| Abstract: | It is difficult to quantify the range in source strength reduction (MdR) that may be attainable from in situ remediation of a dense nonaqueous‐phase liquid (DNAPL) site given that available studies typically report only the median MdR without providing insights into site complexity, which is often a governing factor. An empirical study of the performance of in situ remediation at a wide range of DNAPL‐contaminated sites determined MdRs for in situ bioremediation (EISB), in situ chemical oxidation (ISCO), and thermal treatment remedies. Median MdR, geometric mean MdR, and lower/upper 95 percent confidence interval for the mean were: 49x, 105x, 20x/556x, respectively, for EISB; 9x, 21x, and 4x/110x for ISCO; and 19x, 31x, and 6x/150x for thermal treatment. Lower MdR values were determined for large, complex sites and for sites with DNAPL pool‐dominated source zones. A feasibility analysis of partial DNAPL depletion is described for a pool‐dominated source zone. Back‐diffusion from low‐hydraulic conductivity units within a pool‐dominated source zone is shown to potentially sustain a secondary source for more than 1,000 years, indicating that aggressive source treatment may not reduce the remediation timeframe. Estimated plume response demonstrates there may be no reduction in cost associated with aggressive treatment, and little difference in risk reduction associated with the various alternatives. Monitored natural attenuation (MNA) for the source zone is shown to be a reasonable alternative for the pool‐dominated source zone considered in this example. It is demonstrated that pool‐dominated source zones with a large range in initial DNAPL mass (250 to 1,500 kg) may correspond to a narrow range in source strength (20 to 30 kg/year). This demonstrates that measured source strength is nonunique with respect to DNAPL mass in the subsurface and, thus, source strength should not be used as the sole basis for predicting how much DNAPL mass remains or must be removed to achieve a target goal. If aggressive source zone treatment is to be implemented due to regulatory requirements, strategic pump‐and‐treat is shown to be most cost effective. These remedial decisions are shown to be insensitive to a range of possible DNAPL pool conditions. At sites with an existing pump‐and‐treat system, a significant increase in mass removal and source strength reduction may be achieved for a low incremental cost by strategic placement of extraction wells and pumping rate selection. © 2014 Wiley Periodicals, Inc. |
