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Updated Corrective Action Plan—Tosco(76) Service Station No 11192, Stockton,California June 26 2002 34 Remedial Action Plan Based on the information provided, a review of the remedial progress at the nearby former Unocal site and the various remedial alternatives evaluated in this updated CAP,GR recommends the installation of an ozone microsparge system, presented as Option #5 in the Feasibility Study above Proposed microsparge point locations are shown on Figure 3 The sparge points will target the shallow groundwater zone, and be placed at depths of approximately 30 feet bgs, as described below Once the remedial system is installed and operating for 'period of six months, a system status report will be issued, which will include progress monitoring r6sults Further status reports will be prepared and submitted on a semi-annual basis 4.0 Work Plan for Additional Delineation and Interim Remediation To perform additional vertical and lateral delineation of the MtBE plume, GR proposes to install one onsite shallow groundwater monitoring well, two onsite and two offsite deep groundwater monitoring wells The onsite shallow well will be installed to provide onsite downgradient delineation of the plume The four deep wells will provide additional data for lateral delineation of the MtBE plume in the deeper aquifer GR also proposes to install ten microsparge points to initiate remediation of the predominantly delineated shallow hydrocarbon plume The proposed sparge points will only target the shallow impacted groundwater zone The proposed locations of the wells and sparge points are presented on Figure 3 6{�esx�a Based on thobse ved degradation of MtBE to TBA, possibly related to the DPE pilot test,Tosco and GR believe it is-imperative that a larger OMS be installed to address the known shallow impact onsite If the MtBE has degraded to TBA as rapidly as observed through the introduction of oxygen to the subsurface, then additional oxygen is needed to allow the degradation sequence to continue to completion, thereby mitigating the known impact to shallow groundwater The proposed OMS will immediately address the impact to shallow groundwater while additional investigation and delineation of the impact to the deeper groundwater continues The PHS-EHD has expressed concern about costs associated with an OMS system at this site and requested a smaller or reduced number of sparge points be installed to conduct a pilot test of the technology However,the primary cost for OMS is the operating equipment,and installation costs for additional sparge points are incrementally low Based on previous experience, site lithology and subsurface conditions, Tosco and GR believe it would be prudent to install 10 microsparge points to initiate the interim remediation,rather than 3 or 4 points,which would limit effective remedial effort to a smallerro ion fo h plum"e di 14024508 13 a