The risk of contamination of groundwater depends on three elements:
- the source afforded by a potentially polluting activity;
- the vulnerability of groundwater to contamination;
- the potential consequences of a contamination event.
Risk management is based on analysis of these three elements followed by a response to the risk. This response includes the assessment and selection of options and the implementation of measures to prevent or minimise the consequences and probability of a contamination event.
The hazard depends on the potential contaminant loading. The natural vulnerability of the groundwater dictates the likelihood of contamination if a contamination event occurs. The consequences depend on the value of the uppermost target, which is normally indicated by the aquifer category, and the proximity to down-gradient targets such as a well or ecosystem. Preventive measures may include, for instance: control of land-use practices and in particular directing developments towards lower risk areas; suitable building codes that take account of the vulnerability and value of the groundwater; lining of landfill sites; installation of monitoring networks; specific operational practices; etc. Consequently, assessing the risk of contamination to groundwater is complex. It encompasses geological and hydrogeological factors and factors that relate to the potentially polluting activity. The geological and hydrogeological factors include the vulnerability to contamination and the relative importance or value of the groundwater resource. The factors that relate to the potentially polluting activity are the contaminant loading and the preventive measures.
It is important to stress that site suitability and the risk response measures do not depend on vulnerability alone. Figure 2 provides another example of the combination of two of the main elements (vulnerability and aquifer category) from an area of the Irish Midlands. If we are considering landfill site suitability (i.e. adding a hazard element to the risk assessment), the response measure in the three areas highlighted will vary. These are outlined in DELG/EPA/GSI (1999) and are available elsewhere on the GSI website. They are summarised as follows:
- Poor aquifer / extreme vulnerability: "Acceptable, subject to guidance in the EPA landfill Design Manual or conditions or a waste licence. Special attention should be given to checking for the presence of high permeability zones. If such zones are present then the landfill should only be allowed if it can be proven that the risk of leachate movement to these zones is insignificant. Special attention must be given to existing wells down-gradient of the site and to the projected future management of the aquifer."
- Generally moderately productive locally important bedrock aquifer / extreme vulnerability: "Not generally acceptable unless it can be shown that; there is a minimum consistent thickness of 3m of low permeability subsoil present; there will be no significant impact on groundwater; and it is not practicable to find a site in a lower risk area."
- Regionally important karstic aquifer / extreme vulnerability: "Not acceptable". The responses go-on to recommend that exceptions can be made where the hydraulic gradient is upwards, in groundwater discharge areas, where site investigations do not find zones of significant permeability, or where the waste types are restricted.
The key points to note from this example are:
- Though the vulnerability is mapped as being the same in all three areas, the response measures for landfill vary greatly. In this example, the variation is being driven by variations in the target rather than by variations in the pathway.
- A rating of extreme vulnerability does not necessarily preclude development, even for major developments like landfills.
Click here for Geological Survey Ireland's guidelines for assessment and mapping of groundwater vulnerability to contamination report (2003).