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Regarding the cumulative hydrologic impacts of concurrent ongoing surface and current and proposed underground mining in the permit area, the modeling analysis described above and given in Exhibit 32, Quantity • and Quality Modeling Analyses of Surface Water Resources of the Trout Creek Basin, characterized historical ambient conditions and the anticipated changes due to the proposed expanded underground mining, in terms of streamflow and water-quality changes in streams of the Trout Creek Basin and in an associated stream reach of the Yampa River. An updated assessment of regional cumulative impacts, (see, for example, Kaman Tempo, 1982, Rerraro and Nazaryk, 1983) would have to take into account current mining activities and plans of other mining entities in the Yampa River Basin. Although WATBAL predictions in Exhibit 32 are reasonable, more recent data have been used to evaluate the expected concentrations. A comparison was made between the WATBAL predictions in Exhibit 32 and data generated for the 1993 Annual Hydrology Report. Data for lower Fish Creek are from Station 1003, which is coincident with Node 19 of Exhibit 32 for which WATBAL calculations were made. The weighted means of 1993 measured concentrations of TDS and sulfate (329 mg/I and 155 mg/I, respectively) are less than the WATBAL calculated concentrations for the 1992-2000 water year of 491 mg/I and 196 mg/I, respectively. These results indicated that no additional hydrologic impacts occurred other than those predicted based on the results of Exhibit 32. Details for predicted concentrations and SAR values are in Exhibit 7h, Tables 1-5 which are based on data from AHR, 1993. There was no discharge from the underground mine in water year 1997, the mean discharge and weighted mean TDS and sulfate at Station ] 003 (the equivalent of Node 19) were 6.02 cfs, 666 mg/L and 292 mg/L respectively. The flow value is equivalent to the modeled low flow of 6.9 cfs but the TDS and sulfate levels are higher than the modeled 456 mg/L and 181 mg/L for baseline conditions. The WATBAL model did not predict SAR values, but actual SAR values at Station 1003 on Fish Creek ranged from 0.36 to 4.89 in the 1993 AHR. The maximum SAR value measured in water year 1997 was 1.24. Water with SAR values less than ten is classified as low-sodium water and can be used for irrigation with little danger of developing harmful levels of exchangeable sodium. • Data for lower Trout Creek is from Station 69 of the 1993 AHR. Station 69 is upstream from Node 22, the location for which WATBAL calculations were made. The weighted means of the 1993 measured values for TDS and sulfate (406 mg/I and 219 mg/I) exceed those predicted by the WATBAL model of 338 mg/I and 139 mg/I, respectively. However, because Station 69 is upstream from Node 22 and above the confluence with Fish Creek, these concentrations are not strictly comparable to the calculated concentrations of the WA"I'BAL model. If the effects of dilution were accounted for, the concentrations observed at Station 69 would probably be significantly reduced by the time the Yampa River is reached. For this reason, it is unlikely that the TDS and sulfate concentrations at Station 69 would differ markedly from those predicted by the WATBAL model. There was no discharge from the underground mina in water year 1997, the mean discharge and weighted mean TDS and sulfate at Station 69 where 70.8 cfs, 384 mg/L and 183 mg/L respectively. The flow value is much larger than the modeled baseline mean flow rate of 46.4 cfs, the TDS is equivalent to the mean modeled baseline concentration of 383 mg/L and the sulfate concentration is a little larger than the mean modeled baseline concentration of 158 mg/L. The concentrations of TDS and sulfate immediately below the confluence of Fish Creek and Trout Creek were estimated by calculating a weighted mean using data from Stations 1003 and 69. This calculation yields 367 mg/I for TDS and 186 mg/I for sulfate, which compares to WATBAL predictions of 338 mg/I and 139 mg/I for 7'DS and sulfate, respectively, for lower Trou[ Creek. SAR values along Trout Creek at Station 69 were all below 1.0, which is within the category of low-sodium water. There was no discharge from the underground mine in water year 1997, the mean discharge and weighted mean TDS and sulfate at Station 1005 (site below Fish Creek installed in 1994) were 31.5 cfs, 490 mg/L and 196 mg/L respectively. The flow value is equivalent to the modeled low flow of 23.4 cfs and the TDS and sulfate levels of 420 mg/L and ] 72 mg/L for baseline conditions. • APPBd®9IE~ FEB 0 ~ 2000 TR 99-32 2.05-165 11/15/99