QUANTIFYING NUTRIENT SOURCES AND CYCLING ALONG THE UPPER RIO GRANDE

Water quality often limits the potential uses of scarce water resources in semiarid and arid regions. To best manage water quality one must understand the sources and sinks of both solutes and water to the river system. To address this issue we have performed winter and summer synoptic sampling of the Rio Grande from Del Norte, CO to Fort Quitman, TX since January of 2000. The initial focus was identifying salinity sources using solute ratios and solute isotopes. In the summer of 2001 the scope of work was expanded to investigate nutrient sources and sinks. To date the sampling has demonstrated that water quality, both salinity and nutrient concentrations, degrades with distance downstream. Dissolved Organic Carbon (DOC) concentrations gradually increase with distance downstream, but inorganic nitrogen concentrations are more variable due to localized agricultural and urban/wastewater point sources. Ammonium is quickly removed or converted to other N species whereas nitrate concentrations decrease more slowly following exchange between surface water, hyporheic zone, and riparian soils. In general, during the summer agricultural drains north of Elephant Butte Reservoir have lower concentrations of nitrate and phosphate than the Rio Grande and similar or slightly lower concentrations of ammonium and DOC than the river whereas south of Elephant Butte the drains have higher concentrations of nitrate. This suggests that agricultural diversions are not always a significant source of nutrients to the river and may be removing nutrients from some regions of the river.

Increased spatial and temporal sampling was conducted during the summer of 2004 between Albuquerque and Elephant Butte Reservoir to quantify the relationships between agricultural and urban land use and nutrient loading as well as nutrient sinks within the surface water, hyporheic and riparian systems. These sampling results will also be used to quantify transport distance and uptake length of nutrients within different river reaches. Results of this research will be incorporated into a systems-approach model of nutrient cycling which should be transferable to other arid rivers.