Nitrogen in the Nooksack River Watershed: Comparing Models to Monitoring

The Nooksack River Watershed of northwest Washington and southwest British Columbia faces several issues linked to high nitrogen (N) in surface and groundwater. The Sumas-Blaine Aquifer, a locally important drinking water source, has nitrate levels that exceed the EPA recommended 10 mg nitrate-N/L in many wells. Many tributaries to the Nooksack River have 303(d) listings as 'impaired' for dissolved oxygen, which is tied to high nutrient loading. Based on results from the USGS Sparrow model, the Nooksack River has one of the highest rates of N loading per watershed area to the Salish Sea of all monitored rivers, contributing to eutrophication in Bellingham Bay. Limited nutrient monitoring has constrained knowledge of the severity of nitrogen impacts to surface waters, annual trends, sources of nitrogen, and critical loading times. Between 2018 and 2021, we used automated nitrate sensors (SUNA Seabird and OTT Hydromet ecoN models) that make continuous, real-time measurements in three waterways in the Nooksack Watershed. In addition, we used nitrate grab samples and continuous flow monitoring to estimate nutrient fluxes using the LOAD Estimator (LOADEST) model. We found that seasonal fluxes correlated closely with streamflow, with greatest nutrient loading during wet winter months. The real-time sensors provided high temporal resolution of fluxes (every 15 min.) and high accuracy of nitrate concentrations when compared with grab samples. However, they required regular maintenance to prevent breakdown. Compared to the real-time sensors, both LOADEST modeling and grab sample estimates of monthly fluxes were accurate (within 5%) in streams with relatively low variability in flow and concentrations. However, with higher variability, LOADEST required a large number of calibration measurements to adequately estimate actual nitrate fluxes. Continuous monitoring thus offers opportunities for more accurate estimates of nitrogen loading to the Salish Sea.