ARCHIVED REPORTS_2011 REVISED FEASABILITY STUDY

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implementation of a biobarrier would require a substantial amount of additional subsurface characterization along the proposed biobarrier alignment to determine optimal well spacing (horizontal and vertical). The assumed treatment area for this feasibility analysis is based on a limited number of soil borings and groundwater samples from the existing monitoring well network. 3.2 EFFECTIVENESS Biosparging with air or oxygen to stimulate aerobic biodegradation is effective for vinyl chloride although some studies have indicated that cDCE can be degraded aerobically under certain conditions (EPA, 1998; ITRC 1998; EPA, 2000).TCE has not been shown to be degraded aerobically (EPA, 1998; ITRC 1998; EPA, 2000). Aerobic treatment of vinyl chloride alone is not a reasonable objective because untreated TCE and cDCE in groundwater flowing through the biobarrier would later be readily transformed to vinyl chloride due to the highly reducing groundwater environment outside of the conceptualized biobarrier, as discussed in attachment B4. Therefore, there is no benefit to targeting vinyl chloride for treatment unless TCE and cDCE would also be treated by the same process. This is the primary reason for screening out biosparging with air or oxygen as a viable in situ remediation technology for groundwater treatment in the vicinity of the WWTP. As discussed in Section 2.2, aerobic cometabolism is the only option for TCE biodegradation, and this approach has substantial technical and regulatory challenges to overcome. Aerobic cometabolism, which has shown some effectiveness in laboratory and research applications, would involve injecting explosive gases (propane, methane) under high pressures due to the depth below water of the target treatment interval. Because there is little industry experience with aerobic cometabolism, its potential effectiveness as a viable remediation technology for treating groundwater in the vicinity of the WWTP is in question. Aside from the previously discussed treatment limitations, the heterogeneous, layered geology at the site would limit the vertical migration of injected air and lead to a highly complex distribution of air channels, which would also lead to uneven treatment. The presence of the Corcoran Clay at the base of the treatment zone would prevent effective sparging of the lower portion of the intermediate aquifer zone, because the sparge wells could not be operated below the base of COC-affected groundwater. Closely spaced sparge wells (both laterally and vertically)would be required to create a sufficiently dense network of oxygen injection points to target dissolved phase contaminants along the entire biobarrier alignment. 3.3 IMPLEMENTABILITY In Section 3.2, it is shown that biosparging would be ineffective for treatment of all COCs in groundwater within the targeted treatment area because TCE does not degrade under aerobic conditions. Therefore, biosparging should not be implemented on this basis alone. However, AMEC Geomatrix, Inc. \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.1\Attach B1.doc 131-6