Hydrology and Water Quality of the Grand Portage Reservation, Northeastern Minnesota, 1991-2000

The Grand Portage Reservation is located in northeastern Cook County, Minnesota at the boundary between Minnesota, USA, and Ontario, Canada. Between 1991 and 2000 the U.S. Geological Survey conducted a series of studies, with the cooperation with Grand Portage Band of Chippewa, to describe the water resources of the Grand Portage Reservation.

Ground water moves primarily through fractures in the bedrock, probably in three ground-water systems: local, regional, and deep. Lake Superior is thought to be the discharge point for brines in the deep ground-water flow system.

The watersheds in the Grand Portage Reservation are small and steep; consequently streams in the Grand Portage Reservation tend to be flashy. Lake stages rise and fall with rainfall.

The pH of water in the Reservation is generally alkaline (pH greater than 7.0). The alkalinity of water in the Reservation is low. Concentrations of major ions are much greater in ground water than in spring water and surface water.

The ionic composition of water in the Reservation differs depending upon the source of the water. Water from 11 of the 20 wells sampled are a calcium-sodium-chloride type. Water from wells GW-2, GW-7, and GW-11 had much greater specific conductance concentrations of major ions compared to the other wells. Some spring water (SP-1, SP-3, SP-4, SP-6, and SP-8) is calcium-bicarbonate type like surface water, whereas other spring water (SP-5 and SP-7) is similar to the calcium-sodium-chloride type occurring in samples from about one-half the wells. The major chemical constituents in surface water are bicarbonate, calcium, and magnesium.

Measured tritium and sulfur hexafluoride (SF6) concentrations in water samples from springs and wells were used to determine the recharge age of the sampled water. The recharge ages of two of the wells sampled for tritium are before 1953. The recharge ages of the remaining 10 samples for tritium are probably after 1970. The recharge ages of seven SF6 samples were between 1973 and 1998.