Webb County, in semiarid South Texas on the U.S.-Mexico border, is a region confronted by increasing stresses on natural resources. Laredo (fig. 1), the largest city in Webb County (population 193,000 in 2000), was one of the 10 fastest-growing metropolitan areas in the country during 1990–2000 (Perry and Mackun, 2001). Commercial and industrial activities have expanded throughout the region to support the maquiladora industry (manufacturing plants in Mexico) along the border and other growth as a result of the passage of the North American Free Trade Agreement. The Rio Grande currently (2002) is the primary source of public water supply for Laredo and other cities along the border in Webb County (fig. 1). Other cities, such as Bruni and Mirando City in the southeastern part of the county, rely on ground-water supplies to meet municipal demands. Increased water demand associated with development and population growth in the region has increased the need for the City of Laredo and Webb County to evaluate alternative water sources to meet future demand. Possible options include (1) supplementing the surface-water supply with ground water, and (2) applying artificial storage and recovery (ASR) technology to recharge local aquifers. These options raise issues regarding the hydraulic capability of the aquifers to store economically substantial quantities of water, current or potential uses of the resource, and possible effects on the quality of water resulting from mixing ground water with alternative source waters.
To address some of these issues, the U.S. Geological Survey (USGS), in cooperation with the City of Laredo, began a study in 1996 to assess the ground-water resources of Webb County. A hydrogeologic study was conducted to review and analyze available information on the hydrogeologic units (aquifers and confining units) in Webb County, to locate available wells in the region with water-level and water-quality information from the aquifers, and to analyze the hydraulic properties of the aquifers. The purpose of this report is to document the findings of the study. The information is organized by hydrogeologic unit and presented on this and six other sheets.
Few studies have been devoted to characterizing the availability, yield, and quality of water from the aquifers in Webb County. Lonsdale and Day (1937) completed the first county-level reconnaissance investigation of Webb County and the major geologic units. Eargle (1968) revised the stratigraphic nomenclature of the Claiborne Group in Texas. More recently, the geology and ground-water resources of the Carrizo aquifer have been studied by the Texas Water Development Board (TWDB) and the USGS. Marquardt and Rodriguez (1977) compiled information on the locations of water and oil wells and available water-level and water-quality information from wells completed in the Carrizo aquifer in the Winter Garden area (fig. 1) of South Texas. Klemt and others (1976) used information from Marquardt and Rodriguez (1977) to develop a ground-water model for use in assessing the availability of ground water for future development. Mace and others (2000) compiled a database of hydraulic properties for wells completed in the Carrizo-Wilcox aquifer in Texas. Working with a minimal dataset in Webb County (four measurements), Mace and others (2000) estimated the average transmissivity of the Carrizo-Wilcox aquifer in Webb County to be 69 feet squared per day (ft2/d). On a more regional scale, the Texas Gulf Coast aquifer systems, including some of the aquifers in Webb County, were studied by Ryder (1987) and Ryder and Ardis (2002). Baker (1995) constructed hydrogeologic sections showing the stratigraphic framework of the Gulf Coastal Plain in Texas including one section in Webb County. The TWDB evaluated the quality of ground water in the Carrizo-Wilcox, Laredo, and Gulf Coast aquifers as part of a regional evaluation of water quality in Texas counties bordering the Rio Grande (Hopkins, 1995). The TWDB also evaluated the availability and quality of ground water in and around Bruni in response to concerns about the potential for the high concentrations of arsenic and uranium in ground water resulting from commercial mine operations in the area (Adidas, 1991). CH2M Hill (1996, 1999) conducted a local geohydrologic investigation in the Laredo area to assess the feasibility of using ASR to store water in the Laredo aquifer.
Additional information on the geologic units in the subsurface of Webb County is found in numerous references relating to the oil and gas industry in South Texas. The Webb County area has oil and gas fields that produce from formations beneath the aquifers. Detailed field information including production characteristics, type geophysical logs, and location of selected oil and gas fields in Webb County is in Wolbrink (1979) and Corpus Christi Geological Society (1988). Results of other stratigraphic studies that involve the environment of deposition and reservoir morphology of the Wilcox Group and Claiborne Group in South Texas are published in Stapp and others (1986).
A field inventory was conducted during 1996–98 to identify municipal, irrigation, domestic, and stock wells in Webb County. Where possible, a water-level measurement was made at the time of inventory. Information from the well inventory and drillers’ logs was used to identify the aquifer each well was open to or completed in and to select wells for water-quality sampling. Wells open to more than one aquifer were not used for water-level measurement or water-quality sampling. The locations of inventoried and sampled wells used in the study are shown in figure 2, and a summary of the lithologic and hydrologic properties of the hydrogeologic units are shown in table 1.
Geologic sections were constructed using geologic maps, 30-meter digital elevation model (DEM) data, wells with geophysical logs (fig. 3) and drillers’ logs. The geologic sections were constructed to show the distribution and change in thickness and altitude of geologic units in Webb County. Two approximate dip sections (A–A' and B–B'; figs. 6–7) and three strike sections (C–C', D–D', and E–E'; figs. 8–10) were constructed. Probable sand and shale units and aquifer units were correlated between wells with geophysical logs using the spontaneous potential (SP) and resistivity curves.
Using available geologic maps, drillers’ logs, geophysical logs, and
DEM data, contour maps were prepared showing the extent of the aquifers at land
surface and in the subsurface. Subsurface stratigraphic horizons were identified
using type logs from Wolbrink (1979) and the Corpus Christi Geological Society
(1988) and using information provided by geologists who have worked in the area
(Alvin L. Schultz, independent consultant, oral commun., 1998–2000; Richard
N. Hargis, independent consultant, oral commun., 1999; and Amy Vanderhill, Pogo
Producing Co., oral commun., 2000). The geologic information from the DEM data
and the geophysical logs were entered into a series of geographical information
system (GIS) feature datasets. The DEM data and geophysical log data were merged
to create top-of-aquifer and thickness-of-aquifer maps using a software gridding
program. The program interpolated altitudes of the uppermost stratigraphic unit
of each aquifer to create top-of-aquifer maps and subtracted interpolated altitudes
of the bottom of the lowermost stratigraphic unit of each aquifer to create
thickness-of-aquifer maps. Net sand thickness maps were created by estimating
the net sand thickness within an aquifer using the SP and resistivity curves
on geophysical logs. Locations of geophysical log control points used to construct
the maps are shown in figure 3. The average dip of the
aquifers was computed using the average dip of three to four transects across
GIS feature datasets from the northwestern to the southeastern part of the county
for each aquifer. Because of the regional scale of the contour maps and the
limitations of the computer gridding software in displaying complex subsurface
structure, only regional fault zones and surficial faults (University of Texas,
Bureau of Economic Geology, 1976a, b) have been included on the maps. The maps
are intended to be used as a general guideline in determining the top and thickness
of an aquifer.
Hydraulic properties of an aquifer include transmissivity, which characterizes
an aquifer’s ability to transmit water. Hydraulic information from drillers’
reports, including discharge and drawdown, was used to obtain specific capacity,
which in turn was used to compute transmissivity. The specific capacity of a
well is a measure of the productivity of a well and is computed by dividing
the discharge by the drawdown. Aquifer-test and specific-capacity data for Webb
County from which to obtain transmissivity are scarce. Many of the wells that
were inventoried had little or no useful information with regard to hydraulic
properties, and access to existing wells to conduct aquifer tests was limited.
Using specific capacities from available data, transmissivities were computed
for the Gulf Coast, Jackson, Laredo, and Carrizo aquifers using an empirical
method for interbedded sand and shale alluvial aquifers developed by Razack
and Huntley (1991). These estimated transmissivities are shown for each aquifer
in table 2.
The quality of water from aquifers in Webb County was characterized by sampling 36 domestic, irrigation, and municipal wells (fig. 2). Although attempts were made to sample all aquifers in the county, two aquifers (river alluvium and Queen City-Bigford [table 1]) were not sampled because suitable wells could not be located. All samples were collected by the USGS and analyzed by the USGS National Water Quality Laboratory in Denver, Colo. Water samples were analyzed for major inorganic ions and trace elements using the analytical methods documented in Wershaw and others (1987), Fishman and Friedman (1989), Faires (1993), Fishman (1993), McLain (1993), American Public Health Association (1998), and Gabarino (1999). Water samples also were analyzed for nutrients using methods described in Patton and Truitt (1992) and Fishman (1993). Organic compounds, including selected volatile organic compounds, semivolatile organic compounds, base-neutral acid extracts, and pesticides were analyzed using methods documented in Zaugg and others (1995), Lindley and others (1996), and Connor and others (1998). Few organic compounds were detected, and all concentrations were low. Results are discussed in Lambert and Hartmann (1999).
Quality control samples including inorganic blanks, organic blanks, and replicates were collected to ensure the quality, precision, accuracy, and completeness of the water-quality data. Results from the inorganic blanks and the organic blanks indicated that there were some detectable concentrations of aluminum, boron, and zinc, but that the concentrations were less than the background concentrations in the environmental samples. All other analyzed constituents were not detected. These results indicate that there was no outside contamination of the samples resulting from the sampling equipment or the method used to collect the sample. The replicate samples also showed that the sampling method provided consistent constituent results between the samples. Selected analytical results from the 36 wells are presented in this report in tables 3–6 on sheets 3, 4, 5, and 7.
The author thanks all of the property owners and ranch managers who graciously granted permission to enter their property, supplied information, and aided in the collection of field data. Special thanks are extended to the U.S. Department of Agriculture, Natural Resources Conservation Service, in Laredo for their assistance in providing water-use information and landowner contacts in Webb County. The author also thanks Alvin L. Schultz, Richard N. Hargis, and Amy Vanderhill for their assistance in interpretation of geophysical logs and the geologic setting in Webb County; Jerry Woods, Woods Drilling Company, for drilling information; and Peter Van Noort, CH2M Hill, for information from the ASR study in the Laredo area.
The geologic units in Webb County (fig. 4, table 1) can be characterized as a series of northeast-southwest trending interbedded sand and shale sequences that were deposited in fluvial-deltaic and nearshore marine depositional environments, separated by regional transgressive marine shales. These units were part of a highly destructive, wave-dominated delta system in what is now South Texas during the Eocene (Ricoy and Brown, 1977). The geologic units are Tertiary and Quaternary in age and crop out in northeast-southwest trending belts oriented approximately parallel to the present shoreline of the Gulf of Mexico. These outcrop belts reflect the position of the coast during the geologic time during which deposition occurred (Barnes, 1953). The northwestern part of the county is part of the Rio Grande Embayment (fig. 5), a regional subsurface geologic feature that consists primarily of relatively flat-lying, thick-to-massive sandstones interbedded with thinner shale sequences and few major structural features such as faulting. Most of the sandstone beds were deposited in environments where the primary accumulation of sand has been reworked into barrier bars or strand plains oriented parallel to the depositional strike, with minor accumulations concentrated in dip-trending channels. In areas downdip to the southeast, the geologic units are buried under increasingly thick sequences of Tertiary sediments that are influenced by major structural fault zones and salt diapirs (domes) (fig. 5) of the Texas Gulf Coastal Plain and that contain increasing thicknesses of shales (Ewing, 1991). These depositional units have been further modified by subsurface structural elements that developed during the Tertiary period and include the development of syndepositional normal faults (also referred to as “growth faults”) and modified by salt tectonism that formed the salt diapirs (fig. 5) (Ewing, 1991). The Wilcox fault zone in southeastern Webb County was the first growth-fault system that developed parallel to the present-day coast (Ewing, 1991). The Pescadito Dome in central Webb County (fig. 5) is a deep-seated salt diapir that has pushed up and eroded the overlying formations so that the Laredo Formation is exposed at land surface and surrounded by rocks of the Yegua Formation (fig. 4). Another salt diapir, the Moca salt diapir in northeastern Webb County (fig. 5), is associated with the Moca Oil Field. Average regional dips for the Carrizo Sand through the Catahoula Tuff range from 46 to 88 feet per mile (ft/mi), respectively, from northwestern to southeastern Webb County.
The geologic units that form aquifers in Webb County gradually thicken to the southeast toward the Gulf of Mexico along a dip trend (fig. 4). Geologic sections were constructed to show the subsurface distribution and the change in thickness and altitude of geologic units in Webb County. The greatest thickening of the units is in southeastern Webb County in the area influenced by the Wilcox fault zone (fig. 5). In dip section A–A' (fig. 6), the geologic units thicken to the east, and the shale content increases downdip. The increasing shale content is an indication that the sediments downdip were deposited farther out in the basin in a prodelta or nearshore marine environment. Section A–A' also shows the development of the Reklaw Formation, a transgressive marine shale that is equivalent to the base of the Bigford Formation and in Webb County is present only in the subsurface (fig. 6, table 1). Farther to the south along dip section B–B' (fig. 7), the geologic units also become more shaley and dip to the southeast toward the Gulf of Mexico, but the units dip more steeply than the same units to the north along section A–A'. Similar to its configuration in section A–A', the Reklaw Formation in section B–B' increases in thickness toward the Gulf of Mexico in southeastern Webb County (fig. 7).
Along the strike sections, most of the formations are relatively uniform in thickness. The thinnest section of formations is in northwestern Webb County as shown in strike section C–C' (fig. 8). The formations in this area are dominated by sandstones interbedded with shales and lignite. The presence of lignite is an indication that the rocks were deposited in shallow water under reducing conditions in an area such as a lagoon. The formations shown in section C–C' are at deeper altitudes in the southern part of the county and gradually become thinner and shallower in a northerly direction. Farther downdip in the middle part of the county, the formations are slightly thicker than in the area of section C–C'. The formations in the area of section D–D' (fig. 9) are still dominated by sandstones similar to those of section C–C'. The formations have a greater shale content in the area of the Wilcox fault zone and section E–E' (fig. 10). In section E–E', the subsurface formations are much thicker than their equivalents in the northwestern part of the county, lower in altitude than those units to the northeast, and contain a higher percentage of shale.
Webb County receives an average annual rainfall of 20.1 inches (in.) (Ramos and Plocheck, 1995). Recharge to the local aquifers occurs primarily by infiltration of precipitation on the aquifer outcrops and by channel losses where streams intersect or cross the aquifer outcrops. The major aquifers and sources of ground water are the Gulf Coast aquifer in southeastern Webb County, the Laredo aquifer in central Webb County, and the Carrizo aquifer throughout much of Webb County (table 1). Minor aquifers included in this report are the Jackson and Yegua aquifers in eastern Webb County and the Queen City-Bigford aquifer in central Webb County (table 1).
The hydrogeology, hydraulic properties, and water-quality character-istics of the Gulf Coast, Jackson, Yegua, Laredo, and Carrizo aquifers are described in greater detail on sheets 3, 4, 5, and 7. The hydrogeology and hydraulic properties of the Queen City-Bigford aquifer are described on sheet 6. Maps of tops and thicknesses show the structural surfaces and vertical extents of the aquifers in Webb County. Net sand thickness maps show areas with the greatest amounts of sand, which are an indication of areas where there is the greatest likelihood of locating water in sufficient quantities for use. Where available, information on the depth to water in an aquifer, specific capacity and estimated transmissivity of an aquifer, and available water quality for the aquifers also are shown on the sheets. In the discussion of hydraulic characteristics, yields of 100 to 200 gallons per minute (gal/min) are considered small, those of 200 to 500 gal/min are considered moderate, and those greater than 500 gal/min are considered large.
Results of analyses of water samples are shown in tables 3–6 on sheets 3, 4, 5, and 7. In discussing the water-quality characteristics, waters with dissolved solids concentrations less than 1,000 milligrams per liter (mg/L) are classified as freshwater; waters with dissolved solids concentrations ranging from 1,000 to 3,000 mg/L are classified as slightly saline; waters with dissolved solids concentrations ranging from 3,000 to 10,000 mg/L and from 10,000 to 35,000 mg/L are classified as moderately saline and very saline, respectively (Winslow and Kister, 1956).
The Gulf Coast aquifer in Webb County comprises the Catahoula Tuff (Fant member) and the Goliad Sand (table 1). The Gulf Coast aquifer crops out in southeastern Webb County in a band that trends northeast-southwest (fig. 11). The Goliad Sand consists of interbedded sand, caliche, and conglomerate; the Catahoula Tuff is composed primarily of pyroclastic rocks, thinly interbedded bentonitic clays, shaley sandstones, and tuffaceous sandstones (table 1) (Lonsdale and Day, 1937). The outcrop of the Gulf Coast aquifer covers an area of about 290 square miles (mi2) and dips about 66 ft/mi to the southeast toward the Gulf of Mexico. The top (outcrop) of the Gulf Coast aquifer in southeastern Webb County ranges in altitude from about 940 to 550 feet (ft) above NGVD 29 (fig. 12). Most of the Gulf Coast aquifer in the county ranges in thickness from less than 1 ft along the western edge of the outcrop to about 1,300 ft in the subsurface in the eastern part of the county (fig. 13). The thickest section of the aquifer is along the eastern county line. Although the Goliad Sand outcrops throughout most of southeastern Webb County (fig. 4), the greatest thickness of the Gulf Coast aquifer is composed of the Fant member of the Catahoula Tuff. The greatest thickness of sand in the Gulf Coast aquifer (about 360 to about 450 ft) is along the eastern county line (fig. 14).
Recharge to the Gulf Coast aquifer occurs primarily through the direct infiltration of precipitation on the outcrop. The Gulf Coast aquifer in Webb County receives an estimated 15,500 acre-feet per year (acre-ft/yr) of recharge on the outcrop (assuming an effective recharge rate of 5 percent of the average annual rainfall [20.1 in.]). The regional ground-water-flow direction in the Gulf Coast aquifer is downdip to the east and southeast toward the Gulf of Mexico (Adidas, 1991). The depth to water in the aquifer obtained from measurements in inventoried wells ranged from about 64 to 123 ft below land surface (fig. 11). The depth to water is deeper in the area close to the town of Bruni (fig. 1) and a commercial uranium mine where withdrawals for municipal and industrial uses have created a small cone of depression (Adidas, 1991). In general, ground-water flow through much of the Gulf Coast aquifer is slow (Adidas, 1991). Specific capacity for wells open to or completed in the Gulf Coast aquifer ranged from 12 to 180 ft2/d (fig. 15, table 2). Transmissivities estimated from specific capacities ranged from 180 to 1,090 ft2/d (fig. 15, table 2). The specific capacity and estimated transmissivity from well YZ–84–33–102 were not computed because the drawdown was not determined. Because of the relatively small amount of recharge (assumed to be about 1 in. per year) and the low transmissivity, the Gulf Coast aquifer generally yields between 15 and 30 gal/min, with some wells yielding up to 150 gal/min.
Water withdrawn from the Gulf Coast aquifer in Webb County is fresh to slightly saline and is withdrawn for domestic, stock, irrigation, industrial, and public-supply uses. Four wells completed in the Gulf Coast aquifer (fig. 2, nos. 33–36) were sampled for this report. The concentration of dissolved solids ranged from 872 to 1,090 mg/L (table 3), which is greater than the U.S. Environmental Protection Agency (USEPA) secondary drinking water regulation1 (SDWR) concentration of 500 mg/L (U.S. Environmental Protection Agency, 2002). Graphical plots (Stiff diagrams) showing the relative proportion of cation and anion species in water samples indicate that water from the Gulf Coast aquifer is similar throughout the area and is dominated by sodium-chloride ions with a smaller proportion of bicarbonates and sulfates (fig. 16). The sodium percentage (percentage of total cations [in milliequivalents per liter] accounted for by sodium) ranges from 60 to 98. The chloride concentration in well 34 (335 mg/L) exceeded the USEPA SDWR of 250 mg/L.
Analysis of the water samples also indicates the presence of several trace elements including aluminum, arsenic, chromium, copper, manganese, selenium, uranium, vanadium, and zinc. The arsenic concentration in water from well 35 (77 micrograms per liter [µg/L], table 3) was nearly eight times the USEPA maximum contaminant level2 (MCL) of 10 µg/L. The uranium concentration in water from the same well (30 µg/L) exceeded the USEPA MCL of 20 µg/L. Uranium is being mined by injection leaching near Bruni (Adidas, 1991) where the well is located. The leaching agent in such operations can mobilize constituents other than uranium, including molybdenum, vanadium, selenium, iron, lead, and arsenic (Diehl, 2001). In one of the four well-water samples (well 33), the strontium concentration (813 µg/L) was about twice the USEPA lifetime health advisory3 (HA) concentration of 400 µg/L. Concentrations of the minor constituent boron in water from all four Gulf Coast aquifer wells were about two to four times greater than the USEPA lifetime HA of 600 µg/L.
The Jackson aquifer is a minor aquifer that crops out in a north-south-trending band in eastern Webb County (fig. 17) (Lonsdale and Day, 1937). The Jackson aquifer is bounded by the Yegua aquifer at the base and the Frio confining unit at the top (table 1) and is composed of alternating clays and shales interbedded with sandstones and ashy sandstones. Volcanic ash is common in the section (Lonsdale and Day, 1937). The outcrop of the Jackson aquifer covers about 840 mi2 and dips about 46 ft/mi to the southeast toward the coast (fig. 17). The altitude of the top of the Jackson aquifer ranges from about 900 ft above NGVD 29 in the outcrop to about 700 ft below NGVD 29 in the subsurface in southeastern Webb County (fig. 18). The Jackson aquifer is thicker than the Gulf Coast aquifer, ranging from less than 1 ft thick along the western edge of the outcrop to about 2,220 ft in the subsurface (fig. 19). The thickest sections of the Jackson aquifer in the county trend north-south near the center of its area of occurrence (fig. 19), roughly paralleling the trend of the Wilcox fault zone (fig. 5). The Jackson aquifer is dominated by clays and shales (Schultz, 1986). The few sandstones that are present generally are 15 to 50 ft thick and are separated by much thicker sequences of shales. The greatest thicknesses of sand in the aquifer are in the lowermost part of the section and are approximately parallel to the strike of the aquifer. These sands form the oil-producing Mirando Sands in eastern Webb County and neighboring Duval County. The net sand thickness in the Jackson aquifer ranges from about 1 ft along the western edge of the outcrop to nearly 410 ft in the southeastern part of the aquifer (fig. 20). The greatest net sand thicknesses coincide with the thickest sections of the aquifer, oriented primarily parallel to the strike of the aquifer in eastern Webb County.
Similar to the Gulf Coast aquifer, recharge to the Jackson aquifer occurs primarily through direct infiltration on the Jackson aquifer outcrop (fig. 17). The Jackson aquifer receives about 45,000 acre-ft/yr of recharge (assuming an effective recharge rate of 5 percent of average annual rainfall) on the outcrop. The regional ground-water-flow direction is most likely to the southeast along an approximate dip trend, although depth to water could not be determined for this report because no wells were located that could be used to make ground-water-level measurements. Little or no information is available on the hydraulic properties of the Jackson aquifer. Only one well with specific-capacity data was located (table 2). The specific capacity for well YZ–84–33–101 was 17 ft2/d, and the estimated transmissivity was 225 ft2/d. These values are small and similar to the specific capacities and transmissivities computed for wells open to the Gulf Coast aquifer. Large thicknesses of shales and clays represented in the geophysical logs indicate that the rate of flow through the Jackson aquifer probably is low. Yields from the Jackson aquifer are variable (table 1) and depend on the thickness of sand to which a well is open.
The Jackson aquifer generally yields slightly saline to highly saline water used mainly for stock (Lonsdale and Day, 1937). Only one well completed in the Jackson aquifer (fig. 2, no. 32) could be sampled. The dissolved solids concentration in water from that well was 1,480 mg/L (table 4), greater than the SDWR of 500 mg/L. The chloride concentration, 792 mg/L, also was greater than the SDWR of 250 mg/L. The water chemistry of the Jackson aquifer is dominated by sodium and chloride and contains greater concentrations of anions and cations (fig. 21) than the Gulf Coast aquifer (fig. 16). Bicarbonate also is present, but the bicarbonate concentrations are much smaller than those of the Gulf Coast aquifer. Similar to the Gulf Coast aquifer, nutrient concentrations were small, and the boron concentration (1,027 mg/L) exceeded the lifetime HA. Among the trace elements and other metals that were analyzed in the water sample from the Jackson aquifer (and for which the USEPA drinking water standards exist), the arsenic concentration (30 mg/L) exceeded the MCL (10 mg/L), and the strontium concentration (1,083 mg/L) exceeded the lifetime HA (400 mg/L).
The Yegua aquifer underlies the Jackson aquifer and consists of clay and sandy clay interbedded with thin beds of sandstone, secondary gypsum, and some concretionary limestone (Lonsdale and Day, 1937). The Yegua aquifer, which crops out in a north-south band in east-central and central Webb County (fig. 22), is a minor aquifer. The altitude of the top of the aquifer ranges from about 750 ft above NGVD 29 in the outcrop to more than 2,350 ft below NGVD 29 in the southeastern part of the county (fig. 23). The outcrop of the Yegua aquifer covers about 690 mi2 and dips to the southeast at about 64 ft/mi. The Yegua aquifer in Webb County ranges in thickness from less than 1 ft along the western edge of the outcrop to about 1,480 ft in the subsurface (fig. 24). The net sand thickness of the Yegua aquifer ranges from about 1 to more than 410 ft (fig. 25). The thickest net sand sections are downdip in the southeastern part of the county (fig. 25). Sandstones in the Yegua aquifer generally are thin-bedded and stacked and surrounded by thicker sections of clays and shales. Oyster reefs have been observed in the outcrop (Lonsdale and Day, 1937).
Recharge to the Yegua aquifer most likely occurs through direct infiltration of precipitation on the outcrop. Assuming that effective recharge is about 5 percent of the average annual rainfall, the Yegua aquifer receives about 36,900 acre-ft/yr of recharge in Webb County on the outcrop. The regional direction of ground-water flow generally is to the south. The depth to water in wells open to the Yegua aquifer measured for this report ranged from about 94 to 292 ft below land surface (fig. 22). Yields from the Yegua aquifer generally are less than 15 gal/min because there is very little sandstone in the section (Lonsdale and Day, 1937). For this report, no wells open to the Yegua aquifer that had specific-capacity data were located.
Of the aquifers in Webb County, water in the Yegua aquifer generally is the most saline. Water from the Yegua aquifer, used mostly for livestock (Lonsdale and Day, 1937; Winslow and Kister, 1956), is slightly to moderately saline because of abundant gypsiferous clay beds. One well open to the Yegua aquifer (fig. 2, no. 31) was sampled for this report. Concentrations of dissolved solids (4,470 mg/L), sulfate (378 mg/L), chloride (1,772 mg/L), and fluoride (2.4 mg/L) (table 4) exceeded SDWRs of 500, 250, 250, and 2 mg/L, respectively. Water chemistry from the Yegua aquifer is dominated by sodium and chloride with minor amounts of bicarbonate (fig. 21). No concentration of any metal or trace element exceeded its MCL; however, the strontium concentration (503 mg/L) was greater than the lifetime HA of 400 mg/L. The largest boron concentration from any water sample, 4,489 mg/L, more than seven times the lifetime HA of 600 mg/L, was measured in water from the Yegua aquifer.
The Laredo aquifer crops out in the middle of Webb County in a north-south trend (fig. 26). The Laredo aquifer consists primarily of interbedded sandstones and glauconitic sandstones at the base and top of the aquifer. The sandstones are separated by thinned sequences of shale with glauconitic marl, clay, and in the middle part of the aquifer some fossiliferous limestone (Eargle, 1968). Generally, the sandiest parts of the aquifer are at the base of the aquifer, and the percentage of shale in the section increases toward the top of the aquifer. The outcrop of the Laredo aquifer in Webb County covers about 620 mi2, and the average dip of the Laredo aquifer is to the southeast at about 72 ft/mi. The altitude of the top of the Laredo aquifer ranges from about 880 ft above NGVD 29 along the western edge of the aquifer to nearly 3,120 ft below NGVD 29 in the southeastern part of the county (fig. 27). The Laredo aquifer in Webb County ranges in thickness from about 1 ft along the western edge of the outcrop to about 1,510 ft in the central part of the county (fig. 28). The thickest sections of the aquifer are along a north-south trend beneath the eastern edge of the outcrop. The thickest sections of net sand in the Laredo aquifer are in the central and eastern parts of the county (fig. 29). The net sand thickness of the Laredo aquifer ranges from about 1 to 570 ft. Although not shown in figure 29, some of the thickest sands are concentrated in channels, and borehole geophysical logs show that the sands have a fining-upward character.
Recharge to the Laredo aquifer occurs mainly by infiltration of precipitation. The Laredo aquifer is bounded by the overlying Yegua aquifer and the underlying El Pico confining unit (table 1), both of which have a greater shale content than the Laredo aquifer. The Laredo aquifer receives an estimated 33,000 acre-ft/yr of recharge on the outcrop (assuming an effective recharge rate of 5 percent of annual average precipitation of 20.1 in.). The regional flow direction of water in the Laredo aquifer generally is downdip to the east. The depth to water in the Laredo aquifer from water-level measurements ranged from about 12 to 252 ft below land surface (fig. 26). Ground-water flow through the Laredo aquifer is partly dependent on the interconnection between the interbedded sand units within the aquifer. Wells completed in the Laredo aquifer yield small to large (5 to 170 gal/min) amounts of fresh to moderately saline water (Lonsdale and Day, 1937; Winslow and Kister, 1956). Specific capacities computed from 20 tests in the Laredo aquifer were highly variable, ranging from 4.0 to 711 ft2/d, and estimated transmissivities ranged from 85 to 2,735 ft2/d (fig. 30, table 2). The locations of the largest specific capacities and estimated transmissivities generally coincided with the locations of greatest net sand thicknesses.
Water is withdrawn from the Laredo aquifer for domestic, stock, irrigation, commercial, institutional, and public-supply uses. Analytical results from 24 sampled wells open to the Laredo aquifer (fig. 2, nos. 2, 3, 7–16, 18, 20–30) are shown in table 5. The dissolved solids concentrations ranged from 740 to 4,090 mg/L, all greater than the SDWR of 500 mg/L. Analyses of samples from the Laredo aquifer generally indicate that the principal dissolved constituents in the water are sodium and chloride, with lesser amounts of sulfates and bicarbonates (fig. 31). The sodium percentage of cations in the water ranged from 45 to 100. Of the 24 samples, chloride concentrations in 18 samples and sulfate concentrations in 23 samples exceeded the SDWRs of 250 mg/L. The larger sulfate concentrations are in the northern part of the county. The fluoride concentration in one sample (2.1 mg/L, well 30) exceeded the SDWR of 2.0 mg/L. Water samples from the Laredo aquifer generally have some metals and trace elements. Iron concentrations in five of the 24 samples exceeded the SDWR of 300 mg/L. The largest iron concentrations in wells sampled in the county were in the Laredo aquifer. Manganese concentrations in three of the 24 samples exceeded the SDWR of 50 mg/L, and the zinc concentration in one well exceeded the SDWR of 500 mg/L. Strontium concentrations in one-half the samples exceeded the lifetime HA of 400 mg/L; and, as with all wells sampled in the other aquifers, all boron concentrations exceeded the lifetime HA of 600 mg/L.
El Pico Confining Unit, Queen City-Bigford Aquifer, and Reklaw Confining Unit
The El Pico confining unit, composed of a thick sequence of shales, shaley sands, and coals, separates the Laredo aquifer from the Queen City-Bigford aquifer in Webb County (table 1). In northwestern Webb County, the lower part of the El Pico confining unit consists primarily of clays interbedded with thin lenses of sandstone and lignite that grade into interbedded sandstones as distance downdip increases (Lonsdale and Day, 1937). The basal part of the unit becomes sandy enough downdip to be considered part of the Queen City-Bigford aquifer (fig. 6, where the El Pico Clay transitions into the Queen City Sand). The outcrop of the El Pico confining unit (fig. 32) trends north-south in the county and covers about 700 mi2. The average dip of the unit in the county is about 59 ft/mi, and the altitude of the top ranges from about 870 ft above NGVD 29 in the outcrop to about 4,030 ft below NGVD 29 in southeastern Webb County (fig. 33). The El Pico confining unit differs in thickness across the county, ranging from less than 1 ft along the western edge of the outcrop to more than 1,710 ft in southeastern Webb County (fig. 34).
To the west of the El Pico confining unit outcrop is the Queen City-Bigford aquifer outcrop, which covers about 140 mi2 (fig. 32). The outcrop of the Queen City-Bigford aquifer is coincident with the outcrop of the Bigford Formation, as the Queen City Sand, the other component of the aquifer, does not crop out in Webb County. The Queen City-Bigford aquifer is composed of repetitive sequences of thick, massive sandstones of the Queen City Sand and the Bigford Formation that are stacked one on top of the other. The altitude of the top of the Queen City-Bigford aquifer ranges from about 860 ft above NGVD 29 in northwestern Webb County to more than 5,630 ft below NGVD 29 in southeastern Webb County (fig. 35). The top of the aquifer in the area around the Pescadito Dome (figs. 5, 35) is influenced by the structure of the dome. The Queen City-Bigford aquifer generally thickens downdip, ranging from less than 1 ft along the western edge of the outcrop to about 2,170 ft in the eastern part of the county (fig. 36). The greatest thickness of the Queen City-Bigford aquifer occurs along a north-south trend in eastern Webb County, paralleling the depositional strike of the rocks that form the aquifer. Similar to the trend in thickness, the greatest net sand thickness follows a generally north-south trend in eastern Webb County, ranging from about 1 to more than 1,510 ft (fig. 37).
Recharge to the Queen City-Bigford aquifer probably occurs mainly by infiltration of precipitation on the outcrops of the El Pico confining unit and Bigford Formation. The amount of recharge to the aquifer through infiltration on the outcrop of the El Pico confining unit probably is less than that on the outcrop of the Bigford Formation because of the relatively greater amount of flow-impeding shale and clay in the confining unit. According to Lonsdale and Day (1937), the El Pico confining unit yields small amounts water (table 1), which might indicate that the unit has some permeability despite its classification as a confining unit. If the effective recharge on the outcrop of the Bigford Formation is assumed to be 5 percent of the average annual rainfall of 20.1 in., and the effective recharge on the outcrop of the El Pico confining unit is one-half that rate, about 45,000 acre-ft/yr of recharge would enter the aquifer in its outcrop.
Once in the saturated zone, water in the Queen-City Bigford aquifer probably moves to the east, following the downdip trend of the component formations. No water-level measurements in wells open to the Queen City-Bigford aquifer were available; however, water-level measurements were available from six wells in the outcrop and shallow subcrop of the El Pico confining unit. The depth to water in those wells ranged from about 125 to 268 ft below land surface (fig. 32). No wells open to the Queen City-Bigford aquifer had data from which to compute specific capacity and transmissivity, and no suitable wells for water-quality sampling were available.
At the base of the Queen City-Bigford aquifer in central and eastern Webb County is the Reklaw confining unit, a marine shale that is equivalent to the basal part of the Bigford Formation (fig. 6; table 1). The Reklaw confining unit, which does not crop out in Webb County, separates the Queen City-Bigford aquifer from the underlying Carrizo aquifer. The top of the Reklaw confining unit ranges from about 830 ft above NGVD 29 in western Webb County to more than 6,860 ft below NGVD 29 in southeastern Webb County (fig. 38). The Reklaw confining unit thickens in the subsurface toward the southeast in Webb County, ranging from about 1 to about 1,050 ft (fig. 39).
The Carrizo aquifer, the most productive aquifer in the county, underlies the Queen City-Bigford aquifer and (downdip) Reklaw confining unit (table 1). The Carrizo aquifer is a coarser grained, more massive crossbedded sand than the overlying strata. A narrow band of the Carrizo aquifer crops out in extreme northwestern Webb County, and the aquifer is present in the subsurface throughout the rest of the county (fig. 40). The outcrop covers about 18 mi2, and the formation dips to the southeast toward the Gulf of Mexico at about 87 ft/mi. The top of the Carrizo aquifer ranges from about 760 ft above NGVD 29 in northwestern Webb County to 7,800 ft below NGVD 29 in southeastern Webb County (fig. 41). The thickness of the Carrizo aquifer varies within a narrow range throughout much of Webb County (fig. 42). The Carrizo aquifer is thinnest in the outcrop area and thickens to the southeast, ranging from less than 1 ft along the western edge of the outcrop to about 1,250 ft in south-central parts of the county. Generally the Carrizo aquifer is composed of a series of stacked, massive crossbedded sandstones and in some places minor amounts of shale and clay. The net sand thickness map of the Carrizo aquifer shows that the thickest sections of sand occur in several places in the central part of the county (fig. 43). The net sand thickness ranges from about 1 ft in the outcrop area in northwestern Webb County to more than 790 ft in central Webb County.
Recharge to the Carrizo aquifer occurs primarily by infiltration of precipitation on the outcrop; minor or substantial amounts of recharge might occur by downward flow from the overlying Queen City-Bigford aquifer in regions where the formations are hydraulically connected (Klemt and others, 1976). Recharge occurring by infiltration of precipitation on the outcrop in the county is estimated to be only about 950 acre-ft/yr assuming that 5 percent of the average annual rainfall of 20.1 in. recharges the aquifer through the outcrop. (However, the Carrizo aquifer receives substantial recharge through its outcrop outside of Webb County.) Although yields for some irrigation wells in the Winter Garden area northeast of Webb County exceed 1,000 gal/min, yields for Carrizo aquifer wells in Webb County generally range from 150 to 200 gal/min. The Carrizo aquifer is confined throughout much of Webb County. The water levels in the two wells measured for this report were about 162 and 209 ft below land surface (fig. 40). Specific capacities for four wells in the Carrizo aquifer in Webb County ranged from 6.0 to 33 ft2/d, and estimated transmissivities from the specific-capacity data ranged from 115 to 350 ft2/d (fig. 44, table 2). These transmissivities, although probably less than average for the Carrizo aquifer in the county, are compatible with a previous study that indicated Carrizo aquifer transmissivity is less than about 940 ft2/d in the county.
Water from the Carrizo aquifer is fresh to slightly saline and commonly is used for commercial and industrial purposes and public supply in Webb County. Generally the freshest water in the Carrizo aquifer is nearest the outcrop (recharge zone), with increasing dissolved solids concentration farther downdip. Lonsdale and Day (1937) noted that wells improperly completed in the Carrizo aquifer often were contaminated with saline water from the Bigford Formation. Six wells completed in the Carrizo aquifer were sampled for this report (fig. 2, nos. 1, 4–6, 17, 19). The dissolved solids concentration in these samples ranged from 826 to 2,220 mg/L (table 6), all greater than the SDWR of 500 mg/L. The water chemistry of the samples was dominated by sodium and chloride, with minor amounts of bicarbonate and sulfate (fig. 45). The sodium percentage for all samples was 99. The chloride concentration in three of the six samples exceeded the SDWR of 250 mg/L, and in those same samples, fluoride concentrations equaled or exceeded the SDWR of 2.0 mg/L. Neither metal nor trace element concentrations exceeded their respective USEPA standards except for boron, which exceeded the lifetime HA of 600 mg/L in all samples.
Webb County, Texas, is a region confronted by increasing stresses on natural resources. Increased water demand associated with development and population growth have increased the need for the City of Laredo and Webb County to evaluate water sources other than the Rio Grande to meet future demand. The USGS conducted a study in cooperation with the City of Laredo during 1996–98 to assess the ground-water resources of Webb County. A hydrogeologic study was conducted to review and analyze available information on the hydrogeologic units (aquifers and confining units) in Webb County, to locate available wells in the region with water-level and water-quality information from the aquifers, and to analyze the hydraulic properties of the aquifers.
The geologic units that compose the aquifers and confining units in Webb County can be characterized as a series of northeast-southwest trending interbedded sand and shale sequences that were deposited in fluvial-deltaic and nearshore marine depositional environments, separated by regional transgressive marine shales. These Tertiary- and Quaternary-age units crop out in northeast-southwest trending belts oriented approximately parallel to the present shoreline of the Gulf of Mexico. The units gradually thicken to the southeast toward the Gulf of Mexico along a dip trend.
The major aquifers are the Gulf Coast aquifer in southeastern Webb County, the Laredo aquifer in central Webb County, and the Carrizo aquifer throughout much of Webb County. Minor aquifers are the Jackson and Yegua aquifers in eastern Webb County and the Queen City-Bigford aquifer in central Webb County. The Gulf Coast aquifer crops out in southeastern Webb County in a band that trends northeast-southwest. Most of the Gulf Coast aquifer in the county ranges in thickness from less than 1 ft along the western edge of the outcrop to about 1,300 ft in the subsurface in the eastern part of the county. The greatest thickness of sand in the Gulf Coast aquifer (about 375 to about 450 ft) is along the eastern county line. The depth to water in the aquifer obtained from measurements in inventoried wells ranged from about 64 to 123 ft below land surface. Specific capacity for wells open to and completed in the Gulf Coast aquifer ranged from 12 to 180 ft2/d. Transmissivities estimated from specific capacities ranged from 180 to 1,090 ft2/d. The Gulf Coast aquifer generally yields small (less than 15 gal/min) amounts of water in the shallow outcrop and yields greater quantities (30 to 150 gal/min) of water from deeper zones. The water withdrawn from the Gulf Coast aquifer in Webb County is fresh to slightly saline. The water also contains several trace elements; the concentrations of three exceeded Federal water-quality standards in one sample each—arsenic and uranium greater than respective USEPA maximum contaminant levels (MCLs); strontium greater than the USEPA health advisory (HA). The concentration of boron in all four samples, as in all samples from all aquifers, also exceeded the USEPA HA.
The Laredo aquifer crops out in the middle of Webb County in a north-south trend. The Laredo aquifer in Webb County ranges in thickness from about 1 ft along the western edge of the outcrop to about 1,510 ft in the central part of the county. The thickest sections of the aquifer are along a north-south trend beneath the eastern edge of the outcrop. Generally, the sandiest parts of the aquifer are at the base of the aquifer. The thickest sections of net sand in the Laredo aquifer are in the central and eastern parts of the county. The depth to water in the Laredo aquifer ranged from about 12 to 252 ft below land surface. Ground-water flow through the Laredo aquifer is partly dependent on the interconnection between the interbedded sand units within the aquifer. Specific capacities computed from 20 tests in the Laredo aquifer were highly variable, ranging from 4.0 to 711 ft2/d, and estimated transmissivities ranged from 85 to 2,735 ft2/d. Wells completed in the Laredo aquifer yield small to large (5 to 170 gal/min) amounts of fresh to moderately saline water. Water samples from the Laredo aquifer generally show the presence of some metals and trace elements. Of 24 samples, iron concentrations in five, manganese concentrations in three, and zinc concentrations in one exceeded the respective SDWRs. Strontium concentrations in one-half the samples and boron concentrations in all the samples exceeded the respective USEPA HAs.
The Carrizo aquifer is the most productive aquifer in Webb County. A narrow band of the Carrizo aquifer crops out in the extreme northwestern part of the county, and the aquifer is present in the subsurface throughout the rest of the county. The Carrizo aquifer is thinnest in the outcrop area and thickens to the southeast, ranging from less than 1 ft along the western edge of the outcrop to about 1,250 ft in south-central parts of the county. The thickest sections of sand occur in several places in the central part of the county. The net sand thickness ranges from about 1 ft in the outcrop area in northwestern Webb County to more than 790 ft in central Webb County. The water levels in the two wells measured for this report were about 162 and 209 ft below land surface. Specific capacities for four wells in the Carrizo aquifer in Webb County ranged from 6.0 to 33 ft2/d, and estimated transmissivities from the specific-capacity data, although probably less than average for the Carrizo aquifer in the county, ranged from 115 to 350 ft2/d. Carrizo aquifer well yields in the county generally range from 150 to 200 gal/min. Water from the Carrizo aquifer is fresh to slightly saline and generally fresher near the outcrop. Neither metal nor trace element concentrations (except for boron) exceeded their respective USEPA standards.
The minor aquifers in Webb County—Jackson, Yegua, Queen City-Bigford—generally yield less water than the major aquifers because they contain proportionally less sand and more shale and clay. The Yegua aquifer generally is the most saline of the aquifers in Webb County.
Adidas, E.O., 1991, Ground-water quality and availability in and around Bruni, Webb County, Texas: Texas Water Development Board Limited Publication LP–209, 52 p.
American Public Health Association, 1998, Standard methods for the examination of water and wastewater (20th ed.): Washington, D.C., American Public Health Association, American Water Works Association, and Water Environment Federation, p. 3-37–3-43.
Baker, E.T., Jr., 1995, Stratigraphic nomenclature and geologic sections of the Gulf Coastal Plain of Texas: U.S. Geological Survey Open-File Report 94–461, 34 p., 8 pl.
Barnes, J.R., 1953, Preliminary survey of ground-water conditions near Laredo, Texas: Private consultantís report prepared for Laredo Waterworks System, City of Laredo, Texas, 15 p.
CH2M Hill, 1996, Aquifer storage recovery—Feasiblity study, progress report no. 1: Private consultantís report to City of Laredo, March 1996, 9 p. plus attachments.
CH2M Hill, 1999, Aquifer storage and recovery system, step 2 report: Private consultantís report to City of Laredo, January 1999, 26 p. plus attachments.
Connor, B.F., Rose, D.L., Noriega, M.C., Murtagh, L.K., and Abney, S.R., 1998, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of 86 volatile organic compounds in water by gas chromatography/mass spectrometry, including detections less than reporting limits: U.S. Geological Survey Open-File Report 97–829, 78 p.
Corpus Christi Geological Society, 1988, Typical oil and gas fields of South Texas, volume II—Seventy-one field studies: Corpus Christi, Tex., Corpus Christi Geological Society, 416 p.
Diehl, Peter, 2001, Uranium mining and milling wastes—An introduction:
World Information Service on Energy Uranium Project, accessed February 24, 2002,
at URL
http://www.antenna.nl/wise/uranium/uwai.html
Eargle, D.H., 1968, Nomenclature of formations of Claiborne Group, Middle Eocene Coastal Plain of Texas: U.S. Geological Survey Bulletin 1251–D, 25 p.
Ewing, T.E., 1991, The tectonic framework of Texas: Austin, University of Texas, Bureau of Economic Geology, map and text, 36 p.
Faires, L.M., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of metals in water by inductively coupled plasma-mass spectrometry: U.S. Geological Survey Open-File Report 92–634, 28 p.
Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey Open-File Report 93–125, 217 p.
Fishman, M.J., and Friedman, L.C., 1989, Methods for determination of inorganic substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources Investigations, book 5, chap. A1, 545 p.
Gabarino, J.R., 1999, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of dissolved arsenic, boron, lithium, selenium, strontium, thallium, and vanadium using inductively coupled plasma-mass spectrometry: U.S. Geological Survey Open-File Report 99–093, 31 p.
Hopkins, Janie, 1995, Evaluation of ground-water quality in Texas counties bordering the Rio Grande: Texas Water Development Board Limited Publication LP–214, 26 p.
Klemt, W.B., Duffin, G.L., and Elder, G.R., 1976, Ground-water resources of the Carrizo aquifer in the Winter Garden area of Texas, volume 1: Texas Water Development Board Report 210, 30 p.
Lambert, R.B., and Hartmann, C.A., Jr., 1999, Quality of ground water in Webb County, Texas, 1997–98: U.S. Geological Survey Fact Sheet 184–99, 6 p.
Lindley, C.E., Stewart, J.T., and Sandstrom, M.W., 1996, Determination of low concentrations of acetochlor in water by automated solid-phase extraction and gas chromatography with mass selective detection: Journal of AOAC International, v. 79, no. 4, p. 962–966.
Lonsdale, J.T., and Day, J.R., 1937, Geology and ground-water resources of Webb County, Texas: U.S. Geological Survey Water-Supply Paper 778, 104 p.
Mace, R.E., Smyth, R.C., Xu, Liying, and Liang, Jinhuo, 2000, Transmis- sivity, hydraulic conductivity, and storativity of the Carrizo-Wilcox aquifer in Texas—Data and analysis: Austin, University of Texas, Bureau of Economic Geology, CD-ROM.
Marquardt, Glen, and Rodriguez, Eulogio, Jr., 1977, Ground-water resources of the Carrizo aquifer in the Winter Garden area of Texas, volume 2—Records of wells, water levels in wells, chemical analyses of water, and well location maps: Texas Water Development Board Report 210, 467 p.
McLain, Betty, 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of chromium in water by graphite furnace atomic absorption spectrophotometry: U.S. Geological Survey Open-File Report 93–449, 16 p.
Patton, C.J., and Truitt, E.P., 1992, Methods of analysis by the U.S. Geo- logical Survey National Water Quality Laboratory—Determination of total phosphorus by a Kjeldahl digestion method and an automated colorimetric finish that includes dialysis: U.S. Geological Survey Open-File Report 92–146, 39 p.
Perry, M.J., and Mackun, P.J., 2001, Population change and distribution 1990
to 2000—Census 2000 brief: U.S. Department of Commerce, Economics and
Statistics Administration, U.S. Census Bureau, accessed February 14, 2003, at
URL
http://www.census.gov/prod/2001pubs/c2kbr01-2.pdf
Ramos, M.G., and Plocheck, Robert, eds., 1995, The 1996–1997 Texas Almanac:
Dallas, The Dallas Morning News, 672 p.
Razack, M., and Huntley, David, 1991, Assessing transmissivity from specific
capacity in a large and heterogeneous alluvial aquifer: Ground Water, v. 29,
no. 6, p. 856–861.
Ricoy, J.U., and Brown, L.F., Jr., 1977, Depositional systems in the Sparta Formation (Eocene) Gulf Coast Basin of Texas, in Bebout, D.G., and Loucks, R.G., eds., Cretaceous carbonates of Texas and Mexico—Applications to subsurface exploration (Abstracts): Transaction of the Gulf Coast Association of Geological Societies, v. 27, p. 139–154.
Ryder, P.D., 1987, Hydrogeology and predevelopment and flow in the Texas Gulf Coast aquifer systems: U.S. Geological Survey Water-Resources Investigations Report 87–4248, 109 p.
Ryder, P.D., and Ardis, A.F., 2002, Hydrogeology of the Texas Gulf Coast aquifer systems: U.S. Geological Survey Professional Paper 1416–E, 77 p.
Schultz, A.L., 1986, Geology of the First Mirando Sand, South Lopez Unit, Lopez Field, Webb and Duval Counties, Texas, in Stapp, W.L., Dutton, L.A., Weise, B.R., Jones, L.P., and Fergeson, W.G., eds., Contributions to the geology of South Texas—1986: San Antonio, South Texas Geological Society, p. 100–108.
Stapp, W.L., Dutton, L.A., Weise, B.R., Jones, L.P., and Fergeson, W.G., eds., 1986, Contributions to the geology of South Texas—1986: San Antonio, South Texas Geological Society, 487 p.
U.S. Environmental Protection Agency, 2002, 2002 edition of the drinking water standards and health advisories: U.S. Environmental Protection Agency, EPA 822–R–02–038, 12 p.
University of Texas, Bureau of Economic Geology, 1976a, Geologic atlas of Texas, Crystal City-Eagle Pass sheet: Austin, scale 1:250,000.
University of Texas, Bureau of Economic Geology, 1976b, Geologic atlas of Texas, Laredo sheet: Austin, scale 1:250,000.
Wershaw, R.L., Fishman, M.J., Grabbe, R.R., and Lowe, L.E., eds., 1987, Methods for the determination of organic substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources Investigations, book 5, chap. A3, 80 p.
Winslow, A.G., and Kister, L.R., 1956, Saline-water resources of Texas: U.S. Geological Survey Water-Supply Paper 1365, 105 p.
Wolbrink, M.A., 1979, Type field logs in South Texas 1979, volume II—Wilcox (Eocene) trend: Corpus Christi, Tex., Corpus Christi Geological Society, 96 p.
Zaugg, S.D., Sandstrom, M.W., Smith, S.G., and Fehlberg, K.M., 1995, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory—Determination of pesticides in water by C-18 solid-phrase extraction and capillary-column gas chromatography/mass spectrometry with selected-ion monitoring: U.S. Geological Survey Open-File Report 95–181, 60 p.