Modeling Study of Preferential Vertical Transport Zones within Chernobyl Area and Their Role in Radioactive Contamination of Groundwater

The data are presented on occurrence, distribution and characteristic structure, hydrological and migration properties of relief depression forms in Chernobyl Exclusion zone (CEZ) which behave as sites of preferential infiltration and vertical migration of radioactive contaminants. The phenomenon of fast downward transport of the most environmentally dangerous radionuclides (⁹⁰Sr and ¹³⁷Cs) occurring in topographic depressions with specific hydrological, geophysical and geochemical characteristics (higher conductivity and moisture content of unsaturated zone, disintegration of rocks traced to several hundred meters under the depressions, different composition and geochemical properties of soils as compared to background areas) within the highly contaminated area of Chernobyl exclusion zone is studied during continuous field observations. Results of observations were used to construct several models of different scales for predicting groundwater flow and contamination with account for this preferential transport phenomenon. Results of three-dimensional flow and transport modeling at local scale show significant relative increase of vertical infiltration and mass transport within anomalous depressions as compared to background area. At regional scale the role of a single depression is insignificant for areal contamination of groundwater, but taking into account their frequent occurrence within the CEZ, their integral effect may lead to early post-accidental contamination even of deep highly protected aquifers.