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During the study of inputs of sediment-associated pesticides into the San Francisco Bay Estuary, suspended sediments were isolated from large volume water samples collected from the San Joaquin River at Vernalis (see map). Samples were quantitatively analyzed for 21 pesticides and pesticide degradation products. The samples analyzed were collected every few days over an 8-day period corresponding to the onset of winter rains. The onset of rains produced a peak in the hydrograph (blue line) and a preceding wave of entrained sediment material. The samples were analyzed to examine changes in the concentration and distribution of associated pesticides during passage of this wave of suspended sediments.
Observations on Temporal Variability
- The concentration and distribution of pesticides associated with suspended sediments varied widely during the peak of sediment discharge.
- The peak in suspended-sediment concentration preceded the peak in discharge for the San Joaquin River during 1992.
- Even though the mean concentration of pesticides was approximately halved between February 8 and February 13, the sediment load increased by more than a factor of seven, thereby increasing the overall pesticide load.