Pesticide Occurrence as a Function of Land Use, Application, and Hydrology, San Joaquin River, California

Neil M. Dubrovsky, Joseph L. Domagalski, Jo Ann M. Gronberg, Charles R. Kratzer, Kathryn M. Kuivila, and Sandra Y. Panshin

In:
Society of Environmental Toxicology and Chemistry, SETAC 17th Annual Meeting, Washington, D.C., November 17-21, 1996, Abstract Book, p. 69.

Abstract:
Data on the concentrations of 84 pesticides in water in the San Joaquin River system were collected from 1993 through 1994 to determine the relation between temporal and spatial variations in occurrence and causal factors as part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program. Land use in the basin is dominantly agricultural and includes large areas of orchards, alfalfa, corn, cotton, and vegetables. Water samples were collected at the mouth of the basin, and in three subbasins selected to span the range of physiographic conditions and cropping patterns. Method detection limits ranged from 0.001 to 0.050 ug/L. Herbicides were detected more frequently than insecticides. Seven herbicides (simazine, metolachlor, DCPA, EPTC, trifluralin, atrazine, and diuron), but only two insecticides (diazinon and chlorpyrifos) were detected in more than 50% of the samples. The data on spatial occurrence showed clear contrasts among the three subbasins. Data on temporal variation showed seasonal patterns of pesticide occurrence, and radical changes in concentration during single hydrologic events. Temporal and spatial variations in most of the frequently detected compounds can be explained by crop distribution and pesticide application. Pesticide occurrence in the subbasins is also influenced by differences among the major sources of their water, which include rainfall runoff, irrigation return flows, reservoir releases, and subsurface agricultural drainage. A detailed evaluation of the transport of diazinon showed that although less than 1% of the amount applied to crops in 1993 exited the basin in surface water, concentrations in a winter period of high-flow exceeded levels toxic to Ceriodaphnia dubia and could be traced into the Sacramento-San Joaquin Delta.