Pesticide runoff from orchard floors in Davis, California, USA: A comparative analysis of diazinon and esfenvalerate

In the Central Valley of California off-site movement of pesticides in stormwater runoff, particularly by those belonging to the class of organophosphate (OP) pesticides, has significantly contributed to the contamination of the Sacramento and San Joaquin Rivers. There is an increase in the use of pesticides belonging to the pyrethroid class throughout the Central Valley area because these pesticides are hydrophobic and believed to reduce off-site transport. The objectives of this study were to quantify mass runoff of two commonly used dormant-spray pesticides, the OP pesticide diazinon and the pyrethroid pesticide esfenvalerate, from orchard micro-plots (4.5 m 2) and to compare the individual impact on water quality based on runoff patterns and runoff toxicity to three aquatic organisms. Two null hypotheses were tested: (1) no difference occurs between the mass transport of diazinon and esfenvalerate, and (2) pesticide type does not affect toxicity to three model aquatic organisms. A plot retention-tank technique was used in conjunction with artificial rain to establish runoff patterns and runoff concentrations of the two pesticides. Twelve 4.5 m 2 plots were constructed in an orchard in Davis, California, on bare soil. Two separate 2-event rain treatments were applied. Each event consisted of an approximate 2.5-h rain application at a rate of 4.3 cm h −1. The only difference between the two treatments was that treatment 2 allowed the pesticide to soak into the soil (i.e., no runoff occurred) prior to runoff while treatment 1 allowed runoff during both events. Mass transport of esfenvalerate in the runoff was less than the mass transport of diazinon under similar conditions. The runoff containing esfenvalerate was substantially less toxic to the waterflea ( Ceriodaphnia dubia), but slightly more toxic to the fathead minnow ( Pimephales promelas) and the Sacramento splittail ( Pogonichthys macrolepidotus). After soaking the pesticides into the soil, reductions occurred in the mass transport and toxicities of both pesticides. The results suggest that esfenvalerate may be a desirable alternative to diazinon in terms of mitigating aquatic toxicity. Additionally, soaking the pesticides into the soil after application may reduce the mass transport and toxicity occurring in runoff.