Uranium mining was halted in 1968. Operations left behind some 23 separate uranium
tailings dams and 13 waste rock dumps,
[5] poorly designed on unstable hillsides above a town of 20,000 people in an area prone to both landslides and earthquakes, holding a total 1,900,000 cubic metres (67,000,000 cu ft) of material containing radionuclides and heavy metals. No attempt to stabilize or seal the material was done when Soviet mining ceased.
On April 16, 1958, with mining and processing plants still operational, a combination of poor design, neglect, heavy rainfall and a reported earthquake caused the #7 tailings dam at Mailuu-Suu to fail. About 50% of the entire volume of the dam flowed into the swift Mailuu-Suu River, only 30 metres (98 ft) downhill from the breach. The waste then spread about 40 kilometres (25 mi) downstream across the national border into
Uzbekistan then into the heavily populated
Fergana Valley. The Mailuu-Suu River is a tributary of the
Kara Darya, used for agricultural irrigation in the valley.
Some fatalities, building destruction, and contamination of the flood plain were reported as the direct result of the mudflow.
[7] Lack of any public response by officials makes it difficult to identify fatalities from the April 1958 event, especially as distinguished from everyday exposure.
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