Chernobyl-Born Radionuclides In Geological Environment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 29

The areal distribution of 137 Cs and 90 Sr concentrations in the groundwater of
the upper Quaternary aquifer (depths to 30 m) within the Kyiv region as plotted
for 1996 is shown in Figure 2.1.
In the Quaternary aquifer, the maximum groundwater contamination with
137 Cs is observed in the northern part of the region, showing a good correlation
with the surface contamination density (Figure 2.2), which increases nearing the
Chernobyl NPP.
For 90 Sr, which is characterized by its higher migration ability as compared to
137 Cs, relatively high concentrations were found in groundwater not only in the
northern part of the region but also around Kyiv City, where the depression cone
generated by intensive exploitation of the groundwater of deeper (Cenomanian-
Callovian and Bajocian) aquifers has led to an increased recharge intensity of the
Quaternary aquifer and related downward migration of radionuclides. A ground-
water sampling check in the relatively clean area with surface contamination not
exceeding 20 kBq/m 2 and similar hydrogeological conditions did not reveal
significant concentrations of the radionuclides.
At the end of the 1980s, measurable concentrations of the short-lived isotope
134 Cs (half-life two years) were discovered in the valley of the Desna River in
groundwater of several artesian wells springing from the Eocene aquifer (depths
45-65 m) in the close vicinity of Kyiv. In this case it is clear that downward pene-
tration of the radionuclide along the well casing is excluded and there is no other
way of its penetration to the aquifer than vertical downward migration from the
contaminated surface. This fact indicates, first, the Chernobyl-related origin of
the contamination. Second, in order to penetrate to such depths, the downward
velocity of the radionuclide transport should comprise about 10-15m/year or
even more. If extrapolating the possibility of downward radionuclide migration
by natural pathways into the Jurassic aquifer to a depth of 250 m in conditions of
the depression cone being formed due to groundwater exploitation, we obtain a
migration velocity of up to 50 m/year.
Such high velocities testify to the existence of the preferential vertical flow
and transport pathways in the upper sedimentary cover, probably related to disin-
tegration zones of neotectonic activity. Lithological, mineralogical, and grain size
heterogeneities of the covering deposits are also of great importance. Consequently,
the discovery of radioactive isotopes and other contaminants (pesticides) of
undoubtedly surface sources at relatively high depth evidences in favor of the
existence of PFMZ in the upper geological environment.
In 1996-1998, a series of determinations was performed of 137 Cs solid-
phase concentrations in core samples from specially drilled boreholes of depths
down to 100 m within the Kyiv urban area. These measurements showed notice-
able contamination of deposits with concentrations from 1 to 10Bq/kg
[ Shestopalov , 2002]. The corresponding vertical concentration plotted against
depth for one of these boreholes (southwest part of the Kyiv urban area) is
shown in Figure 2.3.