2004-05-20_REVISION - M2000158 (2)

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Mr. Dennis Dubbel -3- May 14, 2004 estimated using the USGS Geohydrology of the Shallow Aquifers in the Fort Lupton-Gilcrest Area, Colorado hydrologic investigation atlas. The invert of the Western Mutual Ditch is about S feet above the regional groundwater table and does not appear to have a direct hydraulic connection to the regional groundwater levels. The construction of asoil-bentonite trench will function as a barrier [o groundwater flow. Except along the south part of Sharkey Lake, impacts to groundwater flow and groundwater elevations from constmction of the soil-bentonite trench should be minimal, because groundwater flow is generally parallel to the east side of the proposed soil-bentonite trench. The soil-bentonite trench along the soutkside of Sharkey Lake will create a barrier that is almost perpendicular to groundwater flow and we expect some groundwater mounding will occur along [he south side of Sharkey Lake. The groundwater elevation would need to rise at the southeast comer of Sharkey Lake to an elevation sufficient to create a gradient from east to west along the south side of Sharkey Lake. The amount of groundwater rise required to create flow from east to west cannot be predicted without additional analyses and modeling. However, based on an assumption that a gradient of 0.0021 fl/fl would be appropriate to redirect groundwater flow to the west, the groundwater elevation would need to rise about 4 feet at the southeast corner of Sharkey Lake. A rise of 4 feet would place the groundwater table about 0 to 2 feet below the existing ground surface. It is possible that the ground surface at the southern end of the stockpile area and south of Sharkey Lake could become unstable under heavy equivalent loads due to this slightly elevated groundwater table. Conclusions Based on this qualitative evaluation, we conclude the following: 1. Impacts to groundwater flow and groundwater elevations from construction of the soil- bentonite [reach around Longhorn and Sharkey Lakes should be minimal and be confined to property owned by Platte Sand and Gravel. 2. A minor raise (less than 4 feet) in groundwater levels should be expected along the south side of Sharkey Lake. The amount of groundwater raise will be highest at the east side and decrease toward the west. 3. Impacts to the groundwater elevations can be easily mitigated by construction of a ditch or drain system along the south side of Sharkey Lake. Recommendations Based on our evaluations, we recommend the following. A surface ditch be constructed along the south side of Sharkey Lake. The ditch should be located I S to 20 feet south of the soil- ben[onite trench and have a depth equal to the greater of 3 feet or 1 foot into the native sand and gravel deposit. The surface ditch should be located and designed to flow either west to the South Platte River or north along the ease side of the lakes and into the swale at the northeast side of Longhorn Lake. V vPf 193 Longhom anJ Shskey Lake~Od 19J Oi-0J-¢ 0eliminary Growdwarcr E.aluarions doc