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Work Plan for Groundwater Monitoring Well Installation, Soil Gas and Groundwater Investigation, and Groundwater Monitoring Army Aviation Support Facility, Stockton, California 1.0 INTRODUCTION Oneida Total Integrated Enterprises (OTIE) is submitting this Work Plan to the Central Valley Regional Water Quality Control Board (CVRWQCB) on behalf of the California Army National Guard (CA ARNG), under Contract W91238-09-D-0017, Delivery Order 0004, Amendment 01 with the United States Army Corps of Engineers (USACE) Sacramento District. It addresses the implementation of groundwater monitoring well installation, soil gas and groundwater investigation, and groundwater monitoring at the CA ARNG Army Aviation Support Facility (AASF) in Stockton, California (hereinafter referred to as the site). This Work Plan was prepared, in part, pursuant to findings from the Versar, Inc., Preliminary Assessment/Site Investigation Update Report(PA/SI Update; Versar, September 2004) and the URS Group, Inc., Site Investigation Report, Underground Storage Tank Location at the Army Aviation Support Facility, Stockton, California (SI Report; URS, June 2008). The primary conclusions associated with these investigations were as follows: • Soil and groundwater adjacent to the former 5,000-gallon and 20,000-gallon underground storage tank (UST) locations at the site have been impacted by the release of petroleum hydrocarbons and related constituents from the former USTs. Additionally, soil and groundwater in this area have been impacted by volatile organic compounds (VOCs), namely trichloroethylene (TCE), although no potential source areas for VOCs were identified at that time. • The lateral extent of soil contamination associated with the former USTs has been delineated to non-detectable levels for VOCs and Benzene, Toluene, Ethylbenzene, and Xylenes (collectively referred to as BTEX) compounds, and to below the United States Environmental Protection Agency (USEPA) Region 9 preliminary remediation goals (PRGs) for lead. • The downgradient and cross-gradient extent of Total Petroleum Hydrocarbons (TPH) as Gasoline (TPH-g), TPH as Diesel (TPH-d), and BTEX compounds in groundwater associated with the former USTs has been defined. • The detection of TCE in groundwater upgradient of the former USTs, and above the primary maximum contaminant level (MCL) of 5 micrograms per liter(Ng/L), represented a groundwater data gap. Based on these conclusions, the SI Report recommended additional groundwater sampling to the southeast of the former USTs to confirm the upgradient (southeast) extent of groundwater contamination. Additionally, the SI Report recommended that the three existing groundwater monitoring wells (MW-1, MW-2, and MW-3) be re-developed, if necessary, and sampled since they had not been sampled since 2004 (and only twice in the last seven years, as of June 2008). Generally concurrent with the investigation and findings documented in the SI Report, the CA ARNG was in discussions with the CVRWQCB regarding potential impacts to groundwater from activities associated with current and former wash racks located along the southern and southwestern margins of the site. It was known that chlorinated solvents were used on the wash racks for cleaning and degreasing purposes, that the wash rack(s) had identifiable cracks, and that surface runoff from the easternmost (former) wash rack periodically discharged to a drainage channel oriented along the southern margin of the site and ultimately into a storm drain OTI E 1