2020-03-24_PERMIT FILE - C1981019A (2)

Home Browse Search

RULE 2 PERMITS The actual resultant spring discharge will likely vary from high flow to low flow periods by an order of magnitude, as measured in the surface water features. Thus, the discharge of groundwater originating as pit infiltration used in the following calculations is assumed to range from 0.06 to 0.6 cfs, which gives a geometric mean of approximately 0.21 cfs (calculated infiltration rate from above). Assuming that 0.06 cfs enters Good Spring Creek during low flow and 0.6 cfs enters Good Spring Creek during peak flow, the pit contribution would be approximately 7% of the base flow and 5% of the peak flow to Good Spring Creek at the NUGSC measuring point or about 3% of both base and peak flows at the LGSC measuring point. This is a maximum value, since the calculated contribution from the pit spoil aquifer is greater than the average measured flow from the potentially affected springs. Thus, the probable reduction in flow will be up to 7% of base flow for 45 years after mining ceases. Potential Surface Water Quality Impacts Potential impacts to the surface water quality from the South Taylor pit operations are considered here. The water quality would be impacted by meteoric water that enters the hydrologic cycle being impacted by contact with the overburden fill. To estimate the impact to surface water quality, existing geochemical and flow data for Good Spring Creek were modified by changing the flow entering from the pit (described above) to have water quality similar to that found in the Streeter Well (completed in backfill in the Streeter Fill) and Streeter Pond discharge. The Streeter Well is located in the Streeter Fill of the existing East Pit, and would appear to represent water quality in direct contact with Colowyo Mine spoils. The Streeter Pond accepts primarily groundwater from the Streeter Fill. Assumptions used include: 1. All pit groundwater will have chemistry similar to Streeter Pond, Streeter Well, or published pit spoil geochemistry 2. All pit groundwater will eventually enter the Good Spring Creek surface water regime 3. The quantity of water entering Good Spring Creek would match assumptions in the Potenital Surface Water Quantity Impacts section. 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 Clark, 1994). TDS concentrations in the Streeter Pond averaged 1,786 mg/L in 2005 and TDS concentrations in aquifers immediately downgradient from nearby 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. Calculated impacts of this groundwater into the alluvial and surface water flow regime at Good Spring Creek are shown here. A calculated spoil pit maximum discharge 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.) South Taylor/Lower Wilson — Rule 2, Page 84 Revision Date: 4/7/17 Revision No.: RN -07