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Phase it Environmental Site Assessment Multi-Modal Station Stockton,San Joaquin County,California Page 30 Low to moderate concentrations of VOCs were detected in four groundwater samples collected from Block 6 A high concentration of acetone was detected in one of the groundwater samples collected from 931 E Weber Avenue As previously indicated, acetone is a common laboratory contaminant and the reported detection is considered suspect The groundwater samples from GBK6-931-1, one of the borings with soil contamination, did not indicate the presence of any VOCs However, the laboratory reporting limit was raised for all constituents in that sample PCE concentrations in groundwater from the Block 6 properties (128 N Aurora Street and 931 E Weber Avenue)may be related to the PCE groundwater plume at Block 5 Further investigation into the potential source of the PCE contamination and the extent of the contamination needs to be performed Air monitoring of residential property basements at Block 6 may need to be conducted in order to determine if contaminants have volatilized 6 3 8 Summary and Recommendation for Prioritization of Environmental Investigations The site perimeter borings indicated that no off-site sources of contamination are moving into the six- block area, and no sources of contamination from within the six-block area are moving out of the area Significant contamination was not encountered in the groundwater of the northern three blocks (Blocks 1, 2, and 3) However, small-sized areas of contamination may be present at any of the six blocks due to the spacing of the field explorations Some fairly well-defined groundwater contaminant plumes were discovered in the southern three blocks (Blocks 4, 5, and 6) At Block 4, a large plume of chlorinated solvents consisting of PCE, TCE, and breakdown products was observed centered on the property at 730 E Channel Street At Block 5, a TCE groundwater contaminant plume was discovered centered on the property at 805 E Weber Avenue A • larger plume of PCE was discovered that extended across properties at 815, 829, and 843 E Weber Avenue, with possible extensions north to the properties at 836 E Channel Street and 147 N Aurora Street, and east onto Block 6 properties All three of these plumes had contaminants detected above the respective MCLs for each contaminant The following is Condor's recommended sequence of events for environmental investigations prior to redevelopment of the site area ➢ Air monitoring in the basements of residences along E Channel Street in the Blocks 5 and 6 areas should be conducted in order to determine if contaminants present in groundwater and possibly soil have volatilized ➢ The UST at 730 E Channel Street (Block 4) should be properly removed according to EHD requirements ➢ The petroleum hydrocarbon impacted soil at 730 E Channel Street (Block 4) related to a former UST should be further evaluated and removed ➢ The large, intermixed, groundwater contaminant plume associated with the 730 E Channel Street (Block 4) property should be further investigated to the north and east Due to the soil and groundwater contamination identified in the Block 4 area, air monitoring of nearby residential property basements may need to be conducted ➢ The groundwater contamination at Block 5 needs to be further investigated with soil and groundwater sampling and possible soil vapor gas surveys ➢ The property at 820 E Channel Street should be further investigated with soil borings once the property is clear of surface obstructions %k CONDOR