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Research about Uvalde County, Texas

Source-linked reports with geographic coverage including Uvalde County, Texas.

5 recordsLinked to original sources
USGS2016

Groundwater ages from the freshwater zone of the Edwards aquifer, Uvalde County, Texas—Insights into groundwater flow and recharge

Tritium–helium-3 groundwater ages of the Edwards aquifer in south-central Texas were determined as part of a long-term study of groundwater flow and recharge in the Edwards and Trinity aquifers. These ages help to define groundwater residence times and to provide constraints for calibration of groundwater flow models. A suite of 17 samples from public and private supply wells within Uvalde County were collected for active and noble gases, and for tritium–helium-3 analyses from the confined and unconfined parts of the Edwards aquifer. Samples were collected from monitoring wells at discrete depths in open boreholes as well as from integrated pumped well-head samples. The data indicate a fairly uniform groundwater flow system within an otherwise structurally complex geologic environment comprised of regionally and locally faulted rock units, igneous intrusions, and karst features within carbonate rocks. Apparent ages show moderate, downward average, linear velocities in the Uvalde area with increasing age to the east along a regional groundwater flow path. Though the apparent age data show a fairly consistent distribution across the study area, many apparent ages indicate mixing of both modern (less than 60 years) and premodern (greater than 60 years) waters. This mixing is most evident along the “bad water” line, an arbitrary delineation of 1,000 milligrams per liter dissolved solids that separates the freshwater zone of the Edwards aquifer from the downdip saline water zone. Mixing of modern and premodern waters also is indicated within the unconfined zone of the aquifer by high excess helium concentrations in young waters. Excess helium anomalies in the unconfined aquifer are consistent with possible subsurface discharge of premodern groundwater from the underlying Trinity aquifer into the younger groundwater of the Edwards aquifer.

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USGS2003

Geologic framework and hydrogeologic characteristics of the Edwards aquifer, Uvalde County, Texas

The Edwards aquifer in Uvalde County is composed of Lower Cretaceous carbonate (mostly dolomitic limestone) strata of the Devils River Formation in the Devils River trend and of the West Nueces, McKnight, and Salmon Peak Formations in the Maverick basin. Rocks in the Devils River trend are divided at the bottom of the Devils River Formation into the (informal) basal nodular unit. Maverick basin rocks are divided (informally) into the basal nodular unit of the West Nueces Formation; into lower, middle, and upper units of the McKnight Formation; and into lower and upper units of the Salmon Peak Formation. The Edwards aquifer overlies the (Lower Cretaceous) Glen Rose Limestone, which composes the lower confining unit of the Edwards aquifer. The Edwards aquifer is overlain by the (Upper Cretaceous) Del Rio Clay, the basal formation of the upper confining unit. Upper Cretaceous and (or) Lower Tertiary igneous rocks intrude all stratigraphic units that compose the Edwards aquifer, particularly in the southern part of the study area. The Balcones fault zone and the Uvalde salient are the principal structural features in the study area. The fault zone comprises mostly en echelon, high-angle, and down-to-the-southeast normal faults that trend mostly from southwest to northeast. The Uvalde salient—resulting apparently from a combination of crustal uplift, diverse faulting, and igneous activity—elevates the Edwards aquifer to the surface across the central part of Uvalde County. Downfaulted blocks associated with six primary faults—Cooks, Black Mountain, Blue Mountain, Uvalde, Agape, and Connor—juxtapose the Salmon Peak Formation (Lower Cretaceous) in central parts of the study area against Upper Cretaceous strata in the southeastern part. The carbonate rocks of the Devils River trend and the Maverick basin are products of assorted tectonic and depositional conditions that affected the depth and circulation of the Cretaceous seas. The Devils River Formation formed in a fringing carbonate bank—the Devils River trend— in mostly open shallow marine environments of relatively high wave and current energy. The West Nueces, McKnight, and Salmon Peak Formations resulted mostly from partly restricted to open marine, tidal-flat, and restricted deep-basinal environments in the Maverick basin. The porosity of the Edwards aquifer results from depositional and diagenetic effects along specific lithostratigraphic horizons (fabric selective) and from structural and solutional features that can occur in any lithostratigraphic horizon (non-fabric selective). In addition to porosity depending upon the effects of fracturing and the dissolution of chemically unstable (soluble) minerals and fossils, the resultant permeability depends on the size, shape, and distribution of the porosity as well as the interconnection among the pores. Upper parts of the Devils River Formation and the upper unit of the Salmon Peak Formation compose some of the most porous and permeable rocks in Uvalde County.

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USGS1997

Geologic framework of the Edwards Aquifer and upper confining unit, and hydrogeologic characteristics of the Edwards Aquifer, south-central Uvalde County, Texas

The stratigraphic units of the Edwards aquifer in south-central Uvalde County generally are porous and permeable. The stratigraphic units that compose the Edwards aquifer in south-central Uvalde County are the Devils River Formation in the Devils River trend; and the West Nueces, McKnight, and Salmon Peak Formations in the Maverick Basin. The Balcones fault zone is the principal structural feature in Uvalde County; however, the displacement along the fault zone is less in Uvalde County than in adjacent Medina and Bexar Counties to the east. The Uvalde Salient is a structural high in south-central Uvalde County, and consists of several closely connected crustal uplifts that bring Edwards aquifer strata to the surface generally forming prominent hills. The crustal uplifts forming this structural high are the remnants of intrusive and extrusive magnatic activity. Six primary faults—Cooks, Black Mountain, Blue Mountain, Uvalde, Agape, and Connor—cross the length of the study area from the southwest to the northeast juxtaposing the Lower Cretaceous Salmon Peak Formation at the surface in the northwestern part of the study area against Upper Cretaceous formations in the central part of the study area. In the study area, the porosity of the rocks in the Edwards aquifer is related to depositional or diagenetic elements along specific stratigraphic horizons (fabric selective) and to dissolution and structural elements that can occur in any lithostratigraphic horizon (not fabric selective). Permeability depends on the physical properties of the rock such as size, shape, distribution of pores, and fissuring and dissolution. The middle 185 feet of the lower part of the Devils River Formation, the upper part of the Devils River Formation, and the upper unit of the Salmon Peak Formation probably are the most porous and permeable stratigraphic zones of the Edwards aquifer in south-central Uvalde County.

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USGS1964

Geology and ground-water resources of Uvalde County, Texas

The principal aquifer in Uvalde County is the Edwards and associated limestones of Cretaceous age. The aquifer underlies an extensive area in south-central Texas extending along the Balcones fault zone from Kinney County eastward to San Antonio, and thence northeastward to Hays County. The hydrologic unit making up the Edwards and associated limestones consists of the Comanche Peak limestone, the Edwards limestone, the Kiamichi formation, and the Georgetown limestone. Other less important aquifers in Uvalde County include the Glen Rose limestone, the Buda limestone, the Austin chalk, and the Leona formation. Uvalde County occupies parts of two physiographic provinces, the Edwards Plateau on the north and the Coastal Plain on the south; the two provinces are separated by the Balcones fault zone. In the Edwards Plateau the formations of the Edwards and associated limestones crop out on the plateau surface; in the Coastal Plain where the formations have been down-faulted, they underlie younger rocks. In both provinces the formations dip gently toward the south and southeast; the dips are greatest in the Coastal Plain. Igneous intrusions have penetrated the formations in many places in the Coastal Plain, and locally have formed barriers to ground-water movement. The aquifers in Uvalde County are recharged by precipitation within the county and in the drainage areas of streams entering the county from the north and west. In the Edwards Plateau, the Edwards and associated limestones are recharged by precipitation that falls on the outcrop. The aquifer is drained by springs at the contact with the underlying Glen Rose limestone where streams have cut through the Edwards at the edge of the plateau. These springs maintain the base flow of the streams that drain the plateau. Most of the base flow and much of flood flow of the streams is lost to the Edwards and associated limestones where they crop out in streambeds in the Balcones fault zone. This stream loss to the aquifer constitutes the greatest part of the recharge in the county, although some of the recharge from the West Nueces River enters the county as underflow from Kinney County. The normal annual recharge to the Edwards in Uvalde County is estimated to be about 200,000 acre-feet. Discharge from the Edwards and associated limestones in Uvalde County can be divided into two .major segments: discharge by underflow from the county to the east and south and discharge to the surface through wells and springs. Discharge by underflow during the period 1934-47, when changes in storage were small, is estimated to have been about 190,000 acre-feet per year. During the drought years 1947-56 the underflow was somewhat less. The discharge to the surface during 1934-47 averaged about 17,000 acre-feet per year. During the 1947-56 drought the rate of discharge to the surface increased principally because of increased use of water for irrigation, reaching a maximum of 58,000 acre-feet in 1956. The chemical quality. of the ground water in Uvalde County ranges between wide limits. Except in the extreme southern part of the county where the water is saline, the water in the Edwards and associated limestones is of good chemical quality except that it is hard. The water in the Glen Rose limestone is saline in many places; the principal objectionable constituents are high concentrations of calcium and magnesium sulfate. The water in the Leona formation is generally of good chemical quality. The water from the other formations varies widely in quality from place to place and no generalizations can be made. Ground-water withdrawals from the Edwards and associated limestones in Uvalde County probably could be maintained indefinitely at a rate of about 200,000 acre-feet per year, provided that withdrawals north and west of the county were not increased. However, continued withdrawals at this rate-would cause wells in structurally high areas to go dry, and underflow into Medina County would cease. Furthermore, saline water might invade the fresh-water part of the aquifer from the south, and perennial spring flow in the Leona River valley would cease.

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