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Radiological Investigations of an Abandoned Uranium Tailings Area¹

ABSTRACT

Tailings have been accumulating in the Elliot Lake area since mining started some 20 years ago. The tailings solids were impounded by construction of dams, resulting in the discharge of a clear and neutral outflow which at that time was considered to be a satisfactory practice. However, acid generated in the tailings by oxidation of pyrite has solubilized certain heavy metals which can escape through the semiporous embankments or as outflow during heavy rainfalls. It is now evident that these areas, some of which have been idle for over 12 years, release acids and certain naturally occurring radionuclides to an unacceptable degree. In most cases the acid tailings outflows are neutralized to pH 8.5 to precipitate these heavy metals. This neutralization will have to be continued indefinitely, unless a better remedy is found.

An investigation is underway at a 16-ha pyrite-containing uranium tailings area to determine what happens to certain radioactive elements. The analysis of solid samples showed the activities of ²²⁶Ra, ²¹⁰Pb, and ²¹⁰Po were lower than could be accounted for by their radioactive decay since disposal. Uranium and thorium also have been significantly leached out of the tailings surface. The collection and analysis of water samples from the water table beneath the tailings area revealed two classes of water: a low-acid class (<1,000 ppm acidity) and a high-acid class. The acidity of the water was directly related to the weathering of pyrite in the surface layers of the tailings. The high-acid water samples generally contained higher concentrations of ²¹⁰Pb, ²³²Th, ²³⁰Th, ²²⁰Th, and uranium than low-acid water samples. The low-acid water samples contained only elevated concentrations of ²²⁶Ra which, if released untreated, would exceed water quality criteria. Seepage samples below the tailings dam were also monitored and analyzed, and may exceed desirable water quality criteria for ²²⁶Ra and ²¹⁰Pb if not treated.

Key Words: uranium tailings • water pollution • radioactivity • ²²⁶Ra • natural radioactivity

¹ Contribution from Elliot Lake Lab., Mining Res. Lab., Canada Centre for Mineral and Energy Technol. (CANMET), Dep. of Energy, Mines and Resour., Elliot Lake, Ontario.

² Research Scientist (present address, Rio Algom Ltd., P.O. Box 1500, Elliot Lake, Ontario, Canada) and Technician, respectively, Elliot Lake Lab.

Received for publication June 17, 1977.