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Surface Water Summar <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 (June - <br />September). The following table outlines those predictions along with <br />this year's observed averages. <br />Stream Site Predicted TDS values (mg/1) This year's average TDS (mg/1) <br />YSGF5 1337 1049 <br />NPDES10 3938 2520 <br />NPDESI2 4291 3150 <br />WSSF3 (Seneca II -W site) 2118 1110 <br />No sites exceeded the PHC predictions. <br />TDS trends are increasing at NPDESI3, NPDES10, NPDESII, NPDESI4, YSS2, <br />YSGF5 and YSSPGI. TDS trends are decreasing at sites NPDESI2, YSSF3, <br />YSSPG2, YSSPG3 and YSSPG4. A recently stable trend has been observed at <br />YSGS. Average flow values this year were below historic averages at all <br />NPDES sites, except NPDESI2. <br />Water discharged from the Yoast Mine is not used directly for irrigation. <br />Indirectly, although, water discharged from the mine is significantly <br />diluted by water from Grassy Creek, Sage Creek, or the Yampa River before <br />it is diverted and used for irrigation. Due to the relative amount of <br />dilution, the ambient water qualities of Grassy Creek, Sage Creek, and <br />the Yampa River are the dominant factors in determining their suitability <br />for irrigation. Water discharged from the Yoast Mine is, however, used <br />for livestock watering. NPDES discharges are suitable for livestock and <br />irrigation, but sometimes exceed water quality standards for aquatic life. <br />15 <br />