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III IIIIIIIIIIIIIIII 999 ,~ Doc Date:12/11/2001 FISH CREEK ALLUVIAL VALLEY FLOOR STUDY ARE?, 1ST HALF 1994 1.0 INTRODUCTION As part of the approval to undermine and subside a segment of Fish Creek and its associated alluvial valley floor Twentymile Coal Company established an extensive hydrologic and subsidence monitoring network. Additionally, Twentymile prepared channel profiles of Fish Creek which began upstream of the mined area and extended through and downstream of the mined area. Also, Twentymile conducted vegetation surveys pre and post mining in areas both within and outside of the mining areas. Finally, the information presented in the first section of this report provides data collected from the various subsidence monuments located within the Fish Creek study area. The undermining and subsiding of the Fish Creek study area has been completed without any significant impacts. The data presented in the following sections indicates that Twentymile successfully subsided a section of Fish Creek and its valley floor without deleterious impacts. An extremely positive note is that the shallow bedrock wells are recovering and are approaching premining • levels. This is the first opportunity that Twentymile has had to show a well recovering after subsiding an area. 2.0 HYDROLOGIC DATA The data for the various hydrologic monitoring stations are presented on tables 1 through 51. In general the majority of the stations have not been impacted by the mining operation. Of the various sites only three have been subsided by Twentymile's mining operation, those being: GWA6, GWAP7, and GWB-2R, this well replaced GWB-2 which was subsided and destroyed. Well GWA6 started subsiding during October, 1993, and exhibited 1.3 feet of subsidence by year's end. At the end of June, 1994 the well had not subsided any further. The water levels recorded for this well appear to reflect the fact that the well has subsided but do not indicate that dewatering of the alluvium has occurred. The water level in the well appears to have stabilized since first being subsided. The water quality is presented in tables 22 through 25, and shows an increase in cadmium and selenium. However, the last measured cadmium concentration for the period is returning toward the premine concentration. The selenium concentration is remaining the same, although the concentration is not problematic at this time. Also, the total recoverable iron concentration is higher than the premine concentration; however, all of the elevated concentrations can be associated with mobilization of these metals in the valley sediments. The downstream station (SW-4) did not experience an increase in these