2007-11-20_REVISION - M1982015 (14)

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7.2 Groundwater Flow Regime and Contaminant Fate & Transport The wells were gauged for depth to water to determine groundwater elevations, and sampled on March 14, and April 21, 2007 for BTEX and TVPH analysis. The laboratory results are illustrated on Figures 6 and 7, respectively. The groundwater gauging and laboratory results are summarized on Tables 5 and 6, respectively. The laboratory reports for the sampling are included in Appendix 4. The slurry wall was completed in early March 2007. Based on previous groundwater elevation surveys as reported in the ICAP, groundwater flowed northerly from the St. Wain River towards Big Lake largely due to the fact that Big Lake was drained and the St. Wain River flow volume was high due to extensive snows over the winter. However, with the completion of the slurry wall and dewatering Sunset Lake, the hydrologic regime is more dynamic than before. Based on the March 14, 2007 elevations, groundwater flow is still generally northerly; however flow from the St. Wain River is obstructed by the slurry wall. Groundwater flow from the area of the release just north of the slurry wall is also northerly since the ponds are both drained. Elevated BTEX concentrations were detected in MW-18 which is near the location of the "sump" in the SHT that was excavated to the east of the pipeline release point. These contaminants are effectively trapped by the northerly groundwater flow traveling from the St. Wain River toward the slurry wall. It is expected that as flow declines through the summer and fall months, groundwater flow should reverse somewhat and begin to flow more downstream. There are 12 wells located to the north of the slurry wall. All of the wells contain elevated BTEX and TVPH concentrations. Since both the quarry ponds have been drained to the north of the slurry wall, groundwater is still flowing northerly toward the hydrostatic low. Due to the dewatering activities, contaminant transport is likely being accelerated and the plume to the north of the slurry wall is growing. If the ponds are refilled, the groundwater flow should reverse to the south and southeast depending on the respective pond elevations. This would tend to trap the contaminants against the slurry wall and possibly minimize growth of the plume. However, refilling of the quarry ponds is not anticipated to be performed in the next several months. 8.0 Air-Sparge and Soil Vapor Extraction Pilot Testing Activities On March 22, 2007, E-21 performed air-sparge (AS) and soil vapor extraction (SVE) pilot testing. Based on the hydrogeology at the site and E-21 personnel experience with successful AS / SVE remediation at nearby locations, it was deemed very likely that this remedial method would be very effective for the site conditions. The site is underlain by a silty and sandy clay to approximately 8 feet bgs and changes to a fine sand which grades into a sandy gravel. Claystone bedrock is present ranging from 18 to 24 feet bgs. On March 12, 2007, one, 2-inch diameter AS, and one, 2-inch diameter, SVE well were ' installed at the site. The wells were 19 and 12 feet deep, respectively. Depth to groundwater at the time of these well installations was approximately 14 feet bgs. The AS well was screened from 17 to 19 feet below the ground surface and the SVE well ' were screened from 5 to 12 feet bgs. The pilot testing wells were placed near the pipeline release area at select distances from surrounding monitoring wells for purposes of collecting sparging and vent pressure to determine radii of influence. 9