English

Abstract

Large scale coal-strip mining has been on-going in southeastern Montana for over three decades. In order to address the potential hydrologic impacts on aquifer systems, extensive long-term monitoring has been conducted since the inception of this mining. As a result, an excellent ground-water database has evolved. This database allows for refinement of key hydraulic parameters in the affected hydrogeologic strata in the vicinity of these mines. In turn, this facilitates prediction of the impacts of future hydraulic/hydrologic stresses imposed on coal bed aquifer systems via any of the following: 1) Mine expansion; 2) Mine development; and 3) Development of coal-bed methane (CBM). One method of refining the understanding of aquifer systems is to use ground-water modeling. To this end, Nicklin Earth & Water, Inc. (NE&W) has developed four separate ground-water models representing coal mines for predicting the impacts of surface mining on aquifer systems in and near the vicinity of coal mines in the northern Powder River Basin. The mine modeling simulations demonstrate that the applicable range of hydraulic conductivity at a mine scale is from 0.1 to 0.3 m/d. The mine simulations also demonstrate that vertical hydraulic conductivity for overlying and underlying strata bounding coal beds is very low. Vertical hydraulic conductivity values in the range of 10 -5 to 10 -6 m/d produced simulation results reasonably consistent with field observations at clustered vertical well sequences. The issue of vertical hydraulic conductivity (or very low vertical leakance) in confining units is a significant issue in estimating CBM production. The very low leakance rates lead to very little contribution of water from the confining units bounding the coals in the Northern Powder River Basin. In general, methods, such as Lohman (1972) and MODFLOW are best employed before the full on-set of methane gas flow begins. Once the on-set of gas flow becomes significant, it has been our experience that both these methods tend to significantly over-estimate actual water production rates during CBM production.