Laserfiche WebLink
NCCI Pit #1, Technical Revision No, 3 Adequacy Review Response, File No. M-2001-107 12/itU2006 <br />-2- <br />The cone of depression/radius of influence from the mining limits has been estimated using the <br />known pumping rate for the first phases of mining dewatering activities, the known depth of <br />dewatering from the normal groundwater level in those phases, an estimated radius of the pit <br />that the mining perimeter of those phases represents, and an estimated value of hydraulic <br />conductivity for the type of soils found in this aquifer. <br />The cone of depression/radius of influence was estimated for two conditions: <br />Full depth dewatering of the pit with no aquifer recharge and the following assumptions: <br />• A constant dewatering pumping rate = 2,500 gpm <br />• Dewatering depth = 27 ft <br />• Radius of the pit = 600 ft <br />• A hydraulic conductivity value of 1000 gpd/sq-ft was assumed for the soil types <br />found on this site. The inflow to the pit through the mining slopes was assumed <br />to be supplied solely by the groundwater present in the aquifer, hence the full <br />hydraulic conductivity value of 1000 gpd/sq-ft was used. <br />2. Full depth dewatering of the pit with an aquifer recharge of 1000 gpm and the following <br />assumptions: <br />• A constant dewatering pumping rate = 2,500 gpm <br />• A constant recharge rate = 1000 gpm through recharge ditches/ponds placed <br />around the perimeter of the pit <br />Dewatering depth = 27 ft <br />• Radius of the pit = 600 ft <br />A hydraulic conductivity value of 1000 gpd/sq-ft was assumed for the soil types <br />found on this site. The inflow to the pit through the mining slopes was assumed <br />to be supplied by both the groundwater present in the aquifer and by the <br />recharge water being added to the aquifer. The net affect is that the <br />groundwater being supplied by the aquifer (removed from the aquifer by the <br />dewatering activities) would be reduced by 1000 gpm or 40% of the total <br />dewatering pumping rate. This factor was applied to the hydraulic conductivity <br />value, which was reduced to 600 gpd/sq-ft for this condition, to compensate for <br />a lesser overall amount of groundwater being removed from the aquifer. <br />The result for condition 1 was the cone of depression/radius of influence extended beyond the <br />mining limit/pit wall approximately 710 feet with a total dewatering depth at the pit of 27 feet. <br />The result for condition 2 was the cone of depression/radius of influence extended beyond the <br />mining IimiUpit wall approximately 360 feet with a total dewatering depth at the pit of 27 feet. <br />These results should be an accurate estimation of what the cone of depression/radius of <br />influence will be at any time during the mining activities. The detailed calculations have been <br />attached for your reference. NCCI's dewatering pumping rate data has been attached for your <br />reference. <br />Through the well permit process for the pit it was determined that three groundwater users were <br />located within 600 feet of the property limits. Attached is the 600 foot spacing field inspection <br />report from Robert Stahl, Colorado Division of Water Resources Water Commissioner, Division <br />1, District 2, that details the wells he identified as being within 600 feet of NCCI's property. Of <br />the three wells identified, one has been abandoned, and NCCI has obtained 600 foot waivers <br />1400 W 122n° Avenue -Suite 120 • Westminster CO 80234 • Ph: 303-457-0735 • Fax: 303-920-0343 <br />