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Techrrteal Revision No. 80 August /9, 1998 Page 6 evaluates worst case discharge from all mines in the basin, to the river, and the cumulative worst case discharge irrstream TDS should not exceed 500 mg/L. Please provide additional clarification which describes the maximum discharge to occur from the West Elk Mine, from all possible discharge sources. The sources include, Ponds MB-/, MB-2R, MB-3, MB-4, MB-5, Loire Pine Gulch and any anticipated discharge facilities in Sylvester Gulch. Refuse pile leachate contributions should also be included. The scenarios provided in Table 59 (TR 80) do not appear to include possible discharge from sources such as Lorie Pirre Gulclr, and Sylvester Grrlch, and the refuse piles. A total maximum discharge rate should be specified, acid it would be helpful to itemize the anticipated contribution from each pottrt source to the total discharge rate. The Table 59 scenarios :rse a TDS value of 1485 mg/L. This vahre may be too low. The maximum anticipated TDS concentration may be 2500 mg/L or higher based upon achral discharge and mine water data available to date. If a lower TDS value is used in the worst case projection, then such a value must be substantia[ed. Please revise or clarify the discharge scenarios presented iii Table 59, as discussed above. The worst cnse scenario is of greatest importance. Include permit page revisions as appropriate. It may be helpful [o review the CDMG CHIS jor the North Fork of the Gunr:ison River (Revised 2/24/97). Discharge variables for the various mines in the North Fork Basin are defined and may be useful as yorr respond to this item. The "worst case" scenario presented in Table 59 of the revised permit text has been revised by increasing the total dissolved solids (TDS) concentration from 1485 mg/L to 2500 mg/L as suggested by the Division. The revised table is enclosed. The discharge rate was kept at 9.5 cfs-representing the very rare occurrence of both primary sedimentation ponds, MB-1 (5.0 cfs) and MB-2R (4.5 cfs), discharging at the maximum rates. This discharge rate could also represent the Lone Pine Gulch pipeline (4.5 cfs) discharging simultaneously with one of the sedimentation ponds. However, it is extremely unlikely that both ponds and the Lone Pine Gulch pipeline would all discharge simultaneously. Furthermore, the other discharge locations (MB-3, MB-4, MB-5, the RPE pond, and SG-1) would not discharge under such extreme conditions. The underdrains for the lower refuse pile have been historically dry, and these underdrains and the RPE underdrains would drain to the sedimentation ponds, if flowing. Therefore, the discharge rate of 9.5 cfs accurately depicts a plausible, worst case scenario. As shown in the revised Table 59, under the worst case scenario, the downstream mixed TDS concentration would be approximately 670 mg/L. This concentration would occur only during the short duration over which the discharges occur (i.e., less than two days to empty both ponds at their maximum discharge rates), or while the North Fork flows are at the 7-day, 10-year (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,