Geography Reference
In-Depth Information
6.3.3. GOD Scheme
GOD scheme is one of the earliest vulnerability index methods. GOD
rating system is an empirical method for quick assessment of vulnerability
incorporating three parameters:
G
roundwater occurrence including recharge,
O
verlying lithology, and
D
epth to groundwater (Foster, 1987). To each
category of these parameters - namely the aquifer type (e.g. confined, semi-
confined, unconfined), the lithology of the overlying aquitard or aquiclude (in
case of a confined or semi-confined aquifer) or the aquifer unsaturated zone
(in case of unconfined aquifer) and the depth to water - a rating value between
0 (not vulnerable) to 1 (highly vulnerable) is assigned. The vulnerability index
is calculated using the following formula (Foster, 1987):
GOD
G
O
D
index
R
R
R
(25)
where, G = groundwater occurrence, O = overlying lithology (only in case of
unconfined aquifer), and D = depth to groundwater, and subscript R indicates
rating of the parameters. Schematic of the GOD system for assessing aquifer
pollution vulnerability index is shown in Figure 7. The index values also
ranges from 0 to 1 and gives the overall pollution vulnerability.
The vulnerability is classified into four classes according the index values
as (i) low (GOD index<0.3), (ii) moderate (0.3<GOD index<0.5) and (iii) high
(0.5<GOD index<0.7) and (iv) extreme (GOD index>0.7).
Table 6. Ranges and ratings for aquifer thickness and contaminant
characteristics for DRAMIC method
Aquifer Thickness
Contaminant Characteristics
Range (m)
Typical Rating
Characteristics
Rating
0-6
9
Stable, easy to infiltrate into aquifer
9
6-15
7
Stable, relatively easy to infiltrate
7
15-25
5
Stable, uneasy to infiltrate
5
25-32
4
Relatively stable, easy to infiltrate
5
Relatively stable, relatively easy to
infiltrate
32-40
3
4
40-50
2
Relatively stable, uneasy to infiltrate
3
>50
1
Unstable, easy to infiltrate
3
Unstable, relatively easy to infiltrate
2
Unstable, uneasy to infiltrate
1
After Wang et al., 2007.
