Salinity Contributions from Geothermal Waters to the Rio Grande and Shallow Aquifer System in the Transboundary Mesilla (United States)/Conejos-Médanos (Mexico) Basin

Abstract

Freshwater scarcity has raised concerns about the long-term availability of the water supplies within the transboundary Mesilla (United States)/Conejos-Médanos (Mexico) Basin in Texas, New Mexico, and Chihuahua. Analysis of legacy temperature data and groundwater flux estimates indicates that the region’s known geothermal systems may contribute more than 45,000 tons of dissolved solids per year to the shallow aquifer system, with around 8500 tons of dissolved solids being delivered from localized groundwater upflow zones within those geothermal systems. If this salinity flux is steady and eventually flows into the Rio Grande, it could account for 22% of the typical average annual cumulative Rio Grande salinity that leaves the basin each year—this salinity proportion could be much greater in times of low streamflow. Regional water level mapping indicates upwelling brackish waters flow towards the Rio Grande and the southern part of the Mesilla portion of the basin with some water intercepted by wells in Las Cruces and northern Chihuahua. Upwelling waters ascend from depths greater than 1 km with focused flow along fault zones, uplifted bedrock, and/or fractured igneous intrusions. Overall, this work demonstrates the utility of using heat as a groundwater tracer to identify salinity sources and further informs stakeholders on the presence of several brackish upflow zones that could notably degrade the quality of international water supplies in this developed drought-stricken region.

Mesilla/Conejos-Médanos Basin: U.S.-Mexico Transboundary Water Resources

“…[m 2 , square meters; m/day, meters per day; m/m, meters per meter; m 3 /day, cubic meters per day; hm 3 Geothermal waters that are associated with the region's known geothermal systems upwell from depths of greater than 1 km to recharge shallower groundwater supplies. The brackish (1800 to more than 4800 milligrams per liter (mg/L) DS concentrations) waters are evidence of deep groundwater circulation and ascend in the northernmost portion of the Basin at Radium Springs, in the west near the East Potrillo Mountains, and along the East Bench along the eastern border zone of the Mesilla Valley [83,84]. Discharge estimates for the East Bench range from 0.01 to 19 hm 3 /year and between 0.5 and 1.6 hm 3 /year for the East Potrillo Mountains [83][84][85].…”

“…The brackish (1800 to more than 4800 milligrams per liter (mg/L) DS concentrations) waters are evidence of deep groundwater circulation and ascend in the northernmost portion of the Basin at Radium Springs, in the west near the East Potrillo Mountains, and along the East Bench along the eastern border zone of the Mesilla Valley [83,84]. Discharge estimates for the East Bench range from 0.01 to 19 hm 3 /year and between 0.5 and 1.6 hm 3 /year for the East Potrillo Mountains [83][84][85]. No estimates are available for the Radium Springs system [83].…”

“…Discharge estimates for the East Bench range from 0.01 to 19 hm 3 /year and between 0.5 and 1.6 hm 3 /year for the East Potrillo Mountains [83][84][85]. No estimates are available for the Radium Springs system [83]. These inflows represent localized upflow within the geothermal systems and tend to be focused along faults, uplifted bedrock horst blocks, and fractured igneous intrusions [83].…”

“…No estimates are available for the Radium Springs system [83]. These inflows represent localized upflow within the geothermal systems and tend to be focused along faults, uplifted bedrock horst blocks, and fractured igneous intrusions [83].…”

“…Groundwater temperatures generally increase with depth at a rate of about 35 • C per km in the Basin but can be affected by groundwater advection in places [98]. For example, temperature gradients are much higher within known geothermal areas, where waters ascend from depths of over 1 km (e.g., East Bench, East Potrillo Mountains, and Radium Springs) (Figure 1) [83,84]. Localized groundwater upflow zones within these geothermal areas can have temperatures exceeding 55 • C, and geothermal waters at Radium Springs are close to 100 • C [83,99].…”