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_ ~S ~ <br />Adequacy Items <br />S}IB AGRA Project E92-7075 <br />Page l0 <br />TABLE A -Surface Water Runoff <br /> <br />Drainage <br />Basin Runoff From <br />2 Yeaz <br />Storm Event <br />(cfs) Runoff From <br />10 Year <br />Storm Event <br />(cfs) Runoff From <br />25 Yeaz <br />Storm Event <br />(cfs) Runoff From <br />50 Yeaz <br />Storm Event <br />(cfs) Runoff From <br />100 Year <br />Storm Event <br />(cfs) <br />A 205 342 417 488 547 <br />B1 112 179 220 251 282 <br />B2 609 1019 1237 1436 1620 <br />C 214 360 439 504 566 <br />D 432 709 856 997 1120 <br />2. Is the entire stream going to be mined? if so, how is it going to impact the <br />entire water balance of the downstream area? Please provide quantitative <br />data to show the water contribution to the overall hydrologic balance for both <br />surface and ground water so the Division can make a determination of the <br />impact to downstream water users. <br />• The area of the stream within the limits of the mining as indicated on Figure C-2 <br />of the original Permit Application will be mined. As part of the surface water <br />control plan, upland drainage will be diverted azound the active mining area. Note <br />that the mining will progress from south to north with reclamation following <br />mining. Accordingly, the impact to the drainage will be in segments with the <br />diverted water rejoining the reclaimed land. <br />The net impact on the hydrologic balance will be a reduction in surface flow to <br />the St. Vrain, a loss to evaporation as a result of collection in surface water <br />bodies in the pit. Water balance calculations for the Dowe Flats hydrologic basin <br />are presented in Attachment 12. From this analysis, the annual runoff from the <br />stream basin (approximately 394 acres) is about 336 ac. ft. and the annual <br />recharge to ground water is estimated at 9.5 ac.ft. The mining operation will <br />impact about 140 acres of the basin, intercepting an estimated 120 ac.ft. of <br />surface runoff. <br />• <br />3. Are there plans to expand the mining beyond the 4th ridge? If not, are there <br />plans to place upland diversions along the maximum extensions of the pit <br />area around the northeast corner of the 3rd ridge to avoid drainage from <br />entering the unnamed drainage channels oo the west side. <br />L^ A G R A <br />Eerth & Environments/Group <br />