The Source History Tool attempts to assist site managers and site consultants better understand and reconstruct long-term source histories. A long-term source history, from the beginning of releases at a site to present time, would help confirm a site conceptual model that shows attenuation is a significant process for both the source and the plume, and generate data that are well-suited for use in predicting future concentration and attenuation trends. Reducing the uncertainty associated with assessing long-term concentration trends before a remedy is selected and implemented, could assist site stakeholders select more appropriate remedies and improve effective risk communication with regulators and the public.
The tool was developed in Microsoft Excel to ensure its applicability to a wide audience. It has a simple interface, and the primary input data for the Tool are soil concentration vs. depth data from a low-permeability zone. Diffusion of contaminant mass into these zones is a function of the concentration gradient in an adjacent transmissive zone, which can change over time if the source attenuates. Diffusion is a predictable process, and the Tool uses a 1-D model based on Fick’s Law that includes vertical advection and degradation to reconstruct a concentration vs. time history from soil data collected at a single timepoint. In addition, the Toolkit provides a Monte Carlo-type approach to analyze uncertainty in the input parameters such as the porosity, apparent tortuosity factor exponent, fraction organic carbon/distribution coefficient, and constituent half-life. With this tool, groundwater practitioners can evaluate the accuracy of the hydrologic measurements that are being used for the reconstruction of source histories.