More Background on the Geology at the Colstrip Site

Past strip mining on the site has altered the shallow geology. The removed material above the coal is dumped back into the depressions left after the coal is mined. The “mine spoil” is highly variable in composition and resulting water quality.

In some areas, the bedrock is believed to be fractured. Fractures can direct groundwater in unpredictable directions.

The shallowest coal seam, Rosebud Coal, has naturally burned in the area in an irregular pattern. The burning has caused variable collapse of the overlying material and variable amounts of baking of the surrounding bedrock. The altered bedrock (“clinker”) can be extensively altered to a hard, highly fractured material.

Because of the formation process, several of the geological materials show large horizontal changes in the parameters that govern groundwater flow.