Sand and gravel aquifers present within glacial deposits are important sources of water in Marshall, Pennington, Polk, and Red Lake Counties in northwestern Minnesota. Saturated thicknesses of the unconfined aquifers range from 0 to 30 feet. Estimated horizontal hydraulic conductivities range from 2.5 to 600 feet per day. Transmissivity of the unconfined aquifers ranges from 33 to greater than 3,910 feet squared per day. Theoretical maximum well yields for 6 wells with specific-capacity data range from 12 to 123 gallons per minute.
Saturated thicknesses of shallow confined aquifers (depth to top of the aquifer less than 100 feet below land surface) range from 0 to 150 feet. Thicknesses of intermediate, deep, and basal confined aquifers (depths to top of the aquifer from 100 to 199 feet, from 200 to 299 feet, and 300 feet or more below land surface, respectively) range from 0 to more than 126 feet. Transmissivity of the confined aquifers ranges from 2 to greater than 210,000 feet squared per day. Theoretical maximum well yields range from 3 to about 2,000 gallons per minute.
Recharge to ground water is predominantly from precipitation that percolates downward to the saturated zone. Recharge to unconfined aquifers in the study area ranged from 4.5 to 12.0 inches per year during 1991 and 1992, based on hydrograph analysis. Model simulations done for this study indicate that recharge rates from 8 to 9 inches per year to unconfined aquifers produce the best matches between model-simulated and measured water levels in wells.
Discharge from ground water occurs by seepage to streams, lakes and wetlands, ground-water evapotranspiration, and withdrawals through wells. In 1990, total ground-water withdrawals in the study area were 6.0 million gallons per day. All of the withdrawals were from drift aquifers.
Numerical models of ground-water flow were constructed to represent two beach-ridge aquifer systems under steady-state conditions. Beach-ridge aquifer systems were simulated in Pennington, Polk, and Red Lake County. Simulated recharge from the infiltration of precipitation accounts for most of the sources of water to the beach-ridge aquifer systems and simulated evapotranspiration accounts for all of the discharge other than ground-water withdrawals. The numerical-model simulations indicate that upward movement of water from underlying confined aquifers to overlying unconfined aquifers is an important component of ground-water flow within the beach-ridge aquifer systems. Simulated long-term, steady-state yields from the unconfined aquifers are generally less than 50 gallons per minute, due to the generally low saturated thickness of the aquifers and the relatively low hydraulic conductivity of the aquifer material.
Water from all the drift aquifers in the study area is very hard (more than 180 milligrams per liter of calcium carbonate). The predominant ions in water from the unconfined and shallow confined aquifers were generally calcium and bicarbonate. Water from the intermediate confined aquifers includes a variety of water types, including calcium bicarbonate, calcium sulfate, mixed calcium-sodium bicarbonate, and sodium chloride type waters. Waters from the deep confined aquifers are predominantly calcium bicarbonate, mixed calcium-sodium bicarbonate, and sodium chloride type waters.
Mean concentrations of calcium and magnesium generally decreased with depth below land surface. Mean concentrations of sodium and sulfate generally increased with depth. Mean chloride concentrations were greatest for the shallow and deep confined aquifers and least for the unconfined and intermediate confined aquifers.
The concentration and percentage (as percent of total cations) of sodium, and concentration of dissolved solids tend to increase from east to west along regional flow paths. Concentrations and percentages (as percent of total anions) of chloride tend to be greater in the western part of the study area than in the eastern part. These trends are probably due to longer residence time of the water in the flow system, and upward leakage of water from the underlying Cretaceous and Paleozoic strata.
Waters from the drift aquifers underlying most of the study area generally are suitable for domestic consumption, crop irrigation, and most other uses. Water from 20 wells screened in unconfined and confined aquifers exceeded U.S. Environmental Protection Agency recommended limits for dissolved solids concentrations. Chemical analyses of waters from the unconfined and confined aquifers generally indicated a potentially low sodium hazard and a medium to high salinity hazard for irrigation.
Water samples analyzed for nitrate had nitrate concentrations below the reporting limit (0.05 milligrams per liter) in 10 out of 23 wells. Two samples had nitrate concentrations greater than 10 milligrams per liter. Pesticide concentrations in water samples from 17 wells screened in unconfined and shallow confined aquifers were below or only slightly above laboratory reporting limits.