Risk and risk assessment
Risk can be defined as the likelihood or expected frequency of a specified adverse consequence. Applied to groundwater, it expresses the likelihood of contamination arising from potentially polluting sources or activities (called the hazard). A hazard presents a risk when it is likely to affect something of value (the target, which in this case is groundwater). It is the combination of the probability of the hazard occurring and its consequences that is the basis of risk assessment.
RISK = PROBABILITY OF AN EVENT x CONSEQUENTIAL DAMAGE
The groundwater protection scheme used in Ireland (DELG/EPA/GSI, 1999) is based on the concepts of groundwater contamination risk and risk management. Relating a groundwater protection scheme to these concepts allows consistent application of a protection policy and encourages a rigorous and systematic approach.
The conventional 'source-pathway-receptor' model for environmental management is useful when applying the risk concept to groundwater protection and vulnerability:
Source: The source is the development and activity that pose a threat to groundwater. Key considerations in assessing the source are the type of contaminant, the contaminant loading, the potential hydraulic loading associated with the contaminant release, and potential the depth of release. The potential point of release of contaminants may be at the ground surface for activities such as land spreading of piggery slurry or 5 m below ground for developments such as storage tanks or landfills. The point of release is a critical reference point for groundwater vulnerability assessment and mapping.
Receptor: The receptor is the water which has to be protected. Wells and groundwater dependent ecosystems are obviously potential targets, but in the Irish Groundwater Protection Scheme the groundwater in the aquifer below a site is also a target in its own right. Note that the water in subsurface layers overlying the aquifer (e.g. in tills, or in peat) is not normally a target. Vulnerability assessments relate to groundwater in the uppermost bedrock or sand & gravel aquifer below a site. This is a fundamental concept which underpins all the issues outlined here. Other targets that occur down-gradient of the site (e.g. wells) are important considerations in the overall risk assessment, but are not considered by the vulnerability assessment per se. Descriptions of the risk to these down-gradient targets that use vulnerability terminology are strongly discouraged.
Pathway: The pathway includes everything between the source and the receptor. It is from the point of release of contaminants through geological materials and layers to the groundwater (receptor). The pathway is determined by the groundwater vulnerability.