NEW CONCEPTUAL MODELS OF HYDROGEOLOGIC PROCESSES IN THE RIO GRANDE BASIN ESTABLISHED BY ENVIRONMENTAL TRACERS (Invited Presentation)

Our binational study of the surface and groundwater resources of the Hueco Bolson/Rio Grande aquifer system establishes new models of fluid movement and groundwater salinization in the aquifer system. At the onset of our studies that began in 2002 and continue, we sampled approximately 170 water wells in urban and rural parts of the system and tested almost all for standard inorganic constituents, halides, and stable water isotopes. Many water wells were also tested for carbon-14, tritium, and sulfur isotopes and a few wells were tested for chlorine-36. New models of groundwater flow and salinity emerged from the studies that had not been described heretofore, but that are reconciled by examining the geological development of the basin. Our new hydrogeologic models include: 1) predevelopment recharge of most of the area underlying Juarez by pre-dam Rio Grande water; 2) predevelopment recharge by pre-dam Rio Grande water at slope-front areas at the border between the Hueco Bolson and the Rio Grande alluvium; and 3) extensive salinization of the Rio Grande alluvium and Rio Grande in the lower valley area due to artesian upwelling and reaction with shallow evaporite units formed in late stage paleo-phreatic playas. These flow and salinization processes that were unknown or misinterpreted previously arise due to the unique relationships between geologic deposits of the area, including: 1) older Fort Hancock deposits consisting of mud rich facies formed prior to entry of the ancestral Rio Grande into the basin; 2) deposition of sand and gravel rich deposits of the Camp Rice Formation formed by the ancestral Rio Grande; 3) deposition of late stage phreatic playa deposits in the lower valley area when the ancestral Rio Grande had been diverted away from the basin due to tectonic uplift; and 4) development and incision of the modern Rio Grande-alluvial aquifer system. Our work demonstrates the important feedback processes between interpretation of hydrogeological processes using environmental tracers, and reconciliation of hydrogeological models by comparisons to unique geological facies associated with complex recent basin sedimentation.