2007-11-20_REVISION - M1982015 (13)

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Bayshore Pipeline Release - Spill Incident No.2006-1029 I August 2007 Groundwater Monitoring and Remediation Progress Report Page 2 of 5 ' The soil cuttings from all the new well installations were containerized in a roll-off for future waste profiling and disposal. The new monitoring wells installed on August 10, 2007 were developed the same day. Monitoring wells MW-29 was developed on August 20, 2007. MW-28 was dry. The well ' development water was containerized in a 275 gallon plastic water tank for future collection and disposal and a licensed waste water treatment facility. ' Groundwater Monitoring Results and Hvdrogeologic Evaluation On August 11, 2007, E-21 personnel gauged and sampled 25 monitoring wells following CDPHE guidelines. Measurements for depth to water, dissolved oxygen, pH, temperature, conductivity and oxidation-reduction potential were recorded. The samples were collected using new disposable PVC bailers and nylon twine. ' On August 11, 2007, water levels were also measured and water samples were collected from both the St. Wain River staff gauges, and a water sample was collected from a pool of water near the staff gauge in Quarry Pond #1. Water table elevations were not surveyed in the ponds, but the water level ' is approximately 7 to 8 feet lower than earlier in the year due to construction dewatering. The staff gauge in Quarry Pond #1 (Sunset Lake) is no longer near the water line in the pond, and the gauge in the southwest corner of Quarry Pond #2 (Big Lake) was removed during earthwork. The water table ' elevations are summarized on Table 2. The groundwater contours for wells on the north and south side of the slurry wall are illustrated on Figures 2A and 2B, respectively. Please note that the water lines prior to pond dewatering are shown ' on these figures. They do not represent the current water table elevation but are being included on the maps for comparison to the current water table elevations. The final ground surface elevation data was not available during completion of this report but will be obtained and shown on figures for ' future reports. There is approximately an 8 foot difference in the water table elevation between the north and south ' side of the slurry wall further confirming the slurry wall has bisected the water table between the ponds and the St. Wain River. Groundwater flow direction on the north and south side of the slurry wall is both generally to the east-northeast and parallel to the slurry wall. The groundwater gradient on the north side of the slurry wall ranges from 0.0011 ft/ft using MW-15 to MW-29, to 0.0029 ft/ft using data from MW-4R to MW-29. The groundwater gradient on the south side of the slurry wall ranges from 0.0012 ft/ft using MW-9R to MW-23 to 0.0018 using data from MW- 21 to MW-23. Thus, the gradients on both sides of the slurry wall are comparable and show an increase in gradient toward the eastern end of the spill area. ' The site is underlain by impermeable shale bedrock ranging from 5 to 22 feet below the ground surface. The bottom of the quarry ponds roughly coincides with the bedrock surface. Although a bedrock contour map has not been produced, it is likely the bedrock generally dips gently to the south and east directing the remaining water on the north side of the slurry wall to the south until it ' encounters the slurry and begins to flow eastward. This aquifer is no longer continuous to the north and since the ponds have been dewatered, groundwater to the north of the slurry wall does not likely extend very far beyond the new wells MW-24, MW-25 and MW-26.