CHARACTERIZATION OF HYDROCHEMISTRY OF THE EDWARDS-TRINITY (PLATEAU) AQUIFER IN SUTTON COUNTY, TEXAS

The primary source of water for residents of Sutton County, Texas is the Edwards-Trinity (Plateau) Aquifer. The hydrogeologic setting of the aquifer in Sutton County is complex and includes (i) a major topographic divide separating the Llano River basin in the east from the Devils River basin in the west, (ii) a north to south subsurface transition from Cretaceous clastic rocks to the Glen Rose Limestone, and (iii) significant structural and elevation variation of the Edwards Limestone from northeast to southwest. Previous conceptual models of recharge in the area suggest that there is significant influence from the less permeable Buda Limestone that caps the topographic divide, and a recent groundwater flow model developed for the Devils River Basin is based partly on the conceptual model that high-permeability flow zones have developed preferentially along existing stream channels.

Using geostatistical and geochemical models, we analyzed recent geochemical data collected by the Sutton County Underground Water Conservation District and historical data from the Texas Water Development Board’s groundwater database in an effort to characterize groundwater quality within Sutton County and to test conceptual models of recharge and flow in the region. Results of the analyses show distinct variations in geochemistry that are likely related to mechanisms of recharge and potential interactions between the Trinity and Edwards strata. Some of the geochemical trends are counterintuitive. For example, calcium and bicarbonate concentrations are highest in proposed recharge areas and are closely aligned with the North Llano and Dry Devils River channels. The data and analyses also suggest that the Glen Rose Limestone may act as a barrier to interformational flow from deeper groundwater.