Valley (RailroadCommission of Texas, 1986).Tourism is also an important minor industry for Cameron, Hidalgo,and Willacy Counties. Brownsville and McAllen are an importantgateway for travel to and from Mexico. In addition, the beach on SouthPadre Island and numerous state parks attract many visitors. Duringthe fall and winter, the Valley is subject to a large seasonal influx ofmany retirees from northern states who prefer to winter in south Texasbecause ofthe mild climate. Known in the Valley as "winter Texans",these visitors usually arrive in October and return to their home statesin April.Numerous ground-water investigations have been conducted on specificareas of the Lower Rio Grande Va.lley, but only one study has beenundertaken covering all four counties ofthe area. In addition, the areahas been included in several regional studies of the Texas Gulf coastalplain and the Rio Grande basin. The results of these studies andinvestigations were published as reports or bulletins by the TexasWater Development Board or its predecessor agencies.PreviousInvestigations3
Eva1uation of Ground- Water Resources in the Lower Rio Grande VaHey, TexasJanuary 1990The principal ground-water study of the Lower Rio Grande Valley wasmade by the U.S. Geological Society (Baker and Dale, 1961) andpublished in two volumes as Bulletin 6014 by the Texas Board ofWaterEngineers. This study covered the geology and ground-water hydrology of the area and addressed the questions of ground-water availability and quality.Two smaller, more recent ground-water investigations have beenconducted by the Texas Department of Water Resources. The firstinvestigation covered ground-water availability and quality in an areanorthwest of the City of Brownsvi1l. in Cameron County (Preston,1983). The second investigation reported the results of test-holedrilling by the agency near Mission in southwestern Hidalgo County(Molofsky, 1985).In addition to the above ground-water studies, county reports havebeen issued for Cameron County (Dale and George, 1954) and StarrCounty (Dale, 1952). Various basinal and regional reports are alsoavailable on the Rio Grande basin and the Texas Gulf coastal plain thatinclude the Lower Rio Grande Valley (Wood, et al., 1963; Baker, 1965;Baker, 1979; Carr, et al., 1985). Geologic mapping of the Lower RioGrande Valley is best available on the McAllen-Brownsville GeologicAtlas sheet published by the University of Texas Bureau of EconomicGeology. Soils surveys published by the Soil Conservation Service ofthe U.S. Department of Agriculture are available for Cameron, Hidalgo, Starr, and WiUacy Counties.The Texas Water Development Board, in accordance with its legislatively mandated data-collection activities, maintains a network of 49water·level observation wells in the four counties of the Lower RioGrande Valley, which are visited annually to obtain water-level measurements. In addition, as part of its water-quality monitoring program, the agency has 774 chemical an alyses of water samples collectedfrom 257 wells.This report was prepared under the general supervision of Robert L.Bluntzer, Henry J. Alvarez, and Dr. Tommy Knowles of the TexasWater Development Board.Acknowledgements7
Evaluation of Ground·Water Resources in the Lower Rio Grande Valley. TexaaJanuary 1990GEOHYDROLOGYGeologyStructureSince the end of the Mesozoic era tens of thousands of feet of erodedmaterial from the North American Continent have been steadilydeposited into the Gulf of Mexico basin in series of cross-cutting andoverlapping layers of fluvial, lacustrian, and eolian sediments onto athick progradational deltaic and continental-slope platform (Galloway,1982). From time to time this basin has subsided in order to accept thistremendous amount of clastic debris. During brief, intermittentperiods of Cenozoic history, when the rate of deposition was less thanthe rate of basinal subsidence, the sea would invade the land surfaceresulting in the deposition of transgressive marine sequences.The post-:E ocene Texas Gulf Coastal Plain forms a . relatively flatsurface, which dips gradually gulfward. Further inland, this plainabuts more resistant Cretaceous and Eocene strata, which form low,strike-parallel escarpments (Figure 3). In the study area, Eocene-agestrata form the Bordas escarpment in western Starr County.The Rio Grande and Houston embayments, along with the San Marcosarch, comprise the major structural elements of the post-Eocene TexasGulf Coastal Plain (Figure 3). Since the Mesozoic, these structureshave influenced sedimentation on the coastal plain. Major faultsystems, such as the Balcones Fault Zone and the Pearsall-LulingMexia Fault Zone, rim the basin, breaking up the upper Cretaceousand Eocene strata. These fault zones also serve as a buffer between theEdwards Plateau of central and western Texas and the Texas GulfCoastal Plain.Deposition within the Rio Grande embayment has been fairly steadysince the end ofthe E
E'V"3.111ationl. of Grou.nd-Wat er Resources irl thE: Lo"rel Rio (;r lnde Valley, Texas . i.e., not obtained from other sources, is freely granted. The Board would . dominating the hydrochemistry. Additionally, high boron and nitrate concentrations appear to be widespread throughout the area. In general, the ground wa ter is un sui table .
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Welcome to the Southern Trust's Annual Volunteer Report for 2015//2016. This report provides an up-date on the progress made by the Trust against the action plan under the six key themes of the draft HSC Regional Plan for Volunteering in Health and Social Care 2015-2018: Provide leadership to ensure recognition and value for volunteering in health and social care Enable volunteering in health .
