HYDROLOGIC PROCESSES IN THE RIO GRANDE BASIN ABOVE EL PASO, TEXAS: A BASIN SCALE VIEW WITH O AND H ISOTOPES

The Rio Grande, like many rivers in arid regions, exhibits reductions in streamflow and degrading water quality with distance downstream as a result of decreasing inflows, increasing evapotranspiration, and the addition of natural and anthropogenic solutes. We have employed a number of geochemical tracers to evaluate how these processes result in the observed basin-scale water and solute balances. For the past five years we have conducted biannual synoptic sampling of the Rio Grande from its headwaters in Colorado to ~150 km south of El Paso, Texas. This period coincides with an increasingly severe regional drought, allowing us to evaluate how water and solute balances change during periods of increasing water stress. During the same period of time a number of studies have focused on groundwaters of the associated alluvial aquifer system, allowing an evaluation of the role of groundwater and surface water exchanges.

This presentation will focus primarily on the O and H isotopes and their implications for the basin-scale water balance. Within the river system our results indicate that runoff from high-elevation areas in Colorado and northern New Mexico -primarily as snowmelt - is the source of river water. This water then exhibits progressive evaporation with distance downstream, with the greatest evaporation occurring at Elephant Butte Reservoir. Studies of several groundwater basins along the Rio Grande indicate that river water is an important source of recharge to the alluvial aquifer system. In addition we are also using multiple environmental tracers to “fingerprint” and quantify solute sources. Tracers currently being used include Cl/Br ratio, 36Cl/Cl, and 87Sr/86Sr, all of which identify saline groundwater as the significant salinity input to the Rio Grande.