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LIGHT, AIR & SPACE CONSTRUCTION ENVIRONMENTAL SERVICES COMPANY State Contractor's License Number 445403 State EPA R.E.A.Number 04072 Office 408-979-0661 6.0 Conclusions and Recommendations LA&S has completed the groundwater monitoring well installation and soil sampling at the 19580 McKinley Avenue site in Manteca, CA. Two USTs (one gasoline and one diesel) were removed in June 2006 and soil excavated from the tank pits was stockpiled for aeration per the Work Plan approved by the EHD. On the basis of dissolved TPHD observed in the tank pits in 2006, EHD required that additional assessment and groundwater monitoring wells be installed, Three monitoring wells were installed near the tank pits and soil stockpile area. Chemical analysis of soil and groundwater samples showed TPHG at 69 ug/l was present at MW-1 and one detection of MTBE at 3.99 ugll was present at MW-3. Soil composite samples from the stockpile showed only very low TPHD concentrations. The laboratory reports that both the TPHG and TPHD did not match the laboratory standards and these are interpreted as suggestive of residual degraded fuel. A 2000-foot radius well search was performed for the site using data from selected State well log reports. The search revealed domestic drinking and irrigation water wells in the area. Most wells in the area are assumed active although this was not verified, and most wells appear to be sealed at least to 20 to 50 feet deep. The residence well uses one on-site drinking water; no well construction information was available for this well. LA&S sampled this drinking water well and no contaminants were detected in that water sample. On the basis of the tank removal and well installation information and soil and groundwater chemical analyses, a significant contaminant problem is not present in our opinion. Soil samples collected from the tank pit excavated soil show only very low TPHD is present. Very low residual dissolved TPHG in MW-1 and MTBE contaminants are present in MW-3. The MTBE is below the State MCL and Secondary MCL in well MW-3. LA&S recommends the following: • A copy of this report should be forwarded to San Joaquin EDH for their review. • The soil stockpile data show only very low TPHD detections in the composite samples. In our opinion, further treatment of this soil is not warranted. Given the very low contaminant detections in monitoring wells and no contaminants were present in the on- site water well, we recommend that this site should be considered for site closure. January 5, 2009 Page 8 of 12 MAIN OFFICE • 1707 LI-I-1-LE ORCHARD STREET, SUITE A • SAN JOSE•CA•95125 MAILING ADDRESS•P.O.Box 36303•SAN JOSE•CA•95158-6303•408/979-0661 •4081979-0621 FAX•1.NM 0%S i;ir-Not t()%I E-MAIL GENERAL AND ENGINEERING CONTRACPOR.HAZARDOUS SUBSTANCE REMOVAL CONTRACTOR•SITE EN VIRONMENTAL ASSESSMENTS WE['LAND AND RIPARIAN HABITAT-ASSESSMENT-DESIGN-RESTORATION-CONSTRUCTION-MITIGATION MONITORING