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Treatability Study Report and Feasibility Evaluation for In Situ Petroleum Hydrocarbon Remediation Field Maintenance Shop #24, 8020 South Airport Way Stockton. California evidence of SPH has ever been identified in site groundwater. Impacts with petroleum hydrocarbons and related constituents are shown with the monitoring program to have migrated to the approximate site margins. The overall conclusion from the ongoing voluntary groundwater monitoring program is consistent with previous site investigations, and is that the site has been impacted with petroleum hydrocarbons and related VOCs at concentrations that exceed potentially applicable regulatory standards. 1.2.3. Conceptual Site Model The chemicals of potential concern (COPCs) for the site are TPH-d, TPH-g, and VOCs including fuel oxygenates, naphthalene, and BTEX. The primary release mechanism for petroleum impacts to groundwater at the site is leaks from the former USTs. An additional potential release mechanism is incidental surface discharge at the former fueling islands. The primary mechanism of impacts to groundwater from these potential releases is the vertical leaching of contaminants downward through coarser -grained units and into groundwater. Because COPCs are known to be present in soils within the vadose zone, they can potentially be released into groundwater as a result of seasonal upward fluctuations in groundwater levels. There is limited potential for surface water infiltration because, as previously mentioned, approximately 90 percent of the surface area at the site is paved. Based on historical investigation performed at the site (Versar, 2004; URS, 2007; OTIE, 2010a), it is known that the subsurface at the site is dominated by fat clays and sandy/silty clays with interbedded fine sands, silty sands, and clayey sands. Above the groundwater table, these coarser units are generally discontinuous, both vertically and laterally. Below the groundwater table, which occurs at approximately 31-35 feet bgs, these units are more laterally continuous and occur at two general depths (approximately 40-45 feet bgs and 50-65 feet bgs). These lithologic conditions would tend to inhibit downward migration from the location of the former USTs, but would favor lateral, advective, downgradient migration of COPCs if and when they reach groundwater. Because COPCs, namely petroleum hydrocarbons and related compounds, have been shown with the current groundwater monitoring program to have migrated to the approximate site margins, these findings suggest that releases from the USTs by one of the mechanisms described above occurred over an extended period before the USTs were removed. The treatability study addressed in this document took into account these hydrogeologic conditions, and was intended to identify what combination of AS/SVE/HVDPE technologies will maximize removal of COPCs from the test area. SVE testing was performed within the vadose zone (approximately 20-30 feet bgs); AS testing was completed within the water table at two different depths (approximately 38-40 feet bgs and 63-65 feet bgs); likewise, HVDPE testing was performed across the impacted vadose/saturated zone using two wells, each screened from approximately 20-60 feet bgs. Figure 3 indicates the approximate area of most significant groundwater impacts, based both on baseline groundwater monitoring activities performed in May 2012, as described below, and on historical groundwater monitoring performed at the site by OTIE, most recently in December 2011 (OTIE, 2012). Figure 3 also shows two lines of section utilized in this report to document complex site subsurface conditions and treatability study results. Cross -Section A -A' (Figure 4), is oriented generally from southwest to northeast, and Cross -Section B -B' (Figure 5), is oriented generally from northwest to southeast. OTIE