Abstract
Research suggests that both the amount and form of dissolved inorganic nitrogen (nitrate versus ammonium) in water affects the abundance, health and species of phytoplankton in the San Francisco Bay Delta (Delta), which subsequently impacts food stocks for pelagic organisms. The Sacramento River (Sac River) is the primary source of freshwater and a phytoplankton seed source for the Delta. Effluent releases from the Sacramento Regional Wastewater Treatment Plant currently contribute high concentrations of ammonium just downstream of Freeport Bridge, which have been purported to negatively impact phytoplankton. Forthcoming upgrades to Regional San’s treatment train will include nitrification and denitrification, which will reduce inputs of ammonium (by approximately 90%) and total inorganic nitrogen (by approximately 70%) to the Sac River. Understanding the rate at which ammonium is converted to nitrate (nitrification) may help us unravel controls on inorganic nitrogen concentrations and form which impact phytoplankton health, both under current and future conditions. Data for this study were collected from two high-frequency (15-minute), in-situ monitoring stations; one at Freeport Bridge (FPT) located 0.16 km upstream of Regional San’s effluent outflow, and a second at Walnut Grove (WGA) located 29 km downstream of Freeport. Both stations report river flow, river velocity and nitrate concentration along with standard water quality measurements. Effluent flow and effluent water quality data was provided by Regional San. These data allowed us to evaluate changes in nitrate concentration over time, estimate effluent derived ammonium concentrations, and determine net changes in nitrate (mg-N/L) as well as rates of change in nitrate (mg-N/L-d) as water travelled between FPT and WGA for the period of record to date (September 2013 to September 2014). Additionally, 25 wastewater discharge holds by Regional San occurred during the study, allowing for evaluation of changes in nitrate concentration in the absence of wastewater. Nitrate concentrations at FTP due to upstream flows were typically below 0.1 mg-N/L, except during storm events when they increased up to 1.1 mg-N/L. Comparison of FPT and WGA station data show that nitrate concentrations vary seasonally, but generally increase as water travels from FPT to WGA. Nitrate concentrations in Regional San’s treated wastewater effluent were below the measured detection limit of 0.1 mg-N/L, while ammonium concentrations were typically 25-35 mg-N/L. Due to continuous changes in both river and effluent flow, wastewater contributions to total river flows are extremely variable over short time periods, ranging from 1-4% in a single day; this means that concentrations of wastewater derived ammonium are also highly variable downstream of Regional San’s discharge point. This emphasizes the importance of knowing the travel time between FPT and WGA, so that water quality measured at WGA can be compared to the appropriate water quality that parcel of water had at FPT. During the period of study, travel time between FPT and WGA was as fast as 0.5 days during high flows and as long as 3 days during low flows. Comparison of nitrate concentrations between the monitoring stations at FPT and WGA, while taking travel time into account, demonstrated that there is typically an increase in water column nitrate as water travels this 18.4 mi stretch, particularly during the warmer summer months. Increases in nitrate were observed even in the absence of wastewater. This suggests that nitrate is entering the water column not only from nitrification of wastewater derived ammonium, but also from benthic sources. It seems likely that wastewater derived nitrogen inputs have accumulated in the benthos over time, and are being released back into the water column. Future changes to nitrogen loadings due to the new regulations may, over time, affect nitrogen storage in the system, leading to decreased nitrogen release from the benthos. We estimated water column nitrification rates by assuming the difference in nitrate measured in the presence of wastewater versus the absence of wastewater is due to nitrification of wastewater derived ammonium. Rates varied seasonally, and were estimated to be 0.026 ± 0.011 mg-N/L-d in the winter and 0.045 ± 0.012 mg-N/L-d in the summer. These values are within the range of previously published nitrification rates. Factors that clearly affect nitrification rates along this river reach include: temperature, abundance of nitrifying bacteria, and availability of ammonium. Residence time can also be important, particularly when populations of nitrifying bacteria increase over time. Extrapolating these nitrification rates, we estimated it can take from 26-92 days for nitrification to draw down ammonium added to the river from Regional San’s wastewater effluent to concentrations considered non-inhibitory to phytoplankton uptake of nitrate. Understanding the pathways and sources of nitrogen and their links to ecosystem health along the Sac River can aid in determining the impact of the Sac River on nitrogen cycling in the greater Delta. The question of whether the Delta will be impacted – either positively or negatively – by anticipated reductions of nitrogen inputs from Regional San’s treatment plant upgrades remains to be seen.