Proterozoic bedrock aquifers are the principal source of water supply in the Grand Portage Indian Reservation. The bedrock aquifers are the (1) North Shore Volcanic Group; (2) Keweenawan Volcanic and intrusive rocks and (3) Rove Formation. Sand and gravel aquifers are a small source of water. The median estimate of transmissivity determined from data for 17 wells completed in bedrock was 17 feet squared per day; the range was from 3 to 500 feet squared per day. Estimated transmissivity did not show a clear trend in variation with depth of bed- rock penetrated by the well borehole. 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. Hydrofracturing of two wells completed in bedrock increased their yield from about 0.25 and 0.05 gallons per minute before hydrofracturing, to about 1.2 and 1.5 gallon per minute, respectively, after hydrofracturing. Although the estimated yield from the two wells was increased by nearly 5 and by 30 times from hydrofracturing, the well yield after hydro- fracturing 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 and dissolved chloride that were higher than their respective SMCLs (Secondary Maximum Con- taminant Levels) established by the USEPA (U.S. Environmental Protection Agency). Concentrations of dissolved arsenic, barium, chromium, copper, silver, zinc, and selenium in water from the nine wells were less than their respective USEPA MCLs (Maximum Contaminant Levels) and USEPA SMCLs. The concentration of dissolved lead in water from the nine wells was less than the detection limit of 100 ug/L (micrograms per liter). Water from an observation well located about 200 feet 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 ug/L). The presence of toluene suggested possible contamination.
Additional publication details
USGS Numbered Series
Water resources of the Grand Portage Indian Reservation, northeastern Minnesota
Water-Resources Investigations Report
U.S. Geological Survey ;
USGS Earth Science Information Center, Open-File Reports Section [distributor],