Methods of Groundwater Vulnerability and Protectability Assessment - Groundwater Vulnerability: Chernobyl Nuclear Disaster - page 4

geological medium containing the assessed groundwater. The appearance of
modern GIS technologies allowed for the development of several, effective
methods which have already been used in different countries.
The DRASTIC method was proposed by Aller et al . [1987] for the U.S.
Environmental Protection Agency (EPA) and was applied in the United States,
Canada, South Africa, and many other countries. The method is based on the
calculation (at each point of the assessed area) of a unified groundwater vulner-
ability index, DRASTIC, as a sum of seven rating indicators ( D, R, A, S, T, I, C )
multiplied by the corresponding weight factors r 1 through r 7 :

DRASTIC=⋅+⋅+⋅+⋅+⋅+⋅+⋅ rDrRrArSrTrIrC

1

,

2 3 4 5 6 7

where D is the groundwater table depth, R the net recharge, A the aquifer media
(determined by lithology), S the soil type (by texture), T the topography (by slope),
I the impact of the vadose zone, and C the aquifer hydraulic conductivity.
Each indicator is assessed by the corresponding local hydrogeological
characteristic in a 10-point scoring system [ Aller et al ., 1987]. For weight coeffi-
cients r 1 - r 7 , determining the relative “importance” of the corresponding indicator,
two sets of values are proposed: (1) r 1 - r 7 = 5,4,3,2,1,5,3 and (2) r 1 - r 7 = 5,4,3,5,3,4,2.
The first set determines the standard DRASTIC index used in most cases for
assessing the intrinsic groundwater vulnerability, and the second set (“agricultural”
DRASTIC) is designed for special vulnerability to contamination with pesticides.
Thus determined, the assessed DRASTIC index can vary within the range of
23-230 (intrinsic vulnerability) or 26-260 (vulnerability to pesticides). The pro-
posed values of weight coefficients to some degree have the judgmental character
and /or are based on experimental results. It is clear from the above formula for the
DRASTIC index that, in case of pesticides, the soil type and slope angle appear to
be more important, but the influence of the vadose zone and the aquifer conduc-
tivity are less important.
Higher values of the DRASTIC index correspond to higher groundwater
vulnerability. In real practical applications, the DRASTIC index usually varies in
the range of 5-200.
For example, Zektser et al . [2004] used DRASTIC (with the second set of
weight coefficients) for building a vulnerability map of the main aquifer in
Castelporciano province (Italy). They obtained the DRASTIC index in the range
26-256 and determined five groundwater vulnerability categories: 26-72, very low;
72-118, low; 118-164, medium; 164-210, high; and 210-256, very high. Denny
et al . [2007] proposed to modify the DRASTIC method in order to incorporate the
structural characteristics of bedrock aquifers with large-scale fracture zones and
faults acting as primary conduits for flow at the regional scale. The methodology
is applied to the southern Gulf Islands region of southwestern British Columbia,
Canada. Bedrock geology maps, soil maps, structural measurements, mapped
lineaments, water well information, and topographic data assembled within a