GENERAL50428 - Laserfiche WebLink

Home Browse Search

Tne effects of subsidence on spring and seep flows are almost impossible to predict. Two recent Utah CH IA reports (Cottonwood Creek Basin and Mud ~~ Creek Basin) provide qualitative discussions which identify potentially affected spring and seep flows. However, quantifying such effects depends on ongoing monitoring as mining progresses. Because of the difficulty in predicting effects, it is recommended that subsidence effects on spring and seep flows be eliminated from consideration in estimating streamflow depletions. Alluvial well supplies Alluvial wells may supply water to both surface facilities and underground operations--or to coal processing plants. In most instances, pumpage from such wells can be considered as depletions from streamflow. Two conditions may exist within valley fill alluvial aquifers: Alluvial ground .voter may either discharge to, and augment, streamflow; or streamflow may infiltrate to, and augment, alluvial ground water. However, in born instances, al'uvial well pumpages can be considered as streamflow depletion. In the first instance, pumpage intercepts water that would otherwise eventually augment streamflow. In the second instance, pumpage induces an equivalent abstraction from streamflow as a result of increased gradients, and Hence flow rates, away from the stream channel. Except in the unlikely instance that alluvial wells are hydraulically isolated from stream cnannels, it is recommended toot alluvial well pumpage be considered as a depletion from streamflow. Alluvial aquifer abstractions into mines Such abstractions usually occur where mine-affected aquifers subcrop beneath valley-fill alluvial aquifers. Ground-water movement between the bedrock and alluvial aquifers may be in either direction, and mining may still affect streamflow either by inducing a reversal of water table gradients that normally are away from mining or interception of ground water that would eventually de tributary to streamflow. Quantifying streamflow depletions from such effects is not nearly so straightforward as for alluvial well pumpage, as discussed above. Effects on streamflow are indirect, may be short term or long term or both, and are difficult to define because of the multiple aquifers affected. Defining an effect on streamflow is furtner complicated by considering toot such mine-induced inflows will normally be pumped out of the mine and -eturned, after treatment, to surface flows. Mine discharges returned to surface f]ows are essentially immediate inflows, whereas induced abstractions from streamflow via aquifer depletions may be very long term. An example of such induced a]luvial aquifer inflow is provided by the Eagle Plo. 9 mine in Colorado. Estimated alluvial aquifer inflow via bedrock subcrops is 35 gallons per minute. However, this inflow, together with deep aquifer inflow, is pumped out of the mine and returned to surface flows ditty no resulting depletion. Mining nos been underway for a sufficient period of time toot inflow has stabilized at the 35 gallons per minute rate. J 1 -f}