Laserfiche WebLink
Augrsr 1992 L}'Pne 71CC • Arulyrir of Srbridenor F~feru on lfjdrolo~ ~ 6R <br />• • The zone of inaeated permeability above longwall operations is typidly 30 to <br />60 times the thickness of the removed stnta <br />• The thickness of the zone of increased permeability is dependent on the thickness <br />of the removed stnta, rate of mining, advance and geologic character of the <br />overburden <br />• After completion of mining, the permeability of the disturbed strata returns to <br />dox to pre-mining conditions <br />3.1 IMPACTS ON GROUNDWATER IN THE FISH CREEK ALLUVIAL VALLEY FLOOR <br />Bedrock units in the study area indude the Lewis Shale on the northwes[ side of the Fish Creek <br />valley, and the Williams Fork Formation on the southern side of the valley. The Williams Fork <br />Formation dips at approzimatdy 7 degrees northwest. The groundwater potentiometric surface <br />for the Wadge coal seam overburden ranges from an elevation of 6,950 co 7,000 feet with <br />groundwater movement generally parallelling the strike of the Williams Fork Formation to the <br />northeast. Avenge hydraulic conductivity of the Wadge coal seam overburden, verified by <br />several test; is 0.4 ft/day. <br />Above the Wadge coal seam, potential water bearing strata in the study area include the <br />sandstones in the immediate overburden of the Wadge coal seam, sandstones between and above <br />the Lennoz coal seam; the Twentymile Sandstone, unconsolidated alluvial xdiments in Fish <br />• Creek, and the reclaimed Mine 2 spoils. Only the Twentymile Sandstone is considered a <br />significant aquifer. The Twentymile Sandstone, however, is xpanted from the coal bearing <br />xquence by a 500 to 600 foot chick layer of marine shale which hu been designated u a <br />`Tongue of Lewis Shale' (Maleki, 198. This shale member functions u an effective aquidude, <br />preventing downward movement of groundwater from the Twentymile Sandstone and isolating <br />the Wadge coal/overburden xquence from downward groundwater communication. <br />The Fish Creek alluvium is composed mostly of fine grained sediment; silt; silty day; and <br />days. Baxd on available information, the thidcneu of the alluvial sediments ranges from <br />approaimacely 15 to 30 feet. The thickness and composition of the alluvial sedimenu will be <br />verified during itutalla[ion of alluvial monitoring wells within the study ara. The alluvial <br />sediments are saturated and the weer table, u measured in the ezisting monitoring wells <br />upstream (6-AY-1) and dowattream (6-A~3) of the study area, flurntates seasonally from 7 to <br />9 feet below the surface (measuring point). <br />A single surface spring hu been identi£ed in the vidnity of Panel 6 u shown on Map 1. This <br />spring originates in the hillslope above the Fish Creek floodplain and is not related to the Fish <br />Creek alluvium but rather appears to disdtarge from an overlying Fish Creek sand unit. <br />Sampling of the spring indicates low flow on the order of 0.001 cfs and relatively good water <br />quality with pH of 8.2 and electrid conductivity of 1,080. <br />Site spedfic geology for the study ara is shown on Figure 2, Cross-Section A-A'. This cross <br />section is parallel to Panel 6, u shown on Figure 1. <br />Longwall operations within the study area, oaur 1,000 to 1,100 feet below the groundsurfaa <br />and total extracted coal thictness is approzimately 8.5 to 9.5 feet. Documented research from <br />the Appaladtian and other coal fields indiates thu lottgwall mining an result in development <br />~' ~' ACZ 1nr. • P.O. Bos 77IOIB ' Srrmnbvat Syringr, Grlorado fW77 • f303J879d260 <br />