Patterns of aquatic toxicity in an agriculturally dominated coastal watershed in California

This study was designed to investigate the occurrence, severity, sources and causes of aquatic toxicity in a coastal river and estuary subject to non-point source pollutant inputs from adjacent agricultural and urban areas. The Pajaro River estuarine system on the central coast of California, USA, receives subsurface tile drain runoff from irrigated cropland, and seasonal surface runoff from agricultural, urban, industrial, and residential areas. Seven sites in the estuary, upstream river, tributary sloughs, and agricultural drainage ditches were selected to identify tributaries that might contribute toxic runoff to the estuary. These sites were each sampled 18 times over an 18-month period, and water samples were tested for toxicity to the mysid Neomysis mercedis, a resident crustacean. Results indicated toxicity in 78% of agricultural ditch samples, 25% of tributary slough samples, and 11% of river and estuary samples. Temporal patterns in the occurrence of toxicity indicated that agricultural ditches and upper river were more important than the freshwater sloughs as sources of toxic runoff to the estuary. Chemical analyses were conducted on samples collected at each site on two occasions. Organophosphate pesticides were detected in samples collected when the river flow rate was low, and persistent hydrophobic organochlorine pesticides were detected after high surface runoff. Three pesticides (toxaphene, DDT, and diazinon) were found at concentrations higher than published toxicity thresholds for resident aquatic species. Toxicity in the estuary was significantly correlated with increased river flow. Chemical causes of toxicity were investigated in two preliminary and four full Phase I Toxicity Identification Evaluations (TIEs) on six separate samples from the agricultural drainage ditches receiving tile drain discharges. The TIE results indicated that multiple compounds were responsible for toxicity in all samples evaluated, and that non-polar and perhaps polar organic compounds were present in toxic concentrations.