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SECTION 6.0 CONCLUSIONS AND RECOMMENDATIONS ® A limited vapor extraction pilot test was conducted by GEOCON in August 1996 (GEOCON, 1996b) to evaluate the feasibility of using SVE as a remedial alternative at the Stockton Yard The results of the SVE pilot test predicted that SVE could be used to reduce ' volatile petroleum hydrocarbon mass in soil (estimated rate of) and that the radius of influence is approximately 35 feet This study was conducted using one extraction well screened across more permeable sand zones and less permeable fine-grained zones The test did not evaluate pore volume exchange rates in less permeable zones, therefore, the radius of influence and mass removal rates may be less than those estimated by Geocon g o The mass of TPH- and benzene in soil (from the GEOCON and Alton Geosczence investigations has been estimated to be approximately 220 kg (485 pounds) and 3 6 kg (8 pounds), respectively o The mass of TPH-g and benzene, in groundwater using September 1999 sampling data, has been estimated to be approximately 2 grams (4 4 x 10' pounds) and 0 7 grams (1 5 x 10" pounds), respectively ' 6.2 RECOMMENDATIONS Based on the above conclusions and the objectives of this report, the following recommendations are provided o To further assess TPH-g and BTEX concentrations with depth across the site, additional groundwater samples may be collected at different depths in the groundwater columns of onsite wells The sampling may be designed to evaluate groundwater at the historical water table depth coincident with the highest historical TPH-g and BTEX concentrations • To vertically delineate petroleum hydrocarbons in groundwater, additional subsurface sampling may be conducted around the former USTs and the wells with elevated TPH-g and BTEX concentrations A Cone Penetrometer Test (CPT) ng equipped with a Hydropunch for groundwater sampling may be used to collect stratigraphic data as well as dissolved petroleum hydrocarbon data Vertical groundwater sampling may be conducted at depths at ' which historical high concentrations of petroleum hydrocarbons in soil and groundwater have been detected ' * To further evaluate natural attenuation at the site, a sampling program may be designed to assess microbial activity and natural degradation of petroleum hydrocarbons in soil and groundwater At a minimum, soil samples may be collected using a CPT ng (or Geoprobe) at depths consistent with the depth at which petroleum hydrocarbons were previously detected in soil and groundwater Soil samples may be collected with depth and analyzed for S IPROJ141040700041SYVARDIMAINTYAROSADOC 6'2 URS Greiner Woodward Clyde