Pesticides Detected in the San Joaquin River Basin, California: Analysis of Existing Data and Design of Monitoring Network

Dubrovsky, N.M. and Domagalski, J.L.

In:
EOS Transactions of the American Geophysical Union, Spring Meeting Suplement, v. 74, no. 16, p. 134-135.

Abstract:
Pesticides are used in the San Joaquin Valley, California, throughout much of the calendar year. Fields and roadways are sprayed with herbicides from November to February, almonds and stone-fruit orchards are sprayed with organophosphate insecticides in December and January, alfalfa fields are sprayed with organophosphate or carbamate insecticides in March and April, and large quantities of various pesticides are used from May through September on a variety of crops, including cotton. The actual dates and amounts of pesticides applied are recorded by the State of California and maintained in a data base. Pesticides were detected in the San Joaquin River and its tributaries during each of these pesticide use periods, in 1991 and 1992. These detections were related primarily to spatial and temporal patterns of pesticide use in addition to rainfall/runoff and irrigation drainage.

As part of the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey, the occurrence of pesticide concentrations in surface-water systems will be correlated with agricultural practices or land use, pesticide-use patterns, and physiography. Surface-water flow is highly managed in the San Joaquin Valley, and success of the program will require detailed knowledge of the locations of field drains and the areas that they serve. Two contrasting sites were selected as indicators of pesticide input into the San Joaquin River. Orestimba Creek, an area of easily erodible soils, receives drainage from the western San Joaquin Valley. During the irrigation season, virtually all flow is from field drainage. The primary cropping patterns are row crops and orchards. An irrigation drain of the eastern San Joaquin Valley initially was selected as an indicator of pesticide inputs from orchards. Soils of the eastern San Joaquin Valley are coarse and field drainage is by infiltration. Although pesticides were detected, discharge from a sewage-treatment plant and urban runoff obscured correlation with pesticide-use practices. As a result, the Merced River is now the preferred indicator of pesticide inputs into the San Joaquin River from the eastern valley. The Merced River was selected because it has perennial flow and receives agricultural drainage.