Tracing Sources of Nitrate and Organic Matter in the Willamette River Basin During Summer Baseflow Conditions using Isotopic Techniques

The effect of land use on water quality at the large river basin scale is poorly understood. In particular, quantifying the sources of water, nutrients, and organic matter to the river and linking these sources to specific land uses and point-sources of pollution is problematic in large basins. The Willamette River in Oregon is the 13th largest river by volume in the USA and drains a watershed of approximately 30,000 km2. The watershed contains more than 70 percent of Oregon's population and is under increasing pressure from continued urbanization. To better understand the sources of water, nutrients, and organic matter to the Willamette River and how these sources are linked to land uses, we conducted a synoptic sampling of the Willamette River and its major tributaries under summer baseflow conditions in late August 2006. We collected samples from the entire length of the Willamette River, from its confluence with the McKenzie, Middle Fork, and Coast Fork Rivers north to its confluence with the Columbia River in Portland, a distance of greater than 275 km. Grab-samples were collected mid-river from the Willamette River at sites selected to provide both uniform geographic coverage plus samples of different sources of nitrate, including feedlot operations, urban sewer and septic drainage, industrial effluent, and agriculture. In addition, near-shore grab-samples were collected from all major tributaries near their confluence with the Willamette River, from the mouths of minor tributaries to the Willamette River that had largely urban or agricultural land use, and from major sub-tributaries, reservoirs, or other sites of local hydrologic interest. All these samples will be analyzed for nitrate, ammonium, and DOC concentrations, conductivity, water-d18O and d2H, nitrate d15N and d18O, and POM d15N, d13C, and C:N. Nitrate concentrations in the Willamette River during the summer are typically much lower than in the winter, and generally show little variation downstream. The turbidity is also lower during the summer, and very low concentrations of POM were observed during our synoptic sampling. We will assess whether nitrate isotopes might be useful at identifying relatively subtle changes in nutrient sources to the river that are not apparent with nitrate concentrations alone. We will also determine whether POM isotopes can identify spatial changes in organic matter sources to the river. Finally, during the winter, high concentrations of nitrate in the Willamette River are correlated with storm events that flush nitrate from soils from different land uses and which presumably represent different sources of nitrate. We plan a second synoptic sampling run in Winter 2007 to explore these mechanisms and seasonal differences in nitrate concentrations.