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 ( RΔNO3) 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 RΔNO3 every 15 min (October 2013 to September 2014). Changes in NO3– concentration were strongly related to water temperature. In the presence of wastewater, RΔNO3 was generally positive, ranging from about 7 µM d−1 in the summer to near zero in the winter. Numerous periods when the WWTP halted discharge allowed the RΔNO3 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 ( RNitrific_E) were estimated from the difference between RΔNO3 measured in the presence versus absence of effluent and ranged from 1.5 to 3.4 µM d−1, which is within literature values but tenfold greater than recently reported for this region. RNitrific_E was generally lower in winter (∼2 µM d−1) than summer (∼3 µM 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.

No abstract