Potential impacts of density-driven pesticide transport upongroundwater contamination: modeling.

Pollution of groundwater resources by accidental pesticide spillage has long been an environmental concern. Spills involving concentrated mixtures of water-soluble pesticides such as acephate (acetylphosphoramidothioic acid O,S-dimethyl ester) may result in very high contaminant concentrations in local groundwater resources. Localized high concentrations in the aqueous phase may actually increase the fluid density above that of ambient groundwater. If local increases in fluid density are substantial, density-coupled fluid flow and chemical transport may result due to the local gravitational buoyancy force. The USEPA supported FEMWATER model was used to investigate impacts of the density-driven force upon water flow and acephate transport across a layered and slightly sloping landscape with a pond located down-slope from a spill site. A hypothetical water-saturated soil consisting of sand, clay, and silt/sand horizons with a surface area of 18 by 36 m was used in this study. A 1 h spillage of an acephate solution (concentration = 790 kg m(-3), density = 1350 kg m(-3)) to the top of the groundwater table was used as a contaminant point source. In general, significant impacts of the density-driven force upon the longitudinal and vertical transport distances of acephate became profound as time elapsed. Specifically, the longitudinal transport distance was shorter and the vertical transport distance was deeper when the density-driven force was employed. Although simulation results suggested that density-driven transport of acephate would be significant, an analysis of water heads and flow velocity vectors revealed that effects of density-driven water flow were minimal except initially in the immediate vicinity of the spill. Simulations further demonstrated that the presence of slowly permeable clay and silt/sand layers beneath the rapidly permeable sand layer delayed the vertical transport of acephate.