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elevation of the water surface in the river. The hydraulic gradient of <br />the groundwater in the backfilled channel is taken to be equal to the <br />slope of the original channel. The discharge through the backfilled <br />channel is given by Darcy's Law as: <br />Qb -AK i <br />wherein: <br />Qb = Discharge of groundwater through backfilled channel. <br />A = Cross sectional area of wetted portion of backfill. <br />K = Hydraulic conductivity of backfill. <br />i = Hydraulic gradient (slope or original channel). <br />The hydraulic conductivity of the coal refuse material was <br />measured in the laboratory using a falling head permeameter. Material <br />was packed in the permeameter to a dry bulk density of 1.33 g/cm3 (83 <br />pcf), thoroughly wetted, and subjected to a falling head test. The <br />hydraulic conductivity was determined to be 1.25 x 10-4 cm/s or 4.1 x l0- <br />6 ft/s. <br />• The cross sectional area of the wetted portion of the backfill was <br />estimated from stream depth data collected in conjunction with stream <br />gaging measurements. These data were collected somewhat upstream of the <br />reach to be backfilled but are believed to be representative of the reach <br />of interest, also. Capillary action will cause the backfill to be wetted <br />to a height above the measured water levels. The height of the capillary <br />fringe is now known but will probably be on the order of 2 feet. The <br />unit area discharge through the capillary fringe will be less than <br />through the lower portion of the wetted backfill. This fact was ignored, <br />however, so as to provide a conservatively large estimate of the flow <br />through the backfill. The top-width of the channel was multiplied by a <br />height of 2 feet to estimate the contributory area of the capillary <br />fringes. <br />The estimated wetted areas of the backfill for stream discharge <br />of 5 cfs and 50 cfs are 34 and 61 sq.ft., respectively. These areas <br />include the contribution of the capillary fringe. The slope of the <br />backfilled <br /> <br />2.05-71 <br />(Revised 05/11/94) <br />