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 continu-ity 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.The purpose of this paper is to describe the influence of faults and related structures called relay ramps on Smith et al., 2015). This paper will explore the mechanism for lateral continuity of flow in a karst setting with complex structures. Implications of this work address the lateral continuity of units and therefore intra-aquifer flow. This has great importance for conceptual models and ultimately, resource management. 14TH SINKHOLE CONFERENCENCKRI SYMPOSIUM 5 itorial review. The authors appreciate the comments and edits of Dr. Daniel Doctor and one anonymous reviewer.

The Blanco River is a very important resource for water supplies in the Hill Country of central Texas. Some communities and properties along the river use the surface water directly. But, the Blanco River is more significant in the role it plays in providing recharge to the karstic Trinity and Edwards Aquifers. Recent studies have given a better indication of the complexity of the interactions between surface water and groundwater in the area. Besides being a water supply to a population of several hundred thousand people, water originally flowing in the Blanco River provides flow to springs that host a number of endangered species. The Blanco River is characterized by alternating gaining and losing stretches due to the presence of springs that discharge water into the river and swallets that drain water from the river. Trinity units outcrop in the western part of the study area, and Edwards units outcrop in the eastern part of the study area. Normal faulting along the Balcones Fault Zone has juxtaposed the older, stratigraphically underlying Trinity units against the Edwards units to the east. The region consists primarily of Lower Cretaceous limestone, dolomite, and marl. One of the more significant springs along the Blanco River is Pleasant Valley Spring. During below-average flow conditions, Pleasant Valley Spring becomes the headwaters of the Blanco River even though the headwaters, under wet conditions, are about 50 km upstream. These studies, summarized in this paper, provide a greater understanding of the surface water and groundwater resources in the area which will help guide policies for groundwater management and preservation of springflows and groundwater supplies.

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