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CO Division of Minerals $ Geology <br />~~ SHB AGRA Project E92-7075 <br />~ ~ B ~ Page 2 <br />2. The mine pit is bacl~lled with a material having a hydraulic conductivity of 100 <br />fdday (4x10-' cm/sec). <br />3. The post-mining surface has a total azea of ponds of about 22 acres, with pond <br />bottoms about 50 feet below final ground surface. <br />The overall water balance was modeled to consider inflow to the mined azea from <br />groundwater recharge up gradient of the mine and discharge as a result of evaporation <br />from the ponds and ground water discharge down gradient of the mine. A net evaporative <br />loss value of 25 inches per year was used based on data provided by the Colorado <br />Climate Center. Over the 22 acre total water surface area, this relates to an equivalent <br />uniform discharge of about 33 gallons per minute. <br />The hydrologic condition of the reclaimed pit was evaluated by numerical simulation <br />using the "MODFLOW" model. For this analysis a one layer, variabie spaced grid with <br />35 rows and 24 columns covering an area of 0.5 square miles was used. The northern <br />grid boundary was set at a constant head (zero drawdown) and all other grids set as <br />active. The flow condition was treated as an unconfined aquifer. <br />From this simulation, a drawdown will occur at the northern end of the pit with the <br />maximum decrease in water surface elevation, relative to the static condition of 30 feet <br />at the pit boundary. The drawdown decreases to zero near the northern most pond area. <br />The water surface follows the pre-mining gradient to near the southern end of the pit. A <br />rise in gradient of from 10 to 24 feet is indicated near the southern end of the pit as a <br />result of the difference in hydraulic conductivity of the unmined strata as compared to <br />the bacl~lled mine pit. This predicted rise will bring the groundwater to within at most, <br />6 feet of the final ground surface. <br />If in the post-closure phase the groundwater rises above levels anticipated, contingency <br />measures would consist of developing drainage channels to divert the surface flows from <br />the south end of the mine area to existing natural drainage features. <br />Question #3: <br />On the vegetation map, is shallow grassland located between all of the ridges? <br />Answer: <br />Yes. It is the intent of the applicant to revegetate these areas with the shallow grassland <br />seed miz. <br />Question #4: <br />Calciphily is a rare plant occurrence on Limestone Ridge..... <br />~.~ A G R A <br />Earth & Emironmente/Group <br />