Laserfiche WebLink
B. Ground Water Assessment <br />The probable hydrologic consequences of mining to the ground water system at <br />the Nucla Mine will be confined to affecting shallow, localized bedrock <br />aquifers. The effects to these regionally insignificant aquifers will be <br />two-fold: disruption of ground water flow systems and formation of spoil <br />aquifers. <br />Mining pits and their subsequent backfilling will disrupt the ground water <br />flow system at the Nucla Mine site. The saturated thickness of the surficial <br />Dakota Sandstone is dependent upon the recharge provided by the Lower Second <br />Park Irrigation Ditch. Leakage and overflow from this ditch appears to be the <br />most significant source of ground water recharge in this area. <br />As mining continues, ground water flow gradients will be directed toward the <br />pits. This will cause measurable drawdowns in the immediate vicinity of the <br />mining activity. Due to the hydrologic characteristics of the Dakota <br />Sandstone, the drawdown effects will not be observed with increasing distance <br />from the mine pits. This limited impact will occur in both the coal and the <br />overburden. Monitoring data collected at the site suggests that mining will <br />not deplete or degrade the underburden aquifers. Wells completed in the <br />underburden generally exhibit seasonal fluctuations which coincide with <br />recharge from the irrigation ditch, No measurable drawdowns attributable to <br />mining have been observed in underburden monitoring wells. <br />The water quality of the overburden; underburden and coal is generally <br />unacceptable for domestic use. These waters are usually high in TDS; <br />magnesium and sulfate; in some cases significantly surpassing recommended <br />drinking water standards. The overburden and underburden waters are usually <br />suitable for livestock use only. The coal aquifer water exhibits sulfate and <br />TDS levels high enough to make it marginally suitable for livestock use. Only <br />limited development of any of these three aquifers has taken place in the <br />study area. <br />Portions of the backfilled mining pits will become partially saturated, <br />forming limited spoil aquifers, as indicated by Spoils Spring No. 1 and well <br />GW-N10, These aquifers, like the Dakota Sandstone aquifers, will be recharged <br />almost exclusively by the Lower Second Park Irrigation Ditch. The <br />consequences of establishing these aquifers are relatively insignificant. <br />Based upon data collected at the mine site, it is apparent that only a portion <br />of the spoils will re saturate. One well (GW-N10) is completed in a backfilled <br />mining pit and has been dry for the period of record (1981-1987). Other <br />portions of the backfilled pit have resaturated, as evidenced by the presence <br />of spoil springs located near NPDES Pond No. 001, Over time, the spoil <br />aquifer will contribute poor quality recharge to both the undisturbed coal <br />downgradient of mining and to the surface system via spoil springs. <br />Due to the permeable nature of the spoil material, the quantity of water <br />provided as recharge to the undisturbed coal will increase over time. <br />However, the water supply to the spoil aquifer and eventually the coal will <br />still be dependent upon recharge from the Lower Second Park Irrigation Ditch. <br />_l g_ <br />