Geography Reference
In-Depth Information
Many approach such as process-based methods, statistical methods, and
overlay and index methods have been developed to evaluate aquifer
vulnerability (Tesoriero et al., 1998). Variability of land vulnerability to
groundwater contamination leads to mapping of groundwater vulnerability
(Piscopo, 2001). In the process-based methods, simulation models are used to
estimate the movement of contaminant in groundwater.
The major drawback in using process-based methods is data shortage and
computational difficulties (Barbash and Resek, 1996).
In statistical methods, statistical terms are used to determine relations
between spatial variables and actual occurrence of pollutants in the
groundwater.
Their major limitations include absence of sufficient water quality
observations, data accuracy, and careful selection of spatial variables (Babiker
et al., 2004). Overlay and index methods resulting in vulnerability indices
mainly depend upon factors, which control the pollutant movement from the
ground surface into the saturated zone. Their main advantage is that
vulnerability assessments can be made at regional scale as some of the factors
such as rainfall, soil type and groundwater depth are easily available over large
areas, which makes them suitable to be used with geographic information
system (Thapinta and Hudak, 2003).
In general, overlay and index methods and statistical methods are used for
contamination assessments at map scales smaller than 1:50,000 (i.e., a large
study area), while process-based simulation models are at larger map scales
(i.e., a small study area) (Rao and Alley, 1993). Overlay and index methods
and statistical methods are used to assess intrinsic vulnerability, while methods
based on simulation models are used to assess specific vulnerability.
6.3. Groundwater Vulnerability Mapping by GIS-Driven
Overlay and Index Methods
The most common approach to quantify aquifer vulnerability at present is
the overlay and index method, whereby the protective effect of the overlying
layers is expressed in a semi-quantitative way (Frind et al., 2006). Overlay and
index methods efficiently determine groundwater vulnerability. These methods
deal with overlaying and aggregation of multiple spatial maps and these spatial
analyses of a group of maps can easily be performed in geographic
information system.
