The transboundary Mesilla Basin/Conejos-Médanos 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 (United States-Mexico Transboundary Aquifer Act, Public Law 109-448). As 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-Médanos aquifer system supplies water for irrigation as well as municipal use in El Paso, Texas; Las Cruces, New Mexico; and Ciudad Juárez, Chihuahua, Mexico (Alley, 2013). The U.S. Geological Survey in cooperation with the Bureau of Reclamation assessed the groundwater resources in the Mesilla Basin and surrounding areas in Doña 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 to facilitate the interpretation of the geochemical characteristics of the groundwater sources and the identification of geochemical groups within the aquifer, physical and geochemical description of the groundwater-flow system and the evolution of the groundwater from source to mixing, location of discharge areas, and definition of some of the salinity issues relative to the Rio Grande.