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Ms Lori Duncan Project No 880-039 IA June 18, 2003 Page 10 ' DISCUSSION Based on the results of this investigation and historical data, impact beneath the site appears mainly in groundwater where TPHg concentrations on-site were reported up to 62,000 ug/l in well MW-4RS Well MW-4RS is located adjacent to the former USTs, ' and is within the area of highest dissolved hydrocarbon concentrations reported at the site In deep well boring MW-4R, soil impact is reported only below the water table (Table 1), and there the resulting concentrations are comparable to groundwater results (Table 2, MW-4RS and MW-4RD) The reported hydrocarbon concentrations in soil are likely the result of dissolved hydrocarbons, indicating that there does not appear to be a residual hydrocarbon source on-site The declining hydrocarbon concentrations with depth in soil in well boring MW-4R is probably an attribute of the hollow-stem auger ' drilling process and not due to the downward migration of TPHg and BTEX hydrocarbons, which tend to concentrate near the top of the water column Figure 3 shows the extent of dissolved TPHg in shallow groundwater at the site, and Figure 4 shows the distribution of soil encountered beneath the site As Figure 3 shows, the majority of dissolved hydrocarbons is located beneath the site in the area of the former USTs, and is generally delineated downgradient of the site, which is expected due to the historically shallow groundwater gradient Oxygenate compound DIPE, ETBE, MtBE, TAME, or TBA, or the lead scavengers 1,2-DCA or EDB were not detected in soil or groundwater during this investigation The results of the feasibility remedial tests indicate that groundwater extraction does not appear viable due to the low flow rates achieved in the test, but the addition of air sparging to the existing SVE operation is a feasible remedial option Figure 5 shows historical vapor concentration extraction trends from wells VW-2S and VW-3S The trends in both wells, as well as the overall system extraction trend, are declining As Figure 5 shows, during the air sparge test on April 8, 2003, vapor concentrations from wells VW-2S and VW-3S, represented by the larger symbols, increased at least ten-fold over the previous year's average concentrations Because the remaining hydrocarbons on-site appear to be in groundwater, the effectiveness of the existing SVE system will k1SACRFP 11COMMONIsacDPti20031Pg188018800391 A Sae doc