Water supply and water quality problems facing the City of El Paso and Ciudad Juarez are complex and interrelated. The twin cities share the water resources of the Hueco Bolson, a Tertiary and Quaternary basin fill aquifer that spans the international border. The binational metroplex is located at the junction between the western edge of Texas and the northernmost part of Chihuahua, Mexico. Over-pumping of the Hueco Bolson aquifer has resulted in drawdown of the water table, encroachment of brackish groundwater, and the early retirement of wells. In response to these issues, Mexican and American universities formed a partnership to study the surface and ground-water resources of the El Paso/Juarez area. Governmental agencies are participating in the project by providing existing data, access to water wells, and other support services. The research team is applying a suite of isotopic tracers to provide an understanding of the spatial dynamics of the aquifers by tracing water from areas of recharge to regions of discharge. The team is also using a variety of geochemical and isotopic tracers to answer questions about increasing salinity in the developed parts of the aquifer. With an increased understanding of the flowpaths of the aquifer systems, the team is addressing stream-aquifer interactions between the groundwater systems and the Rio Grande. By combining an understanding of isotopic and geochemical changes in the river system with the information about the groundwater systems, the team is calculating fluxes of water and solutes from the groundwater system to the river system. Finally, this geochemical and isotopic information is being used by the municipal partners to constrain physical and management models of groundwater to utilize the fresh and saline water resources of the Hueco Bolson more effectively.