2016-10-27_PERMIT FILE - C2009087A (2)

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• The apparently different direction of groundwater flow in the underburden as opposed to flow in the Wadge coal and overburden, as well as the different water -level trends exhibited by the underburden compared to the Wadge coal and overburden at the YOV28- YW28-YWU28 well set, may indicate that the underburden unit acts separately from the Wadge coal and overburden units. Comparison of the potentiometric surface elevations for the overburden, Wadge Coal, and underburden shows the areal patterns of vertical gradients between the three units. The vertical gradient between the overburden and Wadge Coal is upward from the coal to the overburden in the eastern three - quarters of the Permit Area, including most of the area proposed to be mined. Extrapolation of potentiometric data suggests that the vertical gradient is downward from the overburden to the coal west of a line trending generally north -south about one - quarter to one -half mile west of the area to be mined. The vertical gradient between the Wadge Coal and the underburden is downward from the coal to the underburden in the southern half to two - thirds of the permit area, with the head difference increasing to about 400 feet at the southern end of the area to be mined. In the north, the gradient is upward, but the head difference between the two units is not more than about 50 feet. The gradient change from downward to upward occurs along a line generally between the CW23 -CWU23 site and SWU11, and the area with an upward gradient includes the mine portal and the workings north of the portal. The pattern and magnitude of the vertical gradient between the overburden and underburden units are similar to those between the Wadge Coal and the underburden. • Groundwater flow directions in the alluvial aquifers along Grassy Creek and Little Grassy Creek can be inferred as being generally down the valleys of both streams, based on the physical geometry of the alluvial aquifers. The potentiometric surface elevations of the alluvial aquifer and the overburden unit, particularly in the Little Grassy Creek valley and downstream of the confluence of Grassy and Little Grassy Creeks, indicate that upward flow from the overburden to the alluvial aquifer can occur. Because potentiometric surface elevations in the Wadge Coal and underburden units are above the land surface in at least parts of the stream valleys, particularly the Little Grassy Creek valley, those units also may discharge upward into the alluvium; such discharge would be indirect, as it would have to occur through the overburden rocks that separate the deeper units from the alluvium. Groundwater flow in the spoils aquifer at the Seneca mine is interpreted fo originate from infiltration of precipitation and snowmelt as well as upward discharge from the underburden unit (Williams and Clark, 1994). The flow direction in the spoils aquifer is generally through the lower part of the spoils, down dip along the floor of the reclaimed mine pit. The groundwater ponds against the less - permeable bedrock of the undisturbed Wadge coal and overburden in the down -dip low wall of the mine pit (Williams and Clark, 1994) adjacent to Little Grassy Creek, saturating the spoils to the level of the low points in the low wall or to other low- elevation areas where the groundwater can emerge as springs or underflow into surface water features such as ponds or channels. Spoils • aquifer potentiometric surface elevations and the elevations of spoils aquifer springs adjacent to the Little Grassy Creek alluvium indicate that the spoils aquifer has the potential to discharge into the Little Grassy Creek alluvium and in some areas, the Wadge Coal and the overburden unit. PSCM Permit App. 2.04 -46 06/15/09