Review of Modeling Approaches for Pesticide Washoff from Impervious Surfaces

Abstract

Pesticide uses on impervious surfaces and subsequent offsite transport significantly contribute to pesticide detection and aquatic toxicity in urban watersheds. This review evaluates the various methods that currently exist to model pesticide washoff from impervious surfaces. Empirical equations successfully describe pesticide washoff by calibration to a single rainfall event, but lack consistent parameterization with varying set time and repeated rainfall. Partitioning coefficients determined from experimental data could significantly improve PRZM capability in predicting pesticide washoff from impervious surfaces. Highlighted in this review is a new semi-mechanistic approach which incorporates the time-dependence of washoff potential during the dry period after application and washoff dynamics during a runoff event. This review aims to provide information to guide model selection and model development for pesticide registration, regulation, and mitigation for urban pesticide uses. IntroductionPesticide transport in urban watersheds is a function of stormwater hydrology, various processes that control transport in watercourses, and the dynamics of pesticide release and washoff from treated surfaces. While stormwater modeling and pesticide transport in runoff have been extensively investigated, relatively few studies have evaluated pesticide washoff from urban landscapes, especially from impervious surfaces. Impervious surfaces are primary sources of overland flow generation in the urban environment. Impervious surfaces are often directly treated with pesticides in structural pest control applications, paved area applications, and incidental overspray or drift (1, 2). Previous studies suggest that impervious surfaces are the dominant contributors to pesticide movement off-site in urban areas (3-5). Compared to other surfaces such as turf and bare soils, limited knowledge is available on the dynamics of pesticide buildup and washoff on impervious surfaces. The California Department of Pesticide Regulation (CDPR) recently adopted new regulations to protect water quality in urban areas by restricting pyrethroid application amounts and certain contact areas (6). Thus, there is an emerging research need for improved washoff modeling capabilities to evaluate the effectiveness of the regulations and extrapolate the effect of mitigation practices to different conditions.The physical processes and modeling approaches of urban pollutant washoff and runoff have been reviewed in previous studies (7-13). Most of the reviews focus on pesticide transport in overland flow, concentrated flow and/or pipe flow over urban landscapes. This chapter reviews existing modeling approaches for simulating pesticide washoff from impervious surfaces, and introduces a semi-mechanistic model developed based on washoff experiments data. The models discussed here are classified as empirical or mechanistic (or semi-mechanistic) approaches. The empirical models are based on statistical analysis and data fitting and do not explicitly simulate mass t...