Verification of Site Cleanup Using a Dual-Equilibrium Desorption Model

The dual-equilibrium desorption (DED) model was introduced in 2002 illustrating the apparent biphasic nature of desorption of contaminants from soil. This model was applied to a gasoline fueling station investigation in northwestern California in which it was tested against empirical data for dissolved-phase gasoline constituents in groundwater in the attempt to predict remediation end points. In this work, we are further testing the applicability of DED to field-based investigations by comparing its model predictions with flow modeling and advective-dispersive contaminant transport empirical results. The model was tested in two parts. First, groundwater flow was modeled using the Laplace equation for groundwater flow in two dimensions. This was compared to observations and used to describe the distribution of hydraulic heads across the flow field. This distribution was then used as input for the contaminant transport model. The equations for advective-dispersive transport were modified by substituting the distribution coefficient with that predicted in the DED. Predicted concentrations were then compared to concentrations observed in a downgradient monitoring well. Results indicate that this procedure is useful in predicting the timing of spikes in contaminant transport to within six months to one year and that the magnitude of those peaks were under predicted by approximately 2 %. The implication is that contaminant spikes are not transported in coherent packets but in a more chaotic manner, possible due to fractionation of gasoline. While better precision may be desired, this is one of the few methods available for use with chemical mixtures, such as total petroleum hydrocarbons.