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• Mr. Steve Sasson Project 22794-100.001 <br /> August 10, 1998 <br /> Page 9 <br /> evaluation did not identify statistically significant trends for TPHD concentrations in <br /> groundwater samples collected from monitoring wells MW-4, MW-5, and MW-6 <br /> between 1992 and 1995. The absence of concentration trends suggests the plume is stable <br /> and the dissolved hydrocarbon mass is not increasing. The statistical evaluation sheets <br /> are presented in Appendix D. <br /> REMEDIAL ALTERNATIVES EVALUATION <br /> An evaluation of remedial alternatives was requested by the SJCPHS. This evaluation is <br /> presented in Appendix E Implementation of an active remediation approach does not <br /> appear to have benefits based on source removal activities conducted to date, the site <br /> setting, and Iuruted effectiveness of these approaches for remediation of TPHD impacts. <br /> CONCLUSIONS FOR SITE CLOSURE <br /> The SJCPHS low-risk soil case criteria have been achieved through site characterization <br /> and remediation. A case closure checklist form is presented in Appendix F. Applicable <br /> criteria and activities to meet those criteria include the following: <br /> * The leak has been stopped and ongoing sources have been removed or <br /> remediated. <br /> — The USTs were removed in July 1987, and approximately 1,200 cubic yards <br /> of impacted backfill and native soils were excavated. Excavation activities <br /> removed impacted soils to the practical vertical and lateral limits of <br /> excavation. The estimated mass of TPHD removed during overexcavation <br /> activities was approximately 1,100 pounds. <br /> — The estimated mass of TPHD remaining in soil is approximately <br /> 1,400 pounds This estimate is based on data collected during site <br /> assessments and does not include a potential mass reduction by <br /> biodegradation <br /> — The estimated mass of TPHD in groundwater ranges from 108.9 pounds <br /> using 1991 data to 188 pounds using 1995 data. The 1995 data include <br /> sitewide HydroPunch sampling results. It should be noted that the highest <br /> detected concentrations declined from 160,000 µg/L detected in 1991 to <br /> 1,200 gg/L detected in 1995. These estimates are based on data collected <br /> SAC\N\PJZ2794\2794 i 0001 Cs-98*zu==8 Fmcon <br /> Rev 0,8/10/98 <br />