REP34327 - Laserfiche WebLink

Home Browse Search

2.0 HYDROLOGIC DATA INTERPRETATION AND IMPACT ASSESSMENT FOR THE PERMIT AND ADJACENT AREAS ..;:.:r::.:y::-:e:.::•:..;::;.;awn:;::Y ~-:.::n:;:::r:ca.aw.. :x:.-cax<,~sa:~n ~'a~:o::~:::~:.;:.:o.;::;.:x.::::.,:;;a:::.,:;:v<;r:~.;;;m;:»<;.:.;n::.<~~:;:.::.::;::~;:a~.mmm • This section focuses on three areas during the water year (WY). First, it analyzes the hydrologic data gathered. Second, it assesses impacts of mining on the quality and quantity of water in surface streams, springs, groundwater aquifers, mine inflows, and mine dischazges. Lastly, it estimates potential impacts of mining on these waters. Data from the Colorado Agricultural Experimental Station and College of Engineering, Colorado State University indicates that WY 1995 precipitation was 150% of the 1961 to 1990 average. Eazly snowfalls at the first of the WY were followed by a mild winter, a wet spring, while temperatures well above average closed the WY. Precipitation levels can affect surface water and spring flow rate. It also has the potential to influence mine water inflows in areas of low overburden cover (e.g., Sylvester Gulch). 2.1 ASSESSMENT OF MINE WATER The underground mining methods in use at the West Elk Mine can affect groundwater in essentially two ways. First, development mining in azeas where groundwater is pres- ent in or near the coal seam can cause local- ized movement of this water into the mine workings. To date, groundwater movements of this type have occurred in areas of low cover which are highly fractured (near outcrops and near stream channels) and where sandstone channels bearing isolated water immediately overlie the coal seam. Secondly, retreat mining can cause subsi- dence and the fracturing of overlying groundwater formations. Again, water AFiEt-93.210/05/ I S /96(I :38 pm i usually moves into the mine workings. At the West Elk Mine, the permeability of the "F" and "B" seams and strata of the Mesa Verde Formation overlying the "F" seam and located between the "F" and "B" seams is very low, and the azeal extent and quantity of water stored in these units are small. Outing the WY, the mine workings located neaz the outcrop, such as the Lone Pine portal azea, Gribble Gulch, the main portal azea, and along Sylvester Gulch behaved similarly to previous years, relative to inflows. Inflows occurred primazily from the roof in areas of low overburden and neaz the coal seam outcrop. The seasonal pattern of mine inflows observed at the main portal area and along Sylvester Gulch indicates that the colluvium, coal, and bedrock are unconfined and rechazge locally. The area along Sylvester Gulch contains many natu- rally occurring fractures. Water moves down slope through the colluvium and fracture zones and discharges as springs and seeps. Water inflows appear to be controlled by the amount of precipitation (mostly snowmelt), depth of overburden, and location and extent of fracture zones, and not by the extent of the underground workings. Mine water inflows for other areas of the mine are related to site specific geologic and topographic features as well as water-beaz- ing formations. Mine activity is dewatering the coal seam and local azeas of roof and floor fractures during mining. Once initial dewatering is complete, there appear to be no sustained flows. There appear to be no correlation with seasons or precipitation events. Water levels in monitories wells 15 RE~7SED 0927/96