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E.N.RC C. BRow.JR. <br /> f P Fu,.oR R,A � MnnHEW Roououez <br /> Water Boards <br /> Central Valley Regional Water Quality Control Board RECEIVED 5 October 2016 DECEIVE® <br /> OCT 0 6 2016 <br /> ENVIRONMENTAL HEALTH <br /> Ms. Sharon Evans PERMIT/SERVICES <br /> Phillips 66 Company <br /> 1380 San Pablo Avenue <br /> Rodeo, CA 94572 <br /> Semi-Annual Groundwater Monitoring Status Report Fourth Quarter 2015, 76 Service <br /> Station No. 11190, 1206 East March Lane, Stockton, San Joaquin County <br /> California Regional Water Quality Control Board, Central Valley Region (Central Valley Water <br /> Board) staff reviewed the Semi-Annual Groundwater Monitoring Status Report Fourth Quarter <br /> 2015 (4Q2015 Report) dated 25 January 2016, the Semi-Annual Groundwater Monitoring <br /> Status Report Second Quarter 2016 (2Q2016 Report) dated 11 July 2016, and the Well <br /> Installation Report (Well Report) dated 13 July 2016, all submitted on your behalf by ATC Group <br /> Services LLC (ATC). The 76 Service Station No. 11190 is located at 1206 East March Lane in <br /> Stockton (Site). <br /> Petroleum hydrocarbons continue to be detected at elevated concentrations in Site wells. <br /> During the fourth quarter 2015, methyl tert butyl ether (MTBE) was detected at a maximum <br /> concentration of 1,600 micrograms per liter (ug/L) in well MW-7, and tert butyl ether was <br /> detected at a maximum concentration of 4,800 ug/L in well MW-5. During the second quarter <br /> 2016, MTBE was detected at a maximum concentration of 620 ug/L in well MW-7 and TBA was <br /> detected at a maximum concentration of 3,200 ug/L in well MW-5. The water quality objectives <br /> (WQOs) for MTBE is 5 ug/L and is 12 ug/L for TBA. <br /> Concentrations of petroleum constituents in well MW-7 appear to be stable to decreasing since <br /> approximately December 2012. The depth to water in Site wells was increasing between May <br /> 2012 and November 2015, a slight decrease in depth to water was observed during the May <br /> 2016 monitoring event. The increased depth to water and dropping watertable may indicate that <br /> reductions in groundwater concentrations are due to the dropping water table and not <br /> biodegradation of the product. Wells MW-1 through MW-4 are screened from 40 to 55 ft bgs, <br /> while wells MW-5 to MW-9 are screened between 48.75 and 64.5 ft bgs. During the fourth <br /> quarter 2015, groundwater samples were not collected from wells MW-1 through MW-4 due to <br /> insufficient water. Additional delineation of the extent of the petroleum impacts and continued <br /> semi-annual groundwater monitoring is needed to evaluate potential risks from the petroleum <br /> impacts to groundwater in this area. <br /> In the Installation Report, ATC presented results of installation of well MW-10, installed to <br /> assess the lateral extent of soil and groundwater pollution to the northwest of the Site. The <br /> scope of work for installation of well MW-10 was presented in a work plan dated 16 April 2012, <br /> KARL E. LONOLEY.SCD, P.E., CHAIR I PAMEU C. CREEDON P.E., BCEE. EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200,Rancho COEUOva.CA 95670 1 www.waterOoar6e.ca.9ov/centralva1ley <br /> % RECVCUIDFAPEP <br />