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EDMUND 0 BROWN JR. <br />cov,Non <br />CALIFORNIA <br />Water Boards <br />MATTHEW RODRIQUEZ <br />SECRETARY CDR <br />ENVIRONMENTAL PROTECTION <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 (1Q2012 Report) received on 11 June 2012. The 1Q2012 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 Dove #200, Rancho Cordova, CA 95670 I www.waterboards.ca .govicentralvalley <br />Cj RECYCLED PAPER