The ground-water system was studied in the Trail Mountain area in order to provide hydrologic information needed to assess the hydrologic effects of underground coal mining. Well testing and spring data indicate that water occurs in several aquifers. The coal-bearing Blackhawk-Star Point aquifer is regional in nature and is the source of most water in underground mines in the region. One or more perched aquifers overlie the Blackhawk-Star Point aquifer in most areas of Trail Mountain.

Aquifer tests indicate that the transmissivity of the Blackhawk-Star Point aquifer, which consists mainly of sandstone, siltstone, and shale, ranges from about 20 to 200 feet squared per day in most areas of Trail Mountain. The specific yield of the aquifer was estimated at 0.05, and the storage coefficient is about IxlO"6 per foot of aquifer where confined.

The main sources of recharge to the multiaquifer system are snowmelt and rain, and water is discharged mainly by springs and by leakage along streams. Springs that issue from perched aquifers are sources of water for livestock and wildlife on Trail Mountain.

Water in all aquifers is suitable for most uses. Dissolved solids concentrations range from about 250 to 700 milligrams per liter, and the predominant dissolved constituents generally are calcium, magnesium, and bicarbonate.

Future underground coal mines will require dewatering when they penetrate the Blackhawk-Star Point aquifer. A finitedifference, three-dimensional computer model was used to estimate the inflow of water to various lengths and widths of a hypothetical dewatered mine and to estimate drawdowns of potentiometric surfaces in the partly dewatered aquifer. The estimates were made for a range of aquifer properties and premining hydraulic gradients that were similar to those on Trail Mountain. The computer simulations indicate that mine inflows could be several hundred gallons per minute and that potentiometric surfaces of the partly dewatered aquifer could be drawn down by several hundred feet during a reasonable life span of a mine. Because the Blackhawk-Star Point aquifer is separated from overlying perched aquifers by an unsaturated zone, mine dewatering alone would not affect perched aquifers. Mine dewatering would not significantly change water quality in the Blackhawk-Star Point aquifer.

Subsidence will occur above future underground mines, but the effects on the ground-water system cannot be quantified. Subsidence fractures possibly could extend from the roof of a mine into a perched aquifer several hundred feet above. Such fractures would increase down ward percolation of water through the perching bed, and spring discharge from the perched aquifer could decrease. Flow through subsidence fractures also could increase recharge to the Blackhawk-Star Point aquifer and increase inflows to underground mines.