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Surface Water Summa <br />The Yoast Mine PHC (PAP Tab 17, Attachment 17-5) predicts TDS increases <br />to be observed at various stream sites during the irrigation season <br />(June-September). The following table outlines those predictions along <br />with this year's observed averages. <br />Stream Site Predicted TDS values (m /1) This years average TDS (m /1) <br />YSGF5 1337 890 <br />NPDES10 3938 2080 <br />NPDESI2 4291 3345 <br />WSSF3 (Seneca II-W site) 2118 740 <br />No sites exceeded the PHC predictions. <br />TDS trends are increasing at NPDESII, NPDESI3, NPDES14: and Spoil Springs <br />2 and 3. TDS values have decreased all three downstream stream sites <br />(YSS2, YSG5 and YSGF5), NPDESI2 and Spoil Springs 1 and 4. TDS trends <br />have stabilized at NPDES10. Average flow values this year were above <br />historic averages at NPDESII and NPDESI2. <br />Water discharged from the Yoast Mine is not used directly for <br />irrigation. Indirectly, although, water discharged from the mine is <br />significantly diluted by water from Grassy Creek, Stage Creek, or the <br />Yampa River before it is diverted and used for irrigation. Due to the <br />relative amount of dilution, the ambient water qualities of Grassy <br />Creek, Sage Creek, and the Yampa River are the dominant factors in <br />determining their suitability for irrigation. Water discharged from the <br />Yoast Mine is, however, used for livestock watering. NPDES discharges <br />are suitable for livestock and irrigation, but sometimes exceed water <br />quality standards for aquatic life. <br />• <br />15