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

This study was designed to investigate the occurrence, severity, sources an d causes of aquatic toxicity in a coastal river and estuary subject to non- point source pollutant inputs from adjacent agricultural and urban areas. T he Pajaro River estuarine system on the central coast of California, USA, r eceives 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 agricu ltural drainage ditches were selected to identify tributaries that might co ntribute toxic runoff to the estuary. These sites were each sampled 18 time s over an 18-month period, and water samples were tested for toxicity to th e mysid Neomysis mercedis, a resident crustacean, Results indicated toxicit y in 78% of agricultural ditch samples, 25% of tributary slough samples, an d 11% of river and estuary samples. Temporal patterns in the occurrence of toxicity indicated that agricultural ditches and upper river were more impo rtant 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 w hen the river flow rate was low, and persistent hydrophobic organochlorine pesticides were detected after high surface runoff. Three pesticides (toxap hene, 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 Toxicit y Identification Evaluations (TIEs) on six separate samples from the agricu ltural drainage ditches receiving rile drain discharges. The TIE results in dicated that multiple compounds were responsible for toxicity in all sample s evaluated, and that non-polar and perhaps polar organic compounds were pr esent in toxic concentrations. (C) 1999 Elsevier Science B.V. All rights re served.