A 40-year nuclear legacy
Between 1949 and 1989, the Soviet Union conducted 456 nuclear tests at the Semipalatinsk Test Site (STS), an 18,500-square-kilometer area of the Kazakh steppe. Of these, 340 were underground explosions. Many of these subterranean tests took place within the Degelen Mountain complex, where nuclear devices were detonated inside 181 horizontal tunnels drilled into the granite massif.
Each underground blast created an immense cavity of molten rock which, upon cooling, was supposed to trap the majority of the resulting radioactive material. This containment was not always successful. The explosions extensively fractured the surrounding rock, creating new pathways for groundwater to enter the detonation chambers. Over decades, this water has been slowly leaching radioactive isotopes out of the vitrified rock and carrying them into the regional aquifer systems.
The slow migration of radioactive water
The main contaminant in the groundwater is tritium (3H), a radioactive isotope of hydrogen. Because it becomes part of the water molecule itself, tritium moves through the aquifer at the same speed as the groundwater. Monitoring shows this contaminated water is migrating at a rate of about 50 meters per year. Tritium's half-life of 12.3 years means it remains a hazard for many decades. Water samples from some test tunnels in the Degelen mountains show tritium concentrations up to 260 kBq/L.
Other, more dangerous radionuclides are also present. Strontium-90, cesium-137, plutonium isotopes, and americium-241 have all been detected in the soil and groundwater. While these heavier isotopes move much more slowly than tritium, their presence confirms that the underground test cavities are a long-term source of contamination for the Shagan and other local rivers that flow into the major Irtysh River. The slow but steady movement of these plumes means the extent of the contamination is still growing, decades after the last test explosion.