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Probable Hydrologic Consequences <br />Groundwater <br />There aze predominately three aquifers of interest in the Marr permit area: the Sudduth coal <br />aquifer, upland terrace deposits, and alluvial deposits. The underburden at the site is the <br />relatively impermeable Pierre shale, with a permeability of 9.15 gpd/ft'. The overburden <br />sequence is composed of interbedded siltstones and shales of the Coalmont formation with <br />permeabilities ranging from 0.37 gpd/ftZ to 7.5 gpd/ft''. Monitoring is focused on the coal <br />aquifer, alluvial deposits, and two wells that encountered water in the overburden. <br />The physical hydrologic characteristics of the coal aquifer suggest that it is a moderately poor <br />aquifer except where it is faulted, as reflected by petmeabilities of 3.8 gpd/ftZ in a well distant <br />from a fault zone and 16.5 gpd/ftZ in a well near a fault. These two wells suggest yields of <br />less than ten (10) gallons per minute and twenty (20) gallons per minute, respectively. The <br />aquifer quickly becomes confined east of the outcrop, which results in artesian heads on wells <br />completed in this zone. <br />Original recharge on undisturbed 50-foot-wide outcrop within the pemut area was calculated <br />to be 4.75 acre-feet per year, and storage capacity of a 3200 acre area was 240 acre-feet. The <br />only discharge of this aquifer in the permit area appears to be a spring located at the Old <br />Sudduth Mine, which discharges less than one gpm. The maximum mine pit inflow is <br />projected to occur during the mining of Pit 2 and would amount to 126 gpm. <br />The coal aquifer water quality is variable, with some wells showing a sodium bicarbonate <br />water and others reflecting a calcium bicarbonate/sulfate water. It is slightly alkaline, and <br />total dissolved solids range from 183 to 1166 mg/1, averaging approximately 500 mg/1. <br />Dissolved manganese concentrations exceed the domestic standard regularly. This water is <br />acceptable for stock and limited domestic use. <br />Mining of the Sudduth coal aquifer dewatered the coal seam in the vicinity of the none. The <br />quality of water percolating through disturbed overburden was, as expected, poorer than water <br />flowing through the undisturbed coal seam due to the leaching of soluble salts. <br />The overburden physical hydrological characteristics were derived from two USGS wells <br />(DH-3B and DH-4) within the general azea. Permeabilities at these two wells ranged from <br />0.37 to 7.5 gpd/ft2. The two overburden wells (79-1 and 79-2) in the monitoring program <br />were chosen simply because the water was observed in the overburden during drilling. A <br />saline, sodic, sodium bicarbonate water is derived from this aquifer. TDS ranges from <br />796 mg/1 to 1708 mg/l, averaging 1216 mg/1. Dissolved manganese concentrations exceed the <br />domestic standards regularly. A review of water quality analyses conducted as part of the <br />Division's approval of Bond Release Request No. SL-O1 found that mining had not adversely <br />affected groundwater quality. <br />Upland terrace deposits are found sporadically throughout the permit area, where they have <br />not been eroded off. They can achieve thicknesses of up to seventy (70) feet thick within the <br />Marc Mine Permi[ Renewal 04 <br />I I January, 2002 <br />-14- <br />