Quantifying Nitrogen Sources and Cycling Along the Upper Rio Grande

Synoptic sampling of a 1200km reach of the Upper Rio Grande has been performed in January and August from 2000 to present. The objective of this sampling has been to develop seasonal relationships between discharge, land use, and major water quality parameters including salinity and nutrients. In general, water quality, both salinity and nutrient concentrations, degrades with distance downstream. Increased salinity is explained largely by gradual downstream increase due to evapoconcentration punctuated by localized inputs of saline groundwater. Both total dissolved nitrogen (TDN) and Dissolved Organic Carbon (DOC) concentrations gradually increase with distance downstream, however for TDN this trend is punctuated by large, localized inputs primarily from urban areas. Somewhat surprisingly, surface water draining from areas of intensive, irrigated agriculture during the growing season often had lower nutrient and DOC concentrations than the river. Increased spatial and temporal sampling of the 250km reach between Cochiti Dam and Elephant Butte Reservoir was conducted in June, July and August of 2004 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. Summer 2004 data indicate that wastewater treatment plants are the largest and most consistent sources of inorganic nitrogen to the river. In both June and July there was a net removal of nitrogen from the reach as discharge decreased 26%, concentrations decreased 39%, and TDN loads decreased 56%. Interestingly, the diversion of river water for irrigated agriculture reduced discharge 25%, TDN loads 60% and concentrations 47% along the same reach before draining back to the main stem of the river suggesting that agricultural diversions were serving as a sink for nitrogen. However in August, TDN loads were higher in returning drains suggesting that agricultural systems had switched to a net source of nitrogen to the river. Possible reasons for this increase include higher rainfall in august flushing accumulated solutes coupled with increased soil available nutrients associated with the end of the growing system. Both conservative hydrologic tracers and labeled nitrogen tracer work in 2005 will be designed to identify primary controls on nitrogen removal along this reach.