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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 over several years at various stations in Suisun Bay and also covering the principal inputs and outlet (see map). The samples were analyzed for 21 pesticides and pesticide degradation products to provide information about the source and fate of pesticides associated with suspended sediments in the estuary. Where multiple samples were collected and analyzed, the data were averaged to provide a more general picture of pesticide transport.
Although averaging provides general insight into the flow of sediment-associated pesticides into and through the estuary, averaging the data presented in this study does not provide specific information about either the annually weighted level of inputs or the source. There is an intrinsic danger in making interpretations from sample averages which are not discharge-weighted and taken over the annual cycle; the sample averages in this study are not. However, because few data of this type exist, averaging provided the best picture available at this time. Further studies that better quantify inputs are in progress.
Samples were collected at several sites. Sacramento River samples included in the average were collected at Sacramento (see map) on March 10, 1992, December 7 and 12, 1992,and February 9, 1994. San Joaquin River samples were collected as stated on the daily figures and on December 8 and 11, 1992. Suisun Bay samples were collected March 19, 30, and 31, 1994 and January 19, 20, 24, and 25, 1995. (More than one sample was collected in a day) The San Pablo Bay sample was collected February 25, 1995.
Observations on Spatial Variability
- Some of the observed pesticides associated with suspended sediments were expected based on their patterns of use (see table) and their physical-chemical properties (see table; for example, oxyfluorfen). However, some pesticides were observed (for example, thiobencarb) even though their physical-chemical properties or patterns of use suggested little if any should be present.
- For the samples and compounds analyzed, the San Joaquin River suspended sediments apparently carried the highest concentration of associated pesticides.
- For the suite of samples analyzed, the Suisun Bay suspended sediments most resembled the Sacramento River suspended sediments in terms of concentration and distribution of associated pesticides.