Taconite tailings, a waste product from processing of iron ore, have been deposited in a 2.5-square-mile containment basin near Keewatin, Minnesota, The basin, which is bounded by earthen dikes of compacted drift and clayey bouldery till, contains saturated tailings consisting of chert and other silica-rich particles that range from clay to coarse-sand size.

Runoff from the tailings is slight and occurs only after heavy rains and snowmelt. Average discharge from the basin from April 1982 through June 1984 was about 0.6 cubic foot per second. Instantaneous discharge ranged from zero during much of the period to about 140 cubic feet per second following snowmelt in spring 1982. Daily mean discharge from the basin exceeded 20 cubic feet per second on two days during the period of study.

Water levels in wells range from 0 to 25 feet below the tailings surface; seasonal fluctuations range from 2 to 8 feet. Ground water flows radially from a mound in the north-central part of the basin under a hydraulic gradient of 4.7 x 10^-3 feet per foot. Vertical flow also is downward to drift deposits beneath the tailings. Vertical gradients range from 7.0 x 10^-3 to 6.0 x 10^-1 feet per foot.

Saturated thickness of the tailings ranges from about 1 to 35 feet. Estimated horizontal hydraulic conductivity ranges from about 1 to 500 feet per day. Transmissivities range from about 25 feet squared per day in fine tailings to about 350 feet squared per day in coarse tailings. Ground-water recharge from precipitation was 11.8 inches from October 1982 through September 1983. Ground-water outflow as leakage to the underlying drift deposits was 9.9 inches for the same period.

Water collected from wells completed in the tailings and from the drainage ditch at the basin outlet is of a mixed type in which the magnesium concentration only slightly exceeds concentrations of calcium and sodium plus potassium, expressed in milliequivalents, and concentrations of sulfate and bicarbonate, expressed in milliequivalents, are equal. Concentrations of arsenic, fluoride, and nitrite plus nitrate in water from the tailings were notably greater than in water from adjacent aquifers. However, only fluoride, manganese, and nitrite plus nitrate concentrations equalled or exceeded State drinking-water standards. Suspended-sediment concentrations in streamflow ranged from less than 1 milligram per liter during low-flow periods to about 4,600 milligrams per liter following snowmelt in the spring of 1982.

Numerical-model simulations of ground-water flow near the vicinity of the tailings basin indicate that, if areal recharge were doubled during spring and fall, water levels in wells could average about 4 feet above 1983 levels during these periods. Model results indicate that water levels in the tailings could possibly remain about 5 feet above 1983 levels at the end of the year. Water levels in the tailings at the outlet of the basin could be about 1 foot above 1983 levels during the spring stress period and could be nearly 1.5 feet above 1983 levels during the fall stress period. Under these hypothetical climatic conditions, ground-water contribution to discharge at the outlet could be about 50 cubic feet per second during spring and about 80 cubic feet per second during fall.