Methodology of Groundwater Vulnerability and Protectability Assessment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 70

The approximate solution of the initial-boundary problem (4.1), (4.2) can be
obtained numerically by the method of finite differences [ Gladkiy et al ., 1981].
The problems are solved for the corresponding typical regions of the study area
selected in the course of the preliminary area zoning. These regions should be
characterized as well as possible by homogeneity in the area conditions of vertical
flow and transport within the depth interval considered for the geological medium
(soil, unsaturated zone, upper groundwater aquifer, underlying low-permeable
layer, confined aquifer). All available data for the lithological composition of
rocks and deposits, unsaturated zone thickness, relief, infiltration conditions, and
aquifer and confining bed topology and conductivity should be taken into
account. The flow and transport parameters for typical vertical boundary
problems (corresponding to the selected typical regions) are determined by all
these available data, including observational data from the experimental plots,
literature data, and the parametric information obtained in the course of the
inverse problem solutions at typical sites of the study area, which are characterized
by different degrees of PFMZ occurrence and activity. The inverse problems for
typical regions are most frequently solved for determination of the dispersion
coefficient distribution D ( z ) by analyzing the series of direct problem solutions
for the corresponding typical region problem (4.1), (4.2) and choosing the best
one (and its dispersion coefficient distribution) that gives the concentrations C ( z )
closest to the real measured concentrations at different depths of the assessed
interval [ Shestopalov et al ., 2006].

4.2. Vulnerability and Protectability assessment for
upper groundwater (unconfined aquifer)

Assessment of vulnerability and protectability of the upper groundwater
(unconfined aquifer) includes the following three stages [ Shestopalov et al ., 2006].
1. Zoning of the study area to typical “background” regions based on the

analysis of maps for unsaturated zone thickness (or depth of the upper
groundwater table), infiltration conditions, relief, and the preliminary data
for occurrence, character, and activity of PFMZs (depressions, lineaments)
of the upper geological medium.

2. For each chosen typical region the “background” vulnerability or protectabil-

ity assessment is performed for the upper groundwater aquifer (still without
accounting for PFMZs) by the determination of a vertical concentration
distribution C ( z,t ), according to following procedures:
2а. The average (by the typical region) migration parameters are determined
for the corresponding initial-boundary problem of the type (4.1), (4.2),
including the dispersion coefficient D , according to available literature
and other data and in the course of solving the inverse problem using the
observational data for concentrations at the experimental test sites.