Date of this Version
Lake waters of the north-central U.S.A. are classified into five groups, based on increasing specific conductivity and changes in ionic composition from east to west, from Wisconsin through Minnesota to North and South Dakota. The most dilute group of waters has specific conductivities <29 μnhos x cm-1 at 25°C; the most concentrated group has specific conductances that range from 7,000 to 73,000 μmhos. As conductivity increases all major ions increase, but there is a shift in cation dominance from Ca2+ to Mg2+ to Na+, and in anion dominance from HCO3- to SO42-. This shift partly reflects a westward increase in climatic aridity, and partly a westward sequence of glacial drifts from noncalcareous to calcareous and thence to calcareous with abundant sulfur-bearing minerals. Levels of pH, K, Cl, F, B, and SiO2also show a distinct westward increase. Concentrations of NO3- and Mn increase from east to west, but the trend is less distinct. Concentrations of Fe vary widely without any trend over the range of conductivity. Color, mostly from dissolved organic matter, is controlled chiefly by lake depth, except for lakes with extensive peatlands in their drainage basins.