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Mr. Dennis Dubbel -3- May 14, 2004 <br />estimated using the USGS Geohydrology of the Shallow Aquifers in the Fort Lupton-Gilcrest <br />Area, Colorado hydrologic investigation atlas. The invert of the Western Mutual Ditch is about <br />15 feet above the regional groundwater table and does not appear to have a direct hydraulic <br />connection to the regional groundwater levels. <br />The construction of asoil-bentonite trench will function as a barrier to groundwater flow. <br />Except along the south part of Sharkey Lake, impacts to groundwater flow and groundwater <br />elevations from construction of the soil-bentonite trench should be minimal, because <br />groundwater flow is generally parallel to the east side of the proposed soil-bentonite trench. The <br />soil-bentonite trench along the soul): side of Sharkey Lake will create a barrier that is almost <br />perpendicular to groundwater flow and we expect some groundwater mounding will occur along <br />the south side of Sharkey Lake. The groundwater elevation would need to rise at the southeast <br />comer of Sharkey Lake to an elevation sufficient to create a gradient from east to west along the <br />south side of Sharkey Lake. The amount of groundwater rise required to create flow from east to <br />west cannot be predicted without additional analyses and modeling. However, based on an <br />assumption that a gradient of 0.0021 ft/ft would be appropriate to redirect groundwater flow to <br />the west, the groundwater elevation would need to rise about 4 feet at the southeast corner of <br />Sharkey Lake. A rise of 4 feet would place the goundwater table about 0 to 2 feet below the <br />existing ground surface. It is possible that the ground surface at the southern end of the stockpile <br />area and south of Sharkey Lake could become unstable under heavy equivalent loads due to this <br />slightly elevated groundwater table. <br />Conclusions <br />Based on this qualitative evaluation, we conclude the following: <br />Impacts to groundwater flow and groundwater elevations Crom construction of the soil- <br />bentonite trench around Longhorn and Sharkey Lakes should be minimal and be confined <br />to property owned by Platte Sand and Gravel. <br />2. A minor raise (less than 4 feet) in goundwater levels should be expected along the south <br />side of Sharkey Lake. The amount of groundwater raise will be highest at the east side <br />and decrease toward the west. <br />3. Impacts to the groundwater elevations can be easily mitigated by construction of a ditch <br />or drain system along the south side of Sharkey Lake. <br />Recommendations <br />Based on our evaluations, we recommend the following. A surface ditch be constructed along <br />the south side of Shazkey Lake. The ditch should be located 15 to 20 feet south of the soil- <br />bentonite trench and have a depth equal to the greater of 3 feek or 1 foot into the native sand and <br />gravel deposit. The surface ditch should be located and designed to flow either west to the South <br />Platte River or north along the ease side of the lakes and into the Swale at the northeast side of <br />Longhom Lake. <br />V:~0~193 Lonyhom vM Sharhq La4~09193 6t-05-13 Nelimuury Grouodwaier Evahwiom.AOc <br />