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Gea[ogicat rec&nksInc, Page 6 Soil&Groundwater Investigation Report Marci Property Project No. 701,2 July 24,2002 In Figure 4, a column of soil contamination is present under the former UST excavation/dispenser area. In the vicinity of MW-6/MW-106, no soil contamination was detected and Win. Hunter & Associates stated no contamination was present in MW-3. This �- suggests that the contamination migrated downward to impact the groundwater without less permeable soil layers causing lateral dispersion higher in the column. Well MW-6 has contained detectable groundwater contamination but no soil impacts in the overlying or underlying soil. This implies that there is a fluctuating water table smear zone radiating out from the release area. In Figure 5, a similar situation is present. The contaminated soil column under the UST 'r excavation/dispenser area appears to extend toward MW-5 @ 34 feet bgs. Well MW-8 was non-detect for the June 7 event and the groundwater plume does not appear to extend this far to the northwest based on this one sampling event. The groundwater plume appears to extend to the location of MW-104 where soil contamination (1300 mg/kg) was detected at the top of the historical groundwater table range. Soils below this depth were clean and MW-104 is non-detect with a screened interval approximately 29 feet below the water table. The field geologist noted a strong HC odor at 40 feet bgs and a free product interface at 45.5 feet bgs in boring MW-7. The soil concentration of 9720 mg/kg TPH-D @ 45 feet bgs does correlate with the free product noted at this depth, It is likely that the geologist had encountered non-aqueous phase liquids (NAPLs) entrained in the silty soil. 12200 mg/kg TPH-D was detected in soil at 55 feet, then attenuating to 331 mg/kg at 70 feet bgs. Once reaching the groundwater table the diesel contamination spread laterally with the rise and fall of the groundwater table. It has attenuated with travel below the water table to at least the depth of the screened intervals of MW-I and MW-105. 5.2 Recommendations Based on the conclusions outlined above, Geological Technics Inc. recommends the following: • Complete four quarters of groundwater monitoring/sampling to determine contaminant and groundwater trends in the deep screened monitoring wells MW- 104, MW-105 & MW-106. .� 0 The presence of NAPL in boring MW-7 suggests that remediation by natural attenuation will not be sufficient to address residual contamination. A feasibility study should be performed to evaluate alternatives to treat this source of further ,., contaminant leaching to groundwater. • After the groundwater monitoring is complete, evaluate whether or not further investigative work is needed to define adequately the extent of contamination. If so, �. develop a worm plan to fill data gaps between areas of known contamination. V u.. ■.w