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when ambient ground water quality is estimated (January 31, 1994, Commission Regulation 41.5(C)(6)(b)(ii)). Surface Water Impacts The leachate formed in the pits at the Grassy Gap Mine can be expected to discharge to the land surface as spoil springs wherever spoil backfill becomes saturated up to the elevation of the land surface. Spoil springs have been identified at six locations at the mine. Each spoil spring discharges up to a few gallons per minute of leachate that has a maximum total dissolved solids concentration near 4,000 mg/1, although most spoil spring discharges are less than 3,000 mg/1. Spoil spring discharge rates can be expected to peak each year in early summer following maximum infiltration of snowmelt into the pits, as observed at other surface coal mines in the Williams Fork Mountains. The Grassy Gap Mine is on Stream Segment 13e of the Yampa River Basin, as designated by the Colorado Water Quality Control Commission. Surface runoff from disturbed areas at the Grassy Gap Mine can be expected to be alkaline and free of contaminants as there are no leachate-forming materials on the land surface at the mine. The quality of this runoff should be closely similar to runoff from surrounding undisturbed lands and, therefore, does not have the potential to degrade surface waters or alluvial water in Grassy Creek. Surface water discharges from the Grassy Gap Mine were most recently monitored at NPDES outfalls in 1993. These discharges were from sediment control ponds that are fed by spoil springs. Data from 1993 and previous year's show that surface discharges are alkaline, high in dissolved solids, and low in suspended solids. Early springtime (April) discharges usually exceeded the instream pH standard of 9.0, but dropped below this level by the month of May. Metals concentrations consistently are well below the instream standards. SAR is consistently less than 1.0, well below the level of concern for SAR of 10. All significant surface water discharges from the mine (surface runoff and spoil spring discharges) flow to the mine's surface water monitoring site SW-4. Assuming all discharge from SW-4 enters Grassy Creek, the potential impact of spoil springs on Grassy Creek water quality can be assessed by performing a mass-balance calculation using approximate flow and TDS data from SW-4 and from a location on Grassy Creek upstream from the mine. The following mass-balance calculation is for the month of July. Flow and TDS data from site SW-4 are reported in the mine's 1992 AHR as 50 gpm and 1,000 mg/l. Flow and TDS data from Grassy Creek, collected upstream from the mine, are reported in USGS Water Resources Investigations Report 90-4020 (Table 3). The Grassy Creek data are 176 gpm and 903 mg/l. The 903 mg/1 value was calculated by multiplying measured conductivity of 950 umhos/cm by 0.95. The 0.95 conductivity-to-TDS conversion factor is based on the relationship observed in Grassy Gap Mine field and lab samples. Using these values, TDS in the creek would increase from 903 mg/1 upstream from the mine, to 925 mg/1 as Grassy Creek passes the mine, calculated as follows: [(50 gpm) X (1,000 mg/1) + (176 gpm) X (903 mg/1)] / [(50 gpm + 176 gpm)] = 925 mg/l. The calculated increase of 22 mg/1 is a 2.4% increase, a negligible amount, indicating the increase in TDS would not cause material damage. This minimal impact to surface water indicates there also would be minimal impact to alluvial ground water in the Grassy Creek valley. Grassy Gap TOJ 6/5/2009 17