Pesticides Associated with Suspended Sediments in the San Francisco Bay During the First Flush, December 1995

The majority of suspended sediments are transported into estuaries by the “first flush” – runoff from the first major storm of the water year. Pesticides associated with these sediments may represent a significant fraction of pesticides transported to estuaries, where they have different environmental effects than dissolved pesticides. Water and suspended sediment samples were collected at the head of the San Francisco Bay during a peak in suspended sediment concentration, following the first major storm. These samples were analyzed for a variety of pesticides that span a wide range of hydrophobicity. For the 19 compounds analyzed, an average of 10 was found on the samples. Few pesticides were found dissolved in concurrent water samples and at concentrations much lower than would be expected from equilibrium partitioning between the aqueous and sedimentary phases. DDT and its metabolites were observed in all suspended sediment samples in concentrations ranging from 5.1 to 11.1 nanograms per gram dry sediment weight. Other compounds were sporadic in occurrence, but did occur at higher concentrations. For example, oxyflurofen and endosulfan each exceeded 20 nanograms per gram in a single sample. However, there were no significant trends in the total or individual pesticide concentrations, or in the number of compounds observed as the peak in suspended sediment concentration passed the sampling site. Also, there were no significant relations between sediment-associated pesticide concentration and parameters such as KOC of the pesticide, organic carbon content of the sediment, or amount of pesticide applied in the drainage basin, suggesting that observed sediment-associated pesticide concentrations may reflect disequibrium between sedimentary and aqueous phases resulting from long equilibration times at locations where pesticides were applied, and relatively short transit times for reequilibration to occur.