2018-01-03_REVISION - C1994082 (29)

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Grass An evaluation of the water quality impacts of the Yoast Mine on Grassy Creek in the permit application projects no increase in TDS in Grassy Creek downstream from the mine. This conclusion is drawn by estimating the quality of Grassy Creek water after salt loading from Yoast Mine spoil springs, and comparing that projected water quality to current water quality in Grassy Creek downstream from the Yoast permit area. Water in lower Grassy Creek currently has an average TDS of 2584 mg/l, partially due to the influence of spoil spring discharges from the Seneca II Mine. Seneca Property, LLC predicts that lower Grassy Creek TDS will be 2177 mg/1 after addition of Yoast spoil springs. Thus, Seneca Property, LLC concludes that TDS in Grassy Creek will not increase as a result of Yoast Mine operations. Using Seneca Property LLC's assumptions about spoil spring development at the Yoast mine, and the existing baseline water quality data for lower Grassy Creek, the Division estimated the TDS of lower Grassy Creek after establishment of Yoast spoil springs will increase by approximately five percent. Because the quantity and quality of Grassy Creek water is marginal or unsuitable for use as domestic or irrigation water, this projected increase in salinity will not affect any uses of Grassy Creek water. The only current use of Grassy Creek water (livestock watering) will not be impaired by the projected increase in salinity. Sage Creek Life -of -mine plans for the Yoast Mine anticipated coal extraction within the Sage Creek watershed during the second and third permit terms. Preliminary estimates by Seneca Property indicated that TDS concentrations in Sage Creek downstream from Yoast spoil springs could increase from 641 to 2118 mg/l. With the submittal of PR -01 in 1999, Seneca Property, LLC further evaluated the probable hydrologic consequences of Yoast mining operations in the Sage Creek watershed. This evaluation occurred prior to any mine -related disturbance in that watershed. Based on Seneca Property, LLC's evaluation, the Division finds that although the company has predicted that salinity levels in Sage Creek may exceed the Division's conductivity threshold of 1.0 mmho/cm, the effects to ranching/agricultural operations along Sage Creek are deemed to be insignificant. The Division finds that the coal mining operation would not interrupt, discontinue, or preclude farming on the potential alluvial valley floors that have been identified along Sage Creek, downstream from the mining operation. The Division's findings in regard to impacts to alluvial valley floors are presented in Section B, X of this document. The projected electrical conductivity values for Sage Creek were in excess of the suspected level for material damage in the original Yoast Mine application. As 25