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From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
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Bureau Seminar, January 31, 2014

Investigating the Edwards-Trinity Aquifer Interface: Designing a Comprehensive Study of a Complex Regional Karst System

video streaming of this seminar beginning at 8:55am January 31, 2014.

Marcus Gary, PhD, PG
Senior Hydrogeologist, Aquifer Science
Edwards Aquifer Authority

Abstract
The Edwards and Trinity aquifers are critical water resources, supplying high-quality drinking water for over two million people, for agricultural irrigation, and for industry in the San Antonio region of south central Texas. These Cretaceous carbonate aquifers are hydrogeologically juxtaposed by extensive Miocene faulting within the Balcones Fault Zone. The faulting resulted in the younger Edwards Group Limestone (Edwards Limestone) downthrown to the south-southeast, relative to the older Trinity Group. The Trinity and Edwards aquifers are managed separately by various regulatory entities and they have been historically treated as independent hydrologic systems, both scientifically and from a resource management standpoint. A significant interconnection exists between the Edwards and Trinity aquifers based on studies related to upland recharge variability, stream flow gain and loss studies, tracer testing, multi-port monitoring wells, geochemistry, biologic habitat analysis, geophysics, and groundwater modeling (Gary et al., 2011). However, the relationship between the two aquifers is complex, may substantially change laterally across the interface, and is challenging to quantify due to these factors.

The Texas Water Development Board (TWDB) Groundwater Availability Model (GAM) for the Hill Country Trinity Aquifer (Mace et al., 2000) reported that estimates of water leaving the Trinity Aquifer and recharging the Edwards Aquifer ranged from 360,000 acre-ft/yr (Kuniansky and Holligan, 1994) to approximately 5,000 acre-ft/yr (LBG-Guyton and Associates, 1995). The LBG-Guyton estimate excluded contributions from Cibolo Creek. Estimates for the Cibolo Creek area contribution basin ranged from 53,800 acre-ft/yr (Lowry, 1955) to 107,000 acre-ft/yr (Bader et al., 1993). The TWDB revised their estimate of recharge to 110,600 acre-ft/yr from the revised GAM model (Jones et al., 2011). The Edwards Aquifer MODFLOW model produced by the USGS uses a value of 40,298 acre-ft/yr for recharge from the Trinity Aquifer to the Edwards Aquifer (Lindgren et al., 2005). The wide range of estimated values indicates an uncertainty derived from model-generated data rather than empirical data derived from field measurements. Therefore, there is a significant range of uncertainty with regional recharge estimates from the Trinity to the Edwards. In addition, there have been few studies quantifying flow from the Edwards Aquifer to the Trinity Aquifer where "drill-through" wells completed fully through the Edwards pump water from the upper units of the Glen Rose Limestone (top of Trinity Aquifer), and this relationship is poorly understood. Tracer testing by the Edwards Aquifer Authority (EAA) and multiport monitoring wells installed by the Barton Springs Edwards Aquifer Conservation District (BSEACD) indicate that the upper two hundred feet of the upper Glen Rose Limestone and the Edwards Limestone are in hydrologic connection, at least in those areas studied. This presentation discusses the scope of work developed to investigate interformational flow between the Edwards Aquifer and the Trinity Aquifer in the region.

 

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