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water impact caused by the Seneca II Mine. The disturbance of sedimentary rocks <br />during the mining and reclamation process exposes more surface area for the <br />solution of minerals and increases the amount of solute available for transport to <br />surface streams. <br />An increase in TDS concentration of approximately 1500 mg/1 between upstream <br />and downstream stations on Grassy Creek has been observed at the Seneca II Mine. <br />An effect of this magnitude has bz~n appazent throughout the monitoring period, <br />1981 to 1996. This upstream to downstream increase is likely attributable to mining. <br />An increase in TDS from 600 mg/1 prior to mining in the Cow Camp Creek <br />Drainage to approximately 3200 mg/1 has been observed in monitoring data <br />collected from 1981 to 1996. TDS in Bond Creek has risen from S50 mg/1 in 1987 <br />to 1753 mg/I in 1996. The operator has suggested that TDS levels in Bond Creek <br />and Cow Camp Creek will stabilize after mining activities cease, as has been <br />observed in the portion of the Grassy Creek drainage where mining has ceased. <br />A prediction of TDS loading which could be expected to occur in both the <br />Fish Creek and Grassy Creek basins during the months of June through September <br />has been developed by the operator and included in the statement of probable <br />hydrologic consequences. Mean monthly flow and maximum observed TDS <br />concentrations from monitoring of spoil aquifer wells were used in the analysis. <br />Predicted TDS concentrations for downstream locations on both Fish and Grassy <br />Creeks were calculated using the mass-balance approach. <br />Results of the analysis indicate that a maximum TDS of 2341 mg/I would occur <br />during September at a location below the mine on Grassy Creek. A corresponding <br />flow of 0.84 cfs would result. <br />Results of the analysis on Fish Creek predict a cumulative effect from the <br />Cow Camp and Bond Creek drainages to result in an instream TDS concentration of <br />607 mg/1 in Fish Creek below the mine during the month of July. This conclusion <br />was based on assumed dischazge of 0.2 cfs with a TDS concentration of 2245 mg/I <br />from both Cow Camp Creek and Bond Creek. During August and September no <br />flow in the tributaries is expected to reach Fish Creek as surface flow. <br />There is no agricultural use of water immediately downstream of the Seneca II Mine <br />on Grassy Creek. Due to the small quantity of flow contributed to the Yampa River <br />by Grassy Creek, the increased concentration of TDS produced in the Yampa River <br />would be minimal, and not preclude the beneficial uses of this water. An evaluation <br />of the effect of increased TDS concentrations in Fish Creek is discussed in Section <br />XIII of this document entitled Alluvial Valley Floors. <br />29 <br />