Belgrade and Manhattan, Gallatin County

Background

Residential and commercial development continues in the north-central part of Gallatin County, surrounding the communities of Belgrade and Manhattan. Municipal water distribution and wastewater treatment systems are placing an increasing demand on groundwater for potable water supplies. With the estimated population of Gallatin County now over 100,000, and reported growth of about 17% from 2010 to 2016 (http://quickfacts.census.gov), development pressure appears likely to continue in the future.

Purpose

This project will address the effect of increased groundwater use on surface water and groundwater. A numerical groundwater flow model will examine locations for high-yield municipal wells based on hydrogeologic conditions and compare those results to groundwater withdrawals using exempt wells.

Project Scope

Water-level and streamflow data were collected from 2010 to 2015, along with water-chemistry samples, to understand the hydrogeology of the area. Thirty-four monitoring wells and 27 stream gage sites were installed for this study. These data support the development of an analytical flow model and a numerical groundwater model to evaluate the hypothetical scenarios of pumping from high-yield wells on groundwater and surface water.

Results

The ability of the aquifer to support high-yield pumping is highly dependent on the location of the well within the geologic setting. Coarse, thick packages of Quaternary alluvium south of the Central Park Fault are more likely to produce sustainable municipal pumping wells than the finer, thin sediments north of the fault. In addition, the proximity to surface water dictates the timing of pre-stream capture caused by pumping; the nearer the surface water body, the more rapid the connection to groundwater withdrawals.

Project Reports

• Hydrogeologic investigation of the Belgrade-Manhattan area, Gallatin County, Montana: Superposition groundwater modeling report (Sutherland, M., 2023)
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Presentations

• Hydrogeologic Investigation of the Belgrade/Manhattan Study Area, Gallatin County, Montana. Presented at the Gallatin County District Court Spring Water Meeting, May 2016.
  View Presentation

Link to Project Data

Data collected for this project can be accessed through the Ground Water Information Center (GWIC) database:

Download project data from GWIC

Content last updated: 5/9/2022 1:29:36 PM