Source-pathway-receptor linkages

The objective of this phase is to develop a conceptual model for the site, which describes the important source-pathway-receptor linkages for the site and makes a preliminary assessment of their importance. 

Information that has been assembled on potential sources is analysed to identify the types and locations of possible contamination on the site. These are then mapped on to the physical description of the site and an initial identification is made of receptors that might be at risk. Consideration of the possible pathways between potential sources and receptors leads to a preliminary conceptual model for the site, in terms of candidate source-pathway-receptor linkages. The preliminary conceptual model is then tested and refined by observation, during a site reconnaissance. 

Overview. The starting point for risk assessment is knowledge of which source-pathway-receptor linkages are plausible for the site that is being assessed. At this stage, the conceptual model for the site should be completed and agreed with key parties, such as regulators. 

Figure 3 shows a flow diagram for a Phase 2 risk assessment. The first stage of Phase 2 assessment is risk estimation this estimates the potential harm to all receptors within identified source-pathway-receptor linkages. The second stage is risk evaluation this considers the acceptability of potential harm to receptors, and identifies and assesses preliminary options to deal with unacceptable risks. 

The conceptual model uses the Source -Pathway - Receptor Paradigm. The paradigm requires that each of the parameters within the model are documented, estimated, measured or recorded. The model identifies the source of any contamination identifies who or what is affected (the receptor) and identifies how the source may reach the receptor (pathway). The collection of field data, observations and estimates confirms whether a linkage exists between the source and receptor. 

New information gathered from additional desk studies and any site investigation is then collated and used to provide more complete descriptions of the candidate source-pathway-receptor pollutant linkages identified in the Phase la report. Each linkage is considered separately and estimates are made of the extent and strength of the contaminant source, its connectivity to the receptor and the level of hazard that is presented to the receptor. Finally, a preliminary assessment is made of the potential harm that hazards may pose to the receptors. From this analysis it is likely that one or more pollutant linkages will be recognised as the... 

These source-pathway-receptor (SPR) linkages are commonly referred to as pollutant linkages. Guidance on the development of a CSM is contained in a variety of documents for example. Model Procedures for the Management of Land Contamination (Reference 14.29). 

The first step in any assessment of a site affected by ground gas or vapours is to construct a conceptual model. The desk study and site investigation data should be used to develop this. The first part of the gas risk assessment is to use the model to identify any source pathway and receptor linkages from the conceptual model. If there is no linkage then there is no risk. Once the source-pathway-receptor scenarios have been identified the risks associated with each one can be assessed qualitatively (i.e. descriptively). 

Connected with the adoption of a risk-based methodology is the source-pathway-target or contaminant-pathway-receptor philosophy. This philosophy states that, in order for harm to have occurred, or for there to be a risk of harm occurring, contaminants must have been able to reach relevant receptors via specific environmental migration pathways. Where such a relationship exists between a contaminant, a pathway and a receptor at a site, a pollutant linkage is said to exist. 

The overall risk estimation should cover all the pollutant linkages identified, broken into site zones as appropriate. Details should be provided of source concentrations, with estimated confidence limits, the receptor characteristics and confirmation of the viability of the pollutant pathway. If generic guideline values are used, then their applicability should be supported. Clearly, full details need to be recorded of any models and data used to generate site-specific values for unacceptable risk, together with a commentary on assumptions that have been made in their application. 

Risks from contaminated land arise from pollutant linkages that present unacceptable risk to receptors. The purpose of risk management it to make changes that remove these unacceptable risks. This can be done in different ways the source of contamination may be removed or reduced the pathway by which contaminants reach the receptor may be broken or attenuated or the receptor can be removed or modified. 

Ferguson9 has reviewed recent research in human health risk assessment. For there to be a risk to human health, there has to be a source of contamination, one or more pathways along which the contaminants can travel and one or more humans who may be affected by that contamination. If any element of the linkage (contaminant or pathway or receptor) is missing then the contamination is not regarded as posing a risk. 

Risk assessment procedures undertaken on any contaminated site will establish a range of contaminants (hazards), pathways and receptors, will identify possible linkages of concern and will determine a set of remediation objectives. Any remediation subsequently proposed for the site must then be capable of fulfilling these objectives. In general terms the primary aims of any remediation is to break the linkage between contaminant source and the receptor to achieve a reduction of actual or potential risk so that unacceptable risks are reduced to acceptable levels. 

Consultations with other appropriate bodies Review and summary of previous reports, with report references Outline conceptual model with nature and location of gas sources, migration pathways and receptors clearly identified Description of possible pollutant linkages for ground gas Identification of potentially unacceptable risks posed by ground gas, including criteria used to identify those risks Discussion of uncertainties and gaps in information Description and justification of next steps proposed at the site, e.g. carry out site investigation and quantitative risk assessment Yes D/No Yes D/No Yes D/No Yes D/No Yes n/No Yes D/No Yes D/No ... 

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