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Memo to Brandon Heser, Grant Smith,Bruce Humphries <br /> Page 4 <br /> January 5,2017 <br /> conservative assumption. As shown in Table 2,mounding is projected to range from 11 to 16 feet on the <br /> edge of the proposed pond.At a distance of 340 feet,representative of the distance from the proposed pond <br /> to the remediation wells to the south,mounding is estimated to range from 8 to 10.8 feet.We note that this <br /> calculation was completed based on the alluvial aquifer as a whole and we did not differentiate water level <br /> changes expected for the upper and lower sands individually. <br /> The magnitude of projected mounding is in line with the 7 to 20 feet maximum mounding that can <br /> physically occur at the proposed pond location, indicating that the pond may effectively saturate the <br /> unsaturated material immediately beneath the pond. Ground water mounding will be less than 7 to 20 feet <br /> at distances from the proposed pond.The saturation of this material is not a concern,but is a detail that we <br /> note to comprehensively describe the expected ground water flow dynamics that may result at the location <br /> of the proposed pond. <br /> Comparison of Calculated Mounding with Measured Mounding at Existing Pond <br /> Schlage and their consultants maintain a ground water monitoring program that includes water level <br /> measurements in various monitoring wells,including wells located adjacent to the existing settling pond. <br /> These monitoring data provide a reality check against the calculated water level changes because the <br /> functions of the existing and new settling ponds will be identical,the water delivery rate to the new pond <br /> will not change, and ground water mounding at the new pond is expected to be similar to historical <br /> mounding that has occurred at the existing pond. The monitoring data from wells located near the existing <br /> settling pond are presented in Figure 2. The existing pond was constructed in approximately 2010. Water <br /> level trends between 2010 and the present are not obvious but the data appear to show some wells increasing <br /> in water level by approximately 7-8 feet over the lifetime of the existing pond at distances ranging from <br /> 100 to 1100 feet from the pond. These monitoring data suggest that the actual mounding at the proposed <br /> pond will be less than the calculated mounding ranging from 8 to 16 feet.The monitoring data confirm that <br /> some ground water mounding has occurred in the past and will occur in the future,which is expected given <br /> the ground water infiltration that will occur at the pond. <br /> Schlage Ground Water Issues and Contaminant Plume <br /> Geosyntec, consultants for Schlage, provided ground water contour maps for both upper and lower sand <br /> layers showing the ground water gradient,PCE contamination concentrations and monitoring and extraction <br /> well locations. These maps,included as Attachments 2A and 2B with annotations by BBA, indicate that <br /> the proposed settling pond is located on the northwestern boundary of the ground water contamination <br /> plume. Based on the relative locations of the plume and the proposed pond,we conclude that any ground <br /> water mounding created by the proposed settling pond will increase the ground water gradient within the <br /> plume towards the extraction wells and the mapped capture zone.The increased ground water gradient will <br /> cause more ground water to flow towards the extraction wells,thereby accelerating the travel time of the <br /> contaminants to the extraction wells. Again, based on the relative location of the expected ground water <br /> mound,we do not see any potential for the ground water mound to cause the plume to flow away from the <br /> extraction wells and capture zone. <br /> Water Ouality of Infiltration Water <br /> It is important to note that the water that will be delivered to the settling pond is sourced from water supply <br /> wells completed in the same alluvial aquifer. Although the water will have interacted with wash fines and <br /> sand and gravel, we do not expect the chemical composition of the water in the pond to be significantly <br /> different than the water present in the alluvial aquifer beneath the pond,other than it will not contain high <br /> PCE concentrations,similar to what exists in the alluvial aquifer beneath the proposed settling pond. <br />