Priorities of adverse effects
Generic method for assessment of risks
Risks need to be assessed by taking into account for each cluster the pathways between sources and receptors as depicted in the conceptual model (CM) of the megasite (see figure below), i.e.:
- E1: groundwater/the vadose zone - to receptor land surface,
- E2: soil/groundwater - to receptor surface water,
- E3: soil/groundwater - to receptor groundwater,
- other pathways when relevant.
In the WELCOME project the focus of modelling was on E2 and E3. A generic methodology is described and two approaches to assess risks are presented:
- Preliminary assessment of current risks, a procedure using standardized protocols and current concentrations. At many megasites, a complete fate and transport modelling for some or all pathways is not immediately possible because of insufficient input data for the model or the absence of an adequate model. For the preliminary risk assessment the most recent monitoring results can be used, and standardized protocols or generalized calculation rules.
- Comprehensive assessment of current and future risks, a procedure using the fate and transport modelling (Risk assessment > modelling). The mass fluxes and related concentration profiles along the pathways from sources towards the receptors are estimated as functions of time and space. Based on the modelling results, the receptors at risk are identified: the emission is quantified in a mass flux predicted to enter - or a concentration to arrive at - the receptor. Modelling may also yield information on the uncertainty of the estimation. In some cases modelling results can be validated by monitored past and current concentrations along the pathways or at the receptors.
Comparison with environmental standards
Measured and modelled concentrations (or mass fluxes) need to be compared to receptor-specific national or European standards. Concentrations exceeding standards can be assessed as functions of time and space, and subsequently the risks can be quantified and visualized using the previously defined risk clusters.
Derivation of site-specific threshold values and planes of compliance
With regard to the risk management at megasites, site-specific threshold values are to be established and assigned for the risk management zone (RMZ). Within the RMZ, two type of risk reduction measures should be undertaken:
- measures to protect the area outside the RMZ according to the water quality requirements set by the WFD and GWD, and
- measures inside the RMZ safeguarding groundwater and other receptors by taking their functional use into account.
For this, local standards or site-specific threshold values should be derived by technical experts, in interaction with stakeholders and the local authorities. These threshold values are generally concentrations in groundwater or soil, at points in the emission pathway between sources and receptors. Some examples:
- Within the RMZ part of the groundwater may be slightly contaminated, but still be a useful resource for cooling or process water. A local threshold (functional use determined) could then be the concentration value below which this type of use can be maintained at acceptable risk levels.
- Within the RMZ a residential area needs to be protected against contaminated groundwater flowing in its direction: concentrations in soil and groundwater under the houses should be maintained at the value related to an acceptable risk level.
- At the RMZ, the WFD/GWD good quality value concentration of specific compounds for groundwater should be maintained.
From these quality requirements at the receptor, up-gradient, in the direction of contaminant sources, safety concentrations within the RMZ can be deduced, and guarded by a monitoring programme. These safety concentration values and the functional quality requirements for receptors within the RMZ, are called local threshold values. Intervention measures should be implemented in such a way that these local threshold values (quality values at the receptor, and upgradient safety concentrations) are maintained. The points where these site-specific threshold values need to be achieved and monitored are the so-called planes of compliance.
Classification of risk levels
The quantified risks need to be visualized per risk cluster and for different levels of risks. For this, a systematic approach using GIS is proposed. The assessed risks per risk cluster are the base for setting the risk reduction objectives and targets as discussed in the following steps (Finalize clustering). Current and future risks can be separated and taken into account, and used in the prioritization of risk-reduction objectives, which is an important step in selecting appropriate risk reduction scenarios (See: Risk management scenarios).
Output
The output of this step is the quantification of risks related to identified receptors and defined planes of compliance. The receptors at risk are indicated as assessed and visualized per risk cluster. In the case of a preliminary risk assessment, the current risks are visualized. For a comprehensive risk assessment, risks are depicted as they are predicted to evolve in time and space.
