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�V\\ EwewoxG. BRown Je. <br /> �1Z�I c <br /> x_ <br /> `- Mnnnew RmRlouez <br /> Water Boards `./ PROTECToa <br /> Central Valley Regional Water Quality Control Board Rp]�y C IgyV LD R�lJs�tl li �I�.o-� <br /> 26 July 2016 JUL 2 7 2016 <br /> ENWRON ,IENTAL HEALTH <br /> PERMIDSER6CES <br /> Mr. Steve Azevedo <br /> West Clay Properties/Knife River Corporation <br /> P. O. Box 6099 <br /> Stockton, CA 95206 <br /> Project Status, West Clay Property, 639 West Clay Street, Stockton, San Joaquin County <br /> California Regional Water Quality Control Board, Central Valley Region (Central Valley Water <br /> Board) staff reviewed the Additional Site Assessment Report— Third Quarter 2015 (3Q2015 <br /> Report) dated 10 November 2016, and the Groundwater Monitoring Report— First Quarter 2015 <br /> (1 Q2015 Report) dated 30 September 2015, both submitted on your behalf by Advanced <br /> GeoEnvironmental, Inc. (AGE). <br /> Elevated concentrations of petroleum hydrocarbons continue to be detected in Site wells. <br /> During the first quarter 2015, petroleum hydrocarbons were detected at maximum <br /> concentrations of 9,100 micrograms per liter (ug/L) total petroleum hydrocarbons as gasoline <br /> (TPH-G), 1,900 ug/L benzene, 56 ug/L methyl tert butyl ether (MTBE), and 71 ug/L <br /> 1,2-dichloroethane (1,2-DCA). The water quality objectives (WQOs) are 21 ug/L for TPH-G, <br /> 1 ug/L for benzene, 5 ug/L for MTBE, and 0.5 ug/L for 1,2-DCA. <br /> Groundwater pollution has not been adequately delineated to the northeast of well MW-29, see <br /> comments 1 and 2 below. Ozone injection was performed at the Site from 17 November 2006 <br /> until fourth quarter 2010. Since shut-down of the ozone injection system, select monitored wells <br /> show stable to decreasing trends. Many of the Site's monitoring wells have not been sampled <br /> since before shut-down of the ozone injection system. Additional post remedial monitoring is <br /> needed to establish trends and plume stability. <br /> Central Valley Water Board staff have the following comments on this project: <br /> 1. AGE identified five hydrogeologic units (HU): HU1 between approximately 18 to 40 feet <br /> below ground surface (ft bgs), HU2 between approximately 31 to 72 ft bgs, HU3 between <br /> approximately 62 to 90 ft bgs, HU4 between approximately 83 and 170 ft bgs, and HU5 <br /> between approximately 166 and 191 ft bgs. Site monitoring wells are screened in HU1, <br /> HU2, HU3, and HU4. Petroleum hydrocarbons are not detected in monitoring wells <br /> screened in HU4. However, the vertical extent of contamination has not been evaluated in <br /> the direction of well MW-29, screened from 40 to 60 ft bgs, located down-gradient, <br /> northeast, of the former UST No. 3 source area located on the southern end of the subject <br /> property. During the first quarter 2015, petroleum hydrocarbons were detected in well MW- <br /> 29 at maximum concentrations of 9,100 ug/L TPH-G, 1,500 ug/L benzene, and 41 ug/L <br /> KARL E. LONGLEv ScD. P.E.. CHAIR I PAMELA C. CREEDON P.E., 8CEE, EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200.Rancho Cordova.CA 95670 1 www.waterb°ards.ca.gov/centralvallev <br /> �d omva.rn=Wash <br />