Evaluating the natural attenuation of transient-source compounds in groundwater

A hybrid framework, combining deterministic fate and transport modeling with statistical methods, has been developed to examine the impact of the temporal variability of contaminant source on the evaluation of natural attenuation at a landfill site. A one-dimensional analytical solute transport model based on the superposition principle was used to describe the migration of contaminants downgradient of a time-varying source. The stochastic component of the measured contaminant source was determined after the removal of any systematic trends from the source. Statistical tests were employed to verify the assumptions made with regards to the nature of the trend and stochastic components. Monte Carlo techniques were used to generate synthetic time-varying contaminant sources conditioned on the properties of the stochastic component. Transport properties such as the advective groundwater velocity and dispersion coefficient were estimated by calibrating the transport model to the measured chloride data. The results of transport simulations indicated that the measured decrease of dichloroethane concentrations at downgradient locations cannot be explained by physical processes (e.g., dispersion) alone. Therefore, degradation was represented in the model as a first-order process. The degradation rate constants, estimated by a nonlinear least squares regression, were 2.5E-4 d(-1) for 1,1-DCA and 5.0E-4 d(-1) for 1,2-DCA. Numerical experiments indicated that the rate constant estimate is dependent on the nature of the source trend, location of downgradient observations, and the time of observations. It was also observed that the impact of the stochastic component of the source diminishes with travel distance of the contaminant from its source.