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Eum u G.5.u—J. MAnecv,Rama.Ez Water Boards et-arc.9AD--POPHMENTK PAOTC01,0'. Central Valley Regional Water Quality Control BoardE 1 C'T AIN APR 22' 2014 21 April 2014 Ralph Lee White Peter Bui & Kimberly Hoang 2201 East Eighth Street 4004 Maddie Circle Stockton, CA 95206 Stockton, CA 95209 WORK PLAN AND DOCUMENTS REVIEW FOR RALPH'S SQUARE, 2211 SOUTH AIRPORT WAY, STOCKTON, SAN JOAQUIN COUNTY, RB CASE#390995 California Regional Water Quality Control Board, Central Valley Region (Central Valley Water Board) staff reviewed the Groundwater Report-First Quarter 2014 (Report) and the Soil Vapor Investigation Work Plan (Work Plan). The Report and the Work Plan were received as an email attachment on 15 April 2014 and electronically in GeoTracker on 10 April 2014, respectively from your consultant Advanced GeoEnvironmental, Inc. (AGE), The Report provides the 4 March 2014 groundwater monitoring results of MW-1 through MW-11. The data show methyl tert butyl ether(MTBE) concentrations have generally declined from one to three orders of magnitude since the last complete round of sampling in October 2008, with the exception MW-5, which increased from 17,000 micrograms per Liter(ug/L)to 28,000 ug/L. The data show the MTBE plume has migrated at least 50', and is undefined to the east. The MTBE plume is defined to the west by MW-1, to the north by MW-7 and MW-8,to the south by MW-9, to the southeast by MW-10, and vertically in the source area by MWA 1. The San Joaquin County Environmental Health Department (SJCEHD) approved a work plan for a groundwater investigation to define the downgradient extent of the plume in a letter dated 27 August 2009. In the Report, AGE proposed to complete the delayed investigation by installing one soil boring (B-5) 80'to the east of the Ralph's Square building, which is about 60' east of the USTs pit. B-5 would be located about 20' east of a residence on Phelps Street. AGE proposed to collect one grab groundwater sample from B-5. AGE recommended submitting a new sensitive receptor survey, stating a water supply well has been confirmed in City of Stockton Park to the northeast. In the Work Plan, AGE proposed installation of five (5) soil vapor wells completed to five (5) feet below ground surface (' bgs), one well in the underground storage tanks (USTs) pit and the remaining four wells between the USTs pit and the Ralph's Square building, to evaluate the threat from vapor intrusion. Two soil borings are proposed to be advanced to 10, bgs with soil samples collected at 5' bgs and 10' bgs, one in the USTs pit and the second 20'to the northeast near MW-2, to evaluate for a bio-attenuation zone. I concur with the tasks proposed in the Work Plan with the conditions stated below. KAnL E. LONCLCY SCO, P.E., CHAIR I PAMELA C. Cnrmoii P.E.,8CEE,cxECLITIVE orriccri 11020 Sun Center Drive#200.Rancho Cordova,CA 95670 1 www,waterboardB.ca.<Iov/centraWalley C1.1—LeD-eE.