English

Abstract

An evaluation of groundwater conditions in the vicinity of coal mines near Colstrip, Montana was conducted in 2001 as part of a cumulative hydrologic impact analysis. This analysis was required due to permitting activities associated with an expansion of the Rosebud Mine. Active coal mining operations at the Rosebud mine have expanded outward as low-cover coal is sought. Much of the remaining, unmined coal nearest to Colstrip and the adjacent coal-fired power plant is relatively high-cover coal, so mining has slowed in the interior area of the coal deposit in favor of lower-cost mining at the edges. Because the interior, high cover coal will eventually be mined according to mine plans, presently inactive pits remain open for future use. As the mine expands, additional information gained through hydrologic monitoring contributes to and increases the understanding of the groundwater system and how it is affected by mining. Overall, groundwater flow directions are similar to pre-mine conditions. Where coal has been mined out, mine pit backfilled spoil maintains the groundwater flow formerly occurring in the coal seam. However, groundwater head distribution has changed significantly in some areas, and some of these head differences may be retained in the post mining groundwater environment. As mining operations have expanded, data suggests recharge to the principle coal seams may be more localized than was previously thought, and groundwater recharge from the Little Wolf Mountains southwest and upgradient of the coal deposit is limited. Drawdown of groundwater levels in the McKay coal seam, which lies beneath the mined Rosebud coal exceeds that observed in the Rosebud coal at many sites. While this phenomenon has previously been attributed to the presence of inadequately plugged coal exploration boreholes, data suggests this explanation is less likely near recent mining activities.