2013-10-04_GENERAL DOCUMENTS - M1999120

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(iA.Nisi- R I,t iJAIV h .%,ssO ci,Vrj.,.s 1 /rdrogeologiral and Environmental Consultants Wright Water Engineering Report, Response to Comments by DMG in Adequacy Review of Ft. Lupton Sand and Gravel Mine Apulication 0414999). January 3, 2005 The Wright Water Engineering, Inc. (WWE) report responds to the Division of Minerals and Geology (DMG)) comments related to the potential impacts of constructing a slurry wall around aggregate pits operated by L.G. Everist (LGE). DMG was concerned that emplacement of a slurry wall would affect surface water, groundwater, and nearby wetlands along the South Platte River near Fort Lupton Colorado. Response to WWE Comment #7 LGE constructed a slurry wall around their aggregate mining pits in April of 2004 and initiated a groundwater monitoring program to evaluate the impact to groundwater levels surrounding the pits. Because this report was published so soon after (January 2005) the slurry wall was completed, GL &A believes that the monitoring program had not been conducted over a sufficient length of time to adequately determine the true impact to the hydrologic system. The report erroneously concludes that mining operations would have little or no negative effect on surface water flows and groundwater levels in the South Platte River Alluvium. More recent modeling studies from the USES (Arnold et al., 2010) show a mounding effect where groundwater levels rise 2 to 4 feet on the north and west sides of lined pits across the river from the Bachofer property. Another modeling simulation shows groundwater levels rising 4 to 6 feet in the same area around the pits. Although the mounding occurs on the west side of the river the impact to the landowners located on the east side of the river cannot be ruled out. As stated in the U.S.G.S. report (Arnold, 2010), the South Platte acts a sink to groundwater when water levels are elevated in the alluvium. Hence, mounding can cause increased groundwater discharge to the river, thus contributing to higher base flows in the river. As such, a lined slurry wall will not only impact groundwater levels but will also, affect surface water flows in the river. Earlier modeling studies were also conducted by the USGS (Arnold et al, 2003) to determine the hydrologic effects of aggregate mining in hypothetical sand and gravel and fractured crystalline rock aquifers. Analytical solutions and numerical models were used predict the extent of steady -state drawdown and mounding due to aggregate mining below the water table. The results of the model simulations are very similar to the results determined by the USGS Report in 2010. That is, mounding occurs on the up- gradient side of lined pits and drawdown occurs on the down- gradient side. Response to WWE Comment #8 WWE conducted a cumulative assessment of a lined slurry wall by using a numerical groundwater model (USGS MODFLOW). The model was developed to predict groundwater changes subsequent to the installation of the slurry walls around LGE's pit cells. While the modeling simulations showed an increase of approximately 3 feet in 71