2012-06-20_PERMIT FILE - C2010089 (88)

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82. oeuvre! volley floor iIll,tc' e ;:•' • • • Figure 5 Multiple Sandstone, Siltstone And Coal Aquifers With Differing Directions Of Flow And Ground Water Pressures. (Upper aquifer supports alluvial (qal) aquifer and the alluvial valley floor) Figure 6 "Perched" Alluvial Aquifer With Alluvium (qal) And Gravel Supplied By Upstream Runoff And Seeps Issuing From Upper Coal And Siltstone Aquifer. allIelone 5ond ;lone • son bility of recharge and discharge areas, the post- mining water table might change sig- nificantly. This potential for cumulative impacts from more than one mine necessi- tates the initial qualification on size of 1,000 hectares. If appropriate water quality is maintained after reclamation, the valley area shown in Figure 4 would appear reclaimable at least with regard to reestablishment of the ground water system in terms of flow, depth to water, quantity of flow, and quality. Deposition of spoils by draglines and return of the post mining topography to an average elevation about the same as before mining should result in an aquifer transmissivity* and a depth of water about equal to that prior to mining. Of re- maining concern would be the reestablishment of the capillary zone ** and the field capacity * ** of the rock /soil in the root zone. These do appear to have been re- established in some mined areas outside alluvial valley floors where adequate soil has been redistributed on the regraded areas. Figure 5 portrays a multiple "coal aquifer" system which is also considered to be possible in certain areas of the interior West's coal regions. The figure shows again a gaining stream where the unconfined upper coal - sandstone aquifer supplies water to the alluvial valley floor. The lower coal aquifer is confined by the shale which permits only minor leakage of water. The direction of leakage depends upon the relative pressures in the two coal - sandstone aquifers. Similar to the case shown in Figure 4, mining and reclamation of only the upper coal seam (as shown in Figure 5) might result in little disturbance to the alluvial aquifer system provid- ing the regional hydrologic system and water quality were maintained. However, min- ing the lower coal seam (as shown in Figure 5) could result in a combination of aqui- fer water pressures which could be either lower or higher than that existing prior to mining. If the elevation of the water table were changed significantly, vegeta- tion formerly dependent on the upper aquifer water for growth could be "waterlogged" or "waterstarved ", depending on whether water pressure in the underlying aquifer were either significantly higher or lower than that of the upper aquifer. The dis- charge to the alluvium from the bedrock aquifer is typical of the area near Gillette, Wyoming (USGS, 1974). Figure 6 presents a case where the alluvial aquifer subirrigating the alluvial valley floor is separated from regional sandstone -coal aquifers by the clayey shale stratum. Mining of these areas would result in disruption of the natural semi-con- fining layer of clayey shale. If this layer were not restored, it is conceivable that the alluvial aquifer would lose or gain water from the backfilled area of the mined -out coal leading to a change in the elevation of the water table. Again, whether a loss or gain occurs depends on the potentiometric level of the water into underlying strata. Figure 7 depicts in more detail an alluvial aquifer containing lenses of rela- tively impermeable strata which create "perched" water tables which support grasses and other vegetation suitable as forage. Mining of coal beneath such an aquifer would most likely break these lenses and, in the situation portrayed, permit the perched water table to drop. It would be necessary to deposit spoiled overburden in a selective manner and to reform compact lenses in order to attempt reestablishment of the critical functions of the alluvial valley floor in this situation. It is the opinion of some experts consulted during this analysis that recreation of the rela- tively impermeable strata would be impossible. Studies by_itahn_(1976).indicata that the t uck s reoer- damned spoils at the Big Horn Minn have a laboratory coefficient of permgebility_ of 0.16 meters /day (4gpd /ft ). This is essentially equal to that of the undisturbed sandstone overburden measured nearby. Since this is at the high end * Transmissivity: The rate at which water in an aquifer is transmitted through the saturated thickness of an aquifer with a unit width and a unit hydraulic gradient. ** Capillary zone: The zone in which water migrates upward a characteristic distance from the water table or saturated zone, where the distance is dictated by the struc- ture of the pore spaces and chemistry of the fluid and the solid. * ** Field capacity: The amount of water held in the soil after the gravitational water has drained away.