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 & the aquifer report. 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:

click on image for larger version	►	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 is divided into 5 main vulnerability classes, the details of which are provided in the vulnerability report.



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.
click on images for larger versions	►	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

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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.

¹ Please note the following amendments to the Vulnerability Report

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