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Former ARCO Station #6100 -2 - 23 February 2017 25775 South Patterson Pass Road Tracy, San Joaquin County The submitted SCM includes proposals for the following: • Advance three (3) confirmation soil borings via hollow stem augers to 70 feet below ground surface (bgs), in areas of higher TPHg and benzene concentrations to determine the effectiveness of remedial actions. • Rebound vapor sampling to determine continued effectiveness of remedial system. • Continued groundwater monitoring if it is determined that the system should be shut down. I concur with the proposed remedial system vapor rebound testing. Please include the results of the rebound testing in the first quarter 2017 Groundwater Monitoring Report and Remediation Progress Report, due 30 April 2017. However, I do not concur with the proposed assessment activities at this time, and a Revised Work Plan is needed. Below are my comments on the scope of work as proposed in the CSM. 1. Section 5.2.2 of the CSM indicates that boring B-49 is to be located in the vicinity of VW-2 and VW-3. However, included Figure 2 shows the location of B-49 near wells EW-2 and EW-3. By 31 March 2017, please submit a Revised Work Plan, which clarifies the intended B-49 location. 2. Boring B-50 is proposed next to VW-1. Shallow soil sampling in this vicinity will provide confirmation of shallow soil concentrations reported in sample L2 from 6 feet bgs which exceed the Volatilization to Outdoor Air and Utility Worker thresholds for benzene as presented in Table 1 of the Low Threat Closure Policy (LTCP). However, benzene and ethylbenzene concentrations in soil sample L2-1-2.5 from 2.5 feet bgs exceed the Direct Contact thresholds listed in LTCP Table 1. Additional shallow confirmation soil sampling is needed in the vicinity of L2-1-2.5. Therefore, please include a proposal for collection of shallow soil in the vicinity of L2-1-2.5 in the Revised Work Plan due 31 March 2017. 3. In order to properly assess the nature of the release, soil samples collected in the vicinity of the former waste oil tank are required to be analyzed for total petroleum hydrocarbons as gasoline (TPHg), TPH as diesel (TPHd), total oil and grease (TOG), volatile organic compounds (VOCs), semi-VOCs (SVOCs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and Title 22 metals. Soil samples AT-1 A and AT-1 B were collected at 13 feet bgs and 6 feet bgs, respectively, from within the former waste oil tank cavity, however these samples were not analyzed for the required waste oil constituents, and collection and analysis of additional soil samples is needed to address this data gap. Therefore, in the Revised Work Plan due 31 March 2017, please include a proposal for the collection of soil samples in the vicinity of the former waste oil tank, with analysis to include the waste oil constituents listed above. Please also include a proposal to collect a soil sample from outside the hydrocarbon impacted area and analyze for Title 22 metals in order to establish naturally occurring background metals concentrations. 4. During the March and September 2016 groundwater sampling events, 22 to 23 of the 24 shallow wells, and 7 to 8 of the 13 deep wells were reported as dry or containing