Time-Domain Electromagnetic Data Used in the Assessment of the U.S. Part of the Mesilla Basin/Conejos-Mdanos Aquifer System in Doa Ana County, New Mexico, and El Paso County, Texas

The transboundary Mesilla Basin/Conejos-Mdanos aquifer system was identified as one of the priority transboundary aquifer systems for additional study by the United States and Mexico U.S.-Mexico Transboundary Aquifer Assessment Act of 2006. One of the largest rechargeable groundwater systems by total available volume in the Rio Grande/Rio Bravo del Norte Basin region of the United States and Mexico, the Mesilla Basin/Conejos-Mdanos aquifer system supplies water for irrigation as well as for cities of El Paso, Texas; Las Cruces, New Mexico; and Ciudad Jurez, Chihuahua, Mexico. The U.S. Geological Survey in cooperation with the Bureau of Reclamation assessed the groundwater resources in the Mesilla Basin and surrounding areas in Doa Ana County, New Mexico, and El Paso County, Texas by using a combination of geophysical and geochemical methods. Previously published geophysical data were compiled and these data were augmented by collecting additional geophysical and geochemical data. In October 2012, 12 time-domain electromagnetic soundings were collected near the southeastern part of the study area by the U.S. Geological Survey to provide more information on the area south of where the previously published direct-current resistivity soundings had been collected. Results from these geophysical assessments were used with collected geochemical data facilitated the interpretation of the geochemical characteristics of the groundwater sources and geochemical groups within the aquifer, definition of the groundwater-flow system and the evolution of the groundwater from source to mixing, definition of discharge areas, and identification of some of the salinity issues relative to the Rio Grande.