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Dopaco, Inc. - 3 - 24 July 2013 800 W. Church St., Stockton San Joaquin County Prior to constructing the temporary soil gas wells adjacent to CPT-13, two (2) soil samples would be collected at 5' bgs at the bottom of the soil vapor borings. Soil and groundwater analyses include TPH as gasoline by USEPA Method 8015M and volatile organic compounds, including toluene, by USEPA Method 8260B. Soil gas will be analyzed by Method TO-15 for volatile organic compounds (VOCs), including toluene. The Addendum also added two additional soil vapor probes screened at 10' bgs adjacent to the 5' probes, with associated soil samples from the bottom of the 10' bgs borings, in order to better define the soil contamination prior to the over-excavation. Addendum Figure 3 showed an approximately 20 square feet area identified as the footprint for remediation of Toluene above a the proposed soil cleanup level of 9.3 mg/kg (ESL). Confirmation soil analyses for volatile organic compounds, including toluene, will be performed using USEPA Method 8260B. If VOCs in composite samples from excavated soil are above the Toluene ESL, soil will also be analyzed for California Code of Regulations Title 22 metals by USEPA Method 6010 and TPH by USEPA Method 8015M, and any required analyses for disposal at a permitted landfill facility. A final Remedial Work Plan will provide details for the size of the final excavation and the number and location of confirmation analyses from sidewalls and bottom of the excavation. Comments We concur with the Work Plan and associated tasks described in the Addendum with the following modifications and scheduling changes. 1. Toluene was detected at E2- from 42' to 47' bgs and AB1+ from 32.5' to 37.5' bgs. Benzene was detected in sample EO from 30 -35' bgs. The existing groundwater monitoring wells DW-1, DW-2 and DW-3 are screened across the sand zone found between 30 and 40 feet bgs. First groundwater is found underlying the Site at about 14 feet bgs. Additional depth-discrete groundwater monitoring wells are needed in the each water bearing zone that is impacted to characterize the extent of impacts horizontally and vertically. Groundwater impacts should be defined to background levels or at a minimum to the water quality objective; 42 ug/L for Toluene and 1 ug/L for Benzene. Due to the concerns for heaving sands encountered at this Site, a conductor casing should be used for wells advanced within the area of known contamination to avoid cross-contamination. Please present a Work Plan proposing an adequate number of groundwater monitoring wells to characterize the extent of the plume horizontally and vertically at this Site. 2. Soil concentrations detected at 15' — 20'bgs at boring locations identified as CO and AB1+, detected elevated levels of naphthalene, benzene and xylene at concentrations that may be a risk to water quality. Therefore, naphthalene, benzene and xylene should be added to the list of analytes for all soil, soil vapor and groundwater collected at this Site until it can be demonstrated that residual levels of these chemicals do not threaten human health or the environment.