Pesticide Concentrations in Surface Water as a Function of Agricultural Land Use in Five Small Watersheds, Western San Joaquin Valley, California

Sandra Y. Panshin, Joseph L. Domagalski and Neil. M. Dubrovsky

In:
EOS, Transactions of the American Geophysical Union, v. 75, no. 44, Supplement, p. 246.

Abstract:
The occurrence of a wide variety of dissolved pesticides in surface water from five small watersheds in the western San Joaquin Valley was studied in 1992 and 1993. Agriculture is the predominant land use in the valley portion of each watershed; however, significant differences among the types of crops, types of pesticides, and timing of pesticide application in the different watersheds cause spatial and temporal contrasts in pesticide concentrations in surface water. Runoff of winter precipitation from watersheds with the largest proportion of orchard acreage had the highest concentrations of diazinon, a pesticide applied heavily to dormant orchards during December and January. Two pesticides applied to alfalfa in the spring, carbofuran and chlorpyrifos, showed strong seasonality; none were detected in surface water in the months prior to application. Molinate, a pesticide used exclusively on rice, had the highest concentrations (up to 4 ug/L) in irrigation runoff from the watershed with the most rice acreage and exhibits strong seasonal dependence. Drainage from cotton fields in the southern part of the study area had high concentrations of cyanazine during winter and summer. A rigorous evaluation of the spatial and temporal distribution will be needed to link pesticide occurrence to pesticide application and enable agricultural managers to design mitigation measures.