Declining groundwater levels in the Hueco Bolson have raised concerns about the amount of available groundwater and the potential for water-quality changes. Fresh groundwater stored in the aquifer is bordered by regions of brackish-to-saline groundwater, and as the water levels in the Hueco Bolson have declined the local and regional hydraulic gradients have been altered such that the brackish-to-saline groundwater regions have encroached upon and degraded the quality of the freshwater zones (Hibbs 1999; Heywood and Yager, 2003). To help delineate the freshwater and saline groundwater transition zone, 20 time-domain electromagnetic soundings were collected to provide resistivity structure data in the area. This dataset consists of all collected raw data as well as processed results for each sounding.