Assessing Dewatering Requirements and Groundwater-Surface Water Interactions Associated with a Coal Mine in Alaska

An environmental impact study is currently being developed for a proposed coal mine located near Anchorage, Alaska. The project area encompasses approximately 5,000 acres on the west side of Cook Inlet and involves three hydrologic basins discharging into the Chuit River. In order to assess potential impacts to water resources associated with proposed mine development, a groundwater flow model was developed to (1) estimate dewatering requirements over the mine life, (2) evaluate potential impacts to nearby streams from mine dewatering and infiltration, and (3) assess post-mine reclamation impacts and recovery times to groundwater and surface water resources.

The geology of the site consists of semi-consolidated coal-bearing sedimentary deposits of the Tyonek formation overlain by younger unconsolidated sediments that include glacial drift (poorly sorted) and alluvium along stream reaches. The Tyonek formation within the mine area is offset by two local faults that do not extend upward into the younger unconsolidated deposits. There are six hydrogeologic model units that were constructed to replicate the area hydrogeology (from top to bottom): Glacial Drift and Alluvium, Upper Mineable Coal Sequence, Interburden, Lower Mineable Coal Sequence, Sub Red 1 Sand and Lower Coal Sequence.

The model was calibrated under steady-state and transient conditions, with results indicating that the model can properly simulate average and seasonal water levels and baseflows over the entire model domain.

A challenge for regulatory approval was demonstrating that reductions in stream flows could be mitigated to protect flora and fauna. The model estimates indicate that sufficient groundwater will be available to mitigate potential impacts to stream baseflows throughout the mine life.

This presentation will explain site hydrogeology, conceptual site model, and numerical model development, as well as the results that will be incorporated into the ongoing mine EIS.