Defining Risk Clusters
The scale and complexity related to soil and groundwater contamination at the megasite require a systematic approach for an appropriate assessment and management of the risks. For this purpose the risk management zone (RMZ) is subdivided into the so-called risk clusters (see figure below). A risk-cluster is a geographical subdivision of the risk management zone (RMZ) with source-pathway-receptor sequences that can be grouped together into one unit for which jointly: 1) risks can be quantified and, 2) risk management scenarios can be developed. Risk clusters form the units on which the megasite risk management plan will be based. The risk clusters are used in further development and evaluation of risk management scenarios, and the selection of the final risk management scenario for the megasite of concern.
The risks clusters are related to a source-pathway-receptor sequence. The conceptual model and the RMZ are therefore the initial basis for defining risk clusters. The second information source used is the set of megasite characterization data organized in a database related to the GIS, which holds information on:
- the contamination situation of soil and groundwater (sources);
- the geological and hydro-geological characteristics of the site (pathways);
- the receptors.
Clustering is an iterative, cyclical process, depending on the information available at different stages of the management process. The formation of preliminary risk clusters done by technical experts requires an analysis of the single elements of the source-pathway-receptor sequences and makes use of the GIS to create maps of the risk clusters. The preliminary risk clusters are subsequently presented to the group of stakeholders (GOS), who have to agree (and possibly suggest adjustments) on the subdivision of the megasite into these clusters. Also a prioritization of the different risk clusters can be carried out in interaction with the GOS. In the following two steps (see: Risk Assessment -> Modelling) and (see: Risk Assessment -> Determining risks) the risks are further quantified. The results of these steps are again presented to the GOS and the final risk clusters are established in the final step of this IMS risk assessment phase (see: Risk Assessment -> Finalize clustering).
During or after defining the preliminary risk clusters, additional site characterization may be needed for sources, pathways and receptors, to gather sufficient data for the fate and transport modelling and the quantification of risks. For data gathering and evaluation the same tools should be used as before (See: Risk assessment -> Megasite characterization).
Output
The subdivision of the RMZ into risk clusters is done. They are used in identification, overview maps, and prioritization of risks in terms of manageable units, and form the basis for the megasite management plan to be further developed. The preliminary risk clusters defined at this stage need to be fine-tuned and adjusted in the next steps of the IMS.
