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January 20, 2022

California Water Science Center scientists Dina Saleh and Joseph Domagalski discuss water-quality data collected in the Sacramento–San Joaquin Delta.

In recent years, it has been suggested that forms of nutrients and their associated effects may cause changes in productivity that affect food webs in the Sacramento–San Joaquin Delta (Delta). Specifically, the potential effects on the ecosystem from the nutrient ammonium, found in wastewater discharge, is a major concern.

The Delta is part of the largest estuary on the west coast of North America. The Delta is an essential source of water for both urban and agricultural users. The majority of the Delta is agricultural land, but also home to many bird, mammal, and fish species.

The Delta receives most of its freshwater from the combined flows of the Sacramento and San Joaquin rivers. Water management for Delta flow and water quality requires that numerous decisions need to be made regarding reservoir releases, diversions, aquatic species management, and environmental flows to protect water quality.

The presence of ammonium in the Delta has prompted the California Central Valley Water Quality Control Board to issue new discharge requirements for one of the most significant dischargers of wastewater to the Delta: the Sacramento Regional County Sanitation District’s treatment facility.

To better understand the future effects of these requirements on the amount of nutrients entering the Delta, USGS scientists used two water quality models, coupled with long-term monitoring, to evaluate historical nutrient loads and trends in the Sacramento and San Joaquin rivers and to identify the main sources of these nutrients to the Delta. The water quality models used were 1] Weighted Regression on Time Discharge and Season (WRTDS) ; and 2] SPARROW (SPAtially Referenced Regressions on Watershed attributes).

Water-quality data were collected from the Sacramento River at Freeport and the San Joaquin River near Vernalis. These data were used to examine trends in concentrations and loads of nutrients that entered the Sacramento–San Joaquin River Delta (Delta) from upstream sources for the years 1970 to 2019. The sources of nutrients in these two watersheds were also assessed.

To learn more, read the full article in San Francisco Estuary and Watershed Science.