2009-08-13_PERMIT FILE - C1981019A (4)

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RULE 2 PERMITS The South Taylor Pit will likely have geochemical characteristics similar to the water quality in the Streeter Well, the Streeter Pond, and other spoil pit aquifers (Williams and Clark, 1994), since the lithology is relatively homogenous across the area. The TDS in the Streeter Well is 3,750 milligrams per liter (mg/L), and TDS in pit spoil wells nearby average 3,400 mg/L (Williams and Clazk, 1994). TDS concentrations in the Streeter Pond averaged 1,786 mg/L in 2005 and TDS concentrations in aquifers immediately downgradient from neazby pit spoils averaged 1,796 mg/L (Table 2.04.7-31). Wells located a half mile downgradient from pit spoil averaged 900 mg/L (Williams and Clark, 1994). An estimate of TDS loading from backfilled spoils discharge into Good Spring Creek was developed based on a simple mass balancing based on the projected increased TDS of the water contributing to Good Spring Creek. Calwlated impacts of this groundwater into the alluvia] and surface water flow regime at Good Spring Creek aze shown here. A calculated spoil pit maximum dischazge estimate of 0.06 cfs enters Good Spring Creek during base flow, and 0.6 cfs enters during peak flow. Therefore, a maximum of 7% of the base flow and 5% of the peak flow to Good Spring Creek at the NUGSC sampling point would be contributed from the pit outflow at steady state. (These percentages are approximately twice what the springs above NUGSC actually contribute to the creek flows.) To project the potential impact to Good Spring Creek, a weighted TDS loading between the historic low flow at NUGSC (0.85 cfs and 1,050 mg/L TDS) (Table 2.04.7-34) and the projected spoils (0.06 cfs and 3,400 mg/L (worst case) and 1,796 mg/L (likely case) TDS; Table 2.04.7-31) was performed. • Worst case (pit spoil aquifer TDS concentrations): ((0.85 cfs x 1050 mg/L) + (0.06 cfs x 3400 mg/L))/0.92 cfs = 1192 mg/L Reasonable case (groundwater immediately downgradient from pit spoil): ((0.85 cfs x 1050 mg/L) + (0.06 cfs x 1796 mg/L))/0.92 cfs = 1087 mg/L Thus, the base flow of Upper Good Spring Creek is calculated to have between 37 and 142 mg/L increase in total dissolved solids, or an increase of between 3.5% and 13.5% caused by the projected contribution from the pit springs. The increase in TDS in the base flow at Lower Good Spring Creek (with the base flow of 1.8 cfs and TDS of 1187 mg/I placed into the above calculations) would be between 20 mg/L and 71 mg/L, or between 1.6% and 6% of TDS increase. Peak flow TDS increases would be less than these values. ~~ South Taylor/Lower Wilson -Rule 2, Page 119 Revision Date: 3/3 0/07 Revision No.: PR-02