Laserfiche WebLink
<br />Page 2 of 3 <br />in the report. It can be inferred that the discontinuation of active pit dewatering raises local water table (relative to <br />that during mine operation) by eliminating drawdown. <br />The USGS report also discusses several other variables that may contribute to both observed and predicted groundwater <br />and surface water elevations. The following points are relevant to Mr. Bachofer's claim: <br />1. The water table hi the study area is highest in the summer and early fall as a result of increased aquifer recharge <br />from infiltration of irrigation water applied to fields, seepage from unlined ditches (e.g., the adjacent Platteville <br />Ditch), and greater precipitation (Ref. p. 12; USGS 2010). The report notes that seepage losses from agricultural <br />ditches are likely substantial and that aquifer outflows to the South Platte in the study area during irrigation <br />season can be 35 - 243 cfs (Ref. pp. 23 - 24, Figure 17C; USGS 2010). Groundwater-level rises during the <br />irrigation season measured during various years from 1955 to 1991 ranged from 0 to 9 feet with a mean of about 3 <br />feet (Ref. p. 18; USGS 2010). The location of Bachofer property places it downgradient of the Platteville Ditch <br />and therefore subject to higher water tables during the early summer when flows in the South Platte are higher as <br />a result of snowmelt runoff in the mountains. <br />2. Unlined pits tend to lower the water table due primarily to evaporation of open water (Ref. p. 1; USGS 2010). <br />The Deep Lake pit is currently unlined. This pit is adjacent to the South Platte and updgradient of the lined Ft <br />Lupton East pit. The Deep Lake pit may mitigate some of the potential groundwater mounding in the immediate <br />vicinity. <br />3. Increased urbanization in the USGS study area between 1957 and 2000, as well as the Denver metro area has <br />resulted in increased runoff contributing to the South Platte. (Ref. Figures 17A, 17B and 17C; USGS 2010). <br />Urbanization may be a significant factor resulting in the increased flooding of the Bachofer property. <br />4. The report makes a point in the introduction that isolating the hydrologic effects of individual pits is difficult due <br />to the number and complex interactions of pits (Ref. p. 2; USGS 2010). <br />Additional Considerations <br />The South Platte is a constantly changing river. The same conditions that make it a viable resource for sand and gravel <br />extraction cause these fine and coarse sediments to be deposited and remobilized frequently with changing flows. Aerial <br />photos (Ref. Mapquest, 2010) (See Exhibit 1) show the presence of large sand bars visible in aerial photos downstream of <br />property. Sand bars can reduce previously available cross-sectional flow area and restrict river flow causing higher water <br />elevations immediately upstream. Some of the experienced flooding may be directly attributable to the presence of these <br />sand bars. <br />Conclusions <br />There are many factors having the potential to increase the flooding frequency of Mr. Bachofer's property. Mr. Bachofer <br />claims the flooding is a direct consequence of groundwater mounding upstream of slurry walls constructed for Everist <br />Fort Lupton Mine pit reclamation. This suggests the premise that the flooding his property experiences is either a result of <br />the increased groundwater elevation causing higher flows in the South Platte - enough to cause flooding, or groundwater <br />is mounded on the east bank of the Platte to the extent that active seeps are created and flood the property and/or the <br />groundwater is artesian as a result of the mounding. Based on the USGS model results, neither scenario is very likely. <br />The relatively short distance along the west bank of the South Platte (approximately 1,000 feet) that may experience 2 to 4 <br />feet of mounding (Ref. Figure 34; USGS 2010) would have to contribute a significant flow (relative to upstream aquifer <br />contributions) to result in any significant water level increase. As to mounding of groundwater on the east bank of the <br />South Platte River, the model results show no mounding on the east side of the river (Ref. Figure 34; USGS 2010).