Models for leaching of pesticides in soils and groundwater M.M. Hantush a, * , M.A. Marin ˜o b,c , M.R. Islam d a Subsurface Protection and Remediation Division, National Risk Management Research Laboratory, ORD, US EPA, 919 Kerr Research Dr, Ada, OK 74820, USA b Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA c Department of Civil & Environmental Engineering, University of California, Davis, CA 95616, USA d Mantech Environmental Technology, Inc., 919 Kerr Research Dr., Ada, OK 74820, USA Received 14 December 1998; received in revised form 16 July 1999; accepted 24 September 1999 Abstract Models are developed which describe leaching of pesticides in the root zone and the intermediate vadose zone, and flushing of residual solute mass in the aquifer. Pollutants’ loss pathways in the soil, such as volatilization, crop uptake, and biochemical decay, are emphasized, and the effect of local dispersion and nonequilibrium transport in mobile–immobile phases is analyzed. It is shown that the effect of diffusive transfer on the leached mass fractions is dependent on the volume fraction of the immobile phase, adsorption, the apparent mass-transfer rate coefficient, the first-order decay rate in the immobile zone. Error analysis indicates that complete-mixing models can be used to simulate leaching fractions of the pollutant when the Peclet number, P r , is greater than one. However, ignoring the effect of dispersion may underestimate leaching significantly when P r  1, such as for volatile compounds. Potential application of the models to the design of groundwater protection zones is investigated, analysis and application results demonstrate the dependence of the size of protective buffer on potential loss pathways in the soil environment, the size of the source area, and aquifer geometric, hydraulic and biochemical properties. Potential use of the models for the management of pesticides is also investigated with implication on hazardous waste land treatment.  2000 Elsevier Science B.V. All rights reserved. Keywords: Hydrology; Ground water; Solute transport; Models; Volatile organic compounds; Waste disposal; Aquifer; Pesticides; Soils; Evapotranspiration; Leachate; Unsaturated zone; Aquifer vulnerability; Water quality; Two-phase models 1. Introduction Pesticides used in crop production are a major source of nonpoint-source pollutants to ground water, and their discharge to the nation’s surface water may be a contributing factor toward the decline of the living resources and the deterioration of the ecosystems. A recent survey by the US EPA (USEPA, 1993) found pesticide detection in 16 606 wells in 45 states, with concentrations in 10 000 of these exceeding health advisory limits. Cost-effective assessment tools are needed to regulate the use of agricultural chemicals, identify areas which are potentially vulnerable to nonpoint-source pollution, and support ecosystem restoration goals by improving the nation’s water quality. Physically-based environ- mental simulation models can be cost-effective tools for resources managers as an alternative to costly and prolonged field monitoring strategies. Environmental fate and transport simulation models vary in their complexities and purposes, from simple lumped- parameters to distributed parameters. The latter are Journal of Hydrology 227 (2000) 66–83 HYDROL 3877 www.elsevier.com/locate/jhydrol 0022-1694/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0022-1694(99)00166-3 * Corresponding author.