Civil Engineering Reference
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(retardation), dispersion, and radioactive decay are considered. As the basic
parameter of groundwater vulnerability assessment,
Rogachevskaya
[2002] used
the radionuclide full residence time
T
c
in the hydrogeological system as deter-
mined above by equations (1.10) and (1.11):
TTR
c
=
,
(1.12)
w
where, as before,
T
w
is the residence time in the hydrogeological system of a
nonsorbed chemicals moving with groundwater flow velocity, and
R
is the retar-
dation factor determined by equation (1.9). During construction of the resulting
groundwater vulnerability map, the zoning map of protective properties for the
unsaturated zone is overlaid with the map of radionuclide residence time in
groundwater determining the self-cleaning aquifer ability.
Based on results of experimental studies of
137
Cs migration in areas
contaminated after the Chernobyl Nuclear Power Plant (NPP) accident in Russia
(Bryansk region) and experiments with artificial radionuclide injection at special
observation plots, Rogachevskaya came to the conclusion that the soil is not a
perfect protective barrier against radionuclide migration from the soil surface to
groundwater. The share of “fast migration component” of the nonsorbed con-
taminant appeared to be near 10%. This part is determined by fast migration
pathways such as “breakthrough” pores of the unsaturated zone and local
“migration windows.” Of key importance are the relief microforms which
influence the infiltration and depot properties of the soil and unsaturated zone.
The above conclusions are in agreement with our representation of the
existence and importance of preferential flow and migration zones (PFMZs)
of different scales in the geological medium. The assessed share of PFMZs in
the total groundwater contamination (10% from total initial contamination)
determined just on the local site scale (without accounting for larger PFMZs
such as depressions and lineaments) is rather significant. Moreover, the effects
of the landscape type (forested, meadow, plowed, etc.) also provide important
input into the assessment of groundwater protectability.
Polyakov and Golubkova
[2007] also used the water exchange time and retarda-
tion factor. However, they estimated the residence time of a nonsorbed tracer (or
water exchange time),
T
w
,
using a nonsorbed radioactive tracer (tritium). The tritium
concentration was measured, and the time
T
w
was determined by “input” and
observed tritium concentrations according to the methodology proposed by
Maloszevski and Zuber
[1996]. As an “input function,” they used historical data on
tritium concentration in atmospheric precipitation starting from 1953 (when nuclear
weapon tests were conducted in the atmosphere). The authors accounted for the
retardation factor and lifetime of the radionuclide. They developed a score
assessment system for groundwater vulnerability as applied to the area of Azov-
Kuban artesian basin (score range 0-7) corresponding to the average water exchange
time from over 1000 years to 5 years and determined tritium concentrations from
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