POTENTIAL OF THE MADISON AQUIFER FOR A CITY WATER SUPPLY IN BOZEMAN, MONTANA

The municipal water supply for Bozeman, Montana was provided by two reservoirs, but one was drained in 1985 due to concerns over the dam’s safety. The City is now faced with a choice about how to replace the 6,000 acre-feet of water that was lost when the reservoir was drained. This research provides a preliminary assessment of groundwater use from the Madison aquifer, which outcrops in the same drainage basin as the now defunct reservoir.

The Madison aquifer is a limestone and dolomite aquifer that is over 1000 feet thick and contains karst features and collapse breccia and is highly fractured. The recharge of the aquifer was estimated by measuring stream losses where streams cross the Madison aquifer. Two streams show combined losses averaging 1,400,000 gallons per day, while another stream loses all of its water, adding 220,000 gallons per day of recharge to the aquifer. The aquifer is discharging approximately 1 million gallons per day from a spring. Tritium analysis of the spring water yielded 14 tritium units and suggests a short residence time. Fractures and karst are likely conduits for the rapid transmission of spring water.

The regional geologic structure consists of Laramide-age folds in the Precambrian and Paleozoic rocks that have been uplifted along the range-front normal fault of the Gallatin Range. The geologic mapping of this area was done in the late 1950s and was in need of revision, so field mapping was performed throughout the study area. It was determined that the Madison aquifer is at the crest of a large anticline and is highly fractured along the hinge line. A stream runs along the hinge line and the fractured aquifer accepts significant recharge from the stream. The anticline plunges south, toward the high peaks of the Gallatin Range.

Potential drilling locations have been identified for the City. These drilling locations were constrained by the geology, terrain and recharge locations. This research sets the stage for the City of Bozeman to make a well-informed decision about whether groundwater should be a part of the water supply in the future. Moreover, this research can serve as a model for water resources development in other municipalities in the Rocky Mountains.