Ground-water appraisal of sand plains in Benton, Sherburne, Stearns, and Wright counties, central Minnesota

Surficial-sand aquifers in 960 square miles of central Minnesota have been studied to determine the occurrence, availability, and suitability of the surficial aquifer as a source of water. The aquifer is being increasingly developed for irrigation.

During the drought of 1976, nearly 24,000 acre-feet of ground water was withdrawn for irrigation, more than double that of the previous year. The number of irrigation pumping centers more than doubled from 1975 to 1977. Nearly all water is pumped from drift aquifers, except in the eastern parts of Sherburne and Wright Counties, where Paleozoic sandstone beds are a reliable source.

Physical and hydrologic properties of the surficial aquifer were determined by test augering, pumping tests, and laboratory sieve analyses. The aquifer is predominantly medium to coarse sand with lesser amounts of gravel in much of the study area. The Sauk River valley in Stearns County is nearly 50 percent poorly sorted gravel of irregular thickness. Saturated thickness of sand in the Maine Prairie area locally exceeds 100 feet, and transmissivity exceeds 40,000 feet squared per day. Similar deposits in Sherburne County exceed 80 feet in thickness, transmissivity exceeds 30,000 feet squared per day, and wells theoretically could yield 2,000 to 3,000 gallons per minute. Theoretical well yields of less than 100 gallons per minute can be expected where saturated thickness is less than 20 feet and transmissivity is less than 5,000 feet squared per day. Pumping tests indicate horizontal to vertical ratios of hydraulic conductivity ranging from 2-27:1.

Average annual precipitation is 27 inches, about 8 of which is recharge to the surficial aquifer. Regional ground-water movement is toward the Mississippi River, which transects the area. Tributary streams and lakes act as controls for local flow systems. At extreme low flow in August 1976, mainstem gains in streamflow in the Elk, St. Francis, Sauk, and Mississippi Rivers averaged 0.4, 0.4, 0.2, and 2.5 cubic feet per second per river mile, respectively.

Ground water is of the calcium bicarbonate type and is suitable for most uses. Relatively high nitrate and chloride concentrations occur in a heavily irrigated area in Sherburne County.

Surficial aquifers in Sherburne County and the Maine Prairie area of Stearns County were simulated by two-dimensional digital ground-water-flow models. Calibration was achieved by matching calculated water-table heads and streamflow gains with observed field values. Aquifer responses to pumping stresses under present and hypothetically expanded development were determined for average and below average recharge conditions.

Irrigation withdrawals for 1977 totaling 15.6 cubic feet per second from 96 pumping centers were included in the Sherburne steady-state model. Increasing withdrawals to 52.2 cubic feet per second from 153 pumping centers would lower regional water levels as much as 8 feet within a few years at normal recharge rates.

Irrigation withdrawals in 1977, totaling 2.0 cubic feet per second from 19 pumping centers, were included in the Maine Prairie steady-state model. Increasing withdrawals to 10.8 cubic feet per second from 42 pumping centers would lower regional water levels as much as 18 feet at normal recharge rates.

Both modeled areas will support additional withdrawals, but caution must be exercised because lowering ground-water levels will also lower lake levels and reduce streamflow. In some areas, aquifer dewatering will reduce individual well yields.