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A4ounlain Coa! Company 1997 Annual Hydrology Report {Ves! E/k Mine <br />• Assessment of Mining Impacts to Hydrologic Resources <br />Surface Water Impacts <br />While MCC maintains a comprehensive network of streamflow gaging stations throughout the <br />permit and lease azeas, to date few of the monitored streams have been directly within the <br />influence of mining (Map 2). None of the South of Divide watersheds-defined as those <br />drainages tributary to Minnesota Creek-have been subject to mining influences through WY97. <br />Therefore, the following discussion focuses on assessing the impacts to the North Fork as a <br />receiving stream. <br />North Fork Water Quality Data <br />As noted previously, there were three discharge permit excursions from the sedimentation ponds <br />during WY97. To determine the effect these excursions had on the North Fork, a mass balance <br />calculation was performed using the worst conditions present in WY97. Due to the 14 SEHG <br />fault inflows, there were dischazges from pond MB-2R that exceeded the discharge limit for <br />TSS. A maximum value of 132 mg/L was recorded during low flow conditions (January 1997). <br />The discharge rate from pond MB-2R is typically 4.5 cfs. <br />MCC took additional water quality samples on the North Fork during this period (January 21 and <br />January 26, 1997) to assess potential impacts from the fault inflows. North Fork flows during <br />this period were 110 cfs and the upstream TSS concentration ranged from 2 to 11 mg/L. Mixing <br />• the pond discharges with the North Fork water caused the TSS concentration to increase from 11 <br />to ] 6 mg/L on January 21 and increase from 2 to 7 mg/L on January 26. These minor increases <br />are confirmed by the downstream North Fork samples that show TSS increases of only ] mg/L <br />on both occasions. TSS values in this range are not detrimental to the water quality of the North <br />Fork. <br />Impact of TDS on the North Fork <br />WWE evaluated the concentrations of the individual parameters that collectively constitute the <br />TDS in mine discharges, and determined that the pond discharge water is predominantly sodium <br />carbonate/sodium bicazbonate in nature. Aside from high concentrations of sodium, none of the <br />other parameters, including the heavy metals, are found in significant concentrations. There are <br />no applicable numeric standards or criteria for sodium. <br />WWE has reviewed the salinity (TDS) loading analysis in the most recent version of the North <br />Fork Cumulative Hydrologic Impacts Analysis (CHIA), in the context of MCC's WY97 <br />discharges. In the 1997 CHIA, CDMG calculated the cumulative TDS (salinity) loading to the <br />North Fork from coal mines located in the North Fork valley. The assumptions used for MCC's <br />contribution were a dischazge rate of 1,000 gpm (2.23 cfs) and a TDS concentration of 2,500 <br />mg/L. When assessing WY97, the total volume discharged from ponds MB-1, MB-2R, MB-3, <br />MB-5, and FW-1 was approximately 591 acre-feet, which translates to an average continuous <br />dischazge rate of approximately 366 gpm (0.82 cfs). Based on the 1997 Discharge Monitoring <br />Reports for the ponds, the flow-weighted, maximum TDS concentrations were as follows: MB-1 <br />= 2,450 mg/L; MB-2R = 2,190 mg/L; MB-3 = 330 mg/L; and MB-5 = 3,100 mg/L. <br />• Consequently, the annual salinity loading to the North Fork from the West Elk Mine for WY97 <br />was 2,440 tons, which is well below the value assumed in the CHIA as shown in Table 9. <br />831-031.450 Page Wrig r aver ngineers <br />