Brian A. Smith

Groundwater Flow in the Edwards Aquifer: Comparison of Groundwater Modeling and Dye Trace Results

The Edwards Aquifer of central Texas is a karst aquifer developed in faulted and fractured Cretaceous-age limestones and dolomites. Numerous groundwater models have been developed for the three segments of the Edwards Aquifer since completion of the first major flow model in 1979. Groundwater models have helped refine our understanding of the relationships among flowpaths, recharge, groundwater pumping, and springflow. Tracer test studies have been performed on the Edwards Aquifer since 1982. These studies have also brought about a better understanding of aquifer flowpaths, yet the results of groundwater modeling and dye trace studies do not have a high level of agreement. A comparison of the two types of studies has indicated the strengths and weaknesses of each method. Groundwater models of the Edwards Aquifer have been effective in simulating spring discharge and determining water budgets. The models have simulated water levels reasonably well, but there are significant discrepancies between measured and simulated water levels in areas that are more strongly influenced by conduit flow. Tracer testing is the best method for measuring rates of flow from a recharge feature, or a well, to springs and other wells. However, tracer tests provide little useful information about water levels or water budgets. Traditional groundwater models are poor tools for simulating contaminant transport and delineating areas for source-water protection of mature karst aquifers with well-developed conduit networks.

Relay Ramp Structures and Their Influence on Groundwater Flow in the Edwards and Trinity Aquifers, Hays and Travis Counties, Central Texas

The Cretaceous Edwards and Middle Trinity Aquifers of central Texas are critical groundwater resources for human and ecological needs. These two major karst aquifers are stratigraphically stacked (Edwards over Trinity) and structurally juxtaposed (normal faulting) in the Balcones Fault Zone (BFZ). Studies have long recognized the importance of faulting on the development of the karstic Edwards Aquifer. However, the influence of these structures on groundwater flow is unclear as groundwater flow appears to cross some faults, but not others. This study combines structural and hydrological data to help characterize the potential influence of faults and relay ramps on groundwater flow within the karstic Edwards and Middle Trinity Aquifers. Detailed structure contour maps of the top of Walnut Formation in the study area were created from a geologic database (n=380) comprised of primarily geophysical and driller’s logs. The data were then contoured in Surfer® (Kriging) with no faults. Structure contour surfaces revealed detailed structural geometries including linear zones of steep gradients (interpreted as faults) with northeast dipping zones of low gradients (interpreted to be ramps) between faults. Hydrologic data (heads, dye trace, geochemistry) were overlaid onto the structure contour maps in GIS. Results for the Middle Trinity Aquifer suggest relay ramps provide a mechanism for lateral continuity of geologic units and therefore groundwater flow from the Hill Country (recharge area) eastward into the BFZ. Faults with significant displacement (>100 m) can provide a barrier to groundwater flow by the juxtaposition of contrasting permeabilities, yet flow continues across fault zones where ramps exist, or where permeable units are juxtaposed with other permeable units. In the Barton Springs segment of the Edwards Aquifer the primary flow path defined by dye tracing and heads is coincident with the Onion Creek relay ramp dipping to the northeast. This work addresses the lateral continuity (intra-aquifer flow) of the Edwards and Trinity Aquifer systems, which has importance for conceptual models and ultimately resource management. Introduction The Cretaceous Edwards and Middle Trinity Aquifers of central Texas are critical groundwater resources for human and ecological needs (Figure 1). These two major karst aquifers are stratigraphically stacked and structurally juxtaposed in the Balcones Fault Zone (BFZ) (Figure 2). However, the role of faulting and related structures on groundwater flow is not clearly understood due to the stratigraphic and structural complexity. The purpose of this paper is to describe the influence of faults and related structures called relay ramps on Brian B. Hunt Barton Springs/Edwards Aquifer Conservation District, 1124 Regal Row, Austin, Texas, 78748, brianh@bseacd.org Brian A. Smith Barton Springs/Edwards Aquifer Conservation District, 1124 Regal Row, Austin, Texas, 78748, brians@bseacd.org Alan Andrews Barton Springs/Edwards Aquifer Conservation District, 1124 Regal Row, Austin, Texas, 78748 Douglas A. Wierman The Meadows Center for Water and the Environment, Texas State University, 601 University Drive, San Marcos, Texas, 78666, dawierman@aol.com Alex S. Broun Hays Trinity Groundwater Conservation District, P.O. Box 1648, Dripping Springs, Texas, 78620, albro33@aol.com Marcus O. Gary Edwards Aquifer Authority, 900 E. Quincy, San Antonio, Texas, 78215, mgary@edwardsaquifer.org

Cover-Collapse Sinkhole Development in the Cretaceous Edwards Limestone, Central Texas

89 of friable, highly altered, clayey limestone consistent with epikarst in-filled with terra rosa providing a cover of the feature. Dipping beds, and fractured bedrock support proximity to the mapped fault zone. Geophysics and surface observations suggested a lateral pathway for stormwater flow at the junction between the wet pond’s impermeable geomembrane and compacted clay liner for the retention pond. The collapse appears to have been caused by stormwater down-washing poorly consolidated sediments from beneath the SWRP and into a pre-existing karst conduit system.

Groundwater Flow Systems in Multiple Karst Aquifers of Central Texas

Introduction With limited surface water, central Texas is fortunate to have the Edwards and Middle Trinity karst aquifer systems that provide a variety of groundwater resources. The karstic Edwards Aquifer has been recognized for decades as a vital groundwater resource, and thus many studies have been published from Hill and Vaugh (1898) to recent (Hauwert and Sharp, 2014) that characterize the nature of the aquifer and its groundwater flow system. However, few studies have focused on the karstic nature and groundwater flow system of the deeper part of the Middle Trinity Aquifer.

Recharge and Water-Quality Controls for a Karst Aquifer in Central Texas

The Edwards Aquifer is a prolific karst aquifer system in Central Texas that provides drinking water to about 2 million people. Because a significant portion of the water recharging the Barton Springs segment of the Edwards Aquifer enters the subsurface through caves and enlarged fractures in the bed of Onion Creek, the presence of nonpoint source pollution in storm water flowing in Onion Creek can have a direct impact on water quality in the Barton Springs segment of the Edwards Aquifer. To address this concern, the Barton Springs/Edwards Aquifer Conservation District constructed a concrete vault over the entrance to Antioch Cave in the bed of Onion Creek. This structure was designed to prevent entry into the cave of contaminated storm water by closure of two valves on the vault during storm events. When the storm water passes, the valves open and allow the cleaner baseflow water to enter the cave. Results of water-quality sampling at Antioch indicate that the system is capable of significant reduction of nonpoint source pollution entering the aquifer through Antioch Cave. Over a period in 2010 that included five storm events, approximately 1105 kg (2436 lbs) of nitrogen from nitrate/nitrite, 134 kg (295 lbs) of total phosphorus, and 86,385 kg (190,480 lbs) of sediment were prevented from entering Antioch Cave. This amount of sediment is equivalent to about eight dump-truck loads that are prevented from entering the aquifer.

Multi-scale Geophysical Investigations of the Edwards Aquifer and Similar Karst Hydrogeophysics

The Main Barton Springs is a major discharge site for the Edwards Aquifer and is located in Zilker Park in Austin, Texas. The spring discharges into the Barton Springs pool near the diving board at an obvious fault line (Barton Springs fault). The thin bedded unit on the southwest side of the fault is the Regional Dense Member and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is in between 40 and 70 feet. It was geologically assumed that the groundwater recharged from the Barton Spring Segment, which is located several miles to the south?west of the Barton Springs pool area, follows the Barton Springs Fault strike and empties into the pool. To test this hypothesis geophysical surveys [2D and 3D resistivity imaging, natural potential (NP), seismic refraction tomography, induced polarization, and ground penetrating radar] were performed across the Barton Springs fault and in the southern part of the Zilker Park. Only NP surveys were allowed within the boundaries of the pool because of endangered species of Barton Springs Salamander. The purpose of the surveys was multi?folded: 1) to locate the precise location of the Main Springs on the south banks of the Barton Springs pool; 2) to determine the potential location of caves and active flow paths beneath the spring; 3) to characterize the geophysical signature of the fault crossing the Barton Springs pool. Geophysical results altogether, thus, suggest that significant amount of groundwater flow follows the path to the south of the fault along a fracture/fault zone that appears to coincide with the Springs location emptying into the pool within the Georgetown Formation.

Enhanced recharge to the Barton Springs segment of the Edwards Aquifer, Central Texas

The Barton Springs segment of the Edwards Aquifer is a prolific karst aquifer within Central Texas that provides groundwater for more than 60,000 people. Barton Springs is a major recreational attraction for Austin and also is habitat for endangered species. The majority of water recharging the aquifer enters along streams that flow across the recharge zone. Antioch Cave is the largest recharge feature situated in Onion Creek, one of the largest contributing ephemeral creeks for the Barton Springs segment of the aquifer. Therefore, recharge enhancement at Antioch Cave can have significant impacts on the aquifer. Modifications have been made to the entrance of Antioch Cave to increase recharge and to improve the quality of water entering the aquifer. A concrete vault with two 36-in. diameter valves was constructed over the cave entrance. One of the valves is operated by a water-quality monitoring system so that the valve will close automatically when poor quality water from storm runoff is detected. Once the initial storm pulse has passed, the valve will open to allow better quality water to enter the cave. A large screen was installed on the vault to minimize sediment and storm debris entering the cave. During periods of flow in Onion Creek, a significant groundwater mound develops in the aquifer beneath Antioch Cave. This mound increases storage in the aquifer so that during periods of drought, water levels and springflow at Barton Springs can be maintained at higher levels. Increased amounts of cleaner groundwater recharged to the aquifer via Antioch Cave provide a benefit to both the users of the aquifer and the endangered species at Barton Springs. To monitor the effects of recharge through Antioch Cave on the aquifer, a multiport well was installed at the site. This well was completed with seven monitoring zones in the Edwards units.