U.S. Geological Survey Water Resources Investigations Report 02-4023
Water managers are concerned about the increase of ground-water withdrawals from high-capacity wells completed in the uppermost confined aquifers in southern Wadena County. The hydrogeologic units of primary interest in the study area are the surficial aquifer, the uppermost confining units, and the uppermost confined aquifers. The surficial aquifer underlies all but portions of the eastern, western, and south-central parts of the study area, and is as much as 70 ft thick. The thickness of the uppermost confined aquifers ranges from 0 to 72 ft. The thickness of the aquifers is greatest in the south-central and west-central parts of the study area, where thicknesses exceed 50 ft. Depth to the top of the uppermost confined aquifers ranges from 23 to 132 ft. The thickness of the uppermost confining units ranges from 4 to 132 ft.
The regional direction of flow in the uppermost confined aquifers is to the east, southeast, and southwest toward the Crow Wing River in the eastern part of the study area and toward the Leaf River in the western part. Sources of water to the uppermost confined aquifers are leakage of water through overlying till and clay and ground-water flow from adjoining aquifers outside the study area. Discharge from the uppermost confined aquifers is by withdrawal from wells and to the surficial aquifer in river valleys. The theoretical maximum well yields for the uppermost confined aquifers range from less that 175 gal/min to greater than 2,000 gal/min and are greatest in areas of greatest aquifer thickness and transmissivity.
The water budget for the calibrated steady-state simulation indicated that areal recharge to the surficial aquifer is 86.9 percent of the water to the aquifers, with leakage to the uppermost confined aquifers contributing 6.9 percent. The largest discharges from the aquifers are leakage to streams (54.5 percent) and ground-water evapotranspiration (41.4 percent). The simulated transient water budget for 1999 indicated that the principal sources of water to the aquifers were areal recharge to the surficial aquifer and release from storage. The principal discharges were stream-aquifer leakage, addition to storage, and ground-water evapotranspiration.
Results of the steady-state simulation with anticipated increases in ground-water withdrawals indicated maximum drawdowns of 0.3 ft in the surficial aquifer and 0.9 ft in the uppermost confined aquifers due to the anticipated increases in ground-water withdrawals. Model results indicate that the anticipated increases in withdrawals during a drought may lower water levels 2 to 4 ft regionally in much of both the surficial and uppermost confined aquifers. Water-level declines in the surficial aquifer of about 6 ft may occur in Wadena and in the central part of the aquifer south of the Leaf River. Results of the transient simulation indicate that the anticipated increases in withdrawals during a drought would increase seasonal declines in the surficial and uppermost confined aquifers less than 1 and 2 ft, respectively.
Model results indicate that greater than anticipated increases in withdrawals during periods of normal precipitation will have minimal effects on ground-water levels and streamflow in the area. In the uppermost confined aquifers, for example, water levels may decline an average of 0.13 ft regionally, with maximum declines of 0.8 to 2.1 ft near Wadena and Verndale. Greater than anticipated increases in withdrawals would cause decreases in ground-water discharge to streams of about 1.4 percent (2.5 ft3/s) of 1998-99 steady-state conditions.
Description of study area
Methods of investigation
Log data, test drilling, and well installation
Water levels and stream discharge
Theoretical maximum well yields
Modeling of ground-water flow
Vertical hydraulic connection between aquifers
Theoretical maximum well yields in uppermost confined aquifers
Simulation of ground-water flow
Numerical model description
Numerical model calibration
Effects of ground-water withdrawals
Anticipated increases in withdrawals
Anticipated increases in withdrawals during a drought
Greater than anticipated increases in withdrawals
Greater than anticipated increases in withdrawals during a drought
Model limitations and accuracy of results
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