Temporal Distribution of Insecticide Residues in Four California Rivers

Several studies have indicated that certain pesticides are present in California rivers at levels which could be harmful to aquatic life. Generally, these studies have been conducted during certain seasons, but not year-round. Since yearround pesticide concentration data was limited for many California rivers having significant amounts of agricultural runoff, water was collected weekly from one sampling site along each of four rivers for a one-year period. This study was a cooperative effort between the Department of Pesticide Regulation (DPR) and the California Department of Fish and Game (CDFG). The portion conducted by DPR was an investigation of organophosphate, carbamate, and endosulfan residues in surface waters of four rivers: the Sacramento, Merced, Salinas and Russian Rivers. In addition to pesticide residue quantification by DPR, samples were tested for toxicity and ammonia concentration by CDFG on a bimonthly basis. Monitoring began on the Sacramento River in November 1993, followed by the Merced River in June of 1994 and by the Salinas and the Russian Rivers in August 1994. The primary sampling method was collecting composite samples during a 3-day period with an automatic sampler. Equal-width depth-integrated sampling and grab samples were also used. Pesticides were detected two or more times on each river. Diazinon was detected twice on the Sacramento River during periods of increased river flows. Dimethoate was detected twice, methidathion and diazinon were detected three times, and 3-hydroxy carbofuran were detected in samples collected from the Merced River. Each detection came during increased flows and with the exception of the dimethoate detections, the detections occurred during the rainy season. Chlorpyrifos was detected during the first major runoff event of the rainy season along the Salinas River. Samples were also collected monthly at the Salinas River Lagoon. Diazinon and dimethoate were detected in one sample collected in early summer. Samples collected from the Russian River yielded two detections: diazinon during the first runoff event in the fall and dimethoate during a winter rain event. CDFG found no apparent relationship between the presence of pesticide residues and mortality observed in the samples tested for toxicity.