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c= runoff coefficient;NRCS Soil Survey, <br /> shown in Appendix 1 <br /> Runoff ( i(A x c) i = rainfall intensity(inches/day); <br /> Volume 12 NOAA Atlas 14 Point Precipitation <br /> Frequency Estimate:Walsenburg,CO <br /> A=drainage area(acres);measured Map C-2 <br /> Runoff _ 4.6(27.0 x 0.75) <br /> ) = 7.8 acre-feet <br /> Volume 12 <br /> Since the required storage volume of 7.8 acre-feet is so much less than the available storage <br /> volume of 13.5 acre-feet, the dump sump area is more than adequate to contain stormwater <br /> runoff from the dump slope area. There is no threat of discharge from the dump area during a <br /> 100-yer 24-hour flood event, the anticipated worst-case scenario. <br /> The site will not affect existing water rights. The pit will not expose groundwater and will not <br /> store stormwater for more than 72 hours. After reclamation, stormwater encountered in the <br /> mining area will continue to infiltrate or evaporate. <br /> Two principal ways that gravel pit mining could affect the water quality of the area downstream <br /> are through poor sediment controls within the site causing increased sedimentation downstream <br /> and by fuel leakage from a ruptured tank. Water encountered in the pit will not be released from <br /> the site. The only water that will be found within the disturbed area is stormwater runoff and <br /> currently held Edmundson Ranch water that will be used for processing and for dust control. <br /> Once excavation of new phases begins, all water from the disturbed area will drain to the bermed <br /> active mine area or the processing area sump and will not leave the site. Each backfilled mined <br /> out phase will include a perimeter stormwater control berm that serves to isolate the mined phase <br /> with respect to stormwater. These phase specific perimeter berms will only be removed once that <br /> phase is topsoiled, seeded, and deemed reclaimed. Water that falls on the dump will be collected <br /> via each lift's catch bench and funneled to the eastern dump sump. Fuel leakage is an <br /> unanticipated problem because all fuel tanks on site have secondary containment as well as strict <br /> SPCC Plan procedures for spill prevention and control. <br /> Bradford Mesa Quarry ,, ,,a..,.,,,, ,.. ,,,�.r• e< <br /> January 2020 G-3 <br />