2018-05-18_PERMIT FILE - C2009087A (3)

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RULE 2 - PERMITS spillway, and discharges perennially, due in part to increased recharge and outflow from the existing Seneca II spoils, as described in Section 2.04.7. Mine dewatering flows are not expected to have a significant negative effect on surface water quality or quantity, because the inflow water will be used operationally for dust suppression or other consumptive use. If surplus dewatering flows are introduced to surface water, they would improve water quality over that presently occurring in lower Little Grassy Creek, by diluting the Seneca II spoil water that dominates water quality at this location. As noted in the groundwater discussion above, mine dewatering flows (expected inflow rates) will peak at about 18 gpm after 3 months of mining, decrease to 3 gpm after 4 months, then gradually increase to about 45 gpm after 26 months. Of this, approximately 0.06 to 1.8 gpm was estimated as the contribution from the Seneca II spoils aquifer (poor water quality, no more than 10% of the total). As reported in Section 2.04.7, the summer low flow of Little Grassy Creek at the outflow from Pond 002, which will ultimately receive any excess mine outflows, ranged from 0.06 cfs to 0.4 cfs (28 to 180 gpm). Spring high flows ranged from 3.0 to 4.2 cfs (1300 to 1900 gpm). If not used consumptively, mine dewatering discharges could thus represent a nearly twofold increase in summer low flows, but would have negligible effect (4% or less) during spring high flows. Water quality consequences of hypothetical mine inflow contributions would be positive, as Little Grassy Creek at Pond 002 is heavily affected by high -TDS water from the Seneca II spoil aquifer, and averages nearly 4000 mg /L TDS. Addition of mine discharges containing upwards of 90% overburden groundwater ( <1000 mg/L TDS, per section 2.04.7) would represent an improvement in overall water quality. Table 2.05.6 -T1 presents mass balance calculations that estimate the likely mine inflow composition, and the effects of mine inflow discharges on water quantity and quality at the Pond 002 outfall to Little Grassy Creek, and in Grassy Creek below mining at YSG5. Mine inflow parameters were computed using 45 gpm total inflow, iron and TDS concentrations from the highest single concentration among all of the spoil wells, and average concentrations for overburden groundwater. Mixing calculations were performed for a variety of downstream assumptions, testing mixing effects under the lowest and highest measured flows and concentrations at NPDES2 and YSG5. From the table, it can be seen that addition of mine inflows will categorically dilute in- stream TDS levels, with resulting concentrations ranging from 48% to 100% of present values. As demonstrated in Table 2.05.6 -T1, total recoverable iron concentrations at NPDES2 may increase for those samples where ambient iron concentrations were low, but never approach the 1 mg /L in- stream water quality standard in the cases where hypothetical mine inflows may increase concentrations. Exceedance of the iron standard, when it occurs, is caused by existing conditions, and the degree of exceedance is reduced by the addition of mine inflows. PSCM Permit App. 2.05 -88 Revision 03/05/10