Pesticide transport modeling to evaluate diazinon runoff with infiltration enhancement and soil water management

Pesticide runoff from dormant-sprayed orchards is a major water quality problem in California's Central Valley. During the past several years, diazinon levels in the Sacramento and San Joaquin Rivers have exceeded water quality criteria for aquatic organisms. Orchard water management, via post-application irrigation, and infiltration enhancement, through the use of a vegetative ground cover, are management practices that are believed to reduce pesticide loading to surface waters. This study evaluated data collected in field experiments conducted in Davis, CA to measure the effectiveness of these management practices in reducing the toxicity of stormwater runoff.; Statistical analyses of diazinon runoff from experimental plots considered the effectiveness of ground treatment and soil water management practices in reducing pesticide runoff as compared to conventionally managed plots. Treatments using a vegetative ground cover significantly reduced peak concentrations and cumulative pesticide mass in runoff for first flush experiments compared with bare soil treatments. Post-application irrigation was found to be an effective means of reducing peak concentrations and cumulative mass in runoff from bare soil treatments, but showed no significant effect on vegetated treatments.; Numerical and analytical transport models were developed and tested for pesticides spray-applied to fields with a vegetative cover. This study improved upon previous models by adding chemical reactions that represent more completely the pesticide exchange processes between vegetation, soil, and overland flow and including reactions that are consistent with the short time spans encountered during experimental rainfall/runoff events.; The transport model was used to evaluate several key questions growers need to consider prior to implementing management practices intended to mitigate diazinon runoff. Model runs showed that increasing the organic content of surface soil by using vegetative ground covers had very little effect on diazinon runoff. Also, infiltration benefits associated with vegetative covers were offset when pesticides were applied to mature ground cover, because crop foliage released diazinon more quickly than soils. Model results of post-application irrigation scenarios found that the amount of irrigation required to meet assumed compliance levels is much greater for vegetated plots than bare soil plots, because of high rates of foliar washoff.