HR: 09:50h
AN: H31I-08 INVITED     [Abstracts]
TI: Synoptic Sampling of the Rio Grande: A River Basin-Scale View of Hydrologic Processes and Water Quality
AU: * Hogan, J F
EM: jhogan@hwr.arizona.edu
AF: SAHRA - Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85745
AU: Phillips, F M
EM: phillips@nmt.edu
AF: epartment of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801
AB: The Rio Grande, like many arid region rivers, 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. Since 2000 we have conducted biannual synoptic sampling of the Rio Grande from its headwaters in Colorado to ~150 km south of El Paso, Texas to evaluate how these processes result in the observed basin-scale water and solute balances. This period coincides with a severe regional drought, allowing us to further basin response to periods of increasing water stress. We employed multiple environmental tracers, including O/H isotopes, Cl-, and the Cl/Br ratio to help identify dominant hydrological processes and the causes of salinization. Our O and H isotopic 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. At the same time, the total dissolved solids content (TDS) increases from less than 50 mg/L in headwaters of Colorado to over 2000 mg/L south of El Paso, Texas. These salinity increases have previously been attributed to evapotranspirative concentration and to flushing by irrigation water of salts accumulated by pre-irrigation evapotranspiration. Water balance estimates and our O and H isotope results indicate that evapotranspiration alone is not sufficient to explain the salinization. Furthermore, increases in salinity were not a simple function of distance downriver, but rather occurred in a series of steps. Many of these increases are localized at the southern ends of the sedimentary basins comprising the Rio Grande Rift, suggesting that the salts are from discharge of deep, saline, ground water where it is forced to the surface by bedrock highs, rather than due to flushing by irrigation. Associated stepwise increases in the Cl/Br ratio, from about 50 in the headwaters to over 1,000 at the distal end of the river basin, are consistent with an influx of subsurface saline waters.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1806 Chemistry of fresh water
DE: 1871 Surface water quality
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005