Interpretation of the aquifer and vulnerability maps
The movement and concentrations of contaminants in the
subsurface is an important element in the source-pathway-receptor
risk assessment process and has been defined at a national-scale,
reconnaissance level by the Aquifer and Vulnerability Maps. The
maps represent simplifications of the complex processes involved in
subsurface contaminant transport. Their features are described in
detail in the vulnerability
report¹ 
(1.99mb) & the
aquifer
report (1.48mb). SEPA has
simplified the features into vertical and horizontal pathways as
follows:
- Vertical Pathway (represented by the Vulnerability Map)
and
- Horizontal Pathway (represented by the Aquifer Map).
Vertical Pathway (Vulnerability Map)
While the Aquifer Maps represent how groundwater
(and hence contaminants dissolved in groundwater) moves in the
aquifer itself, the Vulnerability Map represents the strata
overlying the aquifer.
The thickness and permeability of the overlying strata
influences the movement of contaminants from surface sources of
contamination to the underlying aquifer:
- Thick, low permeability strata (e.g. thick clays) tend to
provide more attenuation capacity than thin, highly permeable
deposits (e.g. thin, sandy strata);
- For intergranular aquifers, the depth to the water table is
also important, with deeper unsaturated zones allowing more
contaminant attenuation.
The Vulnerability
map (244k pdf) is divided into 5
main vulnerability classes, the details of which are provided in
the vulnerability
report¹ (1.99mb)
Horizontal Pathway (Aquifer Maps)
Aquifers in Scotland have been classified according to their
productivity and groundwater flow mechanisms, the key features of
which are described in the Aquifer
Productivity Table.
As well as assessing aquifer productivity, the maps can be used
to assess contaminant activity in the subsurface. The ‘type’ of
aquifer will often determine the rate of flow (and therefore the
rate of contaminant movement) and the capacity for attenuation.
- Fractured
aquifers often allow relatively rapid movement of groundwater in
fractured zones and offer little opportunity for attenuation of
contaminants.
- Aquifers with
interganular flow tend to provide a relatively higher level of
attenuation. The rate of flow tends to be slower relative to flow
in highly fractured aquifers.
Combined use of Aquifer and Vulnerability
Maps
Aquifer and Vulnerability Maps should be used in
combination when undertaking a risk assessment of groundwater in
Scotland, since contaminant migration is influenced by both
vertical (Vulnerability Map) and horizontal (Aquifer Map)
pathways.