Water samples were collected periodically from 124 wells completed in sand-plain aquifers in Hubbard, Morrison, Otter Tail, and Wadena Counties, Minnesota, to determine baseline water quality, provide data for evaluation of trends, and to investigate seasonal variations in concentrations of selected chemical constituents during a 3-year study that began in October 1979. Results of the study show that the water is predominantly a calcium bicarbonate type with low to moderate concentrations of dissolved solids (77 to 710 milligrams per liter), and that it generally is suitable chemically for most uses. Concentrations of most constituents are below limits for domestic consumption recommended by the Minnesota Pollution Control Agency and the U.S. Environmental Protection Agency. Concentrations of nitrite plus nitrate nitrogen, iron, and manganese in localized areas, however, exceed the recommended limits. Comparison of current data to historical data indicates that concentrations of nitrite plus nitrate nitrogen and chloride have increased in local agricultural areas.
Data indicate that concentrations of certain chemical constituents vary seasonally and annually with changes in ground-water levels. This relationship suggests that chemicals infiltrate the land surface and percolate to the water table during major recharge events.
Site-specific investigations at the Staples Irrigation Center near Staples, Minnesota, indicate that, in addition to seasonal variation, nitrite plus nitrate nitrogen and chloride concentrations decreased with depth while iron concentrations increased. Although no direct quantifiable relationship between concentration and depth was found, mean nitrite plus nitrate nitrogen and chloride concentrations were both 15 milligrams per liter near the top of the aquifer and were 0.1 and 3.5 milligrams per liter, respectively, near the bottom; mean concentrations of dissolved iron were 460 ug/L near the top of the aquifer and 3840 ug/L near the bottom. Data show that the stratification with depth of nitrite plus nitrate nitrogen, chloride, and iron exists at least several hundred feet from the source area throughout most of the year. Chemical stratification appears to be greatest during periods of little recharge.
