The Chernobyl disaster remains the major and most detrimental nuclear catastrophe which completely altered the radioactive background of the Northern Hemisphere. It happened in April 1986 on the territory of the former Soviet Union (modern Ukraine). The catastrophe led to the increase of radiation in nearly one million times in some parts of Europe and North America compared to the pre-disaster state. Air, water, soils, vegetation and animals were contaminated to a varying degree. Apart from Ukraine and Belarus as the worst hit areas, adversely affected countries included Russia, Austria, Finland and Sweden. The full impact on the aquatic systems, including primarily adjacent valleys of Pripyat river and Dnieper river, are still unexplored.
Substantial groundwater contamination is one of the gravest environmental impacts caused by the Chernobyl disaster. As a part of overall freshwater damage, it relates to so-called “secondary” contamination, caused by the delivery of radioactive materials through unconfined aquifers to the groundwater network It proved to be particularly challenging because groundwater basins, especially deep-laying aquifers, were traditionally considered invulnerable to diverse extraneous contaminants. To the surprise of scientists, radionuclides of Chernobyl origin were found even in deep-laying waters with formation periods of several hundred years.
History
Subsurface water was especially affected by radioactivity in the 30-km zone of evacuation (so called “exclusion zone”), surrounding the Chernobyl Nuclear Power Plant, or CNPP (Kovar&Herbert, 1998). The major and most hazardous contaminant from the perspective of hydrological spread was Strontium-90. This nuclide showed the most active mobility in subsurface waters; its rapid migration through groundwater aquifer was first discovered in 1988-1989 Other perilous nuclear isotopes included Cesium-137, Cesium-143, Ruthenium-106, Plutonium-239, Plutonium-240, Americium-241 The primary source of contamination was the damaged 4th reactor, which had actually been a crash site and where concentration of Strontium-90 initially exceeded the admissible levels for drinking water in 103-104 times. The reactor remained an epicenter of irradiation even after the emergency personnel built “Sarcophagus”, or “Shelter”, a protective construction aimed to isolate it from the environment. The structure proved to be non-hermetic, permeable to rainfall, snow and dew concentrations in many parts of 1000 m2 area Additionally, high amounts of cesium, tritium and plutonium were delivered to groundwater due to leakage of enriched water from the 4th reactor while building of the “Shelter” was in progress As a result, considerable amounts of water condensed inside the “Shelter” and absorbed radiation from nuclides-containing dust and fuels. Although most of this water evaporated, some portions of it leaked to groundwater from the surface layers under the reactor chambers.
Other sources of groundwater contamination included: radioactive waste dumps on the territory of “exclusion zone”; cooling water reservoirs connected with aquifer; initial radioactive fallout which took place in first hours after the accident; and forest fires that led to accelerated spread of contaminated particles on soils of the surrounding area On the whole, the researchers recorded the probability of accumulation of nearly 30% of the overall surface contamination in the underground rock medium. This discovery demonstrates hazardous scales of radionuclides underground migration on the one hand, but the important function of igneous rock as protective shield against further spread of contaminants.