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Messrs Larson and Tromm• September 23, 2014 <br /> 1600 W. Durham Ferry Road, Tracy, CA Page 2 of 2 <br /> than 12-feet (sic) bsg." The following analytical data are from excavation sidewall samples and <br /> pre-excavation hand auger boring HB-4 samples, all collected directly adjacent to the building: <br /> Ethylbenzene <br /> Sample ID Depth (feet) TPHg (mg/kg) Benzene (mg/kg) (mg/kg) <br /> SW Sidewall 4.5 26,200 7.27 73.4 <br /> SW Sidewall 10 2,610 3.06 28.3 <br /> SE Sidewall 9.5 11,800 7.79 93.2 <br /> HB-4 5-5.5 9,800 39 155 <br /> HB-4 945 14,000 208 300 <br /> It appears to the EHD that a plume of significantly impacted soil extends under the building, a <br /> view that the consultant shared as shown in the accompanying figure from a 2010 report. This <br /> plume of impacted soil may pose a greater threat of vapor intrusion than groundwater (which is <br /> unassessed under the building), and it appears highly likely that a 10-foot bioattenuation zone <br /> with TPH<100mg/kg does not exist where most needed. This plume of unremediated impacted <br /> soil may be the source of groundwater contamination in the MW-2 area. <br /> The EHD has recently approved an insitu air sparge/soil vapor extraction (IAS/SVE) pilot test in <br /> soil underlying the building; installation of the wells for the pilot test will provide opportunity to <br /> assess soil and groundwater directly under the portion of the building thought to overlie <br /> impacted soil and groundwater. The EHD will evaluate the effects on impacted groundwater in <br /> the MW-2 area and take such actions as seems appropriate to comply with the CUF <br /> recommendation. <br /> Questions or comments should be directed to Vicki McCartney at (209) 468-9852 or to Nuel <br /> Henderson at (209) 468-3436. <br /> Sincerely, / <br /> Nuel Henderson, PG <br /> Engineering Geologist <br /> 2014 Third Review Summary Report Response CUF Claim No 8475.doc <br />