The Mille Lacs Lake study area is a 960 mi2 area containing the ground-water and surface-water drainages to both Mille Lacs Lake and the first 12 miles of the Rum River. Within this study area, available ground water occurs in saturated, overlapping, discontinuous, partially-connected, glacially-deposited (hereinafter, glacial) aquifers and in bedrock aquifers. No extensive glacial aquifer could be delineated. Surficial aquifers generally are less than 30 feet thick, but may exceed 78 feet. Lake Onamia is hydraulically connected with surficial aquifers on its north and south sides. Ground water enters Lake Onamia from an adjacent surficial aquifer with a hydraulic head of 1.56 feet near its eastern shore.
Glacial aquifers buried beneath till form the uppermost confined (hereinafter, buried) aquifers and generally are from 3 to 15 feet thick, but may be as much as 118 feet thick. These buried aquifers generally occur beneath 10 to 60 feet of till and clay, but it may be as thick as 208 feet. Individual aquifers can be partially buried by till and therefore may contain surficial and buried areas. Most buried aquifers are isolated or only partially connected to other aquifers. Recharge water moves horizontally and vertically through other aquifers and through confining units to reach these buried aquifers. Discharge from these aquifers is through well withdrawals, flow to surface-water bodies, and leakage to other aquifers. Buried aquifers may yield as much as 500 gallons per minute in some locations. In most areas, the specific capacity of these aquifers is less than 1 gallon per minute per foot but may reach 41.6 gallons per minute per foot.
Ground water from all aquifers is of the calcium magnesium bicarbonate type. Iron, manganese, and sodium in this ground water frequently exceeded U.S. Environmental Protection Agency drinking water standards and health advisories. The sodium health advisory was exceeded in 15 percent of samples.
