WORK PLANS_FILE 1 - Laserfiche WebLink

Home Browse Search

Geological Tectmics Inc. Page 4 Soil Vapor Extraction Pilot Test Work Plan Project No. 770.2 September 17,2003 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 6, a similar situation is present. The contaminated soil column under the UST excavation/dispenser area appears to extend toward MW-5 @ 34 feet bgs. Well MW-8 was non-detect for the June 18, 2003 event and the groundwater plume does not appear to extend this far to the northwest based on this fifth consecutive 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 during the installation of boring MW-7. The soil concentration of 9,72,WA--'r m 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. In soil at 55 feet, 12,200 mg/kg TPH-D was detected, which attenuated 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-1 and MW-105. Figure 9 illustrates the shallow aquifer TPH-D concentrations for the June 18, 2003 event. V 3.0 SOIL VAPOR EXTRACTION TEST The site presents challenges for the selection of a clean up alternative. Assuming an approximate depth of 15 feet bgs for the bottom of the former UST excavation, a vertical column of 48 feet of impacted soil remains above the water table at 63 feet bgs. Soil layers of variable penneability are present to inhibit the vertical migration of diesel fuel and cause lateral migration or pooling of the contaminant. This case is demonstrated in the apparent NAPL present at 45 feet bgs in MW-7 where a fine to coarse-grained sand unit lies above a silt unit. Removing the contamination from this highly permeable zone would be relatively easy using soil vapor extraction (SVE) if the contaminant were gasoline. However, diesel fuel is much less volatile than gasoline and the only BTEX component present above laboratory detection limits is xylene. GTI proposes to perform a soil vapor extraction pilot test utilizing higher vacuum pressures (up to 26" Hg vacuum) than used with standard SVE. This is similar to dual phase