Nitrate in Surface Water in the San Joaquin Valley Drainage Basin, California: A Retrospective Assessment

Charles R. Kratzer

In
Marston, R.A., and Hasfurther, V.R., eds., Effects of human-induced changes on hydrologic systems: American Water Resources Association Summer Symposium, Jackson Hole, Wyoming, June 26-29, 1994, Proceedings, p. 1164.

Abstract:
Nitrate in surface water was assessed for the San Joaquin Valley drainage basin as part of the U.S. Geological Survey National Water Quality Assessment Program. The assessment was made using data from National Water Information System and STORET data bases. A data base representative of ambient surface-water conditions was developed by excluding sites representing or directly influenced by small subsurface agricultural drains, wastewater treatment plant effluents, major water-supply systems, and reservoirs.

Nitrate concentrations were compared among sites representative of the west and east sides of the San Joaquin Valley, and the Sierra Nevada. Concentrations were highest in the west side of the valley [median values 1.5 to 19.2 mg/L (milligrams per liter) as nitrogen]. The east side of the valley had lower concentrations (0.10 to 0.84 mg/L), and the Sierra Nevada had the lowest concentrations (less than 0.10 to 0.10 mg/L). Nitrate concentrations in the lower San Joaquin River primarily were determined by a combination of relatively concentrated inputs from west side agricultural drainage, east side wastewater treatment plant effluents, and runoff from dairies; and relatively dilute inputs from major tributaries on the east side of the valley.

Flow-adjusted nitrate concentrations have increased steadily in the lower San Joaquin River since 1950. This trend can be attributed to many factors, including increased subsurface agricultural drainage, fertilizer application, wastewater treatment plant effluent, and runoff from dairies. On the basis of loading estimates for these sources, increased subsurface agricultural drainage probably is the most significant source of the increase in nitrate concentrations in the lower San Joaquin River.