In cooperation with the Bexar-Medina-Atascosa Counties Water Control and Improvement District No. 1, Bexar Metropolitan Water District, Texas Water Development Board, and Edwards Aquifer Authority
Hydrogeology, Hydrologic Budget, and Water Chemistry of the Medina Lake Area, Texas
By Rebecca B. Lambert, Kenneth C. Grimm, and Roger W. Lee
Water-Resources Investigations Report 00–4148
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Purpose and Scope
Methods of Analysis
Estimated Ground-Water Recharge Methods
Summary and Conclusions
A. Hydrologic Data
B. Hydrologic Budgets
C. Water-Quality Data
|1.||Map showing hydrogeologic sections A–A' and B–B' and showing subdivisions of the Edwards and Trinity aquifers, Medina Lake area, Texas (1.10 MB, 30" X 25")|
|2.||Map showing locations of water-level and water-quality sites of the Edwards and Trinity aquifers, Medina Lake area, Texas (1.57 MB, 25" X 25.5")|
|1.||Location of the Medina Lake area|
|2.||Regional ground-water-flow units of the Edwards aquifer, Texas|
|3.||Location of hydrologic budget data-collection sites and Quihi monitoring well (TD–69–40–102), Medina Lake area, Texas|
|4.||Daily evaporation rates for Medina and Diversion Lakes, Medina Lake area, Texas|
|5.||Relation of ground-water outflow (GWout) to Medina Lake stage for selected hydrologic budget periods, Medina Lake area, Texas|
|6.||Relation of monthly average Quihi well water-level altitude to monthly average Medina Lake stage, Medina Lake area, Texas, October 1993–February 1997|
|7.||Relation of ground-water outflow (GWout) to Diversion Lake stage for selected hydrologic budget periods, Medina Lake area, Texas|
|8.||Relation of Medina/Diversion Lakes combined ground-water outflow (GWout) to Medina Lake stage for selected budget periods, Medina Lake area, Texas|
|9.||Relation of estimated monthly average recharge from Medina/Diversion Lakes using three methods to monthly average Quihi well water-level altitude, Medina Lake area, Texas, December 1995–July 1996|
|10–11.||Trilinear diagrams of:|
|10.||Waters for surface-water and lake-water samples, Medina Lake area, Texas|
|11.||Waters for ground-water samples in the Edwards and Trinity aquifers, Medina Lake area, Texas|
|12.||Relation between deuterium (dD) and del oxygen (d18O) in water samples from Medina Lake, Medina River, and from selected wells and springs, Medina Lake area, Texas|
|13.||Relation of 1/strontium to 87Sr/86Sr in aquifer and lake water, Medina Lake area, Texas|
|1.||Summary of lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards and Trinity aquifers, Medina Lake area, Texas|
|2.||Hydrologic budget data-collection sites in the Medina Lake area, Texas|
|3.||Summary of hydrologic budgets for Medina, Diversion, and Medina/Diversion Lakes (combined), Medina Lake area, Texas|
|4.||Water-level and water-quality data-collection sites and isotopic data, Medina Lake area, Texas|
|5.||Comparison of estimated average losses (recharge to the Edwards aquifer), Medina Lake area, Texas, December 1995–August 1996|
|6.||Comparison of mixing fractions of endmember Medina Lake water and endmember Edwards aquifer water calculated from deuterium and del oxygen data and from NETPATH mixing models in selected wells and springs, Medina Lake area, Texas|
|7.||Results of NETPATH simulations of mixing water from Medina Lake with water from the Edwards aquifer, Medina Lake area, Texas|
A three-phase study of the Medina Lake area in Texas was done to assess the hydrogeology and hydrology of Medina and Diversion Lakes combined (the lake system) and to determine what fraction of seepage losses from the lake system might enter the regional ground-water-flow system of the Edwards and (or) Trinity aquifers. Phase 1 consisted of revising the geologic framework for the Medina Lake area. Results of field mapping show that the upper member of the Glen Rose Limestone underlies Medina Lake and the intervening stream channel from the outflow of Medina Lake to the midpoint of Diversion Lake, where the Diversion Lake fault intersects Diversion Lake. A thin sequence of strata consisting primarily of the basal nodular and dolomitic members of the Kainer Formation of the Edwards Group, is present in the southern part of the study area. On the southern side of Medina Lake, the contact between the upper member of the Glen Rose Limestone and the basal nodular member is approximately 1,000 feet above mean sea level, and the contact between the basal nodular member and the dolomitic member is approximately 1,050 feet above mean sea level. The most porous and permeable part of the basal nodular member is about 1,045 feet above mean sea level. At these altitudes, Medina Lake is in hydrologic connection with rocks in the Edwards aquifer recharge zone, and Medina Lake appears to lose more water to the ground-water system along this bedding plane contact.
Hydrologic budgets calculated during phase 2 for Medina Lake, Diversion Lake, and Medina/Diversion Lakes combined indicate that: (1) losses from Medina and Diversion Lakes can be quantified; (2) a portion of those losses are entering the Edwards aquifer; and (3) losses to the Trinity aquifer in the Medina Lake area are minimal and within the error of the hydrologic budgets.
Hydrologic budgets based on streamflow, precipitation, evaporation, and change in lake storage were used to quantify losses (recharge) to the ground-water system from Medina Lake, Diversion Lake, and Medina/Diversion Lakes combined during October 1995–September 1996. Water losses from Medina Lake to the Edwards/Trinity aquifers ranged from -14.0 to 135 acre-feet per day; Diversion Lake ranged from -1.2 to 93.1 acre-feet per day; and Medina/Diversion Lakes combined ranged from 36.1 to 119 acre-feet per day.
Monthly average recharge during December 1995–July 1996 was estimated using an alternative method developed during this study (current study method) and compared to monthly average recharge during December 1995–July 1996 estimated using the existing USGS method and the Trans-Texas method. Recharge to the Edwards aquifer estimated using the current study method was about 69 and 73 percent of the recharge estimated using the USGS and Trans-Texas methods, respectively. The USGS and Trans-Texas methods overestimated recharge from Medina Lake compared to the recharge estimated with the current study method when Medina Lake stage was between about 1,027 and 1,032 feet above mean sea level and underestimated recharge from Medina Lake when lake stage was between about 1,036 and 1,045 feet above mean sea level. The USGS and Trans-Texas methods underestimated recharge from Diversion Lake compared to the recharge estimated with the current study method when Diversion Lake stage was greater than 913 feet above mean sea level and overestimated recharge from Diversion Lake when lake stage was less than 913 feet above mean sea level.
The water quality of Medina Lake and Medina River and in selected wells and springs in the Edwards and Trinity aquifers was characterized during phase 3 of the study. Environmental isotope analyses and geochemical modeling also were used to determine where water losses from the lake system might be entering the ground-water-flow system. Isotopic ratios of deuterium, oxygen, and strontium were analyzed in selected surface-water, lake-water, and ground-water samples to trace the isotopic “signature” of the lake water as it mixes with the ground water and to determine the fraction of lake water and ground water in selected Edwards aquifer wells. Isotopic data and geochemical modeling were used to show that lake water is moving into the Edwards aquifer in two fault blocks in the eastern Medina storage unit. One fault block is bounded on the north by the Vandenburg School fault and on the south by the Haby Crossing fault, and the second fault block is bounded on the north by the Diversion Lake fault and on the south by the Haby Crossing fault. In selected Edwards aquifer wells located southwest of Medina Lake and west of Diversion Lake, the proportion of lake water ranged from about 10 to 45 percent. Geochemical modeling using NETPATH confirms the degree of mixing between lake water and aquifer water shown by the isotopes.
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