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I., NswsoxE <br /> NA GA <br /> YANA xC;A <br /> Water Boards a , ,:P"APY <br /> EMVIPOIIMEMiRL xU*EGiIO.V <br /> Central Valley Regional Water Quality Control Board <br /> 30 November 2022 <br /> Joshua Hernandez-Zavala <br /> San Joaquin County General Services — Capital Projects <br /> 44 N. San Joaquin Street #590 <br /> Stockton, CA 95202 <br /> RESPONSE TO SOIL GAS WORK PLAN, PUBLIC WORKS, <br /> 1810 EAST HAZELTON AVENUE, STOCKTON, SAN JOAQUIN COUNTY, <br /> LUST CASE #390489, GLOBAL ID # T0607700379 <br /> Central Valley Water Board staff reviewed the Soil Vapor Monitoring Well Installation <br /> and Sampling Work Plan (Work Plan), dated 11 November 2022. Condor Earth <br /> (Condor) submitted the Work Plan on your behalf for the Public Works underground <br /> storage tank (UST) petroleum release case at 1810 East Hazelton Avenue in Stockton <br /> (Site). <br /> The Site source area (former USTs), is located in the vicinity of the current USTs, <br /> between the Solid Waste Office to the north and the Maintenance Building to the south. <br /> In this area, dissolved benzene concentrations in groundwater exceed 1,000 <br /> micrograms per liter (ug/L), presenting a potential vapor intrusion risk to these <br /> structures. As such, in the Work Plan, Condor proposes to install three soil gas wells in <br /> this vicinity to assess the petroleum vapor intrusion risk to these structures. <br /> Central Valley Water Board staff concurs with the proposed scope of work, as long as <br /> the following comments are incorporated: <br /> 1 . Condor states that soil gas wells will be "installed approximately 5 to 6 feet below <br /> ground surface" (bgs). However, it is unclear whether this is intended to mean <br /> that the probe will be installed in a sand filter pack set between 5 and 6 feet bgs, <br /> or if total depth will be drilled to somewhere between 5 and 6 feet bgs. <br /> The 2015 joint-agency guidance Advisory—Active Soil Gas Investigations <br /> (Advisory) recommends that soil gas wells not be installed shallower than 5 feet <br /> bgs in order to avoid possible barometric pressure effects and/or breakthrough of <br /> ambient air through the soil column. As such, for clarification purposes, Central <br /> Valley Water Board staff recommends that you install the soil gas wells as shown <br /> MARK BRADFORD, CHAIR I PATRICK PULUPA, EsO., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200, Rancho Cordova,CA 95670 1 www.waterboards.ca.gov/centralvalley <br />