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The mined coal seams are not significant aquifers since there are <br />no known wells drawing water from coals in the area, and coal <br />ground water in not a significant component in the regional <br />hydrology. <br />Nevertheless, the following analysis was performed to estimate the <br />drawdown impacts of coal mining within the coal seams to be mined. <br />It is difficult to estimate accurately the drawdown impacts of coal <br />mining within the seams because the flows are small and sensitive <br />to assumptions, the effects of local features such as fractures may <br />out weigh distributed flow, and leakage from enveloping strata <br />cannot be readily quantified. Coals have low transmissivity <br />(estimated to be on the order of 30 sq. ft./day, or 224 <br />gal/ft/day). Mine inflows from the coal seams are very small, less <br />than 100 gallons per minute in either seam excluding local flows <br />associated with fractures (which draw ground water from strata <br />above and below the seams). Enveloping siltstones leak water to <br />the coal seams as the latter are dewatered, diminishing the <br />spreading of drawdown in the seams. <br />Gross aquifer impacts are also difficult to estimate for local <br />coals because there is no potentiometric basis. There are no <br />monitoring wells screened in the mined seams to determine drawdown <br />outside mined areas. There is typically a sharp discontinuity in <br />potentiometric head outside a mined panel, the destressed zone <br />being dewatered and unsaturated, so that computing drawdown as if <br />due to a very large well is not very practical. <br />McWhorter (1981) proposed empirical methods for computing flow into <br />coal faces, and distance/drawdown relations for leaky coal seams, <br />from sparse data. McWhorter gave the following equation relating <br />drawdown sx at distance x from a mined face where the drawdown at <br />the face is so and where ~ is the leakage factor": <br />s <br />sX <br />i <br />()=(T~. Ka) 2 <br />a <br />T~ is the coal transmissivity, and b and KB are thickness and <br />hydraulic conductivity for the leaky s~rata or "aquitard". While <br />it is difficult to determine these parameters, the analysis was <br />performed for a leakance range (represented by the envelope strata <br />hydraulic conductivity) which is thought to represent bounding <br />conditions. <br />For coal transmissivity, T~, equal to 30 ft2/day (from page 204.7- <br />19), so equal to 700 feet (initial head is approximately 700 feet <br />