Pesticides Detected in the San Joaquin River Basin, California: Results of an Intensive Fixed-Station Sampling Design Developed for the National Water Quality Assessment Program

Joseph L. Domagalski

In:
EOS, Transactions of the American Geophysical Union Spring Meeting Supplement, v. 74, no. 16, p. 129.

Abstract:
The objective of the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey is to describe the status and trends of the Nation's water quality. To accomplish this objective, individual study units were divided into subareas with relatively distinct environmental characteristics and land use. Indicator sites on small streams within a subarea were selected to represent pesticide inputs for a particular land-use/physiographic zone. Sites on larger rivers were selected as integrators of the output from multiple subareas.

Questions were raised concerning the proper sampling frequency and protocols required to best describe the temporal distribution of pesticides. The San Joaquin River basin, California, was selected as a prototype study unit to help answer those questions. Samples were collected at three sites from April through August 1992 during the irrigation season. There was no rainfall during this period. An indicator site on Orestimba Creek, which drains a part of the western San Joaquin Valley, was sampled tri-weekly during irrigation season for 6 weeks followed by a 6-week period of weekly sampling and then another tri-weekly sampling period. An integrator site on the San Joaquin River was sampled weekly during the entire sampling period. Another indicator site, an irrigation drain of the eastern San Joaquin Valley, was sampled at the same frequency as the integrator site. Data for the tri-weekly sampling periods for Orestimba Creek showed considerable variability in the types of pesticides detected and their concentrations. Mathematically "compositing" data for individual pesticides sampled tri-weekly preserved trends in the data and indicated that auto-samplers might be appropriate if logistical difficulties can be resolved. Weekly sampling at the integrator site was sufficient to address questions of pesticide occurrence; however, rainfall would have produced short-term variation in concentrations. The implication for the San Joaquin NAWQA Program is that frequent sampling of indicator sites is necessary in this irrigation-dominated agricultural setting in order to determine the temporal distribution of pesticides transported to the San Joaquin River.