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i <br /> Former Unocal No. 0187 Page 2 of 2 <br /> 437 East Miner Avenue August 19, 2014 <br /> Stockton, California 95202 <br /> The EHD approves the pressure grouting of monitoring wells U-7 through U-9 and U-11; <br /> however, apply twenty-five psi of liquid grout for ten minutes instead of five; the EHD does not <br /> approve pressure grouting the off-site wells U-16, U-18, U-20 and U-23 because of the <br /> petroleum hydrocarbon contamination detected in soil samples associated with these wells. <br /> Instead these wells must be over-drilled. Also destroy all sixteen soil vapor probes (SVP-1A/B <br /> through SVP-8A/B) by over-drilling since high concentrations of contaminants of concern were <br /> detected in soil gas samples collected from these probes in 2009. <br /> The EHD does not approve the destruction of the remaining wells by detonation because this <br /> well destruction method does not comply with the San Joaquin County Well Standards. Instead, <br /> these wells must either be over-drilled or pressure grouted based on the associated soil and <br /> groundwater analytical data. If ARCADIS can provide technical justification why the wells with <br /> known soil and/or groundwater contamination can not be over-drilled, then the EHD will <br /> consider detonation as an alternative. However, the EHD requires that the entire well casing be <br /> lined with charges from total depth to a depth recommended by the Welenco technician so that <br /> the entire casing will be destroyed prior to forming the 'mushroom cap'. <br /> There are three sets of 'nested' wells [(NP-1-22, NP-1-65, NP-1-111) (NP-2-22, NP-2-60, <br /> NP-2-116) and (SP-1A, SP-1B)] associated with this site. The EHD recommends that augers <br /> larger in diameter than the original eight-inch augers be used and centered on the deepest <br /> casing (which is off-center in the original borehole). This will allow the augers to reach and <br /> remove all well casings, screens, filter packs and grout sealants installed during construction of <br /> the wells. <br /> Please submit a work plan addendum to the EHD by 15 October 2014 that addresses the <br /> destruction of the monitoring and remediation wells according to the San Joaquin County Well <br /> Standards. <br /> Please contact Gabriel Gonzalez at (209) 468-3442 or gagonzalez at7sicehd.com if you have any <br /> questions concerning this letter. <br /> Sincerely, <br /> Gabriel Gonzalez, EHS Nuel Henderson, PG <br /> Environmental Health Specialist Engineering Geologist <br /> c: Ms. Cori Condon, PG, CHg, California Regional Water Quality Control Board Central Valley <br /> Region, 11020 Sun Center Drive, Suite 200, Rancho Cordova, California 95670 <br /> Mr. Justin Sobieraj, PG, ARCADIS U.S. Inc., 100 Smith Ranch Road, Suite 329, San Rafael, <br /> California 94903 <br /> Mr. David Irey, Deputy District Attorney, SJCADA, 222 East Weber Avenue, Room 202, <br /> Stockton, California 95202 <br />