Geology Reports⌕ Search

Geology topics

T. A. Small

Publications and source records attributed to T. A. Small.

10 recordsLinked to original sources
USGS1994

Geologic framework and hydrogeologic characteristics of the Edwards Aquifer outcrop, Comal County, Texas

All of the hydrogeologic subdivisions within the Edwards aquifer outcrop in Comal County have some porosity and permeability. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; hydrogeologic subdivision III, the leached and collapsed members, undivided; and hydrogeologic subdivision II, the cyclic and marine members, undivided, of the Person Formation. The two types of porosity in the Edwards aquifer outcrop are fabric selective, which is related to depositional or diagenetic elements and typically exists in specific stratigraphic horizons; and not fabric selective, which can exist in any litho strati graphic horizon. Permeability, the capacity of porous rock to transmit water, depends on the physical properties of the rock such as pore size, shape, distribution, fissuring, and dissolution. Two faults, Comal Springs and Hueco Springs, completely, or almost completely, offset the Edwards aquifer along much of their respective traces across Comal County. Porous and permeable Edwards aquifer limestone is juxtaposed against impermeable upper confining units in these areas. These faults completely, or almost completely, offset the Edwards aquifer and are thought to be barriers or partial barriers to groundwater flow where the beds are juxtaposed. In Comal County, the Edwards aquifer is probably most vulnerable to surface contamination in the rapidly urbanizing areas on the Edwards aquifer outcrop. Possible contamination can result from spills, leakage of hazardous materials, or runoff onto the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone.

Texas↗
USGS1993

Ground-water conditions in Pecos County, Texas, 1987

A comparison of 1987 water levels with historical (1940-49) water levels in the Edwards-Trinity (Plateau) aquifer indicated that water levels declined more than 50 feet in three locations in the Leon-Belding irrigation area, in an area north of Fort Stockton, and in a well east of Bakersfield. Maximum measured declines were 54 and 82 feet in the Leon-Belding irrigation area. The maximum measured rise was 55 feet in one well in east-central Pecos County. The chemical quality of water in the Edwards-Trinity aquifer of Pecos County varied greatly during 1987. Most wells in the eastern, southern, and southwestern parts of the county had water with a specific conductance of 1,000 μ S/cm (microsiemens per centimeter at 25 °C) or less. Three areas that had anomalously large specific conductances in ground water in north-central Pecos County are associated with water issuing from Santa Rosa, Diamond Y, and Comanche Springs. Specific conductance in water from wells and springs ranged from 311 μ S/cm in south-central Pecos County to 9,600 μ S/cm in the north. Dissolved sulfate concentrations ranged from 17 to 2,300 mg/L (milligrams per liter), and dissolved chloride concentrations ranged from 12 to 1,400 mg/L. Dissolved-solids concentrations ranged from 251 to 5,580 mg/L. Total nitrite plus nitrate concentrations (considered to be all nitrate for this report) ranged from less than 0.1 to 8.9 mg/L. Chemical water types range from calcium bicarbonate to calcium sulfate to sodium chloride. Historical (1940-49) and 1987 dissolved-solids concentrations were compared to identify potential changes in water quality. In some local areas, dissolved-solids concentrations decreased as much as 1,630 mg/L. The increase in dissolved-solids concentrations in water from wells and springs ranged from 5 to 4,894 mg/L. Maximum increases in dissolved-solids concentrations were 3,290 mg/L in water from Comanche Springs and 4,894 mg/L in water from Santa Rosa Springs. The increases may represent a mixing of Edwards-Trinity water with moderately saline water from underlying rocks of Permian age, or an accumulation of salts from surface-water sources. Comanche Springs, dry since 1961, began flowing again in October 1986, following several weeks of record or near-record precipitation in Fort Stockton and the Trans-Pecos region. Accelerated recharge from the increased precipitation, combined with a cessation of irrigation pumpage in August 1986, probably were responsible. The springs ceased flowing in May 1987, following the start of irrigation pumpage in February 1987. Correlation between flow from Comanche Springs and water levels in Fort Stockton city well no. 2 in the Leon-Belding irrigation area indicates that the springs are unlikely to flow when the depth to water in this well exceeds about 232 feet.

Texas↗
USGS1993

Reconnaissance investigation of the geology and hydrogeology of Lackland Air Force Base, San Antonio, Texas

An investigation at Lackland Air Force Base and Lackland Air Force Base Training Annex (Medina Base) was conducted from May to September 1988 to delineate the subsurface geology, to describe the hydrogeology within the study area, and to determine possible migration pathways for contaminants. Data from this investigation supplement data collected in conjunction with other Air Training Command studies conducted under the Installation Restoration Program. The geologic formations in the vicinity of Lackland Air Force Base and Medina Base consist of sedimentary rocks of Cretaceous, Tertiary, and Quaternary age. The lithology of these formations consists primarily of limestone and marl, with lesser amounts of gravel, sand, silt, shale, and clay. The formations that comprise the geologic setting at Lackland Air Force Base and Medina Base include the Navarro Group of Late Cretaceous age, the Midway Group and Uvalde Gravel of Tertiary age, and the Leona Formation and fluviatile terrace deposits of Quaternary age. The age of the faulting within the study area has not been determined accurately, but the faulting might have occurred during intervals from Early Cretaceous to Holocene time. During these tectonic episodes, uplift of the Edwards Plateau might have occurred. This uplift could have enhanced erosion of the Cretaceous age Edwards Group, which is north of the study area, causing deposition of alluvial deposits south of this tectonic activity. The Leona Formation is composed of limestone gravel deposits forming terraces in the valleys of present streams. In general, these terraces are topographically lower than those formed by the Uvalde Gravel. In contrast, the fluviatile terrace deposits are above flood level along entrenched streams such as Leon and Medio Creeks. The Leona Formation is generally above the level of these fluviatile terrace deposits. A review of the hydrogeologic literature indicated that the Navarro and Midway Groups do not yield water to wells in Bexar County. However, recent studies by the U.S. Geological Survey at Kelly Air Force Base indicate that the uppermost beds of the Navarro Group can be saturated but yield little or no water to wells. Each stream terrace deposit of the Leona Formation and the fluviatile terrace deposits are separate water-bearing units. At Lackland Air Force Base, shallow ground water is present in the Leona Formation and the flow probably is toward Leon Creek. At Medina Base, the fluviatile terrace deposits are present along both banks of Medio Creek and probably are separate water-bearing units. Furthermore, ground-water flow in each of these units probably is toward Medio Creek. The Uvalde Gravel, present only at and west of Medina Base, is not a source of shallow ground water. However, during periods of precipitation, the formation can readily absorb precipitation and surface runoff. Water drains quickly through the formation because of its topographically high position and substantial hydraulic conductivity. Major pathways of potential contaminant migration off the bases include the local streams of Medio and Leon Creeks, and to a lesser extent, the shallow ground water beneath the bases. Although the Uvalde Gravel is not a source of shallow ground water at Medina Base, it drains water quickly, and wastes that might be buried in the gravel could be a potential source of contamination during brief ground-water recharge periods resulting from major precipitation.

Texas↗
USGS1984

Carbonate geology and hydrology of the Edwards Aquifer in the San Antonio area, Texas

Regional differences in the porosity and permeability of the Edwards aquiifer are related to three major depositional areas, the Maverick basin the Devils River trend, and the San Marcos platform, that existed during Early Cretaceous time. The rocks of the Maverick basin are predominantly deep basinal deposits of dense, homogeneous mudstones of low primary porosity. Permeability is principally associated with cavernous voids in the upper part of the Salmon Peak Formation in the Maverick basin. The rocks of the Devils River trend are a complex of marine and supratidal deposits in the lower part and reefal or inter-reefal deposits in the upper part. Permeable zones, which occur in the upper part of the trend, are associated with collapse breccias and rudist reefs. The rocks of the San Marcos platform predominantly are micrites that locally contain collapse breccias, honeycombed, burrowed mudstones, and rudist reef deposits that are well leached and very permeable. The rocks of the San Marcos plat form form the most transmissive part of the Edwards aquifer in the San Antonio area. Karstification of the rocks on the San Marcos plat- form during Cretaceous time enhanced the permeability of the aquifer. Permeability of the Edwards aquifer is greatest in particular strata (lithofacies) which have been leached in the freshwater zone. Ground water moves along vertical or steeply inclined fractures that are passageways by which water can enter permeable strata. Water moves from the fractures into beds formed by collapse breccias, burrowed wackestones, and rudist grainstones that have significant secondary porosity and permeability. Water has selectively dissolved sedimentary features within those rocks to increase the size of the openings and the degree of interconnection between pore voids. Recognition of the hydrostratigraphic subdivisions provides a basis for defining the nonhomogeneity of the aquifer and determining its storage characteristics. The aquifer is considered to be a faulted and multilayered aquifer in which lateral circulation is mainly through very permeable, hydrostratigraphic subdivisions that are hydraulically connected at places by openings associated with steep-angle, normal faults. The Edwards aquifer is vertically displaced for its entire thickness at places along major northeastward trending faults. At these places, ground-water circulation is diverted either southwest or northeast.

Open-File Report↗
USGS1983

Hydrostratigraphic subdivisions and fault barriers of the Edwards aquifer, south-central Texas, U.S.A.

The karstic Edwards Limestone within the Balcones Fault Zone of south-central Texas forms a productive confined aquifer that consists predominately of dense carbonate rocks and contains several layers of highly permeable and porous honeycombed rocks that have been produced by the leaching of evaporitic, tidal flat or reefal deposits. Fractures have hydraulically interconnected these layers at some places. Faults, however, commonly place rocks of very high-permeability opposite rocks of very low permeability, thus creating a lateral discontinuity and a flow barrier. At places, fault barriers probably cause partial to almost complete blockage of groundwater flow normal to the fault.

Texas↗
USGS1980

Hydrochemical data for the Edwards aquifer in the San Antonio area, Texas

This report presents hydrochemical data that was collected as part of the investigations of the Edwards aquifer in the San Antonio area, Te xas, during 1970-78 and indicates other sources of available data. The report includes the results of chemical analyses of 159 water samples from 123 well s and springs; tritium analyses for 242 water samples from 120 wells and springs; isotope and redox-potential analyses of 31 water samples from wells, springs, and streams; and calculated dissolved carbonate, partial C02 pressures, and saturation indices of selected minerals in 98 water samples from 81 wells, springs, and streams. The water types and hydrochemical facies are given for six zones of the aquifer.

Texas Department of Water Resources Report↗