Probabilistic Analysis of Pesticide Transport in Shallow Groundwater at Oatland Island, Georgia

The publicly available analytical contaminant transport analysis system (ACTS) was used to examine the fate and transport of organochlorine pesticides in shallow groundwater at the Oatland Island Education Center, Oatland Island, Georgia. Specific objectives of the analysis included: (1) estimating the probability of impacting coastal wetlands located 800 feet (244 meters) downgradient of the pesticide source area, and (2) developing reference tools (probability curves) for evaluating future groundwater monitoring results at key site monitoring wells. The groundwater contaminant plume consists of four different isomers of benzene hexachloride (BHC). Total BHC was selected as the reference contaminant for the contaminant, fate and transport model simulations. Probabilistic model results for each specific site location and time period were generated by conducting two-stage Monte Carlo simulations for eight model parameters using 10,000 realizations for each probability distribution. Model results are considered representative of all four BHC isomers detected at the site. According to simulation results at the wetlands boundary, the probability of total BHC concentration in groundwater exceeding the analytical detection limit of 0.04 micrograms per liter (μg/L) is less than 2%. The detection limit of 0.04 μg/L is a default compliance standard for BHCs in groundwater, in accordance with applicable Georgia Environmental Protection Division rules. Model simulation results were used to develop a series of probability curves for four different site locations — three downgradient monitoring wells and the downgradient wetlands boundary — along the centerline of the contaminant plume. These probability curves are useful in evaluating future groundwater monitoring results and guiding environmental and regulatory decision-making for the site.