OCCURRENCE AND ACCUMULATION OF PESTICIDES AND ORGANIC CONTAMINANTS INRIVER SEDIMENT, WATER AND CLAM TISSUES FROM THE SAN-JOAQUIN RIVER ANDTRIBUTARIES, CALIFORNIA

A study was conducted in 1992 to assess the effects of anthropogenic a ctivities and land use on the water quality of the San Joaquin River a nd its major tributaries. This study focused on pesticides and organic contaminants, looking at distributions of contaminants in water, bed and suspended sediment, and the bivalve Corbicula fluminea. Results in dicated that this river system is affected by agricultural practices a nd urban runoff. Sediments from Dry Creek contained elevated concentra tions of polycyclic aromatic hydrocarbons (PAHs), possibly derived fro m urban runoff from the city of Modesto; suspended sediments contained elevated amounts of chlordane. Trace levels of triazine herbicides at razine and simazine were present in water at most sites. Sediments, wa ter, and bivalves from Orestimba Creek, a westside tributary draining agricultural areas, contained the greatest levels of DDT (1,1,1-trichl oro-2,2-bis[p-chlorophenyl]ethane) and its degradates DDD (1,1-dichlor o-2,2-bis[p-chlorophenyl]ethylene) and DDE(1, 1-dichloro-2,2-bis[p-chl orophenyl]ethyle). Sediment adsorption coefficients (K-alpha, and bioc oncentration factors (BCF) in Corbicula of DDT, DDD, and DDE at Oresti mba Creek were greater than predicted values. Streams of the western S an Joaquin Valley can potentially transport significant amounts of chl orinated pesticides to the San Joaquin River, the delta, and San Franc isco Bay. Organochlorine compounds accumulate in bivalves and sediment and may pose a problem to other biotic species in this watershed.