Fluvial loadings of selected organonitrogen and organophosphorus pesticides to Chesapeake Bay

The annual loads of nine organonitrogen and organophosphorus pesticides were determined from March 1992 to February 1993 above the fall lines of the Susquehanna, Potomac, and James Rivers, the three largest tributaries of Chesapeake Bay. Sampling at the fall lines of the three tributaries was conducted during base- and storm-flow hydrologic conditions, and surface water concentrations of the organo-N/P pesticides were measured using large-volume liquid-solid extraction and gas chromatography/mass spectrometry. Annual loads of the organo-N/P pesticides were directly correlated with field application rates and stream discharge with the exception of alachlor, which had relatively low fall line loads in all three tributaries. The influxes of pesticides and other organic contaminants contributed by the tributaries are being compared with identified nonpoint sources in Chesapeake Bay.