Discrete and Continuous Nutrient Monitoring and Modeling of Long-Term Trends to Support Management Decisions in a Complex Estuarine Environment: Riverine Nutrient Inputs to the California Delta

The two major rivers of the Central Valley of California (Sacramento and San Joaquin) drain to ecologically sensitive environments: the California Delta and the San Francisco Bay. The Delta has several water quality impairments which include forms of nutrients (primarily nitrate and ammonium), various contaminants, and associated ecological effects such as impacts on fish populations. Nitrogen sources include agricultural and urban regions, and wastewater treatment facilities. In recent years, ammonium from a large wastewater facility has been implicated in decreased primary productivity and alterations of the Delta food web including a shift from diatoms to green algae, which is less desirable as a food resource for fish. In response, regulators have mandated ammonium removal and the treatment facility upgrades will eventually remove all of the ammonium and most of the nitrate from the effluent. This large change in nitrogen load to the Delta may also have potential effects on the ecosystem. In order to understand nutrient dynamics and how nitrogen stoichiometry may change, a multi-step approach has been in place to understand nutrient sources, concentrations, loads, and associated trends. Specific methods include high frequency monitoring of nitrogen species, especially nitrate and to a limited degree ammonium, discrete sampling of nutrients at key inputs to the Delta, load and trend analysis of nutrients, and spatially referenced modeling of nutrient sources throughout the upstream portion of the Delta watershed. We present results of riverine sources and concentration and load trends above the major wastewater treatment facility on the Sacramento River and the lower San Joaquin River in order to understand how concentrations of nitrate and ammonium and their ratios will change after the facility upgrade. Upstream of the wastewater facility, flow normalized nitrate concentrations and loads have shown only a relatively small negative change in 40 years of monitoring while ammonium had a four-fold decrease from 1980 to 1990 with no trend since. Ammonium has also similarly decreased in the lower San Joaquin River. By 2023, nitrate will be the dominant form of nitrogen entering the Delta once the treatment plant upgrade is completed.