RECENT GEOLOGIC MAPPING AIDS IN HYDROGEOLOGIC INVESTIGATIONS: EXAMPLES FROM THE HAMILTON HEIGHTS, RAVALLI COUNTY, AND THE WEST RED LODGE BENCH, CARBON COUNTY, MONTANA

Detailed geologic mapping of alluvial and valley-fill materials is critical to the development of conceptual models used to design ground-water studies. New geologic mapping has revised the understanding of stratigraphic controls on aquifer distribution in many areas and refined overly general conceptual models of hydrogeologic frameworks.

On the Hamilton Heights bench, located on the east edge of western Montana’s Bitterroot Valley, recent geologic mapping has shown that earlier assumptions of homogeneity of the geologic materials and aquifers were too generalized. The new mapping has resulted in better monitoring network design by allowing monitoring points to be selected on the basis of specific positions within the subsurface geologic setting. This new distribution of data collection points confirms that there are distinct shallow and deep ground-water flow systems. This distinction has implications for current water well drilling, future subdivision development, and land use change.

In the Red Lodge area, a new geologic map detailing the different Quaternary deposits and their thicknesses allows re-interpretation of previously collected static water-level data based on the proper identification of ground-water source (underlying Tertiary sandstones and coals of the Fort Union Formation vs. overlying thin Quaternary gravels). This reinterpretation challenges earlier assumptions that the bench was a “one aquifer” system. It is instead a “two aquifer” system; one comprised of the underlying Fort Union Formation, potentially tied to deep flow systems and one comprised of the thin overlying gravels recharged by irrigation water and more susceptible to surface influences.