The Grand Portage Indian Reservation Tribal Council needs information about the availability and quality of the ground water in the Reservation to develop, protect, and manage this resource for future use. The U.S. Geological Survey, in cooperation with the Grand Portage Indian Reservation Tribal Council, did a three-year study of the ground water in the Reservation to provide this needed information. This report presents the results of that study.

Presently, ground water from bedrock is the principal source of supply for municipal, commercial, and residential water use. The bedrock aquifers are the (1) North Shore Volcanic Group basalt, (2) Keweenawan Volcanic and intrusive rocks, which are gabbro and diabase, and (3) Rove Formation argillite, slate, and graywacke. Sand and gravel aquifers are a small source of ground water.

The storage coefficient of the bedrock aquifers was estimated to be 1 x 10^-4, which is a small value typical of confined, fractured rock aquifers. The median estimate of transmissivity determined from data for 17 wells completed in bedrock was 20 feet squared per day; the range was from 3 to 500 feet squared per day. Reported yield of 19 wells completed in bedrock had a range of 1 to 100 gallons per minute and a median of 7 gallons per minute. The median yield of 11 wells completed in the North Shore Volcanic Group was 16 gallons per minute; the median yield of 8 wells completed in the Keweenawan Volcanic and intrusive rocks and Rove Formation was 4 gallons per minute.

Geophysical logs and televiewer images of two wells completed in bedrock indicated the boreholes penetrated many fractures. Hydrofracturing of the two wells increased their yield from about 0.05 and 0.25 gallons per minute to about 1.5 and 1.2 gallons per minute, respectively. Although the estimated yield from the two wells was increased by 30 and by nearly 5 times after hydrofracturing, the well yield after hydrofracturing was still small.

Water types determined from analyses of water from nine wells completed in bedrock were sodium-chloride, calcium-chloride, sodium-bicarbonate, and calcium-bicarbonate. Water from three wells had concentrations of dissolved solids (800 to 3,110 milligrams per liter) and dissolved chloride (410 to 1,600 milligrams per liter) that were higher than their respective Secondary Maximum Contaminant Levels of 500 and 250 milligrams per liter established by the U.S. Environmental Protection Agency. Water from two wells had concentrations of dissolved iron (1,600 and 1,300 micrograms per liter) that were higher than the Secondary Maximum Contaminant Level of 300 micrograms per liter. Water from an observation well located about 200 feet downgradient from an abandoned landfill and screened from 79 to 84 feet below land surface in a gravel aquifer had a trace amount of toluene (0.2 micrograms per liter). The presence of toluene suggested possible contamination.