Promise and problems of real-time nitrate monitoring for watershed N budgets

Watersheds worldwide face issues linked to high nitrogen (N) in surface and groundwater, including drinking water contamination and eutrophication of near shore marine systems. The Nooksack Watershed in Whatcom County, WA has limited nutrient monitoring, which constrains our knowledge of trends in N impacts, sources and critical loading times. The high cost of available sensors has limited the number of sampling locations, and some sensors have had mixed, unverified results. A more cost-effective and extensive monitoring network will help inform regional activities addressing nutrient loading issues.

We tested a low cost, real-time OTT ecoN UV Nitrate Sensor to characterize nitrate levels in surface waters. The ecoN was installed next to an existing USGS gauge station and more expensive SUNA nitrate sensor, which, with related grab samples, we used to verify ecoN measurements. Following testing, we installed two additional ecoN sensors, one in Kamm Creek, a high nitrate tributary of the Nooksack River, and one in the Nooksack River itself, to characterize nitrate loading to Bellingham Bay, a critical area for hypoxia, harmful algal blooms, and tribal shellfish harvest.

Following initial setup, the SUNA and ecoN data tracked each other well, though with an offset of +0.2 mg-N/L of the ecoN relative to the SUNA. Grab samples confirmed the accuracy of both sensors within +/-10%. We found that the "optional" automated wiper was essential to prevent fouling of the ecoN. Subsequent deployment in the Nooksack River matched well with grab samples, even at low concentrations of nitrate with high loads of glacial silt. However, wiper difficulties and high nitrate concentrations in Kamm Creek have prevented accurate measurements there. Once resolved, we will compare stream nitrate-N loads based on sensor data with a typical load-calculating model using monthly grab samples to determine whether sensor data provide better information for managing stream water quality in this climate.