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Recharge to the caved material from the undisturbed overburden unit was not included in the calculation since it <br />amounts to a negligible portion of the total. <br />.The pore volume of the caved material, which is likely to exist after mining, is estimated to be about 186,000 acre- <br />fi (V). This number is derived ir. Dewaterine and Potentiometric Drawdo~vn in Bedrock Units. This is based on a <br />24 square mile mining area, a thickness for the caved zone of about 100 feet and an effective porosity of 10 <br />percent. After re-establishment of potentiometric levels, the recharge rate to the Wadge overburden should be <br />similar to pre-mining conditions which has been estimated at 12.3 acre-fi per year (R). <br />These values were assigned for this calculation. Solving for "t" in the above equation for leaching of 0.6 and ].0 <br />pore volume results in time factors of 9,]00 and 15,000 years, respectively. When taking into account the higher <br />TDS values measured in the mine workings, this calculation indicates that if ground water flow rates through the <br />caved material are about 12.3 acre-fi per year then it will lake at least 14,000 years of ground water TDS levels to <br />start declining significantly and 23,000 years to return to baseline levels. <br />As potentiometric levels in [he Wadge overburden recover to pre-mining conditions, lateral ground water flows in <br />the unit from the recharge areas in the southern and western margins of the Twenrymile Park Basin to the major <br />discharge area in the northeast margin of the basin will also tend to be re-established. This will result in <br />movement of lower quality ground water from areas directly overlying mined zones into the undisturbed <br />overburden units to the east of [he proposed mine area. <br />The rate of movement of the plume of lower quality water will be dependent on the configuration of the <br />potentiometric surface in the post-mining situation and the permeability of the undisturbed overburden units. As <br />indicated earlier, recovery of potentiometric levels is expected to take about 350 years. The relatively high <br />permeability of the rubblized and fractured zones, which are likely to exist over mined areas, should result in lower <br />.hydraulic gradients through these zones than existed in the pre-mining situation. Potentiometric levels within <br />thisarea under fully recovered conditions is expected to show a relatively small drop from the major recharge area <br />south of the underground mine workings to the undisturbed area to the west of the workings. Asa "worst case" we <br />could expect apost-mining potentiometric level of about 6900 feet (the potentiometric level in the spoils to the <br />south of the mine) [o occur at the downdip extremities of mining. The post-mining potentiometric level in the <br />discharge area for the overburden unit is equivalent to the land surface elevation in this area, which is at about <br />6600 feet. The predicted potentiometric levels, at the recharge and discharge areas govern the hydraulic gradient <br />across the undisturbed part of the overburden unit. As it turns out, these "worst case" predicted potentiometric <br />levels are about the same as presently exist in the unit. Consequently, the flow rates through this part of the unit <br />once potentiometric levels, are fully recovered are expected to be similar to baseline conditions. This flow rate has <br />been calculated to be in the order of ] 1,000 gpd over approximately an 8000 foot flow width and 75 foot thickness, <br />which yields an average flux of about 0.0025 fUday. if the effective porosity of the unit in this part of the basin <br />were taken as 0.01 then actual flow velocities in the order of 0.25 ft/day would be expected. <br />This calculated ground water flow velocity is the maximum rate that the plume of lower quality water from the <br />disturbed part of the unit would be expected to move into the undisturbed part of the unit would be expected to <br />move into the undisturbed part of the unit. The closest proximity of mining to the discharge area is about 10,000 <br />feet so [hat once potentiometric levels have been re-established which may take 360 years, it may take an <br />additional 110 years before any discharge of the lower quality water occurs to alluvial deposits on surface streams. <br />Effects on Alluvial and Surface Water Quality As a Result of Bedrock Discharee <br />Discussions in Effects on Ground Water Qualiri of Aouifers indicate that the Wadge overburden unit will be the <br />~nly bedrock unit in which water quality may be affected by the proposed mining operation. The calculations in <br />e previous sections predict that it will take about 360 years for potentiometric levels in the Wadge overburden <br />unit to fully recover and an additional 110 years before lower quality water in this unit (which results from <br />TR99-32 2.05-145 ~li ~,~®iVWnr F[B 0 3 20D0 1/3/00 <br />