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* energy fuels corporation Mr. Stanley V. May August 28, 1979 Page Two i r The mine plan called for removal of all topsoil in accordance with OSM and State regulations in the late fall of 1978, and construction of the box cut and commencement of mining in late 1978. The topsoil was removed on schedule; however, due to unforeseen safety problems with groundwater and overburden stability, the excavation was delayed until late spring 1979 pending geotechnical investigations. As a result, the watershed surface at the time of the violation alleged in your letter was not in the design condition. It was in the poorest hydrological con- dition with respect to runoff in any phase of mining: removed topsoil with no spoil cover and no box cut to intercept surface runoff. This phase of mining, although usually of very short duration, produces the most runoff because the remaining cover is a smoothed clay with very low infiltration rates. Not only will most surface water run off this type surface, but the sediment which is pvt into suspension consists of extremely small clay-sized particles, which require lengthy detention times to settle. Ay the time the delay of excavation was known, the ground was frozen and reconstruction of the sedimentation pond was not possible before the summer of 1979. However, the pond was slightly overdesigned and no serious impacts were anticipated with respect to water quality. When spring runoff did occur, the pond worked well for most of the runoff period. As a result of unusually heavy snowfall the previous winter and rapidly warming spring temperatures coupled with heavy spring rains, runoff and associated TSS concentrations remained extremely high for several days. This factor, combined with the high percentage of very small clay-sized particles, produced a condition which was in excess of the pond's design criteria, and for a brief period, TSS concentrations were above the 30 mg/1 level. •