Groundwater Vulnerability and Protectability to Chernobyl-Born Radionuclide - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 87

After the assessment of average contaminant concentration in the ground-
water taking into account PFMZs according to formula (4.5) (see Section 4.1), a
map has been drawn of the cover groundwater protectability of the Quaternary
aquifer with respect to 137 Cs within the study area. It represents the distribution
in the area of the groundwater protectability index ε 1 = -log с ( z 1 * ,t* ), determined
by equation (4.6), where c ( z,t ) is the dimensionless relative concentration (see
Figure 5.5A).
The map depicted in Figure 5.5 (Legend A) reflects the attenuation capacity of
deposits in the unsaturated zone, with no reference to distribution of the soil
contamination. Hence, it corresponds to the assessment of the intrinsic cover
protectability of the upper Quaternary aquifer.
Further on, to obtain an approximate assessment of the groundwater occur-
ring deeper in the middle aquifer body, a repeated procedure determining the
average relative concentration of radionuclides has been performed in the same
regions (gradation zones) as for the preliminary cover aquifer protectability vulner-
ability assessment. For this purpose, the predicted concentration values have been
taken at depths z 1 *, corresponding to the middle point (in terms of thickness) of
the aquifer body (see Section 4.1). For these concentration values the corresponding
values of the groundwater protectability index ε 1 have been calculated and are
shown in Figure 5.5B.
As seen in Figure 5.5, the radionuclide penetrates into the aquifer throughout
most of the study area; however, its relative concentration is different, as is shown
by the gradations of the concentration range: 0-2%, 2-5%, 5-10%, 10-20%, and
over 20% of the surface concentration (in the liquid soil phase).
Overall, the Kyiv region is characterized by comparatively low values of the
groundwater cover protectability index for the Quaternary aquifer. It varies from
below 0.7 to 2 with corresponding relative 137 Cs concentrations from 20% to 1%
of the soil liquid-phase concentration. The minimum values of the aquifer pro-
tectability index correspond to the northern territories of Ukrainian Polesye,
valleys of the Dnieper and its tributaries (Figure 5.5). The highest aquifer pro-
tectability index ε > 1.5 ( c < 3%) corresponds to elevated, often forested areas with
maximum thicknesses of the unsaturated zone or depth of the groundwater table.
Characteristic for these areas are loamy and sod-podzolic soils and sandy-loamy
compositions of the unsaturated zone. Within the Kyiv conurbation area, the
upper aquifer protectability index varies from below 0.7 to 1 (corresponding to
relative concentrations of 137 Cs from over 20% to 10%). It is sometimes higher at
elevated areas of the Dnieper right bank (the watershed area east of the Irpen
River), reaching 1.5 ( c = 3%).
The contaminant relative concentration in the groundwater of the middle
aquifer body (Figure 5.5, Legend B) appears to be lower than at the groundwater
table depth. For the corresponding gradations it varies within 0-0.1%, 0.1-1%, 1-5%,
5-10%, and over 10% of the upper soil liquid-phase concentration. Corresponding
values of the groundwater protectability index ε vary from below 1 to over 3.