Thirty-nine years of effluent had saturated the lake with nasty isotopes, including an estimated 120 megacuries of long-lived radiation. In contrast, the Chernobyl incident released roughly 100 megacuries of radiation into the environment, but only about 3 megacuries of Strontium-90 and Cesium-137. A delegation who visited Lake Karachay in 1990 measured the radiation at the point where the effluent entered the water, and the needles of their Geiger counters danced at about 600 Röntgens per hour–enough to provide a lethal dose in one hour. They did not linger long.
The Worldwatch Institute calls it the “most polluted spot” in history. Damn Interesting has the story. http://www.damninteresting.com/?p=973