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Rifle Gravel Pit #1 June 2021 <br /> Coverage under a Nationwide Permit No. 7 will also be obtained for the outfall to the <br /> Colorado River. Scott also submitted a Watershed Permit Application to the City of Rifle to <br /> address the dewatering discharge within their Watershed Protection Area. <br /> 8.1.1 Dewatering Pumping Information <br /> The rate of groundwater discharge (pumping) associated with dewatering is expected to <br /> approach about 7 cfs. Mr. Bill Hahn (Hahn Water Resources, LLC) predicted the anticipated <br /> dewatering operations (through modeling) however this is an estimate based upon borings <br /> and groundwater level data. Mr. Hahn presented one possible dewatering configuration that <br /> might be employed for dewatering associated with the first stage (year) of mining. Figure <br /> 122 shows the estimated drawdown following one-year of dewatering using 3 wells in the <br /> vicinity of the Stage 1 mining limits. <br /> In this case, the dewatering might be accomplished using 3 wells plus temporary drains and <br /> pumps to capture incidental inflows, pumping at a combined rate of about 3 cfs. The number <br /> and size of the dewatering pump(s)will be determined once mining commences, however, <br /> Mr. Hahn assumed the wells would be drilled to a depth of 30 feet, constructed with nominal <br /> 8-inch to 12-inch diameter casing and screen, equipped with 10 horsepower pumps and <br /> motors to pump 400 to 700 gallons per minute (gpm) and 2 to 4 centrifugal ditch pumps <br /> capable of pumping up to several hundred gpm of incidental pit inflows. <br /> The dewatering permit terms and conditions will require that this information be documented <br /> in the application. Attachment C includes a copy of Mr. Hahn's reports. <br /> Water depletions will occur during mining operations from water lost with mined aggregate <br /> and water used for dust suppression. On-site sourced water will not be used for potable <br /> water demands. After mining is completed water depletions from the pit will be solely from <br /> evaporation of the exposed groundwater in the finished gravel pit pond. <br /> Augmentation of the exposed and captured groundwater is required per Colorado's Revised <br /> Statutes to protect downstream senior water right holders and will be addressed by Scott <br /> prior to subsurface mining. <br /> 8.2 Aquifers <br /> The groundwater aquifer/water table depth across the site varies from 3 feet to 8 feet below <br /> the surface. The aquifer is alluvial in nature and connected to the Colorado River. See <br /> Attachment B for information regarding the water depths at five boring locations. <br /> 8.3 Groundwater <br /> It is anticipated that the dewatering activities prior to mining will limit the exposure of <br /> groundwater and thus loss of groundwater to evaporation, during mining. The potential for <br /> this exposure of groundwater, however, will necessitate augmentation for the life of the <br /> operation. Augmentation—via a decreed augmentation plan or a substitute water supply <br /> plan (SWSP)—will be in place with the SEO prior to beginning subsurface mining activities <br /> to prevent potential injury to downstream water rights. The depletions from any captured or <br /> exposed groundwater from the Rifle Pit mining operations will have to be replaced in time, <br /> amount, and location during times of a downstream senior call. <br /> 34 <br />