Although pesticides were detected at sites on the larger rivers, such as the San Joaquin River, those concentrations generally were lower than in the agricultural drains. Much of the water-quality data used for this retrospective report is for organochlorine, organophosphate, and carbamate pesticides. Considerably fewer data are available to assess the occurrence and significance of herbicides in surface water. In general, pesticides were detected more frequently in water samples collected during the irrigation season at sites on the west side of the valley. This greater frequency of occurrence can be attributed to the fine-grained soils of that region.
Pesticides enter the drains either dissolved or carried by soil particles.
The pesticide detections depend on application patterns and water management. In contrast, pesticides were detected less frequently at surface-water sites east of the San Joaquin River during the irrigation season. These fewer detections probably can be attributed to the coarser grained soils in that area, which lead to more rapid infiltration of irrigation water and less runoff. Pesticides can enter surface water on the east side of the valley during the winter, especially during storms. Pesticides are detected primarily in water samples collected at surface-water sites on the west side of the valley during the time of zero rainfall.