The dynamics of sediment-associated pesticides in the Sacramento and San Joaquin Rivers and the San Francisco Bay Estuary are being studied by the U. S. Geological Survey (USGS). These two rivers feed the San Francisco Bay Estuary by draining California's Central Valley. The Central Valley is an area of intense agricultural activity in which more than 500 different pesticides are used (California Department of Pesticide Regulation, 1994). Thus, suspended sediments borne by these rivers are potentially an important mechanism for transport of pesticides into and through river and estuary systems.
Pesticides associated with suspended sediments may affect the ecosystem differently than their dissolved counterparts and need to be studied separately. Suspended sediments and associated organic material support filter-feeding pelagic and benthic communities, so pesticides associated with these sediments represent a pathway for entry into the food chain distinct from the more commonly studied dissolved pesticides. Other reasons suspended sediments may have different effects than their dissolved counterparts are that suspended sediments have a longer residence time in estuaries than water, and inputs of sediments occur in a pulse, typically only once a year (Meade, 1972). This pulsed introduction may increase exposure times to pesticides or provide a mechanism for exposure during biologically sensitive times.
Equilibrium partitioning models indicate that many pesticides will be concentrated on suspended sediments (DeToro and others, 1991). However, we may expect that associated pesticides will be observed at higher levels than equilibrium models suggest. For example, when a short transit time exists between agricultural fields, where many sediments originate, and estuaries, where they may accumulate, pesticides associated with suspended sediments may not have time to reach equilibrium prior to deposition (for example, Domagalski and Kuivila, 1993).
Complicating the study of sediment associated pesticides in the San Francisco Bay Estuary is that sediment flow through rivers and estuaries is highly seasonal and episodic in nature (Meade, 1972). The result is high temporal variability in sediment concentration and flux. Pesticides associated with these sediments consequently may exhibit large changes in concentration and distribution. Other complications include the seasonal variation in pesticide applications, the residence time of sediments in the drainage, and the effect of tidal and non-tidal circulation on sediment transport and erosion.
This work is funded by the USGS Toxic Substances Hydrology Program and USGS Ecosystems Initiative. Pesticide-use data were obtained from the California Department of Pesticide Regulation (Sacramento, Calif. 95814) computer database. The authors wish to thank D.S. Baston, B.J. Jennings, T.J. Wilson, and R.G. Linville of the USGS for invaluable assistance in the laboratory. J. DeRose, B.J. Jennings and D. Copeland of the USGS were indispensable in the field effort.
California Department of Pesticide Regulation, 1994, Pesticide use data: California Department of Pesticide Regulation.
DeToro, D.D., Zarba, C.S., Hansen, D.J., Berry, W.J., Swartz, R.C., Cowan, C.E., Pavlou, S.P., Allen, H.E., Nelson, T.A., and Paquin, P.R. , 1991, Technical basis for establishing sediment quality criteria for nonionic organic chemicals using equilibrium partitioning: Environmental Toxicology and Chemistry, v. 10, 1541-1583.
Domagalski, J.L., and Kuivila, K.M., 1993, Distributions of pesticides and organic contaminants between water and suspended sediment, San Francisco Bay, California: Estuaries, v. 16, no. 3A, p. 416-426.
Meade R. H., 1972, Transport and Deposition of Sediments in Estuaries: Geological Society of America, v. 133, no. 1, p. 91-120.
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by David H. Schoellhamer, U.S. Geological Survey, Sacramento, California, USA