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Rifle Gravel Pit #1 January 2021 (Amendment to File No. M-2020-008) <br /> alternatives that avoid a minor and temporary filling of wetlands, which have been minimized <br /> to the extent possible by careful selection of the proposed pipeline route. <br /> An aquatic resources report and wetland delineation will be submitted to the ACOE in <br /> support of a nationwide permit application 39 (NWP 39) for Commercial and Institutional <br /> Developments. The NWP 39 application will encompass the initial pit development stages <br /> (Stages 1-3) and the dewatering pipeline. All impacts from the dewatering pipeline are <br /> temporary; in accordance with NWP 39 conditions, during installation in wetland areas <br /> heavy equipment will work on mats to minimize surface disturbance, and topsoil will be <br /> segregated and reapplied following pipeline installation. No changes are proposed to the <br /> existing hydrologic conditions on the Shideler's, therefore the wetland conditions will quickly <br /> re-establish in the pipeline route. The NWP 39 will be acquired prior to construction. <br /> Coverage under a Nationwide Permit No. 7 will also be obtained for the outfall to the <br /> Colorado River. SCI also submitted a Watershed Permit Application to the City of Rifle to <br /> address the dewatering discharge within their Watershed Protection Area. <br /> 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 10 <br /> shows the estimated drawdown following one-year of dewatering using 3 wells in the vicinity <br /> 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 /> 18 <br />