SITE INFORMATION AND CORRESPONDENCE

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Casa de Oasis -2- 26 May 2015 1700 E. EI Dorado St., Stockton San Joaquin County probes after vapor sampling. Upon completion of the soil vapor sampling, the borings are proposed to be grouted to surface with neat cement. Additionally AGEI proposed collecting one indoor air sample in a summa canister over a 24-hour period. The indoor air sample is proposed to be collected inside the Community Center room at three to five feet above floor surface. One additional summa canister sample is proposed to be collected outdoors, south of the Community Center, for measurement of ambient air quality. Central Valle Water Board staff concur with the proposal to investigate soil vapor in the area of EI Dorado Street and 3 Avenue in Stockton, with conditions. 1. We are concerned that there is no possibility for repeatable data from temporary soil probes. Since atmospheric conditions may result in variable soil vapor concentrations over time, additional soil vapor sampling may be required to fully assess threats at this site. Therefore please install soil vapor wells that can be resampled in the locations where soil gas is detected. 2. AGEI proposed sampling VOCs by USEPA Method 8260B. Since the former USTs are the primary concern for vapor intrusion, laboratory analyses should also include; benzene, ethylbenzene, toluene, xylenes, methyl tert butyl ether(MTBE), tert amyl methyl ether, di-isopropyl ether, ethyl tert butyl ether,tertiary butyl alcohol, 1,2 dichloroethane, ethylene dibromide, and naphthalene by TO-14 and naphthalene by TO-17.. 3. The State Water Resources Control Board Low Threat Closure Policy contains criteria for soil vapors measured with and without oxygen levels in soil. The presence of oxygen in soil at 4% or greater at 5' bgs indicates a bio-attenuation zone which changes the acceptable levels of benzene, ethybenzene and naphthalene in soil gas. The oxygen content in soil gas, reported as a per cent, needs to be measured to determine if a bio-attenuation zone exists. Oxygen levels need to be measured at 5 feet below ground surface in all borings. 4. Figure 2 shows seven (7) soil vapor locations, however, the Scope of Work described on page 2 of 4 in the Work Plan indicates twelve (12) soil vapor sample locations are proposed in the Work Plan. Central Valley Water Board staff concur with the seven (7) locations for sampling of soil gas as depicted in Figure 2. Please limit the investigation to only those seven soil vapor locations in Figure 2. 5. With the exception of the deeper 10' bgs boring near SV3, all soil vapor wells need to be installed and screened a minimum of 5 feet below ground surface. 6. The Indoor Air sampling is not needed at this time. Many contributing factors can make indoor air sample results difficult to interpret, for example; fresh paint, new carpeting and cleaning products stored in a building. For this reason we recommend postponing the collection of indoor air samples until, or if, soil gas sample results show a significant threat to indoor air pollution. The results from the soil gas investigation are due in a Report by 30 July 2015.