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(TA\SER LUJAN cL ASSO('IATE'S <br />J pydrogeological and Environmental Consultants <br />Modeling results indicate that existing mining conditions (based on the 2008 <br />aerial survey conducted by L.G. Everist) result in a maximum change of <br />approximately 2 feet in the water surface elevation compared to pre -mining <br />conditions for the 100 -year flow event. At cross-section 3193.73, which transects <br />the Bachofer property, there is 1.86 -foot change in water surface between these <br />topographic surfaces for the 100 -year flood event. <br />• The modeled surface water elevation for the 100 -year event using the 2008 <br />mining surface is 4875.81 and is well above the previously established 100 -year <br />flood plain elevation of 4874 determined by Weld County. <br />Model simulations were also conducted for lower flows that include the 5 and 10 - <br />year events. The 5 -year event (7,900 cfs) approximates flow at the Bachofer <br />property, which has been recently flooding the lower level of his house. The 5 - <br />year event water surface (4873.91 feet) for current mining conditions is very close <br />to the 100 -year permitted flood plain elevation of 4874 feet. The model results <br />show the 100 -year pre -mine water surface to be 4873.95. <br />The 10- year event (10,009 cfs) simulates peak flows that occurred during flood <br />events in September 2013 (10,300 cfs) and in the spring of May 2015 (10,500 <br />cfs). Flooding occurred in the Bachofer house (Bachofer, Oral Comm, 2015) <br />during both of these events and model results validate these events with water <br />surface elevations above 4874 feet. Simulations using pre -mining topography <br />show that the water surface at 4872.66, which is well below the established flood <br />plain level at the Bachofer property. This indicates flooding of the residence is a <br />result of alterations of the original floodplain morphology and topography caused <br />by mining and pit -lining activities. <br />Groundwater mounding has resulted from slurry wall construction, constriction or <br />restriction of subsurface flow, and has caused higher water levels than previous <br />and historic levels in the river alluvium upgradient of the slurry walls. As a result, <br />it has induced greater groundwater flow into the river. The results of the surface <br />water model should be considered conservative since they don't reflect the total <br />local discharge to the river from groundwater. <br />-14- <br />