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San Joaquin County OPpUA C Environmental Health Department DIRECTOR Donna Heran, REHS 600 East Main Street N Stockton, California 95202-3029 PROGRAM COORDINATORS Robert McClellon, REHS Jeff Carruesco, REHS, RDI cq• .A Website: www.sjgov.org/ehd Kasey Foley,REHS Phone: (209) 468-3420 Linda Turkatte,REHS Fax: (209)464-0138 June 1, 2011 Mr. Dung Thanh Phan and Ms. Nhuan Thi Phan 3524 Morningside Drive Stockton, California 95219 Subject: California Stop 2224 Manthey Road Stockton, California 95206 Dear Mr. and Ms. Phan: The San Joaquin County Environmental Health Department (EHD) has reviewed Semi-Annual Monitoring Report — January 2011 (report), dated 22 April 2011, prepared and submitted by your consulting firm Advanced GeoEnvironmental, Inc. (AGE) on your behalf: In the report AGE recommends that all eight monitoring wells currently at this site be sampled semi- annually for methyl tertiary-butyl ether (MTBE) and tertiary-butyl alcohol (TBA) "due to the shifting nature of the dissolved plume." The EHD has reviewed the historical analytical data for groundwater collected from the eight monitoring wells and directs that monitoring wells MW-1 through MW-4 and MW-6 through MW-8 be sampled semi-annually and the groundwater samples to be analyzed for total petroleum hydrocarbons quantified as gasoline (TPH-g); benzene, toluene, ethylbenzene, and total xylenes (BTEX), MTBE, TBA, and tertiary- amyl methyl ether (TAME). In light of the low concentrations of contaminants of concern detected in groundwater collected from MW-5, this well should continue to be sampled annually for the same constituents listed above. The EHD is omitting the analyses for 1,2- dichloroethane (1,2-DCA), 1,2-dibromoethane (EDB), di-isopropyl ether (DIPE), and ethyl tertiary-butyl ether (ETBE) based on the historical data. By letter dated 30 March 2011, the EHD directed that a work plan be submitted by 05 July 2011 for the installation of six additional monitoring wells as initially suggested by the EHD by letter dated 15 October 2008, and recommended by AGE in the Corrective Action Plan dated 27 October 2010. Based on the Rose Diagram illustrating the historical groundwater flow direction to be to the southeast, the EHD recommends that one additional monitoring well, screened between 10 and 30 feet below surface grade, be installed southeast of MW-1 and down gradient of MW-2 to define the MTBE plume down gradient of the source area. Please include this additional well in the work plan. Please contact Vicki McCartney at (209) 468-9852 or by email at vmccartney a.sicehd.corn if you have any questions concerning this letter.