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wells completed in the Wolf Creek Coal. The last water quality data collected from this aquifer was during the 1985 water year. • Trout Creek Sandstone Aquifer. Well GW-S5-TC serves as the potable water supply well for the Seneca II Mine complex. Water from this well has a Na-SOgHC03 water type, and TDS concentrations have been relatively consistent since this well was first sampled in 1980. This aquifer is stratiaraphically beneath the aquifers discussed above and is unaffected by mining activities. 3. Surface Water Data Water quality data collected from surface water sites, NPDES sites, natural springs and spoils springs at the Seneca II Mine are presented in Appendix 7-4A. Detailed discussions of this information are provided in the Seneca II Mine Annual Hydrology Reports which are submitted each spring to the CDMG. A copy of the current NPDES permit (CO-0000221) for the Seneca II • Mine is provided in Appendix 7-5. a. Review of Surface Flow Data. Surface water flow data collected through 1986 is summarized in the 1986 PHC document. Following is a review of surface water flow data collected during the 1987 through 1991 water years. Precipitation. Prior to examining recent surface water flow data, a discussion of precipitation conditions at the Seneca II Mine area is appropriate. Table 7-90 shows precipitation data collected since water year 1979 at Hayden, Colorado and since 1983 at the Seneca II Mine. As has been noted in recent AHRs, the Seneca II Mine/Hayden area has, in general, received reduced precipitation du__*ing the 1987 through 1991 water years when compared with earlier water year totals. This is clearly indicated in Figure 7-44, which shows a plot of total precipitation received at the sites during water years 1979 • through 1991. The average yearly precipitation at Hayden during 7-317 Revised 02/05/97