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s; y; °,CGeoitical7ecFrrtics Inc. Page 2 Former Moresco Property-IRAP Project No. 251.3 April 8,.1997 L 2.0 INTERIM REMEDIAL ACTION After discussions with Ron Rowe of SJCPHSEHD, Geological Technics Inc. recommends that an interim remediation action plan be conducted to achieve -source reduction. In the vadose zone under the former tank location there are present extremely high concentrations of gasoline hydrocarbon compounds in excess of 900,000 ppb benzene. The purpose of this remedial action is: 1. To remove the high concentrations of chemicals. 2. To introduce oxygen into the subsurface to promote further degradation of the chemicals beyond what is removed through engineered means. 3. To protect human health and the environment by removing the highly mobile chemicals to that groundwater resources are protected and the drinking water well is not impacted. The following sections outline the conceptual approach for interim remediation. �I - 2.1 Proposed Extraction Well One vapor extraction well, VEW-1 is proposed. This well is designed to address the heart of the contaminated zone (total depth to approximately 45 feet below grade) is proposed to be installed through the former UST excavation (Site Map Figure 2). The boring is proposed to be installed by drilling and core sampling for geologic description through the screened interval, with no analytical sampling occurring. The boring will be drilled with a 10.25 inch hollow stem auger owned and operated by'V&W Drilling of Rio Vista, CA, C57# 720904. tVEW-1 will be constructed of 4 inch diameter schedule 40 PVC casing with flush thread joints with a screened interval (0.040 inch slot) extending from 25-45 ft below grade. A pea gravel filter pack will surround the perforations. A three foot transition seal consisting of granular bentonite will be installed and hydrated before a surface seal consisting of neat Ik cement (<4%,bentonite) grout installed using a slurry pump and tremie pipe. See Figure 4 for a schematic of well construction. 2.2 Proposed Recharge Wells Three air-recharge wells (AR-1, 2 & 3) are proposed. 'These recharge wells are designed to aid in introducing oxygen to the subsurface and to induce horizontal vapor movement across the site are proposed to b� installed around the former UST pit. The proposed locations are approximately at the lateral extents of the contaminated soil zone (refer to the Site Map in Figure 2). The borings are proposed to be installed by drilling and core sampling for geologic description through the screened interval, with two samples from each boring submitted for analytical evaluation. The borings will be drilled with a 10.25 inch hollow stem.auger owned and operated by V&W Drilling of Rio Vista, CA, C57# 720904. The air-recharge wells will be constructed of 3 inch diameter schedule 40 PVC casing with flush threads and a screened interval (0.040 inch slot) extending from 25-45 ft below grade. A pea gravel filter pack will surround the perforations. A three foot transition seal consisting of granular bentonite will be installed and hydrated before a surface seal consisting of neat cement (<4% bentonite) grout installed using a slurry pump and tremie pipe. The recharge wells will be finished with breather caps and covered by a lockable monument. The recharge ,I wells will also be fitted with control valves so that they can be cycled on and off and