Using Paired In Situ High Frequency Nitrate Measurements to Better Understand Controls on Nitrate Concentrations and Estimate Nitrification Rates in a Wastewater-Impacted River

We used paired continuous nitrate (NO3) measurements along a tidally affected river receiving wastewater discharge rich in ammonium (NH4+) to quantify rates of change in NO3 concentration (RAN()) and estimate nitrification rates. NO3 sensors were deployed 30 km apart in the Sacramento River, California (USA), with the upstream station located immediately above the regional wastewater treatment plant (WWTP). We used a travel time model to track water transit between the stations and estimated RAND every 15 min (October 2013 to September 2014). Changes in NO3 concentration were strongly related to water temperature. In the presence of wastewater, R-Delta NO3 was generally positive, ranging from about 7 mu M d(-1) in the summer to near zero in the winter. Numerous periods when the WWTP halted discharge allowed the R-Delta NO3 to be estimated under no-effluent conditions and revealed that in the absence of effluent, net gains in NO3 were substantially lower but still positive in the summer and negative (net sink) in the winter. Nitrification rates of effluent-derived NH4 (R-Nitrific_E) were estimated from the difference between RAND, measured in the presence versus absence of effluent and ranged from 1.5 to 3.4 mu M d(-1), which is within literature values but tenfold greater than recently reported for this region. R-Nitrific_E was generally lower in winter (similar to 2 mu M d(-1)) than summer (similar to 3 mu M d(-1)). This in situ, high frequency approach provides advantages over traditional discrete sampling, incubation, and tracer methods and allows measurements to be made over broad areas for extended periods of time. Incorporating this approach into environmental monitoring programs can facilitate our ability to protect and manage aquatic systems. Plain Language Summary The advent of in situ, high frequency nutrient sensors provides new opportunities to identify nutrient sources and quantify rates of transfer between different nutrient pools, improving our ability to assess how anthropogenic nutrient inputs affect aquatic ecosystems. We demonstrate how deployment of paired nitrate sensors along a tidally affected river which receives treated wastewater effluent rich in ammonium allowed us to quantify changes in nitrate concentration and, by identifying periods when effluent inputs were halted, estimate nitrification rates. Accurately quantifying nitrification rates enables us to approximate how much time it takes to deplete the ammonium pool and thus assess the fate and effects of effluent-derived nitrogen. In the Sacramento River, ammonium inputs from a wastewater treatment plant were rapidly nitrified and accounted for a large portion of the nitrate inventory entering the downstream estuary. Continuous measurements also revealed that a substantial benthic source of nitrate exists in the river reach below the treatment plant. This in situ approach avoids artifacts and logistical constraints associated with traditional approaches such as bottle incubations and tracer studies, providing environmentally relevant rates at high temporal and spatial resolution. Incorporating this approach into environmental monitoring programs will facilitate our ability to protect and manage aquatic systems.