54 research outputs found
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Report of Investigations No. 131 Origin and Diagenesis of Cap Rock, Gyp Hill and Oakwood Salt Domes, Texas
UT Librarie
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Nitrate contamination of groundwater in southern Runnels County, Texas
Nitrate concentrations in the groundwater in southern Runnels County, Texas, ranged from less than 0.5 mg/l to 3,580 mg/l. Only ten percent of the water samples analyzed contained less than the U. S. Public Health Service recommended limit for nitrate in drinking water (45 mg/l). The major sources of nitrate contamination are soils beneath or near barnyards or septic tanks. Average total nitrate concentration in barnyard soils was 26,000 pounds of nitrate per 15 acre-feet, whereas the average total nitrate (NOâ) concentration in cultivated fields and pastures was 4,100 lb. NOâ / 15 acre-feet and 3,900 lb. NOâ / 15 acre-feet, respectively. Nitrates appear to be added to the groundwater by two mechanisms. First, large volumes of water from cattle excrement can enter the aquifers by easy drainage down poorly cased water wells. Second, extensive terracing has caused an appreciable rise in the potentiometric surface with subsequent dissolution of nitrate caliches from the soils by groundwater. Groundwater flow is restricted to solution cavities and fractures in the limestones. Aquifer tests indicate transmissivities on the order of 10,000 gpd/ft, and coefficients of storage on the order of 10â»â”. Numerous poorly cased water wells, unplugged seismic shot holes and abandoned oil wells have interconnected the thin limestone aquifers and have permitted extensive contamination of the aquifer system. Early improvement of future groundwater quality cannot be expected because of the vast quantities of nitrate still in the barnyard soils. Water importation or desalination may prove economically feasible for human consumption, but not for agricultural needs.Geological Science
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Determining the source of nitrate in groundwater by nitrogen isotope studies
Nitrogen isotope ratios of ammonium and nitrate ions from soil and water samples can be analyzed reproducibly with an experimental error of approximately ±1â°/ââ. Two isotopic ranges of soil nitrate are found in the soils of southern Runnels County, Texas. Nitrate from the decomposition of animal waste nitrogen has a ÎŽNÂčâ” of +10â°/ââ to +22â°/ââ. The isotopic ratio is controlled by the volatilization of isotopically light ammonia gas during the decomposition of urea in urine. Nitrate derived from the mineralization of organic nitrogen in cultivated soils has a ÎŽNÂčâ” of +2â°/ââ to +8â°/ââ. In southern Runnels County the major source of nitrate in groundwater is natural soil nitrate. The isotopic composition of groundwater nitrate beneath cultivated 15 fields corresponds with ÎŽNÂčâ” of natural soil nitrate. Groundwaters beneath farmhouse-barnyard complexes have a higher average ÎŽNÂčâ”, indicating the addition of animal waste nitrate. Eleven samples of groundwater from Macon County, Missouri, have ÎŽNÂčâ” of +10â°/ââ to +19â°/ââ indicating that the waters are contaminated with nitrate from animal wastes. Nitrates in groundwaters from the Upper Glacial aquifer in Queens County, New York, appear to be from an animal waste source, whereas nitrates in groundwaters from the Magothy aquifer in Nassau County, New York, appear to be from either natural soil nitrogen or artificial fertilizer.Geological Science
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Evaluating the Potential of East Texas Salt Domes for Isolation of Nuclear Waste
Since January 1978, the Bureau of Economic Geology has been evaluating the potential for using a salt dome in the East Texas Basin as a repository for nuclear waste isolation. This report is a brief summary of work accomplished within Year I.
Using the dome selection criteria of Brunton and others (1977), Kreitler and others (1978) selected Oakwood, Keechi, and Palestine salt domes as possible sites for a nuclear waste repository. The problem of depositing nuclear wastes into an East Texas salt dome contains two critical questions: (1) Are the domes still growing (tectonic stability)? (2) Are the domes dissolving, and what is the rate of dissolution (hydrologic stability)? These two questions are being asked on a dome-specific and regional scale. The long-term suitability of a dome cannot be ascertained until it is placed in a regional context. This necessitates regional as well as site-specific studies.
The approach includes three subprograms: (1) subsurface geology, (2) hydrogeology, and (3) surficial geology and remote sensing. The subsurface geology program investigates dome size and shape, the geology immediately around the dome, and the infilling of the East Texas Basin over geologic time and how this basin filling affected the growth of the domes. The surficial geology and remote sensing program addresses the problem of potential dome growth during the Quaternary. Typical questions examined in this program include: have Pleistocene terraces been uplifted or warped, has there been any fault movement in the Pleistocene, and are there any movements reflected in lineation patterns around domes anomalous to regional patterns or indicated by drainage networks? The hydrogeology program evaluates the hydrologic stability of the domes with the following objectives: What are the rates and directions of regional groundwater flow? What are the ages of these groundwaters? How does groundwater flow around a salt dome? What are the rates of salt solution? Does the caprock prevent salt dissolution?
This document represents a progress report and is not a final statement on the Bureau of Economic Geology's position on the suitability of salt domes in the East Texas Basin for waste isolation. The observations and ideas presented in this document therefore represent a status report and may be subject to change as more information and concepts are developed.Bureau of Economic Geolog
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Land-surface Subsidence and Active Faulting in the Texas Coastal Zone
Land-surface subsidence characterizes much of the Texas Coastal Zone, but is most common in the upper part of the Zone and especially in the greater Houston area. The degree of subsidence ranges from acute to that discernible only with precise instrument leveling. A number of factors can result in surface subsidence; however, the major cause in affected portions of the Texas Coastal Zone is the lowering of pressure heads due to the withdrawal of ground water. In local areas, subsidence has also been a function of the removal of oil and gas, and the solution mining of sulfur and salt. Other causes, significant in other areas, are insignificant in the Texas Coastal Zone.Bureau of Economic Geolog
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Report of Investigations No. 133 Three-Dimensional Ground-Water Modeling in Depositional Systems, Wilcox Group, Oakwood Salt Dome Area, East Texas
UT Librarie
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Numerical Modeling of Regional Ground Water Flow in the Deep-Brine Aquifers of the Palo Duro Basin, Texas Panhandle
A conceptual hydrogeologic model of the Palo Duro Basin, Texas Panhandle, subdivides the basin into three hydrogeologic units: the shallow Ogallala and Dockum aquifers, the Permian evaporite aquitard, and the deep confined, underpressured Permian and Pennsylvanian brine aquifer. The first permeable units beneath the thick Permian evaporite section are Wolfcamp strata composed of carbonates, shales, and arkosic sand and gravels (granite wash) with average effective permeability values of 8.9, 0.0001, and 8.6 md, respectively. Groundwaters in the Wolfcamp aquifer flow to the northeast toward the semi-impermeable, granitic Amarillo Uplift. This anomalous hydrologic condition (flow toward a low-transmissivity barrier) may result from the presence of highly permeable granite-wash deposits that flank the uplift and function as "hydrologic sinks."
A two-dimensional, vertical-averaging finite-element model, incorporating the different lithologies and their different permeabilities as well as leakage through the overlying evaporite aquitard, has been used to simulate the observed potentiometric surface of the Wolfcamp aquifer. The conditions that best simulate the observed Wolfcamp potentiometric surface are a combination of specified head and no-flow conditions along the uplift, permeability values greater than 260 md for the granite-wash deposits that flank the uplift (in contrast to the average value of 8.6 md), and an increased permeability value of 50 md for the highly porous carbonate zone. The best estimate of the vertical permeability of the evaporite aquitard is 0.00008 md.
Treating the whole deep-brine aquifer as a single permeable unit beneath the evaporite aquitard, groundwater flow is to the northeast toward the uplift with a slightly larger west-to-east component than that found when considering only flow in Wolfcamp strata. The conditions that best simulate the averaged potentiometric surface are those from the best simulation of Wolfcamp strata, with increased permeability values of 260 md for the Pennsylvanian granite-wash close to the uplift and 250 md for the high-porosity Pennsylvanian sandstone.Bureau of Economic Geolog
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Wellhead Protection Strategies for Confined-Aquifer Settings
Improper management of contamination sources has resulted in numerous cases of groundwater contamination of public water supply wells. One approach toward preventing contamination of public water supplies is to protect the areas that recharge precipitation and surface water to the aquifer near the wells. This zone of protection is referred to as a wellhead protection area (WHPA). The potential for contamination is typically less in a confined aquifer than in an unconfined aquifer. Nevertheless, contamination of confined aquifers has occurred. Wellhead protection areas should be developed for all aquifer settings.
A confined aquifer is an aquifer overlain by low-permeability strata. The presence of the low permeability material reduces the risk of a surface contaminant reaching a producing well. The potential for contamination of a confined aquifer is controlled by two factors: (1) The presence of permeable pathways (for example, faults, fractures, permeable sands, or unplugged abandoned boreholes) that permit contaminant migration and (2) the existence of appropriate hydrologic conditions (for example, downward flow) that cause contaminants to migrate through the low-permeability strata.
Confined aquifers occur pervasively from coast to coast in the United States. The coastal plain aquifers along the Atlantic Ocean and Gulf of Mexico represent some of the largest confined aquifer systems in the United States. There are numerous other smaller aquifers which exhibit confined conditions.Bureau of Economic Geolog
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Sources of Shallow Saline Ground Water in Concho, Runnels, and Tom Green Counties
Soil and groundwater salinization, causing vegetative kill areas and water well contamination, are major concerns for farmers not only in Texas but also in many other states across the U.S. In various regions, a combination of natural processes and agricultural activities is primarily responsible for salinization. Moreover, in West Texas, pollution hazards associated with oil exploration and production further complicate the challenge of identifying the sources of soil and groundwater contamination.
This study focused on Concho, Runnels, and Tom Green Counties in west-central Texas (see Figure 1) due to the widespread occurrence of soil and groundwater salinization in these areas. Additionally, both natural and man-made sources of salinization could be active in this region.
The terrain in this area varies, with hilly terrain present in southern, western, and northern Tom Green County, where remnants of the Edwards Plateau rise to approximately 2,500 feet above sea level. Plains and river valleys dominate most of the remaining area, with the lowest surface elevations (approximately 1,500 feet) found in river valleys to the east. Major drainage systems in the region include the Colorado River in Runnels County, the Concho Rivers in Tom Green and Concho Counties, and several surface-water reservoirs just west of San Angelo.
The average annual rainfall in this area is approximately 21 inches, which is nearly one-third of the net lake evaporation.Bureau of Economic Geolog
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Hydrologic Characterization of the Saline Frio Formation, Victoria County, Texas Gulf Coast: A Case Study
Pressure data gathered from drillstem tests (DSTs) and bottomhole pressure measurements in onshore oil and gas wells along the Texas Gulf Coast were used for evaluating pressure regimes and their influence on the migration potential of formation fluids. The data were used to construct potentiometric surfaces and residual potential surfaces and to assess the effects of depressurization caused by hydrocarbon production.
This technique was utilized for preliminary geohydrologic characterization of the Frio Formation of the Texas Gulf Coast. Included in this report are the results of such an analysis for Victoria County, Texas, as a sample case study. Pressure data were evaluated for reliability, and a screening and classification system was applied to closely monitor the quality of data used for generating potentiometric surfaces. Preceding the hydrologic analysis, steps were taken to review the available geologic information in the context of Tertiary Texas Gulf Coast formations.
An evaluation of the regional pressure-depth plots reveals multiple overlapping pressure regimes. Also indicated is an area of extensive depressurization attributable to hydrocarbon production. The potentiometric surfaces also reflect depressurization that results in local variations in flow directions and a general trend of flow toward the oil and gas fields. Potentiometric surfaces for the deeper sections of the Frio reflect the high equivalent hydraulic heads in the geopressured region.Bureau of Economic Geolog
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