Ground-water/surface-water interaction in nearshore areas of Three Lakes on the Grand Portage Reservation, northeastern Minnesota, 2003-04
The U.S. Geological Survey (USGS), in cooperation with the Grand Portage Band of Chippewa Indians, applied three techniques to assess ground-water/surface-water interaction in nearshore areas of three lakes (North, Teal, and Taylor) on the Grand Portage Reservation in northeastern Minnesota. At each lake, analyses of existing aerial photographs, in-situ temperature measurements of shoreline lake sediment, and chemical analyses of surface water and pore water were conducted. Surface-water and pore-water samples were analyzed for major constituents, nutrients, and stable isotopes of oxygen and hydrogen. Bulk precipitation samples were collected and analyzed (1) for nutrient concentrations to determine nutrient input to the lakes through atmospheric deposition and (2) for stable isotope ratios of oxygen and hydrogen to determine a meteoric waterline that was needed for the stable isotope analyses of surface-water and pore-water samples. Total nitrogen concentrations in the precipitation samples ranged from 0.51 to 8.4 mg/L (milligrams per liter) as nitrogen at the North Lake precipitation station and from 0.42 to 2.3 mg/L as nitrogen at the Grand Portage precipitation station. Oxygen-18/oxygen-16 and deuterium/protium isotope ratios for the bulk precipitation samples lie relatively close to a meteoric waterline for northern Wisconsin, except for the ratios for samples collected on May 20, 2004. Analyses of existing aerial photographs, nearshore lake-sediment temperatures, and seasonal isotope ratios of surface-water and pore-water samples were the most valuable data for identifying locations of ground-water inflow and surface-water outseepage. Analyses of existing aerial photographs of the three lakes indicated the location of potential inflow channels and lineaments identifying potential ground-water inflow locations for pore-water sampling. Lake-sediment temperatures at potential ground-water inflow locations ranged from 4 to 16 ºC, varying between lakes, seasons, and climatic conditions. Major constituent chemistry was valuable at Taylor Lake, and to a limited extent at North and Teal Lakes, in confirming results from the isotope and lake-sediment temperature data. Ground-water inflow to North Lake likely occurs along the southwest and south shores, and along portions of the west, southeast, north, and northeast shores. Relatively cool lake-sediment temperatures along the southwest, south, west, and southeast shores, and in isolated beaver channels along the north and northeast shores of North Lake indicate potential ground-water inflow at these locations. Both localized ground-water inflow and surface-water outseepage occurs along portions of the north, northeast, southeast, and south shores, varying seasonally. Conflicting evidence for ground-water flow conditions exist for the northwest and north-northwest pore-water samples. Only minor differences in the major constituent concentrations were seen between the surface-water and pore-water samples from the North Lake area with the exception of iron and manganese concentrations. Ground-water inflow likely takes place along the south-southwest and north shores of Teal Lake, with a mixture of ground-water inflow and surface-water outseepage occurring in other areas of the lake. Cooler lake-sediment temperatures occurred along the south-southwest, west, and northwest shores, portions of the north shore, and in channels identified in aerial photographs throughout the lake, indicating potential ground-water inflow at those locations. Warmer lake-sediment temperatures along the northeast and portions of the southwest and northwest shores of Teal Lake indicate potential locations where surface-water outseepage or little ground- and surface-water interaction occurs. The major constituent concentrations were higher in the pore-water samples collected from the south-southwest and northeast shores of Teal Lake, indicating ground-water inflow. Cation adsorption, cation exchanges with hydrogen ions, and chelation with organic materials occurring in the fen surrounding the lake likely resulted in the low dissolved calcium, magnesium, and sodium concentrations in north, northwest, and west pore-water samples from the Teal Lake area. Pore-water samples from the south-southwest, north, and southwest shores of Teal Lake had isotopic compositions that plotted closest to the meteoric waterline, indicating that little evaporation or transpiration occurred in these samples and that ground-water inflow may be occurring at these locations. Surface-water outseepage from Teal Lake likely occurs along the northeast shore even though major constituent concentrations were high. Major constituent concentrations may be high because of a nearby beaver dam. Ground-water inflow to Taylor Lake likely occurs at the north and south pore-water sampling sites. Higher major constituent concentrations and the least evaporative isotope ratios were found in pore-water samples along the south, north, and west shores of Taylor Lake, indicating potential locations of ground-water inflow. However, a combination of warmer and cooler lake-sediment temperatures along the west lowland indicated that ground-water inflow and surface-water outseepage may occur at that location. Surface-water outseepage likely occurs from Taylor Lake along the south shore through a surface-water drainage channel to a downgradient bog. Warmer lake-sediment temperatures along portions of the south and southeast shores indicate that surface-water outseepage may occur at those locations. Both ground-water inflow and surface-water outseepage may occur along the west, southeast, and east shores of Taylor Lake, varying seasonally and with local precipitation. Knowledge of general water-flow directions in lake watersheds and how they may change seasonally can help water-quality specialists and lake managers address a variety of water-quality and aquatic habitat protection issues for lakes. Results from this study indicate that ground-water and surface-water interactions at the study lakes are complex, and the ability of the applied techniques to identify ground-water inflow and surface-water outseepage locations varied among the lakes. Measurement of lake-sediment temperatures proved to be a reliable and relatively inexpensive reconnaissance technique that lake managers may apply in complex settings to identify general areas of ground-water inflow and surface-water outseepage.