Title
Groundwater Use in the Clark Fork Basin, Montana
Abstract
The Groundwater Information Center database (GWIC) has records of 85,400 wells in the Clark Fork Basin, most (73,200) have a reported use of “domestic”, that is, they are wells that serve individual households. Other wells provide water for irrigation, municipal, industrial/commercial, and stockwater use. Although the number of wells seems large, groundwater accounts for only 7 percent of the water withdrawals in the basin. Surface water dominates the water withdrawals; an estimated 1,510 million gallons per day (MGD) of surface water is withdrawn (93 percent), primarily for irrigation use, as compared to 112 MGD of groundwater. Of the groundwater withdrawn, most is used for irrigation, municipal water, and industrial uses. Domestic use accounts for only 8 percent of groundwater withdrawals.
In the Clark Fork Basin, there are generally two types of aquifers: (1) shallow water table and (2) deeper confined to semi-confined. The extent of basin-fill aquifers generally coincides with the extent of basin-fill deposits. Near-surface sand and gravel deposits (mostly Quaternary alluvium) coincident with the floodplains of main-stem streams contain very permeable alluvial water-table aquifers that store and yield large volumes of water. These aquifers are generally in hydraulic connection with adjacent streams. The deeper confined-semi-confined aquifers occur in layers of sand and gravel separated by silt and clay.
Pumping groundwater will remove water from aquifer storage. Long-term water-level data, collected as part of the Montana Groundwater Assessment Program, show changes in groundwater storage in the basin-fill aquifers across the Clark Fork Basin. The data show that groundwater fluctuations in basin-fill aquifers vary with regard to the timing and magnitude of recharge and discharge. Most long-term trends are stable, indicating little or no depletion of basin-fill aquifer storage. Water levels in parts of the highly utilized deep aquifer in the Flathead Valley show long-term declines.
In groundwater systems, water removed by pumping is derived from aquifer storage, and some combination of increased recharge and/or decreased groundwater discharge (as base flow to streams). During the months of December, January, and February the steady low flows shown in stream hydrographs represent the “base flow” period, when groundwater contributes most, if not all stream flow. Annual December-February flows were averaged to provide a measure of the base-flow rate at stations on the Clark Fork, Bitterroot, Swan, and Blackfoot Rivers. The base-flow graphs show variability from year to year but flat long-term trends.
Start Date
5-3-2010 12:00 PM
End Date
5-3-2010 2:00 PM
Document Type
Poster
Groundwater Use in the Clark Fork Basin, Montana
The Groundwater Information Center database (GWIC) has records of 85,400 wells in the Clark Fork Basin, most (73,200) have a reported use of “domestic”, that is, they are wells that serve individual households. Other wells provide water for irrigation, municipal, industrial/commercial, and stockwater use. Although the number of wells seems large, groundwater accounts for only 7 percent of the water withdrawals in the basin. Surface water dominates the water withdrawals; an estimated 1,510 million gallons per day (MGD) of surface water is withdrawn (93 percent), primarily for irrigation use, as compared to 112 MGD of groundwater. Of the groundwater withdrawn, most is used for irrigation, municipal water, and industrial uses. Domestic use accounts for only 8 percent of groundwater withdrawals.
In the Clark Fork Basin, there are generally two types of aquifers: (1) shallow water table and (2) deeper confined to semi-confined. The extent of basin-fill aquifers generally coincides with the extent of basin-fill deposits. Near-surface sand and gravel deposits (mostly Quaternary alluvium) coincident with the floodplains of main-stem streams contain very permeable alluvial water-table aquifers that store and yield large volumes of water. These aquifers are generally in hydraulic connection with adjacent streams. The deeper confined-semi-confined aquifers occur in layers of sand and gravel separated by silt and clay.
Pumping groundwater will remove water from aquifer storage. Long-term water-level data, collected as part of the Montana Groundwater Assessment Program, show changes in groundwater storage in the basin-fill aquifers across the Clark Fork Basin. The data show that groundwater fluctuations in basin-fill aquifers vary with regard to the timing and magnitude of recharge and discharge. Most long-term trends are stable, indicating little or no depletion of basin-fill aquifer storage. Water levels in parts of the highly utilized deep aquifer in the Flathead Valley show long-term declines.
In groundwater systems, water removed by pumping is derived from aquifer storage, and some combination of increased recharge and/or decreased groundwater discharge (as base flow to streams). During the months of December, January, and February the steady low flows shown in stream hydrographs represent the “base flow” period, when groundwater contributes most, if not all stream flow. Annual December-February flows were averaged to provide a measure of the base-flow rate at stations on the Clark Fork, Bitterroot, Swan, and Blackfoot Rivers. The base-flow graphs show variability from year to year but flat long-term trends.