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Soil Boring Results and VEG/ISCO Work Plan Field Maintenance Shop #24, Stockton, California October 2016 1.0 INTRODUCTION ECM Consultants (ECM) has prepared this Vapor Energy Generator (VEG) technology and In- situ Chemical Oxidation (ISCO) Work Plan for submittal to the Central Valley Regional Water Quality Control Board (CVRWQCB) on behalf of the California Army National Guard (CA ARNG). This work plan details the proposed remedial action activities for the implementation of in-situ thermal treatment via VEG and ISCO to remediate soil and groundwater impacts observed the CA ARNG Field Maintenance Shop (FMS) #24, located at 8010 South Airport Way in Stockton, California (Figure 1), and hereinafter referred to as the Site (Figure 2). The proposed VEG and ISCO remedial actions are detailed in Section 3.0. The results of soil sampling and analysis conducted in the source area in July 2016 are also included in this work plan. The findings of historical soil and groundwater investigations along with the 2013, 2014, 2015, and 2016 ISCO post-treatment results indicate that site soil and groundwater beneath the former gasoline and diesel underground storage tank (UST) locations were impacted by the release of petroleum hydrocarbons and related compounds from the former USTs. The findings also indicate that after implementation of ISCO injection remedial actions, a residual source of petroleum hydrocarbons and related compounds still remain in the site soil and groundwater in the immediate vicinity of the former USTs. Dissolved phase petroleum hydrocarbons and related compounds have migrated downgradient to areas within the site margins. The CA ARNG determined that additional soil investigation was required beneath the former UST concrete pad in the vicinity of monitoring well FMS-MW6 to address potential vadose zone influence. The results of the soil borings are provided in this report and used to support the layout and design of the VEG implementation to further reduce vadose zone soil impacts. In addition, to further reduce the concentrations of petroleum hydrocarbons and related compounds in groundwater in the vicinity of the former USTs (source zone) an additional round of ISCO injections is proposed after VEG implementation. 1.1 Site Description and History The site is located at 8010 South Airport Way, Stockton, California (Figure 1). FMS #24 is one of four state-owned divisional areas operated by the CA ARNG. The other divisional areas include the Army Aviation Support Facility (AASF), the Armory, and the Combined Support Maintenance Shop (CSMS) (Figure 2). FMS #24 consists of a one-story maintenance building composed of work bays, supply areas, a latrine, offices, a vehicle inspection rack, and hazardous materials storage sheds. A concrete wash pad and holding tank, former USTs, associated piping, and fuel dispensers were previously removed from the site. The primary source for petroleum hydrocarbons and related compounds are the former diesel and gasoline USTs (Figure 3). The former 10,000-gallon gasoline and 5,000-gallon diesel USTs were single-walled steel tanks installed sometime in the early 1950s, when the FMS began site fuel operations. The USTs and associated piping were removed in 1996 (Versar, 2004). Soil samples collected during UST and piping removal indicated that releases of diesel fuel and gasoline impacted site soils. Multiple investigations and studies have been conducted at the site since 2004. The investigations or studies conducted include: site investigations by Versar in 2004, by URS in 2007, and by OTIE in 2010; treatability studies of soil vapor extraction (SVE), air sparging (AS), and high vacuum dual phase extraction (HVDPE) by OTIE in 2012; and membrane interface probe (MIP) studies by OTIE in 2013 and ECM in 2014. Previous remedial actions include multiple rounds of in situ chemical oxidant (ISCO) injections. Several rounds of ISCO have been conducted to reduce the concentrations of petroleum Page 1 •