A groundwater vulnerability screening methodology for Northern Ireland

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 report is in two parts: first, a description of the groundwater screening methodology is made. This methodology was originally developed for use in Scotland, but has now been adapted for use in Northern Ireland, taking into account local data availability. Second, the creation of the GIS-based 1:250 000 scale groundwater vulnerability map using suitable data is described. 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 Northern Ireland. It is not intended as a complete solution to risk assessment and should be used as a regional guide to the possible 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. Factors that can influence attenuation include: • 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 to the water table 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 Northern Ireland showing significant areas of Classes 4 and 5 within upland and certain other regions. This reflects the common occurrence of igneous and metamorphic rocks within these areas where the potential for attenuation of contaminants in the pathway is very limited