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. - v ATC • A S S O C I A T E S I N C Analytical results are summarized in Table 1 Laboratory reports and chain-of-custody documentation are presented in Attachment 2 CONCLUSIONSIRECOMMENDATIONS Although benzene was not detected above the laboratory detection limit of 10 ppb, a review of historical groundwater data suggests that benzene continues to exceed the 10 ppb California Primary Maximum Contaminant Level (MCL) for drinking water A review of historical groundwater data (Table 1) indicates petroleum hydrocarbons continue to exhibit concentrations exceeding applicable water quality objectives for groundwater at the subject site and threatens the nearby Supply Well Natural Attenuation of the residual petroleum hydrocarbons at the project site should occur over time, however, shallow groundwater at the site and the documented soil contamination at the soil/water interface appear to reverse and threaten mass reduction of dissolved petroleum hydrocarbons in the vicinity of MW4, prolonging natural attenuation Due to benzene levels currently in well MW4 and with the well's close proximity to the municipal supply well, it remains questionable whether passive bioremediation would meet regulatory criteria for the Sprotection of groundwater quality ATC recommends the Workplan dated October 30, 1997 and subsequent addendum dated December 15, 1997 (which was approved by PHSIEHD) be completed This work included three phases The first phase includes well installation, obtaining permits and a property access agreement, installing three vapor wells, two monitoring wells, and an air sparge well, and collecting soil and groundwater samples The second phase includes preparing a workplan containing a summary of well installation activities and proposed pilot testing, and, conducting soil and groundwater pilot testing Upon obtaining results of the pilot tests, ATC will prepare a Corrective Action Plan (CAP), the third phase of work Topics discussed during the September 3, 1998 project meeting, including the indefinite postponement of a proposed deep monitoring well, the re-evaluation the construction of the proposed wells, a longer pilot test than originally proposed, and a groundwater receptor survey should be addressed in a modified workplan and submitted for PHSIEHD approval Following the submittal and subsequent PHSIEHD approval of a modified workplan, ATC, on behalf of Ms Donna Sartim-Gardner, will apply for State funding pre-approval W 1625821quart1yl2gr-1999 doc 5