GROUNDWATER MODELING OF THE FOUR CORNERS STUDY AREA GALLATIN COUNTY, MONTANA

A numerical groundwater flow model was developed as part of the Four Corners, Montana Ground Water Investigation Program project. The primary purpose of this project was to evaluate the effects of the conversion from irrigated agricultural land to residential and commercial uses on groundwater and surface water resources. Subdivisions, rural residential and commercial development are transforming agricultural landscape of this community four miles west of Bozeman. The numerical model design was based on a conceptual model of the study area including an analysis of water budget components, geologic data, aquifer properties, potentiometric maps, and surface water flow and stage measurements. Streams and canals interact with groundwater in the model interior, and precipitation and irrigation contribute aquifer recharge in the model.

The model was used to replicate baseline hydrogeologic conditions described in a previous USGS study (Hackett et. al, 1960). Comparison of model results showed little change in water table elevations from 1960 to 2011, a conclusion also drawn by Slagle in the early-1990s in comparing the preceding 45 years. Three additional scenarios were modeled to predict possible future changes in stress on the system 1) a decrease in recharge to the overall system; 2) expansion of urban development (with decreasing agricultural use); and 3) a hypothetical aquifer storage and recovery system.

The model results agree with the other study findings that, although groundwater elevations have not significantly changed, the groundwater flow system is highly dynamic and individual stresses likely have a greater impact than can be discerned from the valley-scale potentiometric surface. If future climatic conditions create reduced flow conditions entering the Four Corners area, and residential/commercial development continues to intensify the demand on groundwater and a decrease in agricultural recharge, flow through the aquifer will continue to decrease.