PESTICIDES IN THE SAN-JOAQUIN RIVER, CALIFORNIA - INPUTS FROM DORMANTSPRAYED ORCHARDS

Rainfall-induced runoff mobilized pesticides to the San Joaquin River and its tributaries during a 3.8-cm rainstorm beginning the evening of 7 February and lasting through the morning of 8 Feb. 1993. Two distin ct peaks of organophosphate pesticide concentrations were measured at the mouth of the San Joaquin River. These two peaks were attributed to contrasts between the soil texture, basin size, pesticide-use pattern s, and hydrology of the eastern and western San Joaquin Valley. The fi ne soil texture and small size of the western tributary basins contrib uted to rapid runoff. In western valley streams, diazinon concentratio ns peaked within hours of the rainfall's end and then decreased becaus e of a combination of dilution with pesticide-free runoff from the nea rby Coast Ranges and decreasing concentrations in the agricultural run off. Peak concentrations for the Merced River, a large tributary of th e eastern San Joaquin Valley, occurred at least a day later than those of the western tributary streams. That delay may be due to the presen ce of well-drained soils in the eastern San Joaquin Valley, the larger size of the Merced River drainage basin, and the management of surfac e-water drainage networks. A subsequent storm on 18 and 19 February re sulted in much lower concentrations of most organophosphate pesticides suggesting that the first storm had mobilized most of the pesticides that were available for rainfall-induced transport.