Characterizing hydrostratigraphic communication between the various unconsolidated sediment aquifers of the East Flathead region in northwestern Montana through hydrogeophysical surveys

Abstract:

Ongoing development in the Flathead Valley in northwest Montana is increasing the demand for both residential and commercial water. The Ground Water Investigation Program (GWIP) of the Montana Bureau of Mines and Geology has been conducting an ongoing investigation to provide a detailed understanding of the interconnection between the groundwater and surface waters on the east side of Flathead Valley. In particular, evaluating the continuity and permeability of the confining layer at the quaternary-tertiary boundary will help determine the extent to which the deep and shallow aquifers are connected and allow for the development of a calibrated numerical groundwater flow model. This model will be a powerful management tool capable of forecasting the impact of increased groundwater development scenarios on the watershed’s aquifers and surface waters. The study is monitoring groundwater levels in the various shallow and deep aquifers; measuring stream flow to determine where water is gained or lost into the subsurface; installing wells and performing aquifer tests to evaluate the hydrologic connections between the aquifers and surface water; and collecting geochemical water samples to evaluate the connection between the aquifers. In cooperation with the Department of Geophysics at Montana Tech, this study incorporates hydrogeophysical data including magnetotelluric and transient electromagnetic profiles and soundings at five pilot sites which are integrated into the ongoing investigation by GWIP to characterize the complex hydrostratigraphy of the Flathead Valley. Logs and core from deep wells at two of the sites (100 m and 200 m deep) will be used to geologically constrain the interpretation of the geophysical data and improve our understanding of hydrostratigraphic connectivity in the Flathead Valley.