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