Pesticides in the San Joaquin River, California: Inputs from Dormant Sprayed Orchards

Joseph L. Domagalski, Neil M. Dubrovsky, and Charles R. Kratzer

In:
Journal of Environmental Quality, v. 26, no. 2, p. 454-465.

Abstract:
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 distinct peaks of oganophosphate 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 patterns, and hydrology of the eastern and western San Joaquin Valley. The fine soil texture and small size of the western tributary basins contributed to rapid runoff. In western valley streams, diazinon concentrations peaked within hours of the rainfall's end and then deceased because of a combination of dilution with pesticide-free runoff from the nearby Coast Ranges and decreasing concentrations in the agricultural runoff. Peak concentrations for the Merced River. a large tributary of the eastern San Joaquin Valley, occurred at least a day later than those of the western tributary streams. That delay may be due to the presence of well-drained soils in the eastern San Joaquin Valley, the larger size of the Merced River drainage networks. A subsequent storm on 18 and 19 February resulted in much lower concentrations of most organophosphate pesticide suggesting that the first storm had mobilized most of the pesticides that were available for rainfall-induced transport.