As early as July 1947, a decision was taken to release liquid radioactive waste from the plutonium extraction plant at Mayak into the River Techa, whose waters flow eventually into the Ob river system and the Arctic Ocean. Tens of thousands of people living along the Techa and Iset rivers were told nothing of this decision until the rivers and their flood-plains had become heavily contaminated.

In November 1951 large-scale discharges into the Techa were halted, after an estimated 2.75 million curies of radioactivity had been poured into the river. In December, the Soviet authorities began evacuating 8000 people from 20 local settlements.

From this time, low and medium-level liquid wastes were poured into a system of ponds centred on the small Lake Karachay. Eventually, the lake became the dumping site for some 120 million curies of radioactivity, out of a total of more than a billion curies that the Mayak combine is estimated to have released into the environment. For purposes of comparison, the amount of radioactivity released in the Chernobyl disaster was about 140 million curies.

However, the deadliest component of either is the concentration of Caesium-137. Chernobyl contained around .085 EBq. Karachay contained around 3.6 EBq, 42 times the amount.