2020-08-21_REVISION - C1994082 (36)

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Alluvium, additional downgradient chemical changes, and dilution and mixing from shallow bedrock units which discharge to the alluvium. Seneca Property, LLC predicts that the maximum TDS increase in the Annand Draw Alluvium will be 58% immediately below there-saturated spoil, attenuating to less than 5% near the confluence of Lower Grassy Creek and Scotchman's Gulch. The predicted TDS increases in the Grassy Creek and Armand Draw Alluvium will not preclude use of the alluvial groundwater for livestock watering, although the low well yields in these areas may already render these aquifers useless for these purposes. In the Sage Creek Alluvium, the predicted TDS increase immediately below the re saturated spoil is 99%, but attenuates to 0.1% within 1000 feet. The high hydraulic conductivities of greater than 1 ft./day measured in the Sage Creek alluvium, in combination with the large area of alluvial material, provides for a relatively high volume of flux in the Sage Creek Alluvium. Contributions from tributary alluvium and shallow bedrock units will provide dilution of the spoil discharge as it mixes with much larger volumes of groundwater in the Sage Creek Alluvium. The point at which the TDS increase in the Sage Creek Alluvium is predicted to attenuate to 0.1% is just above a potentially sub-irrigated field. Therefore, the impacts to the field are predicted to be negligible. The minimal increase in downgradient TDS levels will also not affect the suitability of the groundwater for livestock watering uses. Hydrologic monitoring will continue to be implemented by Seneca Property in order to verify the predicted impacts. The hydrologic monitoring program is outlined in Tab 15 of the PAP. Surface Water The probable hydrologic consequences of coal mining at the Yoast Mine in regard to surface water include contributions of sediment to streams from storm runoff, and impacts to stream water quantity and quality during and after mining. Potentially affected streams and drainages include Annand Draw, Grassy Creek, and Sage Creek. The most significant impact of mining on surface water will be the effect of spoil spring discharges from the reclaimed mine on surface water quality. Seneca Property LLC's predictions of the magnitude of these impacts is discussed below. Sediment Loading All runoff from areas disturbed during the life of mine will pass through one of five sedimentation ponds prior to leaving the permit area. These ponds are Ponds 010, 011, 012, 013, and 014 (Pond 011A is upstream and in series with Pond 011). Discharges from the ponds are required to meet NPDES effluent limits, and should, therefore, not contribute additional suspended solids to stream flows near the permit area. Since drainage channel reconstruction and revegetation is complete at the mine, more runoff from the mine area will occur than during mining, when pits were in place and maximum disturbance had not yet occurred. However, sediment loads in the increased runoff should be no larger than those coming from the permit area during mining, which was treated effectively by the existing sediment ponds. 18