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RULE 2 PERMITS <br />acre ft. At the estimated pre -mining groundwater flow rate through the pit area of 50 ac-ft/yr, this would <br />require about 130 years to re -saturate assuming no flow to the north out of the pit backfill. <br />The infiltration rate into the mine backfill may be higher than under pre -mining conditions because of the <br />substitution of the highly stratified pre -mine bedrock aquifers with the homogenous backfill aquifer. The <br />pre -mining groundwater recharge rate from infiltration in the Collom area is estimated to range from <br />about 0.11 in/yr in the southern portion of the area to about 1.1 in/yr in the northern areas where bedrock <br />units outcrop (WMC, 2006). The backfill area is expected to cover about 825 acres. If infiltration into the <br />backfill increases to 3 in/yr (about 20% of precipitation) then an additional amount of groundwater <br />recharge will be available to saturate the pit backfill. Under this condition, it is estimated that the total <br />amount of recharge to groundwater would be about 230 ac -ft per year and the time to re -saturate the <br />backfill would decrease to about 30 years, again assuming no outflow of groundwater to the north. <br />Groundwater will flow down -dip in the bedrock units to the north from the pit backfill as the backfill re - <br />saturates. If it is assumed that the flow rate out of the backfill at the north pit wall is equal to the pre - <br />mining flow rate at this location, then there will be an annual average groundwater flow of about 50 ac -ft <br />per year. At the higher groundwater recharge rate into the backfill of about 230 ac-ft/yr as described <br />above, this would result in a time to re -saturate of about 40 yrs. Lower infiltration rates into the backfill <br />would increase the time to re -saturate the backfill. The estimated range of times to re -saturate the backfill <br />up to the 7150 ft elevation varies from about 30 to 130 years. <br />Potential for development of springs from pit backfill <br />If the saturated thickness of the backfilled area of the pit increases as described above, then the <br />groundwater flow rate to the north potentially will be higher than the natural groundwater flow rate <br />because of the higher hydraulic head. This may result in a groundwater elevation in the highwall area of <br />the pit backfill that is higher than the pre -mining groundwater level elevation of about 7150 ft. <br />Little Collom Gulch intersects the north wall of the pit at about elevation 7300ft. If the water level in the <br />backfill increases to the 7300 ft elevation, then a spring could develop in Little Collom Gulch where it <br />intersects the pit highwall. An evaluation of the time that would be needed to re -saturate the backfill to <br />the elevation and the potential spring flow quantity is made based on the information in WMC (2005, <br />2006) and the information presented above. <br />The time re -saturate the backfill up to the 7300 ft elevation will largely depend on the infiltration rate into <br />the backfill. It is expected to be about 40 years for the maximum infiltration rate of 3 in/yr into the <br />backfill considered above. <br />The likelihood of a spoil spring developing is considered to be low. Based on the estimates described <br />above, an infiltration rate of less than about 2.5 in/yr into the backfill would not result in a saturation level <br />in the backfill high enough to form a spring. It is unlikely that the effective infiltration rate will be greater <br />than 2.5 in/yr. It is more likely to be in the range of 1 to 1.5 in/yr, which is similar to the value of 1.1 <br />in/yr estimated for the upper portion of the watershed in the regional groundwater model (WMC, 2006). <br />If a spring develops at this location, the flow will likely re -infiltrate into the valley fill in Little Collom <br />Gulch and not flow down the stream channel as a surface flow. There is a significant thickness of <br />unsaturated valley fill in lower portion of Little Collom Gulch. The water level in well MLC -04-01 near <br />the mouth of Little Collom Gulch is at 46 ft below ground surface. Therefore, it is unlikely that a spoil <br />spring would result in surface water flow down Little Collom Gulch. <br />Collom — Rule 2, Page 121 Revision Date: 1/6/17 <br />Revision No.: TR -109 <br />