2022-05-10_GENERAL DOCUMENTS - M1993041

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Dowe Flats Quarry Site Description and Future Mining Impact Analysis 2.3 Areas Subject to Flooding After the 2013 flood, The Colorado Water Conservation Board re-mapped the 100-year flood zone. This boundary has been adopted by the Boulder County Commissioners for use in regulatory decisions and is available online from the Boulder County website (Boulder County 2020b). The Dowe Flats property is not within the 100-year flood zone. No activities proposed in the permit amendment application will occur within the 100-year flood zone. 2.4 Lakes There are no lakes within the Dowe Flats permit area. No activities proposed in the permit amendment application will impact any lakes. 2.5 High Ground Water Areas The primary source of ground water in the Dowe Flats permit area is an unconsolidated shallow alluvial aquifer of Quaternary age that ranges between 12 and 30 feet thick. The well yields in this aquifer are classified by the Colorado Geological Survey(CGS)as being small to inadequate (less than 15_gallons per minute) (Hall et al. 1980). The Niobrara Formation and Pierre Shale, both Cretaceous-age sediment formations, underlie the unconsolidated alluvium. These formations are both classified by the CGS as aquifers with inadequate well yields of less than one gallon per minute. Previous mining through these aquifers has yielded little to no production of groundwater into the mine pit.No activities proposed in the permit amendment application will have new impacts on groundwater quantity or quality. 2.6 Topography Dowe Flats is in a triangular-shaped valley that resulted from erosion of less-resistant geologic strata in the interior of a plunging syncline. Ridges of more resistant beds surround the valley on three sides. The Hi-Cal Ridge that runs down the western edge of the current mine area transects the western half of the larger valley. In general,the valley floor has low topographic relief and slopes toward the south at an average 2%gradient. The highest point on the Dowe Flats property at the north end of the Hi-Cal Ridge is approximately 5,470 in elevation and the lowest elevation at the south end of the property near Highway 66 is 5,240 feet. The Dowe Flats property currently has more topographic relief than it did before mining or than it will when mining is complete. At the south end of the Hi-Cal Ridge, there is a large overburden stockpile where the crusher facilities currently sit. There is another large overburden stockpile to the southeast of the Hi-Cal Ridge,referred to as Mt. George on the site plan map. There are also three pits in the current mining area and several growth media stockpiles around the outer edges of the property. Based on the current reclamation plan, the overburden and growth media stockpiles will be redistributed to fill in the mining pits and entire property will be graded to tie into the existing undisturbed topography and gently slope toward the south except for a 20-acre depression on the northeast side that will be developed into a wetland. 2.7 Current Land Use The property included in the permit area of this application currently operates as a permitted mine site. The types of activities occurring within the Dowe Flats property include: • Stripping Habitat Management, Inc. 3 May 2, 2022