SITE INFORMATION AND CORRESPONDENCE_FILE 2

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page 2, 6131 Pacific Ave. Extensive investigative work has been completed at this site. Unfortunately, definition of the contaminant plume, in soil or groundwater, has not been made. The work plan proposes to do one soil boring for vertical definition, but does not state to what minimum depth the boring will extend. The work plan also proposes to install one additional monitoring well for down-gradient delineation of the contaminant plume in groundwater. Again, no information on the proposed depth is provided. The QMR reported a groundwater flow direction that ranges from southwesterly to southeasterly. The included site map, labeled Plate 2, depicts this. Obviously, site conditions fluctuate, and it would therefore not be possible to define the down-gradient extent of the groundwater contaminant plume with just one well. The deepest soil boring advanced on site, SB-4, was drilled on July 2, 1997 and was located in the northern dispenser island area. Total depth was 71-feet below surface grade (bsg) and contamination in soil was detected in the sample collected at this depth. Borings SBA and SB-3, completed on the same date as SB-4, were advanced to only 11.5 feet bsg. In March 1990 eight geoprobe borings were advanced in the area of the new UST's. Only one, B-8, was advanced to a depth greater than 20-feet bsg. Boring B-8 was located between the new tank farm and the northern dispenser island area (approximately 25-feet east of boring SB-4). Soil samples for analysis collected at 25 and 45-feet bsg both evidenced petroleum hydrocarbon contamination. Seven soil borings have been advanced in the area of the former tank pit. Of these, two were converted to vapor extraction wells (VEW-1 and VEW-2). VEW-1 was advanced to 20-feet bsg, no soil samples were collected. The remaining six borings, B-2 through B-5, GEO-2 and VEW-2 all had detectable levels of contamination in the deepest soil sample analyzed. Depths of these samples ranged from 40-50 feet bsg. Down-gradient wells MW-4, MW-6 and MW-7 were completed on September 16-17, 1997. All three had detectable levels of contamination in soil samples collected from the bottom of the boreholes. Depths of these samples ranged from 55-61 feet bsg. A soil vapor extraction (SVE) pilot test was conducted by AEGIS Environmental in late 1991 to early 1992. No real conclusions were drawn from this test, and since site conditions and available data have changed considerably since that time, ground water levels have risen approximately 24 feet, it is likely that another SVE test will need to be performed. As per the Underground Storage Tank Regulations, feasibility studies must be completed in preparation for submittal of a Corrective Action Plan (CAP). The work plan as submitted is not approved. Please prepare a detailed work plan that, when implemented, will completely define the contaminant plumes at this site. Include in the work plan a proposal for enough vapor extraction and air sparge wells to perform feasibility testing. If you have any questions or comments please call Lori Duncan at(209)468-0337. Donna Heran, REHS, Director Ernvironmental Health Division A-r Lori Duncan, REHS Margaret Lagorio, REHS Site Mitigation Unit IV Supervisor, Unit IV cc: Mark List, CVRWQCB Ron Slate, PO Box 1146, Lodi CA 95241 Vicki Iten, Porter Station Condo Assoc., 8807 Thornton Rd. Suite M, Stockton 95209 Diane Lundquest, Enviros, PO Box 259, Sonoma CA, 95476-0259