The mass of contaminants remaining in the source area(s) at contaminated sites is a critical piece of data in any characterization effort. No rational decisions can be made with respect to judging cleanup timeframes or evaluating the success of any aggressive source area remediation technology without this information. Standard characterization techniques involving borings and sediment samples can be extremely expensive, especially if the source area is deep, and they are fraught with uncertainty, particularly if significant portions of the source area mass reside in non-aqueous phase liquids (NAPLs). We propose that accurate estimates of source area contaminant mass can be made relatively inexpensively using hydraulic means with existing characterization infrastructure when combined with a reliable mathematical model for the source area contaminant release function.
Tuck and Ferguson, in a separate presentation, demonstrate that the stretched exponential equation provides a powerful tool for projecting contaminant release rates in heterogeneous porous media. This equation was used to generate source area release models for alachlor and two isomers of hexachlorocyclohexane, a-HCH and b-HCH, from a clay source area based on historical concentrations observed in wells penetrating the clay. Standard calculations of ground water flux combined with the source area release models yielded mass estimates in the source clay that compare very favorably with estimates based on sediment data taken from cores. A comparable approach is currently being used to estimate DNAPL mass remaining in the subsurface in the M-Area at the DOE Savannah River Site using data from the operating pump-and-treat system. Significant cost savings can be realized when sufficient historical data are available. Alternatively, hydraulic testing can be designed to acquire the requisite data. Data can also be collected during early stages of remediation when any type of recovery operation is involved. Cost comparison data for mass estimate techniques will be presented.