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