Overview of Water Quality in Selected Basins
Lake Superior Basin. Most of the streams in the Lake Superior Basin have near-pristine water quality, but wood processing facilities and urban centers have increased concentrations of some pollutants in the St. Louis and Rainy Rivers. Nitrogen, sulfate, chloride and suspended sediment concentrations are generally low. Low alkalinity values and dissolved solids concentrations are the result of low solubility of the thin soils and crystalline bedrock in the area.
Red River Basin. In the Red River Basin, dissolved solids and alkalinity are high because of geologic and climatic conditions. Sediments left behind by ancient glaciers and lakes are 200-300 feet thick. During low flow periods, highly mineralized groundwater seeps into the river and increases the load of dissolved solids. Concentrations of dissolved chemical constituents normally fall during spring runoff and after thunderstorms because of dilution. Total nitrogen and phosphorus concentrations are elevated in the south due to agricultural run-off. Some persistent pesticides, such as atrazine and 2,4-D, also have been detected although pesticide concentrations rarely approach drinking or ambient water-quality standards.
Minnesota River Basin. In the Minnesota River, alkalinity is generally high because of calcareous (limestone) glacial sediments. The river is turbid (carries large amounts of sediment) and high in sulfate and carbonate. As in the Red River Basin, highly mineralized groundwater contributes to the riverís dissolved solids load. Contamination from agricultural chemicals also is a major problem.
St. Croix River Basin. The St. Croix is relatively unpolluted. It is slightly colored by humic matter from bog sources, but has low turbidity. Levels of water quality variables are good relative to other waters in the region and generally fall within standards set for the St. Croix River. Concentrations of a few metals, such as mercury, iron, and manganese, exceed standards, but iron and manganese are high because of natural sources.
Mississippi River Basin. In the Mississippi River, concentrations of chemical constituents generally increase from the source to the Iowa border. Differences in agricultural and urban development cause nitrogen concentrations to differ greatly among the sub-basins that make up the upper river system. Headwater streams drain an area that has little agriculture. Areas with more agriculture have higher concentrations of associated pollutants, but these generally do not exceed the limit for untreated drinking-water.
Trends in Water Quality. Trends in stream water quality over time are detected by statistical analysis of monitoring data. No clear statewide trends exist for Minnesota rivers over the last 20 years. Trend analyses show local increases in concentration of some constituents and decreases in others (USGS, 1999). Increases are probably due (primarily) to population pressures, unimproved treatment facilities, nonpoint control practices, and agricultural activities. Increases in chloride concentrations in the Minnesota River probably resulted from road deicing and may be associated with population increases (USGS, 1993). A decreasing trend in dissolved nutrients in the Minnesota River may be attributable to reductions in agriculture activities because of the droughts of the 1980s. Increasing dissolved oxygen concentrations in the Mississippi River south of Minneapolis-St. Paul, for example, resulted from reduced input of oxygen-demanding substances from the metropolitan sewage treatment facility and better control of urban nonpoint runoff.