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Ea.-G. Einov;JA <br /> l:�I LcvErt on <br /> cwuronar. Monr.f Pv R000arouez <br /> Nater Boards ENVIAUYYEfl lA PlOii it 1; <br /> Central Valley Regional Water Quality Control Board <br /> I <br /> 6 August 2018 I <br /> Ralph Lee White Peter Bui & Kimberly Hoang <br /> 2201 E. 8"' Street 4004 Maddie Circle <br /> Stockton, CA 95206 Stockton, CA 95209 <br /> i <br /> 3 <br /> i <br /> E <br /> RALPH'S SQUARE, 2122 S. AIRPORT WAY, STOCKTON, SAN JOAQUIN COUNTY, CASE# <br /> 390995 <br /> I <br /> t <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff reviewed the } <br /> Groundwater Monitoring Report—First Quarter 2018 (Monitoring Report) dated 25 April 2018, <br /> submitted on your behalf by Advanced GeoEnvironmental Inc. (AGE)for the Ralph's Square <br /> unauthorized petroleum release at 2122 S. Airport Way in Stockton (Site). In the Report, AGE presents <br /> March 2018 groundwater monitoring and sampling data, and recommends that the Site be considered <br /> for case closure under the Low Threat Underground Storage Tank Case Closure Policy(LTCP). <br /> i <br /> Central Valley Water Board staff concurs with AGE's conclusion that the Site meets the LTCP's <br /> Petroleum Vapor Intrusion and Direct Contact and Outdoor Air Exposure media specific closure criteria. P <br /> However, Central Valley Water Board staff does not concur with closure at this time and requests that <br /> you conduct additional Site investigation to assess for the presence of petroleum hydrocarbons <br /> downgradient from the Site. <br /> t <br /> Monitoring data indicate that samples from well MW-2 contained up to 380,000 micrograms per liter <br /> (pg/1) methyl tertiary butyl ether(MTBE) in 2008. Groundwater monitoring was suspended from 2008 <br /> through 2014 and after resuming groundwater monitoring in 2014, the detected concentration of MTBE <br /> in groundwater appears to have decreased significantly. However, due to an absence of monitoring <br /> data, the fate of the MTBE remains unclear. <br /> t <br /> Based on the recent MTBE concentration decreases, it is likely that MTBE has mobilized beyond the <br /> on-Site monitoring wells. In addition, recent groundwater sampling indicates that up to 46,100 pg/l s <br /> tertiary butyl alcohol is present in groundwater at the Site (MW-5). However, groundwater samples <br /> collected from downgradient monitoring wells MW-12 through MW-17 have not contained petroleum <br /> hydrocarbons above their respective detection limits since commencing sampling in January 2015 <br /> indicating that the mobilized MTBE has not yet reached these wells. As a result, the nature and mobility <br /> of the residual petroleum hydrocarbon plume remains unknown and the residual plume length unknown. <br /> By 12 October 2018, Central Valley Water Board staff requests that you submit a Work Plan for the <br /> installation of two monitoring wells downgradient from monitoring well MW-5 along the western edge of <br /> South Phelps Street to assess for the presence of petroleum hydrocarbons in groundwater. Central <br /> Valley Water Board staff also request that you provide a discussion regarding the fate of the MTBE <br /> historically detected in groundwater at the Site. <br /> KARL E. LONGLEY SCD, P.E.,CHAIR j PATRICK PULUPA,E50., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive 9200,Rancho Cordova,CA 95670 1 www.water hoards.ca.0 ovrcen lralva Hey <br /> L,+sMYCLEn o> F.n <br />