Laserfiche WebLink
Ilydrageological and Environmental Convuhants <br />Conclusions <br />• Peak flows in the South Platte River corresponding to approximately 10,000 cfs at <br />the Fort Lupton station did not flood the Bachofer property prior to August 2004. <br />The L.G. Everist pit was lined in April 2004, and as of August 2004, the Bachofer <br />property started flooding and has continued to flood with flows of 6,440 cfs at the <br />Fort Lupton gauge. <br />• The 100 year flood plain elevation at the house that Mr. Bachofer currently owns at <br />7525 Highway 85was originally determined in 1987 to be at 4874 feet. The house <br />was surveyed to be at 4875 feet or 1 foot above the 100 year flood elevation. The <br />fact Weld County and Aurora Water would like to revise the flood plain elevation, <br />suggests that this level has risen with time. <br />• WWE monitoring program has not been conducted over a sufficient length of time to <br />adequately determine the true impact to the hydrologic system. The report <br />erroneously concludes that mining operations would have little or no negative effect <br />on surface water flows and groundwater levels in the South Platte River Alluvium. <br />• Modeling studies from the USGS (Arnold et al., 2010) show a mounding effect <br />where groundwater levels rise 2 to 4 feet on the north and west sides of lined pits <br />across the river from the Bachofer property. Arnold states in report, the South Platte <br />River acts a sink to groundwater when water levels are elevated in the alluvium. <br />Hence, mounding can cause increased groundwater discharge to the river, thus <br />contributing to higher base flows in the river. As such, a lined slurry wall will not <br />only impact groundwater levels but will also, affect surface water flows in the river. <br />• WWE's numerical model had the capability using the stream routing package in <br />MODFLOW to simulate changes to surface flow in the South Platte River. <br />However, using this package was not considered in the modeling scenarios and as a <br />result the impact to surface flows was not estimated. <br />• WWE wrongly concludes that their modeling results are conservative because <br />recharge to groundwater from Little Dry Creek and irrigation ditches was not <br />included in the model. While this statement is true for predicting groundwater <br />declines, it is an incorrect assumption for water level mounding. On the contrary, <br />the opposite is true in that the modeling results will underestimate the mounding <br />effect because there will be more water added to the system if recharge is included. <br />WWE's modeling study shows a rise in groundwater levels of 3 feet on the west side <br />of mining operation due to the slurry wall construction. Although this value is <br />similar to the one in Arnold's report (2 to 4 feet), WWE appears to ignore the impact <br />of these results on the South Platte River system. WWE says that the mounding <br />effect will be mitigated by Little Dry Creek, which will act like a drain allowing <br />higher groundwater levels to discharge into the creek. Similar to the Little Dry <br />Creek, the South Platte will also act like a drain as mounded groundwater will flow <br />18 <br />