Application: Pesticide Transport in Shallow Groundwater and Environmental Risk Assessment

Abstract

An analytical model dynamically linked with Monte Carlo simulation software such as ACTS can provide a relatively rapid, cost-effective way to conduct probabilistic analysis of contaminant fate and transport in simplified groundwater systems. This approach was used to evaluate migration of an existing organochlorine pesticide plume in a shallow, unconfined aquifer underlying a barrier island in coastal Georgia, USA (Anderson et al. 2007) Probabilistic analysis provided an estimate of the likelihood that the pesticide plume would reach coastal wetlands 244 m downgradient of the source area. The contaminant plume consists of four isomers of benzene hexachloride (BHC), also known as hexachlorocyclohexane (HCH). To analyze this problem, the Analytical Contaminant Transport Analysis System (ACTS) was used to simulate two-dimensional, saturated zone contaminant fate and transport. Deterministic simulations using calibrated, single-value input parameters indicate that the contaminant plume in the barrier island shallow aquifer will not reach the wetlands. Probabilistic analyses consisting of two-stage Monte Carlo simulations using 10,000 realizations and varying eight input parameters indicate that the probability of exceeding the detection limit (0.044 μg/L) of BHC in groundwater at the wetlands boundary increases from 1% to a maximum of 13% during the period from 2005 to 2065. This represents an 87% or greater confidence level that the pesticide plume will not reach downgradient wetlands. Based on this outcome environmental decisions for the contamination at the site can be made more reliably by managers.