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III goo <br /> [[-- EDMUNC G. BROWN JR. <br /> COVCnNCn <br /> (1 's <br /> C L 110 A M I♦ ��� {�/�/„L MATTHEW RODRIOUEZ <br /> Water Boards L X-`i*� ��� � ENVIRONMENTAL <br /> N��`\'�A-WOES ENviRONMENTnI FFOTECTION <br /> Central Valley Regional Water Quality Control Board <br /> 6 December 2012 <br /> Belinda May, LTC <br /> Environmental Compliance Officer <br /> 10620 Mather Road <br /> Mather, CA 95655-4176 <br /> FIRST QUARTER 2012 GROUNDWATER MONITORING REPORT AND PETITION <br /> FOR SITE CLOSURE, ARMY AVIATION SUPPORT FACILITY, STOCKTON <br /> CALIFORNIA ARMY NATIONAL GUARD COMPLEX, SAN JOAQUIN COUNTY <br /> Central Valley Regional Water Quality Control Board staff (Central Valley Water Board <br /> staff has reviewed the First Quarter 2012 Groundwater Monitoring Report and Petition <br /> for Site Closure (1 Q2012 Report) received on 11 June 2012. The 1 Q2012 Report <br /> provides the results of groundwater monitoring conducted in the first quarter of 2012 <br /> and requests closure of the Army Aviation Support Facility (AASF). This letter responds <br /> to the request for closure. <br /> The California Army National Guard (CA ARNG) recently completed extensive <br /> investigation of the former underground storage tanks and washrack at the AASF. CA <br /> ARNG's findings are documented in Report on Groundwater Grab Sampling, <br /> Groundwater Monitoring Well Installation, and Soil Gas Investigations (September <br /> 2011) and four quarterly groundwater monitoring reports spanning second quarter 2011 <br /> through first quarter 2012. These findings indicate releases likely occurred from both <br /> the former USTs and washrack. Concentrations of volatile organic compounds (VOCs) <br /> are generally very low in site soils except beneath the former washrack where the <br /> California Human Health Screening Levels (CHHSLs) for vinyl chloride and benzene are <br /> exceeded in a soil vapor sample collected from a depth of 19 feet below ground surface <br /> (bgs). <br /> Groundwater beneath the site has been impacted. Concentrations of total petroleum <br /> hydrocarbons (as both diesel and gasoline), benzene, ethylbenzene, tetrachloroethene, <br /> cis-1,2-dichloroethene, naphthalene, and dissolved lead in site groundwater exceed <br /> potentially applicable water quality objectives. Dissolved lead was detected in all six <br /> monitoring wells at concentrations ranging from 0.9 to 3.8pg/I and likely represents local <br /> background concentrations. Except for one detection of benzene (0.36pg/1) in <br /> monitoring well MW-6, all of the detections occurred in monitoring well MW-4, which is <br /> located in the area of the former washrack. Based on four quarters of groundwater <br /> KARL E. LONGLEY SCD, P.E., CHAIR I PAMELA C. CREEDON P.E., BCEE, EXECUTIVE OFFICER <br /> 11020 Sun Center Drive H2O0,Rancho Cordova,CA 95670 1 www.waterboards.ca.gov/centraIvaIIBy <br /> RECYCLED P—ER <br />