Methodology of Groundwater Vulnerability and Protectability Assessment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 66

and porosities is almost always insufficient for obtaining a reliable solution of
a 3D flow transport modeling, even without accounting for PFMZs.
One way of overcoming this situation is the solution of inverse problems
to assess missing flow transport parameters of the medium using all available
data (e.g., literature) and results of real groundwater contamination studies
(in particular, radionuclides of the Chernobyl origin) at experimental test sites
and areas characterized by different-scale PFMZ occurrences.
Another effective technique for simplifying the groundwater vulnerability
assessment model (taking PFMZs into account) is combining the method of
hydrogeological zoning described in Chapter 1 with the corresponding 1D
modeling and comparing with the observation results. This allows (with a certain
degree of convenience) to reduce the model dimension from 3D to 1D and to con-
sider, for example, 1D vertical transport from the contaminated surface to the
groundwater aquifer. We used this approach in the development of the proposed
methodology for assessing groundwater vulnerability to the Chernobyl-born
radionuclide 137 Cs within the Kyiv region area of the Dnieper basin [ Shestopalov
et al ., 2006]. Let us consider the methodology in more detail.
The methodology is based on 1D mathematical modeling of the vertical
contaminant migration from the soil surface (in the upper groundwater
assessment) or from a given (previously assessed) aquifer into the deeper aquifer.
The downward transport of a soluble contaminant in the unsaturated and satu-
rated zones should be propagated by advection, dispersion, and sorption mecha-
nisms. We neglect the molecular diffusion because its influence, according to
numerous data literature, is insignificant in the scale of the assessment considered
[ Bochever and Oradovskaya , 1972; Lukner and Shestakov , 1986]. The model allows
for the determination of the vertical contaminant concentration distribution at a
given time C ( z,t ) in the infiltrating water at depth z during a given time period
0 < t < t* from the initial contamination fallout and known initial (at t = 0) value
of contaminant concentration C 0 in the liquid phase at the top model boundary
(surface) z = 0.
The 1D modeling approach puts a definite restriction on the groundwater
vulnerability assessment; that is, the vulnerability of a given aquifer is assessed
with respect to the penetration of a contaminant into the aquifer with the infil-
trating water through the overlying covering deposits. The assessment of the pre-
dicted contaminant concentration is performed for a definite characteristic depth
z* , which can be chosen either on the aquifer surface (unconfined groundwater
table or upper boundary of a confined aquifer) or below the aquifer, depending
on whether the protection properties of water-bearing rocks of the assessed
aquifer itself are considered or not.
The forecast time t* for the modeling of the transient contaminant
concentration distribution C ( z , t ) should be comparable with the lifetime of the
given contaminant. For a radionuclide, the characteristic time period is represented
by its half-life period. The forecast period t* is assessed a priori in such a way that