ability. ERH is attractive for volatile and semi-volatile chemicals in heterogeneous settings because its ability to achieve targeted energy delivery is less sensitive to subsurface heterogeneities than steam injection, and the energy delivery and contaminant recovery systems are arguably less complex to design and operate. Conductive heating has likely increased in use because it is the only thermal technology that can achieve in situ temperatures significantly greater than the boiling point of water and that is sometimes a desired operating condition. The study did not provide remediation costs because the cost data reviewed varied greatly and were thought to be unreliable, especially given some of the suboptimal designs.

Most relevant to this report are the post-treatment performance data from in situ thermal treatment sites. Interestingly, post-treatment groundwater monitoring data that could be used to evaluate technology performance were available for only 14 of the 182 sites (8 percent) reviewed by Triplett Kingston et al. (2010a,b, 2012), reflecting the overall industry-wide lack of sufficient post-treatment monitoring at many remediation sites. Most of the sites for which adequate data were available correspond to hydrogeologic setting Scenario C, with little or no performance data available for the other settings. Table 4-3 presents the estimated order-of-magnitude reductions in concentration and mass discharge for the 14 sites that had sufficient data for the analysis. Note that mass reduction data are not provided in Table 4-3 because initial mass in place was rarely known with certainty. For six of the 14 sites (43 percent), at least a 100-fold reduction in mass discharge was observed. For five of the 14 sites, detailed analysis revealed that post-treatment groundwater concentrations ranged from about 10 to 10,000 μg/L and source zone mass discharges ranged from about 0.1 to 100 kg/y.

The following factors should be considered in interpreting the widely varying performance results shown in Table 4-3:

1. As noted by Johnson et al. (2009), the criteria or rationale used to set the duration of treatment operation was usually not documented, and “in most cases it appeared that the duration was determined prior to start-up or may have been linked to a time–temperature performance criterion (i.e., operate for two months once a target temperature is reached in situ). There was little indication that the duration of operation was selected based on mass removal-, groundwater quality-, or soil concentration-based criteria” or performance monitoring.

2. Triplett Kingston et al. (2010a,b, 2012) discovered that treatment system footprints (areas treated) were often smaller than the source zones that had been treated. The main reason for this was that the pre-treatment extent of the source zone was larger than what it was conceptualized to be at the time that the remediation system was



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