Development of a groundwater vulnerability screening methodology for the Water Framework Directive
WFD 28
September 2004

EXECUTIVE SUMMARY

KEY WORDS:
Water Framework Directive, groundwater, vulnerability, aquifers, geology, pollution

A requirement of the EU Water Framework Directive is the assessment of the risk of groundwater contamination within those groundwater bodies identified in each Member State. In order to carry out the risk assessments, knowledge of the vulnerability of groundwater is necessary. The current project is in two parts: first, a groundwater screening methodology was developed that was suitable for use in Scotland, but allowance has been made in the design framework for its adaptation in Northern Ireland, England and Wales. Second, the creation of a GIS-based groundwater vulnerability map of Scotland at a working scale of 1: 100,000.

Groundwater vulnerability is defined as the tendency and likelihood for general contaminants to reach the water table after introduction at the ground surface. All groundwater is to some degree vulnerable and the screening tool produced for the current project is designed to reflect the ability of contaminants to reach the water table surface across Scotland. It is not intended as a complete solution to risk assessment and should be used as a regional guide to the degree of specific site investigation required at any locality.

The screening methodology applies to the situation where contamination from the land surface leaches vertically downwards to the water table within the uppermost aquifer at a particular locality. The groundwater vulnerability assessment is, therefore, influenced by several factors that relate to the pathway element of a typical hazard – pathway – receptor risk assessment. In this case, the pathway is characterised by the geological and hydrogeological characteristics of the soil layer, the underlying superficial deposits and bedrock.

The pathway between the ground surface and the water table can affect the degree of attenuation of contaminants:

• The permeability and clay content of the superficial deposits.
• The thickness of the superficial deposits.
• The mode of groundwater flow in bedrock aquifers (fracture or intergranular flow).
• The permeability and clay content of intergranular bedrock aquifers.
• The depth to the water table in both superficial and intergranular bedrock aquifers.

It is the above factors that determine the vulnerability classification. Vulnerability has been divided into five categories, with Class 1 areas having the lowest risk of groundwater pollution and Class 5 the highest.

One of the main principles adopted for the current methodology was how attenuation could be affected by the nature of groundwater flow. It is assumed that only in geological deposits where there is significant or total unsaturated intergranular groundwater flow that attenuation can occur. Where contaminants move through unsaturated fractured bedrock, the methodology assumes that no attenuation of pollutants can take place.

It is the recognition of the hydrogeological characteristics within the pathway instead of the ‘importance’ of a particular aquifer that results in the final vulnerability map of Scotland showing large areas of Classes 4 and 5. This reflects the widespread occurrence of igneous and metamorphic rocks in the Highlands and Southern Uplands where the potential for the attenuation of contaminants in the pathway is very limited.

Copies of this report are available from the Foundation in electronic format on CDRom at £20.00 + VAT or hard copy at £25.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website