Gallatin Valley, Montana

A three-dimensional groundwater model for the Gallatin Valley was developed by Water & Environmental Technologies (WET) in collaboration with Morrison-Maierle. The Model formulation effort was for developing a basin wide simulation tool that can be used to assess a variety of issues related to surface and groundwater interaction. MODFLOW Unstructured Grid (USG) was the numerical code used in the model development. USG and MODFLOW 6 are recent adaptations of USGS MODFLOW which allow greater flexibility in representing groundwater systems.

The Gallatin Valley is a topographic and structural intermontane basin bounded by folded and faulted sedimentary, metamorphic, and igneous rocks ranging from the Precambrian to the Cretaceous age. The Gallatin Valley is within the Three Forks structural basin which includes the Gallatin Valley, the Madison Valley, and the lower Jefferson River Valley. The primary water-bearing deposits in the Gallatin Valley are quaternary and tertiary sand and gravel deposits. The most conductive and productive are alluvial deposits adjacent to the main stems of the Gallatin River and in lower portions of the valley which often yield copious amounts of groundwater.

The basic steps in developing the model involved the development and application which included data mining and management, conceptual model development, geophysical subsurface interpretation, steady-state model calibration, sensitivity analysis, and final application. The overall domain of the groundwater model is 30 miles N-S and 20 miles E-W.

The selection of MODFLOW-USG allows for local scale (project scale) discretization (vertically and laterally) as well as “pinchouts” without disrupting the regional scale model features. Model pinchouts, model sub-layering, and MODFLOW-USG Transport were applied in one instance to represent transport for a proposed waste-water discharge at one development. The valley model is readily adaptable to other localized or regional types of projects as well.