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Doc Date:12/11/2001 III III I III IIIIII III sss Little variability between the seasons for pH has been detected at SW-N703. During •ation, pH averages 7.84 and ranges from 6.84 up to 8.10 units. After the irrigation season, pH averages 7.88 and ranges between 7.50 and 8.06 units. Similar to previously discussed surface water monitoring sites, the mean of TDS during the non-irrigation season is more than double (3,135 mg/7) the mean for this constituent during the irrigation period (1,371 mg/1). Again, the higher levels of the major ions in Calamity Draw at this site after irrigation stops explains this trend (see Figure 7-31). At Site SW-N103, both TSS and FeT average higher during the irrigation period (57.0 mg/1 and 1.46 mg/1, respectively) than after irrigation has ceased (39.4 mg/1, TSS and 1.05 mg/1, FeT). The mean value of FeD is slightly higher (.04 mg/1) during the non-irrigation period compared to the average FeD (.02 mg/1) determined for the irrigation season. As seen for all previously discussed sites, MnT averages consistently higher (1.24 mg/1) after the irrigation season has ended than during the irrigation period (.33 mg/1). S/~ NPDES Ponds - Flow and Quality.) In accordance to the requirements of the National P ution Discharge Elimination -System (NPDES), Peabody has monitored six sediment ~ainment structures constructed on effected drainages in the vicinity of the Nucla Mine. Monitoring of each pond outfall has been conducted since the completion of each pond as outlined by the NPDES Permit Document (Permit No. CO-0000213, see Appendix 7-6). The location of each NPDES pond (001, 002, 003, 004, 005 and 006) can be found on Exhibit 7-1. Since the structures were completed, no discharges have occurred at Ponds 002, 003, 004 and 005. Because of this, the following discussions of flow and quality focus only on NPDES Ponds 001 and 006. NPDES 001 - Flow. NPDES discharge point 001 is located on the West Return Ditch at the southern most edge of mining disturbance as shown on Exhibit 7-1. Flow has been monitored at this site since 1975, and has been continuously monitored via a water level recorder coupled with a 90° v-notched weir since 1978 (see Tab 15, Hydrologic Monitoring Program). Appendix 7-5 contains the mean daily flow reports for data collected at NPDES 001 since 1978 through January of 1988. The continuous flow data collected from this site demonstrates the seasonal effects of irrigation on the surface water flow regime at the Nucla Mine. ~~l L 1 7-154 Revised 04/11/88