Chernobyl-Born Radionuclides In Geological Environment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 26

groundwater flow parameters and solution of the whole (plane and vertical)
problem is important. In particular, this is explained by the necessity of regional
long-term predictions of groundwater resources and quality, especially in condi-
tions of their intensive exploitation during the postaccidental period.
Starting from the concept of the existence of downward radionuclide
transport from the surface with infiltration water, the geological rocks along the
filtration pathway could accumulate definite amounts of radionuclides, in such a
way being a protection barrier as well as a potential secondary source of
contamination for deeper aquifers.
After the Chernobyl accident, the surface contamination with 137 Cs and 90 Sr
in the vicinity of the Chernobyl NPP reached several MBq per square meter and
several tens of kBq at farther distances.
According to the results of studies implemented soon after the Chernobyl
NPP accident in April 1986 [ Borzilov , 1989; Baryakhtar et al ., 1997], it was
assumed that radioactive contamination coming to the soil surface by atmos-
pheric fallout is concentrated in the upper soil layer, and its lower boundary
gradually deepens with time. It is obvious that different-scale heterogeneities of
the geological medium as well as relief variability result in the existence of
various preferential pathways of water infiltration and radionuclide migration
from the contaminated soil into the groundwater. Such pathways are distinguished
from the average “background” geological medium by the intensity of the
migration process as determined by different velocities of downward radionuclide
migration from the upper soil layers into the unsaturated zone and groundwater
aquifers.
The separate sampling results of radionuclide concentrations in the soil layer
and unsaturated zone show that in the most studied areas these concentrations
are relatively small and usually do not exceed 1% of the initial surface
concentration. For this reason, they are not taken into account in the balance cal-
culations. However, when accounting for a possible occurrence of PFMZs, the
total transported amount of radionuclides into groundwater can become
significant even at a relatively high depth.
This preliminary conclusion has been confirmed by the results of postacci-
dent studies of groundwater contamination with Chernobyl-related radionuclides
90 Sr and 137 Cs [ Shestopalov et al ., 1992, 1996, 1997].
The results of field research and experiments implemented in the study area
of the Kyiv region and CEZ showed that the most probable PFMZ occurrence is
related to closed depressions in the relief, which exert a significant influence on
the intensity of vertical infiltration and transport of radionuclides from the
contaminated surface into groundwater. Their occurrence and activity increase in
the study area from south to north and northeast in relation with changing the
types of geomorphological conditions.
The southern area (to the latitude of Kyiv on the right bank of the Dnieper)
is located in the Dnieper Upland. Its unsaturated zone is composed of black soils