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The unexpected behavior of radionuclides in the
subsurface near the ChNPP has challenged common
wisdom to search for the mechanisms responsible for rapid migration of the
pollutants into the groundwater system. Field
observations and results of preliminary modeling show the important role played
by surface depression zones as areas of anomalously rapid vertical radionuclide
migration to the shallow water table.
Depression zones have hydrogeological, geophysical, and geochemical properties that apparently provide preferential vertical
conduits for water and dissolved contaminants to migrate into
the subsurface.
The depression zones are "collectors"
of radionuclides as overland flow to these zones mobilizes surface
sediments and associated radionuclides that are then redeposited in the
depressions. Consequently, the surface
depressions, while inducing shallow groundwater contamination, may be beneficial
in removing, or scavenging, radionuclides from the landscape. Over time a
non-point source pollution problem is being
naturally converted into a finite number of point sources of contamination. Figure 1 shows a schematic representation of a
typical depression zone and the associated surface catchment
area.
Many depression zones occur within the Chernobyl
Exclusion Zone. They are typically round, elliptical,
or irregular in shape with dimensions ranging from several tens of meters to
several hundred meters in diameter and 3 m to
10 m in depth. They cover about 10% of the CEZ area, though they influence
the distribution of over 60% of surface runoff. Several
of the depression zones have been instrumented, sampled, and geophysically
surveyed by Radioecology Center scientists to
better document their behavior. Radionuclide assessments within these
vertical pathways suggest that the surface depression zones may be responsible
for penetration of a significant portion of
initial and subsequent surface radioactive contamination into shallow aquifers.
In order to understand this hydraulic and
geochemical phenomenon, the vertical mass flux (water and radionuclides)
must be documented as to chromatographic effects, speed of the migration,
effects of organic adsorption, and changes
controlled by rock characteristics and clay mineralogy along with other specific
information. The depression zones are sources of contamination for aquifers from
which water has been extracted for use in and
around the Chernobyl area.
Rock properties are anomalous in these rapid
migration zones because of possible tectonic dislocations,
fracturing, or disintegration of rock materials that appear often as complex and
dense networks. Originally the depression
zones may have been formed as kettles that are characteristic of glaciated
terrain but possibly were complicated by zones of weakness that have become
faults. They could be activated under
influence of suffosion, or other processes. In karst areas in other parts of the
world, such zones are easily recognized and have
been extensively studied. Much less documented are zones
within unconsolidated or highly plastic terrigenous (sandy or clay) rocks, such
as those prevalent in the Chernobyl area. The
depression zones are easily identified by geophysical data (Figure 2).
A more thorough understanding of
the physical and chemical behavior of surface depression zones
in the Chernobyl area is critically important for several reasons. First, there
are significant geotechnical implications for
depression zones related to radioactive waste storage and burial associated
with the continuing aftermath of the Chernobyl accident. Second, the migration
of Cs and 137 Sr to shallow groundwater
within the Chernobyl Exclusion Zone poses an immediate threat to local, 90 rural water supplies. Third, there is a long-term
health risk posed to a large population (i.e., +2 million)
Figure 1.
Schematic representation of a typical depression zone near the Chernobyl NPP.
(Source: Shestapalov, V.M., and A.S.
Bohuslavsky. 2002. "Chernobyl Disaster and Groundwater." A.A. Balkema, Tokyo,
pp.2001-253;
Shestapalov, V.M.,(ed). 2001. "Water Cycle in Hydrogeological Structures and
Chernobyl Disaster." Vol. 1, Kiev, p.242 )
Figure 2.
A section of the shallow deposits of a depression within the Chernobyl Exclusion
Zone imaged by high-frequency (300 MHz) electromagnetic sensing equipment
(SIR-2): Depth = surface to 4 m. Width = 60 m. The depression zone is evident in
the center. persons) via the Kiev water supply from migration of radionuclides
of Chernobyl origin to regional groundwater resources through depression zones.
(Source: Shestapalov, V.M., and A.S.
Bohuslavsky. 2002. "Chernobyl Disaster and Groundwater." A.A. Balkema, Tokyo,
pp.2001-253;
Shestapalov, V.M.,(ed). 2001. "Water Cycle in Hydrogeological Structures and
Chernobyl Disaster." Vol. 1, Kiev, p.242 )
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