ELECTRICAL RESISTIVITY SURVEYS: INITIAL STEPS IN CHARACTERIZING THE SAN SOLOMON SPRINGS AQUIFER, FAR WEST TEXAS

Recent oil and gas discoveries in the southern Delaware Basin have raised awareness of the San Solomon Springs group, a series of six environmentally sensitive karst springs that discharge groundwater from Cretaceous limestones along the northeast flank of the Davis Mountains in Far West Texas. The springs and related groundwater provide water resources for recreation and much of the agricultural activity in the area. The springs also provide habitat for several federally listed endangered species. The San Solomon Springs are located at the far western edge of the Edwards Plateau, one of the largest karst regions in the United States and lie within the boundaries of several regional investigations of the greater Edwards-Trinity Aquifer system. However, a review of historical investigations of the region indicates that the San Solomon Springs are only indirectly associated with the Edwards-Trinity Aquifer. Springs in the Balmorhea area are in fact part of a different hydrologic regime more closely associated with carbonate aquifers in the Apache Mountains, and with recharge areas in the southern Salt Basin almost 100 km to the west.

Personnel with the National Cave and Karst Research Institute have conducted a series of electrical resistivity (ER) surveys over and beyond mapped portions of Phantom Lake Spring Cave, currently the deepest underwater cave in the United States, and one of the San Solomon Springs. Most of the cave is partially or completely flooded with brackish water and appears on ER profiles as a zone of low electrical resistivity. ER surveys show electrically conductive zones indicative of a flooded conduit more than 400 m beyond the farthest downgradient station in the mapped portion of the cave. A dye trace study conducted in 2013 demonstrates that water in Phantom Lake Spring Cave flows at a rate of ~1000 m/day through conduits formed in Cretaceous limestone, eventually discharging from San Solomon Spring at Balmorhea State Park, 6 km east of the cave entrance. Low resistivity anomalies identified on ER surveys conducted west of the park probably represent those flooded karstic conduits, supporting the hydrologic link between Phantom Lake Spring Cave and San Solomon Spring. Additional work will include a more comprehensive dye trace investigation, sampling for water chemistry analysis, as well as a series of water level measurements to better characterize conduit flow within the karstic aquifer that feeds the San Solomon Springs group.