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Memo to Brandon Heser,Grant Smith,Bruce Humphries <br /> Page 3 <br /> January 5,2017 <br /> should be taken by the embankment construction contractor to not intercept ground water and to not <br /> penetrate the layer of fine grained material. We suggest clarifying these concerns in the specifications for <br /> the construction contract. <br /> Estimates of Water Level Changes <br /> In order to estimate water level changes,or mounding,that may result from the proposed settling pond,we <br /> reviewed available information pertaining to the local ground water system. These informational sources <br /> included the following: <br /> 1. U.S. Geological Survey Water-Resources Investigations Report 94-4129, Hydrogeologic <br /> characteristics of the alluvial aquifer and adjacent deposits of the Fountain Creek valley, El Paso <br /> County, Colorado <br /> 2. Borehole data presented in the September 2016 Brierley Report titled Design Report-Daniels Sand <br /> Wash Fines Pit(Brierley Report) <br /> 3. Well records from the State Engineer's Office <br /> 4. Summaries of ground water data provided by Geosyntec,including: <br /> a. Aquifer transmissivity values <br /> b. Aquifer water level data <br /> c. Mapping of PCE contaminant plume concentrations <br /> d. Geologic cross sections <br /> Mounding will occur through downward infiltration from the settling pond into the unsaturated material <br /> that exists beneath the pond. If the hydraulic properties of the subsurface material are such that the <br /> infiltration water can drain away from the pond at a rate greater than the rate of infiltration, then the <br /> subsurface material will remain partially unsaturated.However,if the rate of infiltration exceeds the rate of <br /> aquifer drainage,then the subsurface material immediately beneath the pond may become fully saturated. <br /> It is important to note that because of the limited amount of unsaturated material beneath the proposed pond <br /> location,the amount of mounding that can occur at the proposed pond location is limited by the thickness <br /> of the unsaturated material. Mounding cannot exceed 7 to 20 feet because that is the thickness of the <br /> unsaturated material beneath the proposed pond. As referenced above, we believe the actual limit of <br /> unsaturated material,and therefore potential mounding,to be closer to 7 feet in most locations and not 20 <br /> feet. <br /> We completed analytical calculations using a multi-well spreadsheet superposition model based on the <br /> Theis equation to estimate the amount of water level mounding that may occur beneath the proposed settling <br /> pond. The multi-well spreadsheet model simulates ground water mounding resulting from nine locations <br /> distributed across the proposed pond location.This approach represents the distribution of the aquifer stress <br /> and resulting water level change across the footprint of the proposed pond,and allows for estimates of water <br /> level changes at distances from the proposed pond. The water level changes resulting from each simulated <br /> point are superimposed to estimate water level changes at various distances from the proposed pond. <br /> Aquifer characteristics, assumed pumping amounts,and the resulting water level changes(mounding)are <br /> summarized in Table 2.Note that the amount of water pumped to the proposed pond was estimated based <br /> on the total pumping from the alluvial water supply wells in 2016,not including the well that is used for <br /> dust suppression.Not all of this water is delivered to the settling pond. Some of it is lost and some is reused. <br /> Therefore, the assumption that the water delivered to the pond is equal to well pumping is a very <br />