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4.2.5 Spring SP 4 Hydraulics The recharge source for Spring SP-4 will be interrupted during the mining process. The extent of the recharge area which supplies SP-4 is illustra- ted on Exhibit 44. SP-4 acts as a low point where the water contained by the upper coal aquifer spills into Hayden Gulch (this phenomia is illustrated on Exhibit 44). As mining progresses south the recharge connection for SP-4 will be cut off and Sp=4 will dry up. The low point (SP-4) will still act as a structural outlet for the area once backfilling and reclamation occur (see Exhibit 44). Once backfilling is complete 'and the reclaimed spoil has a chance to recharge (fill up to the spill point -- SP-4). SP-4 will flow again approximately premining discharge rates. The time required after backfilling for SP-4 to return to pre- mining flow conditions is calculated below: • Aerial extent of disburbed recharge area = 325.5 acres • Recharge rate = 3.2"/year • Volume of the spoil (to the spill point) = 14648 ac-ft • • Porosity of the spoil = 18% • Volume of spoil to be resaturated = 2637 ac-ft • Volume of recharge/year = 36.8 ac-ft/year • Number of years before SP-4 starts flowing again = 2637/36.8 = 30 years In summary the recharge rate at the site is approximately 3.2"/year. Spring SP-4 will be interrupted during mining and reclamation but the structure will not be destroyed. SP-4 will return to approximate premining flow conditions in about 30 years after backfilling takes place. The recharge rate for the coal aquifers can be approximated utilizing the water level data variation over the period of monitoring. Assuming that the water level variation is a response to down-dip movement and recharged by snow pack infiltration, the recharge rate can be calculated . as a function of porosity and water level variation. Based on a number