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ArgKrt 1991 Gjpria 71LC'Anahsu glSrGridcnce Fffertr oa Nydralo~ • 8R <br />• to the 1991 AI-IIt, water quality data from the Fish Creek alluvial wells near the Fish Creek <br />borehole shows little impact from the mine discharge. Well 006-AW-2, downgradient from the <br />discharge point, shows a slight increase in conductivity during the baseflow puiod. <br />Previous watu samples from bedrock wells 006-87-1 and 006-88-2 indicated chat the Wadge <br />overburden groundwater (generally asodium-biarbonate system) remained unaffected by mining <br />attivities. These wells (the closest bedrock wells to the study area) have been dewatered by <br />mining activities. <br />Subsidence could potentially impax groundwater quality through the following mechanisms: <br />• Caved overburden materials replay the mined portion of the coal seam aquifer <br />• Fracturing develops a new hydraulic connexion between aquifers with diffuent <br />water quality <br />The first example is applicable only in cases where the coal seam acts u an aquifer, which is not <br />the case in the study arc. The second eumple is more common in cues where several aquifers <br />with different water quality are present. In the study area, interconnection, for e:atttple, <br />between the Twentymile Sandstone and the alluvial aquifer could result in changes in water <br />quality, As discussed in previous sections of this report, however, subsidence impacts would not <br />result in hydraulic communication between the Fish Creek alluvium and Twentymile Sandstone <br />or the Twentymile Sandstone and undulying units. No impacts on the groundwater quality <br />• in the Fish Creek AVF are, therefore, antiapated. This conclusion is mnsistenr with <br />documented ezperience in other coal fields. <br />Documented studies of subsidence effexs on surface and groundwater quality in the Appalachian <br />coal fields concluded that no significant long-term effects on watu quality were observed u a <br />result of mining related rubsidence (Owili-Eger, 1987, Moebs and Barton, 1985, and Schultz, <br />1988). <br />4.0 CONCLUSIONS <br />Regarding potential impacu o[ TCC longwall opentiotts on the loaf rurfaa and groundwatu <br />resources, it can be concluded thu the effects will be very limited due to the depth and <br />lichologic composition of the overburden above the mined Wadge coal seam. Impaxs on rurfaa <br />and groundwatu resources are possible within 340 - 380 feet above the mined coal seam and <br />within an angle of draw of about 17 degrees from the lottgwall panels. Analysis of the geologic <br />composition of the Wadge coal seam ovuburden indicted that only limited sandstone strata <br />above the Wadge and I,ennoz coal seams will be directly impacted by rubsidenoe. This <br />conclusion is co~rmed by limited groundwater inflows to ezisting mine workings. <br />Predixed ground surface subsidence within the Fish Creek AVF would range from 0 to 48 <br />inches during mining in Pane16. This rate of rubsidena would cease short tum changes in Fish <br />Creek stream gradients. The calculated mazimum change in gradient of 0.15 % would not have <br />any signifianc impact on stream flow and/or wuu quality. The dynamic charatter of Fish <br />Creek, with a constantly changing channel cutting through fine grained udiments, would <br />. fadlitate rapid natural adjustments to any temporary changes in stream gradient. <br />ti'~"„ A CZ /ru. ' P.O. Bas 771018 • .Lvmbioat SprinBr, Colorado 80177 • (303J879d260 <br />