AMEC Geomatrix developed a basin-scale groundwater flow model for the project to provide a tool for estimating flows into existing and proposed underground mine workings and to help assess potential impacts to surrounding water resources. The model was developed using the finite element code FEFLOW. The model encompasses approximately 370 square miles around the Cabinet Mountains, and is divided into seven vertical layers. Model boundary conditions include areal recharge, transfer boundaries, specified head boundaries, specified flux boundaries, and discrete elements. Groundwater - surface water interaction is simulated using transfer boundaries. Underground mine workings are simulated with a combination of transfer boundaries and discrete elements.
The hydraulic characteristics of major faults within the project area are not well understood. Of concern to stakeholders is the degree to which faults may: 1) influence flow to underground mine workings, and 2) affect communication between mine workings and overlying surface water features within the wilderness area. The model was used to test different conceptual models of fault hydraulics to address this concern.
Preliminary modeling results suggest impacts to streams and lakes in the Cabinet Mountains will be minimal and within natural variability of surface water flows and levels. The model predicts some stream flow depletion will occur within lower potions of the basins. Flow to the underground workings may be substantial if the permeability of the faults is greater than the surrounding bedrock. However, grouting of underground workings would help limit inflows.