Geography Reference
In-Depth Information
Another possible factor for the vast GIS application in groundwater
quality studies is the significant variations in the water quality over a short
distance within an aquifer. This is, in general, not the case of surface
waterbody where water at point is free to move and mix with water at other
points and this causes relatively less spatial variability especially in stagnant
water of small ponds and reservoirs. Also, the water stored by the surface
waterbodies is mostly the rainwater containing less concentration of the major
ions as the water on surface has the least chances to come across different
geological terrains comprising of certain minerals and other substances. On the
other side, groundwater passing and moving through the subsurface formation
meets different kind of salts, minerals, etc. which are easily dissolved with the
flowing water. Thus, chances of increased concentration of the major ions and
other metal contents are relatively higher for the groundwater as compared to
surface water.
This, perhaps, may be one of the causes that groundwater quality studies
cite large application of the GIS techniques.
Spatial and temporal variability of the water quality is one of main
features of different types of surface and subsurface waterbodies. Water
quality variations over space and time are largely determined by
hydrodynamic characteristics of the waterbody. Water quality of a waterbody
varies over a space in all three dimensions, which are further altered by flow
direction, discharge and time (Meybeck and Helmer, 1992). Thus, one location
measurements in a waterbody may not be appropriately represents the water
quality of entire waterbody. Instead, one network or grid of sampling sites
would be needed to present spatial variations of the water quality. Generally,
one-dimensional samples are collected on a longitudinal profile in case of river
and on a vertical profile in case of pond/reservoir/lake as illustrated in Figures
2 (a,b). Two-dimensional profile sampling is appropriate for observing plumes
of pollution from a source and this is most-suitable for groundwater quality of
aquifers (Figure 2c). Temporal variability of chemical water quality can be
defined into five categories based on time scale as listed in Table 2.
5.1. Characterizing Extent and Patterns of Contamination
In water quality studies, activities begin with field data collection where it
is a common practice to obtain the data from multiple locations and sources.
All the collected data need to be collated and converted into common
format of the water quality database.
