Geology and Hydrology of the Elk River Minnesota Nuclear-Reactor Site

The Elk River, Minn., nuclear-reactor site is on the east bluff of the Mississippi River about 35 miles northwest of Minneapolis and St. Paul. The area is underlain by about 70 to 180 feet of glacial drift, including at the top as much as 120 feet of outwash deposits (valley train) of the glacial Mississippi River. The underlying Cambrian bedrock consists of marine sedimentary formations including artesian sandstone aquifers. A hypothetically spilled liquid at the reactor site could follow one or both of two courses, thus: (1) It could flow over the land surface and through an artificial drainage system to the river in a matter of minutes; (2) part or nearly all of it could seep downward to the water table and then move laterally to the river. The time required might range from a few weeks to a year, or perhaps more. The St. Paul and Minneapolis water-supply intakes, 21 and 25 miles downstream, respectively, are the most critical points to be considered in the event of an accidental spill. Based on streamflow and velocity data for the Mississippi River near Anoka, the time required for the maximum concentration of a contaminant to travel from the reactor site to the St. Paul intake was computed to be about 8 hours, at the median annual maximum daily discharge. For this discharge, the maximum concentration at the intake would be about 0.0026 micro curie per cubic foot for the release of 1 curie of activity into the river near the reactor site.