HYDROGEOLOGIC INVESTIGATION OF THE FOUR CORNERS STUDY AREA GALLATIN COUNTY, MONTANA

The purpose of the Four Corners Groundwater Investigation was to assess the possible hydrologic effects of land use conversion from irrigated agriculture to high-density residential. Subdivisions, rural residential and commercial development are transforming agricultural land in the area of this busy community four miles west of Bozeman, Montana. Commercial water distribution and wastewater treatment systems are replacing irrigated land and traditional well and septic systems.

Water for domestic and commercial use in the Four Corners area is typically obtained from the local alluvial aquifer. Irrigation water is almost exclusively diverted from the Gallatin River and local tributaries. Both applied irrigation water and canal leakage are known to recharge groundwater and affect subsequent late-season surface-water return flows. If irrigation is significantly curtailed groundwater quantities and water levels could be affected.

Management of water resources in this area necessitates definition of groundwater flow directions (including both horizontal and vertical gradients) and the hydrologic relationship between the aquifer and the river. Specifically, the main objectives of the Four Corners investigation were to:

• Determine the extent of alteration to the groundwater system in the Four Corners Area over the last 60 years.

• Correlate groundwater flow changes to land use conversion.

• Document the effects of irrigation and canal leakage on groundwater recharge.

• Develop a numeric model to evaluate effects of future changes and development.

Monitoring wells and stream gauging sites were installed, aquifer tests conducted and water samples analyzed to support the objectives. This study concludes that, although groundwater elevations have not significantly changed, the groundwater flow system is highly dynamic and small changes in hydraulic gradients caused by individual stresses likely have a greater impact to groundwater flow than can be discerned from static water levels. If additional stresses reduce groundwater flow entering the study area, or if development continues to increase demand on groundwater while decreasing agricultural recharge, flow through the aquifer will continue to decrease.