Browsing by Subject "Groundwater flow"
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Item Groundwater flow and recharge within the Barton Springs segment of the Edwards Aquifer, southern Travis and northern Hays Counties, Texas(2009-05) Hauwert, Nico Mark; Sharp, John Malcolm, 1944-The Barton Springs Segment, part of the karstic Edwards aquifer in Central Texas, is a Sole Source aquifer, is habitat to rare karst species, and provides water to a well-loved municipal swimming pool, yet its hydrogeologic properties remain insufficiently understood. For this study, the hydrogeologic characteristics of the Barton Springs Segment were investigated using several approaches, including mapping of hydrostratigraphic units and faults, measurement of upland infiltration, groundwater traces, and aquifer tests. The depositional environment, diagenesis, fracturing, down-dropped and dipping faulted blocks, and subsequent dissolution were determined to play important roles in controlling groundwater flow-path development within the Barton Springs Segment. In particular, downdropped fault blocks create groundwater gradients to the southeast that influence flow in the Edwards outcrop area. Upland internal drainage basins were found to be extremely efficient at conveying recharge to the underlying aquifer. The maturity of natural internal drainage sinkholes can be measured by its bowl volume, which grows in proportion to the catchment area it captures. A 19-hectare internal drainage basin, HQ Flat sinkhole, was monitored for rainfall, evapotranspiration, soil moisture, and discrete runoff to the cave drain. During a 505-day period, 5.5% of measured rainfall entered the cave drain as discrete recharge, 26% of measured rainfall infiltrated through soils on the slopes, and the remaining 68% was lost through evapotranspiration. This amount of upland infiltration is consistent with infiltration measurements in other karst areas and is much larger than the 1% upland recharge of rainfall that was previously estimated. A chloride mass balance indicates that at the adjacent Tabor research site, about 50% of rainfall infiltrates to a 6-meter depth. Dye-tracing and pump tests demonstrated that primary and secondary groundwater flow paths are the major influence on transmissivity within the Barton Springs Segment. Groundwater tracing breakthroughs reveal very high advection and relatively low dispersion. Drawdown response to pump tests indicates a very high degree of anisotropy, controlled by location of groundwater flow paths. Overall the Barton Springs Segment is a mature karst aquifer with highly developed rapid, discrete network for both recharge and groundwater-flow.Item Groundwater flow controls on coastal water quality and global groundwater ages(2015-05) Befus, Kevin Martin; Cardenas, Meinhard Bayani, 1977-; Gleeson, Thomas P; Hesse, Marc A; Paine, Jeffrey G; Sharp, John MHumanity relies on groundwater. But, current consumption may be outpacing groundwater renewal rates, and anthropogenic activities are altering its quality. This dissertation advances the state of knowledge of how local and regional groundwater dynamics affect its quality and quantity. First, I investigate groundwater discharge patterns and fluxes in three lakes in the Nebraska Sand Hills region and on the island of Rarotonga, Cook Islands, to understand the hydrologic connection between groundwater and surface water in these lacustrine and coastal settings. In Nebraska, I use electrical geophysical methods to characterize the spatial signature of groundwater recharge and discharge to and from the lakes using groundwater salinity patterns. On Rarotonga, a detailed field study of groundwater flow at the intertidal zone shows how groundwater flow influences the thermal regimes of nearshore environments, affecting the biota that live and chemical processes that occur near and below this dynamic interface. Next, a dense network of geophysical surveys across the coastal plain and into the lagoon on Rarotonga constrains multiple features of the larger-scale hydrologic system that are primarily controlled by the local carbonate and volcanic geology on the island. Finally, I give the first estimate of the global storage and spatial distribution of groundwater with a mean age since recharge of less than fifty years. I use several thousand two-dimensional groundwater flow and age-as-mass transport simulations parameterized by the best available hydrologic and geologic datasets. This global analysis suggested that ~6% of the groundwater stored in the upper 2 km of the Earth’s crust is younger than 50 years. Comparing this young groundwater storage to current groundwater depletion rates indicates that more than half of the irrigated areas depending significantly on groundwater could have already used up all of the young groundwater and are using groundwater more quickly than the storage is replenished. Together, these studies advance how to quantify groundwater as a renewable resource through the global estimation of groundwater storage associated with certain timespans and by analyzing the implications of groundwater flow on water quantity and quality in field settings.Item Hydrogeologic controls on underflow in alluvial valleys : implications for Texas water law(1988-12) Larkin, Randall G.; Sharp, John Malcolm, Jr., 1944-Groundwater flow in alluvial valleys consists of two components, baseflow and underflow. The baseflow component of the Darcy flux flows normal to the river and contributes to the surface flow. The underflow component moves downstream in the same direction as the river but at a much slower rate. Underflow is important in Texas because the conjunctive use of groundwater and surface water is regulated by controlling the diversion of underflow by wells. Land owners in Texas are legally entitled to unrestricted use of the underground water beneath their property. Stream underflow, however, has been expressly excluded from the definition of underground water. The distinction is important because it allows the State to legally restrict the non-domestic pumpage of groundwater (in an "underflow zone") near streams. Regulators are interested in controlling pumpage near rivers in order to prevent streamflow depletion. Historically, the underflow exemption has not been well recognized by the courts. In large measure, this may be due to the fact that our understanding of underflow in alluvial valleys is incomplete. If the underflow rule is to be successfully implemented, a complete understanding of the nature and occurrence of underflow is imperative. This study was initiated to: 1) determine the hydrogeologic factors that control underflow (and baseflow) in alluvial valley aquifers in Texas and the United States; and 2) to examine the suitability of the underflow criterion as a management tool for the prevention of streamflow depletion by wells. To accomplish this, a data base of 23 alluvial river basins was compiled and a 3-dimensional digital model of a hypothetical alluvial valley aquifer was constructed. Examples from the data base indicate that alluvial aquifers can be classified into three types based on the predominant regional groundwater flow direction: baseflow-dominated, underflow-dominated, and mixed flow. Flow patterns can be transient, however, and respond rapidly to changing river stage if the aquifer and the riverbed are highly permeable. Therefore, the distinction must be made between local, transient underflow and baseflow occurring near the river and regional, steady state underflow and baseflow away from the river. Underflow dominated aquifers are found in classic bedload depositional settings which are characterized by high channel gradient, high width to depth ratio, low channel sinuosity, and low river penetration. Linear regressions performed on the parameter values in the data base verify the validity of the data. The degree of correlation provides the basis for a method of estimating the predominant regional groundwater flow direction in an alluvial aquifer based on geomorphologic and morphometric data. The results from the digital model agree with the findings from the data base. Digital simulations indicate that the amount of underflow is directly related to the channel gradient, the amount of recharge, the aquifer hydraulic conductivity, and the streambed hydraulic conductivity. The riverbed hydraulic conductivity is the most critical hydraulic factor controlling the amount of underflow. The output from the model is 100 percent underflow at low values of riverbed permeability. Both the model results and published field data do not support the existence of a significant local "underflow zone" adjacent to rivers in large alluvial systems. Close to the river, the baseflow component may predominate even in regionally underflow-dominated systems due to the influence of high transverse valley gradients. There are many problems associated with the use of underflow as a management tool. The definition is vague and ambiguous. Underflow can be transient and spatially variable. Texas alluvial systems are baseflow dominated and there is probably no significant "underflow zone" near rivers. Lastly, the presence of underflow has been difficult to prove in court. It is the finding of this study that the underflow criterion is insufficient to prevent streamflow depletion by wells. The underflow rule in the Texas Water Code should be reconsidered, or perhaps abandoned, in favor of criteria that are more justifiable.Item Hydrogeological analysis of groundwater chemistry and sulfate distribution, Blanco and Hays Counties, Texas(2010-05) Andring, Megan J., 1984-; Sharp, John Malcolm, 1944-; Bennett, Philip C.; Helper, Mark A.High concentrations of sulfate in groundwater, up to ten times the amount recommended by the EPA, in Blanco and Hays Counties, Texas, are of concern as groundwater pumping and population increase. The goals of this study are to characterize the chemistry of groundwaters in Blanco and Hays Counties within the context of Texas Groundwater Management Area-9 and to determine chemically and hydrogeologically the explanation for the spatial distribution of sulfate between the Pedernales River, the Blanco River, and Onion Creek. Insights gained by examining sulfate distribution in Blanco and Hays Counties can be applied to the other counties on the Edwards-Trinity Plateau with similarly high concentrations of sulfate in groundwater. Hydrochemical data from the Cretaceous Edwards and Trinity Groups and water level measurements were used to analyze groundwater chemistry and flow. PHREEQC was used to examine whether phase changes in aquifer minerals could explain the observed geochemical patterns. COMSOL was used to develop a simplified groundwater flow model for a cross-sectional area between the Pedernales River and Onion Creek in Hays County. Water levels indicate that groundwater generally flows southeast in the study area and most streams are gaining. The groundwater flow model indicates a zone of slow-moving groundwater beneath the topographic high between the Pedernales River, the Blanco River, and Onion Creek. Chemical analyses of well data show the presence of four groundwater chemical endmembers in Groundwater Management Area-9; a Ca-Mg-HCO3 fresh endmember, a Ca-Mg-SO4 endmember, a Ca-Mg-SO4-Na-Cl endmember, and a Na-Cl endmember. High sulfate waters generally come from the Upper and Middle Trinity aquifers while fresher waters are from the Edwards aquifer. Physical and chemical analyses indicate that the zone of high sulfate in Blanco and Hays Counties may be the result of gypsum dissolution and dedolomitization in the Upper and Middle Trinity aquifers combined with low rates of groundwater flow beneath the topographic high. Groundwater flow analyses are consistent with those for the Groundwater Availability Models published for the region. Chemical analyses, specifically SO4 distributions and Ca/Mg ratios, are consistent with those found by Nance(2010) on the Edwards Plateau, farther west of the study area.Item Investigation on heat transport in hyporheic zone using flume simulation and modeling(2011-08) Chan, Wai Sum, 1984-; Cardenas, Meinhard Bayani, 1977-; Wilson, Clark R.Recent research has shown that groundwater flow in hyporheic zone is critical in major hydrologic, ecological, and biogeochemical processes. Quantitative analyses from the literature show that there is a strong correlation between the diel cycles in pH, water temperature, and other parameters such as trace metal concentrations. There is, however, no controlled experimental data to illustrate how water temperature influences the trace metal concentrations and other parameters. The research study presented here illustrates the mechanism of heat is transported from stream water to groundwater in the hyporheic zone on different bed form. The work will serve as the foundation of future research in understanding the relationship of heat and trace metal concentrations in the sediments.Item Literature evaluation of induced groundwater tracers, field tracer techniques, and hydrodynamic dispersion values in porous media(Texas Tech University, 1981-08) Reid, Gary CarlTracers have been employed to evaluate groundwater flow velocity as well as the physical characteristics of the geologic media through which they pass. A wide variety of groundwater tracers have been reported in the literature. These tracers include dyes, gases, particulate matter, thermal, and molecular and ionic substances. Well tests which include both single and multiple wells have been utilized in conjunction with these groundwater tracers. Single well evaluation includes borehole dilution and pulse technique. Multiple well tests encompass studies conducted in an unstressed well field, pumping and injection well tests, and discharging - recharging well pairs. The media's ability to disperse, or mix the tracer, which is known as the dispersivity of the specific medium, can be calculated by the use of tracers in conjunction with well tests. Dispersivities have been extracted from the literature and include values from several media types including elastics, fractured carbonates, and fractured crystalline rocks.Item Secondary recovery of groundwater by air injection--a finite element model(Texas Tech University, 1987-12) Sathiyakumar, NeelakandanNot available