Simple Modeling Tool for Reconstructing Source History Using High Resolution Contaminant Profiles From Low-k Zones

An innovative but simple analytical modeling tool for reconstructing contaminant concentration versus time trends (i.e., “source history”) for a site using high-resolution contaminant profiles from low permeability (low-k) zones was developed and tested. Migration of contaminants into low-k zones via diffusion (and possibly slow advection) produce concentration versus depth profiles that can be used to understand temporal concentration trends at the interface with overlying transmissive zones, including evidence of attenuation over time due to source decay. A simple transport-based spreadsheet tool for generating source history estimates fit to the profiles was developed and applied to published soil concentration versus depth data from five distinct areas of four different sites contaminated with chlorinated ethenes. Using the root mean square error as an optimization metric, strong fits between measured and model-predicted soil data were obtained in the majority of cases using site-specific values for input parameters. In general, significant improvements could not be obtained by varying these parameters. As a result, the source history estimates generated by the tool were similar to those that had already been generated using more intensive analytical or numerical inverse modeling approaches. This included confirmation of constant source histories at locations where dense nonaqueous-phase liquid was present (or suspected to be present), and declining source histories for locations where source isolation and/or attenuation had occurred. The advantage of the modeling tool described here is that it provides a simpler yet more dynamic method for understanding source behavior over time than existing approaches.