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• Quality Effects of Post-Minin¢ see page on Williams Fork Alluvial Water After the mines refill, water may move out of the coal subcrop into the William Fork Alluvium. While there is nc indication that the flooded mine workings will be under high heads in the subcrop areas, significant heads might occur. Recent excavation of the E Seam area near the SA portal indicates that this area of the mine is not naturally under any hydrostatic head. Therefore, it is expected that the maximum possible head on the No. 5 and No. 6 Mines in the subcrop areas will be approximately 100 feet above ground surface. The mine is presently being dewa[ered at rate of 500 gpm (2000 AIIIt). The rock slopes [o the No. 6 Mine are contributing only a small portion of this inflow (see paee 2.05-42). This inflow rate will decrease as the mine floods. Inflow decreases with the decrease in the head difference in the flooded mine working relative to the premining head. Therefore post-mining discharge will be significantly less than the discharge rates during mining. Since the portalwill be bulkheaded the discharge will be through the coal seam at the subcrop. Possible seepage rates from the seams into the subcrop area were calculated assuming that the water flowed updip from the nearest mine working in each mine to the appropriap subcrop area. The nearest workings for the No. 5 Mine area are 800 feet from the subcrop and for the No. 6 Mine area 1,200 feet away. Using Darcy's Law, a permeability of 2.5 feet per day, aquifer thickness of 12 feet and subcrop width of 1,000 feet produced seepage rates for the E and F Seam subcrops of 19.5 gpm (0.043 cfs) and 13.0 gpm (0.044 cfs), respectively To check if this is a reasonable estimate, a worst case would be if [he portal was not bulkheaded and all inflow could freely exit the mine. As a rough estimate using Darcy's Law (flow rate =cross-sectional area times permeability times gradient) the inflow would be inversely proportional to the piezometric head (gradient = Flowpath divided by head). The average 2000 inflow rate was 500 gpm with an average pumping level of approximately 5770 feet elevatron. The estimated premining piezometric head at the No. 5-Mine portal is 6300 feet. This yields a drawdown (head) of 530 feet. After the mine floods the head at the 6160 elevation portal Hould be approximately ]40 feet. • Since the mine is a very large well the inflow/outflow over a long time period will be approximately proportional to the head. This would yield aworst-case discharge of 130 gpm if there were no bulkhead. Since the portal will be bulkheaded, the flow path will be longer, the effective permeability lower due to the water having to discharge through [he subcrop, the resultant discharge would be substantially less than the free draining rate. Therefore the above estimate of discharge from the subcrop of approximately 31.5 gpm is not likely to be an underestimate of actual discharage. The potential seepage from the coal seams would affect the water quality in the alluvium downstream of the subcrop area. There is approximately 5.5 million square feet of alluvium in that area. Using a recharge rate of threa(3) inches per year, approximately 0.044 cfs of fresh water (assumed to be the same quality as the existing alluvial water) is added on an annual basis. This value is based upon a lysimeter study performed at the Seneca Mine (Bob Williams, USGS, pers. Communication). In addition, the William Fork River water recharges the alluvium in the spring. There is an average water level rise in the alluvium each year of approximately two (2) feet. Assuming a specific yield of 10 percent, this converts to approximately 0.035 cfs of water on an annualized basis. The net impact of seepages can be estimated using the mass balance technique, as shown in this permit. The estimated net effect will be an increase in the SAR from 3.6 to 9.4. This is a worst-case calculation and will only occur after the mine is refilled. Since the bottom areas in the vicinity of the mine that are used for agricultural purposed are flood irrigated, the fields would be irrigated with ]ow SAR water. Since the alluvial water is naturally higher in metals and dissolved solids than the mine discharge water, no other impacts on the water quality are expected. Development mining beneath Big Bottom is not antiupated to impact the AVF as there is not subsidence predicted and water inflow discharge wit] be handled utilizing the existing system and outfalls. n -r(~ 33 Midterm Response 2.05-46 ~P~®,~~ f•I;~ 1 1 5 2~~~ 7/30/01