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ERI 227903 R03 Former Unocal Service Station 5098,Stockton,California September 30, 1999 RECOMMENDATIONS Based upon review of the data, ERI recommends the following • The County should concur with the Regional Board that hydrocarbon impacted soil has been adequately addressed at the site, and should not require any further characterization of soil at the former Unocal property • The County should find that the dissolved hydrocarbons detected in groundwater downgradlent of the former Unocal property are part of a commingled plume originating from both the Unocal and Montgomery Ward sites, and that the groundwater case should be managed as a single commingled plume • Joint quarterly groundwater monitoring and sampling between Unocal and Montgomery Ward should continue at the sites Groundwater samples should be analyzed for TPPHg using Environmental Protection Agency (EPA) Method 8015 (modified) and benzene, toluene, ethylbenzene, and total xylenes (BTEX) using EPA Method 8020 However, since dissolved oxygenated compounds have not been detected in groundwater, analysis for oxygenated compounds will not be performed Quarterly groundwater monitoring and sampling reports should be submitted to the County prior to the following quarters sampling event • An evaluation of groundwater conditions within the deep portion of the water-bearing zone should be conducted beneath the Montgomery Ward site The purpose of this investigation is to evaluate the extent to which petroleum hydrocarbons originating from the Montgomery Ward site are impacting groundwater in the deep water-bearing interval Unocal should replace groundwater monitoring well MW 11 with nested wells screened at discrete intervals within the water-bearing zone to further evaluate the vertical distribution of dissolved gasoline hydrocarbons in groundwater downgradlent of the site The nested wells should consist of a shallow well (MW11S) screened from approximately 30 to 50 feet bgs and a deep well (MW11D) screened from approximately 61 to 66 feet bgs The timing of the replacement of well MW 11 should be coordinated with the deeper investigation at the Montgomery Ward site With the installation of wells MW 10A, MW 14A, and proposed wells MW 11S and MW 11D, groundwater monitoring and sampling of wells MW 10, MW 11, and MW 14 should no longer be performed Wells MW10, MW11, and MW14 should be abandoned following submittal of a Work Plan to perform the well destruction and upon approval of the Work Plan by the County • Upon completion of the proposed soil and groundwater investigation beneath the Montgomery W ara site and in the vicinity of Untx.al well MW 1 i, a risk assessment should be performed to evaluate the impact of the commingled plume upon the Delta College irrigation well • In light of the recommendations provided above, ERI requests that a meeting with representatives of Unocal, Montgomery Ward, and the County should be conducted by November 19, 1999, in order to establish a timetable for implementation of a soil and groundwater investigation beneath the Montgomery Ward site and in the vicinity of Unocal well MW I 1 7