Methods of Groundwater Vulnerability and Protectability Assessment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 6

SUPRA is a regional mapping method for groundwater vulnerability. It
is an index method using the matrix ranging procedure for the indicators
and GIS overlay to obtain the resulting areal groundwater vulnerability. The
method is proposed by Zaporozec [2002] and was applied in mapping the
groundwater vulnerability of northern Wisconsin. The assessment is based on
five indicators:
1. Soil characteristics
2. Unsaturated zone thickness
3. Permeability of vertical sequences in the unsaturated zone
4. Groundwater recharge
5. Aquifer characteristics (lithology, flow regime, recharge)
The resulting vulnerability index is assessed in three stages that correspond
to the assessment objectives and stages of downward migration of contaminants
from the soil surface into groundwater aquifers:
I. Evaluation of the soil capacity to attenuate contaminants
II. Evaluation of the contamination potential of shallow groundwater
III. Evaluation of the contamination potential of deeper aquifers
In the conclusion of each stage, a map is built which can be used alone or in
compiling the combined composite map. Use of the independently evaluated
components (soil, upper aquifer, deeper aquifers) makes the method flexible to
requirements of different users.
The assessment and mapping of vulnerability of deeper aquifers (stage III)
is based on geological and hydrogeological characteristics such as aquifer
deposit lithology, integrity of the overlying confining bed, location, area and
character of the recharge zones, as well as the regional groundwater flow direction.
The evaluation of attenuation capacity of soil can be carried out using the
soil contamination attenuation model (SCAM) developed by Zaporozec [1985].
With its aid the soil attenuation capacity is assessed in relation to contaminant
sources, located within or out of the soil, based on a two-layer model (soil and
subsoil), using characteristic indicators such as the soil texture, pH, depth, drain-
age degree, and content of organic material. Each indicator is assessed by its
score, and the sum of the scores is found using GIS. Depending on the total
score range in the area, it is classified based on the four categories of soil atten-
uation capacity: best, good, average, and least.
The second assessment stage for the upper groundwater aquifer is performed
using GIS based on the three parameters: unsaturated zone thickness, vertical
hydraulic conductivity, and average groundwater recharge assessed by means of
the evaluation of net infiltration. Each of these parameters is assessed using three
gradations (low, medium, and high). After that, for each assessed subarea, the
GIS matrix overlaying procedure is performed successively for these three indica-
tors, in the result of which the groundwater vulnerability is assessed as low,
medium, or high, and a three-color map (green, yellow, red) of the corresponding
vulnerability zones is drawn for the studied area.