Laserfiche WebLink
Adequacy Items <br />SHB AGRA Project E92-7075 <br />_ ~~~ Page 14 <br />• ~- Bi <br />It is the understanding of the Division that there will be a lot of blasting taking place <br />during the mining operation. It is also true that the Niobrara formation is not <br />impervious. What about fractures that would likely be caused by the blast? Are <br />there any plans to dewater the pit? Please provide a detailed ground water modeling <br />to show that the operation will not impact the deep aquifers so the Division can <br />come up with the same conclusions you did. <br />Currently accepted theories on blast induced fractures identify two mechanisms for rock <br />breakage. The first is formation of radial cracks around the blast hole as a result of blast <br />hole expansion and the effects of the compressional shock wave. These fractures aze of <br />limited extent, generally measured in terms of several blast hole diameters. The second <br />and more significant is flexural breakage of rock as it deflects to a free face. Due to the <br />confinement of rock with depth below the pit floor, only the first mechanism will cause <br />fracturing of underlying rock. The limited extent of the fracturing azound the blast holes <br />will not increase the fracturing of the Niobrara Formation with depth to the extent that <br />would affect hydraulic conductivity. <br />The pit will be dewatered as is currently being done at the Lyons Quarry by pumping <br />from open sumps. <br />• g. Has the operator done a water mass balance to conclude as stated in page S that <br />recharge to the unconsolidated valley fill at Dowe Flats is principally from snow melt <br />and rain fall. If not, please provide that data to support your conclusions. <br />Calculations for the water balance of the basin aze presented in Attachment 11. As <br />presented the water balance shows a net deficit of about 120 acre-feet per yeaz. This <br />amount is most likely provided by infiltration from water supply canals crossing the site. <br />Recent monitoring of surface water and shallow groundwater near the St. Vrain Supply <br />Canal has documented the relationship between canal flow, surface water presence, and <br />groundwater levels. <br />b. Why was only one well pulled on the up gradient (D11VIW-1) and three in the down <br />gradient portion of the permit application? Are the tables in G9 the average for <br />the four wells? Please provide in a detailed explanation how the numbers were <br />divided. <br />One upgradient groundwater monitoring well (DFMW-1) was installed to provide <br />background groundwater quality data. Three downgradient wells were installed to be <br />able to compare any potential water quality impacts resulting from mining to the <br />background data of three different downgradient locations. <br />The tables in G-9 present the highest recorded value for each monitoring well. <br />• Attachment 15 contains the complete groundwater quality monitoring data. Please use <br />the tables in Attachment 15 for comparison to drinking water standards. <br />L^ A G R A <br />Earth & Environments/Group <br />