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EDMUND a.BROWN JR. <br /> BWtRXOR <br /> OR°IFOI1111• <br /> Water Boards ° NN°I NID <br /> °.O,I..X,RROr¢nax <br /> Central Valley Regional Water Quality Control Board <br /> 15 September 2016 <br /> Sharon Evans <br /> Phillips 66 Company <br /> 1380 San Pablo Avenue <br /> Rodeo, California 94572 <br /> REVIEW OF GROUNDWATER DELINEATION WORKPLAN, FORMER CIRCLE K STORE <br /> #08671, 8606 THORNTON ROAD, STOCKTON, SAN JOAQUIN COUNTY, CASE#391008 <br /> Staff of the Central Valley Regional Water Quality Control Board (Central Valley Water Board) <br /> reviewed the case file and the 30 August 2016 Groundwater Delineation Workplan(Work Plan)for <br /> the unauthorized petroleum release site at 8606 Thornton Road in Stockton (Site). In the Work Plan, <br /> your consultant, Cardno ATC (Cardno), outlines procedures for advancing five cone penetrating test <br /> (CPT) borings and collecting groundwater samples from various depths to assess the lateral and <br /> vertical extent of the Site's methyl tertiary butyl ether(MTBE) plume. <br /> In May 1998, Tosco Marketing Company(Tosco) upgraded the Site product lines and dispensers <br /> and detected an unauthorized release of petroleum hydrocarbons. Beginning in 2002, MTBE <br /> concentrations peaked in shallow groundwater monitoring wells, screened 25 to 45 feet below <br /> ground surface (bgs) in a roughly west to east sequential pattern. In late 2004, Phillips 66 detected <br /> up to 10,000 micrograms per liter(ug/L) of MTBE in MW-4, and in 2011 Phillips 66 detected <br /> 240 pg/L of MTBE in MW-7,the maximum concentration of MTBE detected in this most <br /> downgradient monitoring well. <br /> Central Valley Water Board staff has the following requests regarding the Work Plan: <br /> 1. Groundwater data indicate that MTBE appears to be migrating downgradient of the Site and <br /> recent MTBE concentrations in Site monitoring wells are low to non-detectable. Central Valley <br /> Water Board staff has concluded that it is unlikely MTBE will be encountered in groundwater in <br /> the proposed onsite CPT borings. We request that the two proposed onsite borings be moved in <br /> the downgradient groundwater flow direction from the current monitoring well network, outward in <br /> a fan shape, and at distances from the Site which provide the best lateral coverage based on the <br /> inferred rate of MTBE movement through the Site monitoring well network; <br /> 2. Cardno proposes to collect samples from each CPT boring at first encountered groundwater, any <br /> identified intermediate water-bearing zones, and between approximately 100 and 110 feet below <br /> ground surface(bgs). Central Valley Water Board staff requests that Cardno analyze at least <br /> four samples for MTBE from each boring with particular attention given to identifying and <br /> sampling intermediate water-bearing zones between 45 and 100 feet bgs and especially around <br /> 65 feet bgs, a previously undelineated depth where Phillips 66 detected MTBE in <br /> December 2001; and, <br /> KARL E. LONOLEY BCD', P.E.,CHAIR I PAMELA C.CREEDON P.E.,BCEE,E%ECI IVE OFFICER <br /> 11020 un EnMF Orivs 0200.BEneho COrtlo e.CA 86610 I Www.watxboErtla.cn.BDV/DenbAlvElley <br /> RmrcvR—IRI <br />