2022-09-29_PERMIT FILE - C1980007 (2)

Home Browse Search

West Elk Mine . The lazgest total iron increase occurred in July, when the river upstream was 0.13 mg/L and downstream from the mine was 0.17 mg/L. While this calculates to a high percentage increase, the absolute increase was of no practical significance and the resulting concentration was well below the stream standazd. • MCC's 1996 discharges to the North Fork did not affect the pH of the river. • In addition to reviewing the water quality data for actual discharge events during 1996, WWE considered a "worst case" scenazio in order to project future PHC's associated with mine dischazges. The worst case scenario assumes the rare situation of: (1) both ponds discharging simultaneously, as occurred in March 1996, and (2) winter low flow conditions in the North Fork. Even under these extreme circumstances, where the dischazge flow comprises nearly one quarter of the downstream flow, the mixed total iron concentration is still less than the stream standard. Also, the mixed TSS concentration of 21 mg/L is well within the natural variability of TSS in the North Fork. The pH was unaffected by pond discharges under the worst case scenario. This worst case scenazio also considered the effects of the increase in the TDS concentration on crops during the irrigation season, which is discussed in more detail in the following section. Impact of TDS on the North Fork TDS levels in the North Fork aze elevated under natural conditions. For example, in April 1996, the TDS concentration in the North Fork upstream from the mine was 316 mg/L; MCC's dischazges increased the TDS concentration to 319 mg/L (based on mass balance computations, see Table 54). As shown in Table 54, all of the TDS concentrations downstream from the mine were below any numeric standazds for TDS that WWE has been able to locate, irrespective of the beneficial use classification that is to be protected (Table 55). For comparison, the highest TDS concentration calculated downstream of the mine during 1996 was 319 mg/L. Even under the worst case scenario, the downstream TDS concentration was calculated to be 426 mg/L (see Table 54), less than the most stringent limit shown in Table 55 (450 mg/L). Under the worst case scenario, as shown in Table 54, the downstream mixed TDS concentration would be approximately 670 mg/L. This concentration would occur only during the short duration over which the dischazges occur (i.e., less than two days to empty both ponds at their maximum dischazge rates), or while the North Fork flows are at the 7-day, 10-yeaz (7Q10) low flow conditions. This is important since TDS is a parameter that is of concern only when the concentration is elevated for significant periods of time (i.e., months to years). Additionally, the primary concern with elevated TDS is for agricultural uses. The 7Q10 low flow is most likely to occur during the winter months, when irrigation is not occurring. There would be no material damage to irrigated agricultural operations as a result of the short- term increases in salinity to 670 mg/L as shown in Table 54. While this value is above the 500 mg/L level, the increased TDS levels would occur for a duration of less than one week. Since adverse salinity impact to crops result from an accumulation of salts in the soil, a minor, short- 1.05-136 Revised June 2005 PRIG; Rev. March 1006; Rev. May 2006 PRlO