POTENTIOMETRIC SURFACE INVESTIGATION OF THE KARSTIC MIDDLE TRINITY AQUIFER, BLANCO RIVER WATERSHED, HAYS COUNTY, TEXAS

Recent studies have increased our understanding of the hydrologic relationship between two large karst springs. Pleasant Valley Spring (PVS) discharges into the Blanco River providing perennial base flows that recharge the Edwards Aquifer. Jacob's Well Spring (JWS) discharges into Cypress Creek about 5 miles NE of PVS, but has become intermittent due to localized pumping. Until now, no detailed synoptic potentiometric investigation has been conducted that includes these two springs. We constructed a synoptic potentiometric map of the Middle Trinity Aquifer during drought (July 2013) conditions to assess the hydrologic relationships of the Blanco River, PVS, and JWS. The study area is about 70 sq mi consisting of Cretaceous-age limestones and dolomites. Fifty-nine wells and springs were measured using an eline and sonic meter.

Results indicate groundwater flow is NW to SE in the study area, approximately parallel to the direction of depositional dip and the Blanco River trend. Potentiometric gradients change from ~15 ft/mi in the recharge areas to ~60 ft/mi in the confined zone SE of the springs and faults. Potentiometric data indicate the upper Blanco River is a direct source area for PVS characterized by losing stretch of river and swallets. A small ridge in the potentiometric surface between the Blanco River and Cypress Creek watersheds suggests a localized hydrologic separation between the PVS and JWS; however, the upper losing stretch of the river could be a source to JWS. Springflow measurements were collected at both springs. Preceding the synoptic event, PVS flows increased in response to a Blanco River flow event (May 2013), while JWS had no response.

Under drought conditions the data suggest the perennial flowing portion of the Blanco River may not be a source of recharge to JWS in Cypress Creek. Management zones specific to the Cypress Creek watershed are needed to help maintain springflows.