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d San Joaquin County DIRECTOR o i N Donna Heran, REHS Environmental Health Department ASSISTANT DIRECTOR 600 East Main Street Laurie Cotulla, REHS Stockton , California 95202 -3029 N � PROGRAM COORDINATORS Carl Borgman, REHS Mike Hu . Ov. oet Lag REHS, RDI Website: wwws � rFOFC� Ig r1hd g Margaret agorio, REHS Phone: (209) 468-3420 Robert McClellon, REHS Fax: (209) 464-0138 Jeff Carruesco, REHS, RDI Kasey Foley, REHS December 2, 2008 Mr. Mike Wallace City of Stockton Office of Economic Development Redevelopment Department Real Property Section 425 North El Dorado Street, Third Floor Stockton, California 95202 Subject: Former San Joaquin County Motor Pool 130 North Hunter Street Stockton, California 95202 The San Joaquin County Environmental Health Department (EHD) has reviewed Addendum to Investigation Summary Report and Groundwater Monitoring Work Plan, Former County Motor Pool, 130 North Hunter Street, Stockton, California, dated November 7, 2008, submitted by Treadwell & Rollo, Inc. (Treadwell & Rollo) on your behalf. In the addendum, Treadwell and Rollo proposes to advance one soil boring to approximately eighty-five feet below surface grade (bsg) instead of installing an additional monitoring well .screened at approximately seventy to eighty-five feet, as recommended by the EHD by letter dated April 28, 2008. The EHD approves the advancement of one soil boring as proposed by Treadwell and Rollo to assess groundwater potentially in a sand unit encountered in soil borings S136 and SB7. The proposed boring will be advanced in the vicinity of former monitoring wells MW- 1 , MW-2, and MW-3 , as illustrated in Figure 1 of the addendum. However, instead of labeling the proposed boring SB- 10, the EHD recommends the boring be labeled SB-8 since historically there exist soil borings SB-2 through SB-7. In addition to analyzing groundwater samples collected from the soil boring for total petroleum hydrocarbons (TPH) quantified as gasoline (TPH-g); benzene, toluene, ethyl benzene, and total xylenes (BTEX), methyl tertiary-butyl ether (MTBE), and 1 ,2-dichloroethane ( 1 ,2-DCA), analyze groundwater samples for di-isopropyl ether (DIPE), tertiary amyl methyl ether (TAME), TPH quantified as kerosene (TPH-k), and as diesel (TPH-d). Treadwell and Rollo has proposed collecting groundwater samples from newly installed monitoring wells using low-flow purging and sampling technique instead of the industry standard of removing three to five well casing volumes of water prior to sampling. Before the EHD will approve the low-flow purge and sampling technique, you are directed to purge and sample all monitoring wells using both techniques; first implementing the low-flow technique followed by the three to five well casing volumes technique. If the results are comparable, the EHD will approve the use of low-flow sampling technique for future sampling events.