Methodology of Groundwater Vulnerability and Protectability Assessment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 68

For mapping purposes, a more convenient index of groundwater protectabil-
ity is the decimal logarithm of the inverse relative concentration,

( ) =− ( )

ε= log c 1 zt

−

*,* log ,*, c zt

because values of relative concentration c ( z , t ), especially for deep aquifers, are
often small (10 −4 and lower). A zero groundwater protectability index (ε = 0)
corresponds to the relative concentration c = 1, that is, initial contaminant
concentration in its source. For example, in the 1D groundwater protectability
model with surface contamination, c = 1 at z = 0. The value of the groundwater
protectability index ε = 1 corresponds to relative concentration c = 10 −1 , ε = 2 to
c = 10 −2 , etc. The range of variation of the index e is determined for the whole
assessed area, and starting from this range, the gradations (intervals of e value)
are then chosen and subscribed to definite subareas in the course of groundwater
protectability mapping. High values of ε (10 and higher) correspond to relative
contaminant concentration c = 10 −10 and lower, which is most common in hydraulic
and physicochemical conditions of cover groundwater protectability of a deep
confined aquifer.
One should note that the introduced indicators [relative contaminant
concentration in groundwater, c ( z* , t* ) , and groundwater protectability index ε],
if taken at a depth of the groundwater table (for the upper aquifer) or upper
boundary of a confined aquifer, characterize the cover vulnerability and protect-
ability. Strictly speaking, this characteristic refers not to the groundwater itself
but to the whole aquifer together with water-bearing rocks. If we want to consider
the vulnerability or protectability of the groundwater of an aquifer in the sense of
groundwater quality, then we must distinguish also that part of potential contam-
ination that comes to deposits and rocks. For this purpose, at least the data of
aquifer thickness, porosity of water-bearing formation, distribution coefficient
for a particular contaminant and rock mineral composition, and hydrogeological
conditions should be taken into account. This can be achieved by including the
whole aquifer vertical profile with all these parameters in the assessment model.
The groundwater vulnerability indicators are then calculated at some depth z*
within the aquifer, for example, in the central point of its vertical cross section or
at the depth of a planned water intake well. The characteristic indicator value can
also be assessed as an average for a given depth interval within the assessed aquifer
or its part.
A more exact characteristic of the covering deposits (and consequently of
cover's groundwater vulnerability) is the product of predicted contaminant
concentration C ( z* , t* ) by the downward net infiltration or groundwater recharge
w ( z* ): P = C ( z* , t* ) ∙w ( z* ), which can be called the predicted groundwater contam-
ination potential at the depth z* . This parameter, unlike the concentration itself,
reflects both capacitive and screening properties of the overlying covering