Contaminant-pathway-receptor

The Pollutant Linkage Concept. A further concept within the definition—the contaminant-pathway-receptor approach—is elaborated in the statutory guidance. This concept has, in recent years, come to represent the predominant intellectual framework for risk assessment in general20 21 and has underpinned the development of contaminated land technical thinking. Unless a particular receptor could be harmed, through a defined pathway, by an identified contaminant, then land cannot be considered to meet the definition of contaminated land. 

The UK follows the widely recognised contaminant-pathway-receptor concept for assessing risks from contaminated land.4-8 This contaminant-pathway-receptor relationship is called a pollutant linkage.7 Receptors may, for example, be humans, surface water, groundwater, buildings, protected ecosystems or property, including livestock and crops. 

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. 

In the initial stages of assessing a site, the contaminant-pathway-receptor philosophy can be used to exclude the site from further consideration on qualitative grounds when one or more of the three components is missing. For example, if the contamination is encapsulated in a sealed on-site cell which prevents the ingress or egress of precipitation, vapours and leachate, and animal or plant contact cannot occur, then no pathways exist and hence no pollutant linkages exist. The land cannot, therefore, be classified as contaminated land. 

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. 

Qnantitative uncertainty analysis is appropriate when it is essential to set priorities among sites, contaminants, exposure pathways, receptors, or other risk factors, given limited resources the consequences of an incorrect decision are high and available or obtainable information is insufficient to conduct a defensible analysis. 

Once the pathways, receptors and sources of contamination have been identified and evaluated, various methodologies can be applied to give a quantitative or qualitative measure of the potential treats to human health, welfare or the ecosystems. In all cases, a series of investigations and data collection activities is required before one can proceed to the site evaluation. 

Many countries, including the UK, have adopted a risk-based approach to contaminated land management and use the familiar source-pathway-receptor approach to risk analysis (see chapter by Quint). Here, we highlight some of the recent contributions towards development of the risk assessment for contaminated land management. 

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. 

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). 

On a global scale, the atmosphere serves as the major pathway for the transport and deposition of contaminants from emission sources to terrestrial and aquatic ecosystem receptors (22, 27). Once a contaminant is airborne, the processes of atmospheric di sion, transport, transformation, and deposition act to determine its fate. These processes are complex and the degree to which they influence the fate of a particular contaminant is dependent on its physico-chemical characteristics, the properties and concentrations of coexisting substances, and the prevailing meteorological conditions, including wind, precipitation, humidity, temperature, clouds, fog, and solar irradiation. 

Collecting and analyzing existing data. Existing data (Table 16.1) are collected and analyzed to develop a conceptual site model that can be used to assess both the nature and the extent of contamination and to identify potential exposure pathways and potential human health or environmental receptors. 

Receptors are those elements of the paradigm that are affected by the potential contamination emanating from the various sources via the different pathways. A receptor is any person, animal, plant, ecosystem, waterbody, protected site, or property. Receptors, in the context of the Historic Mine Site project include ... 

The US EPA characterizes As, Be, Sb, Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl, and Zn as priority metals because of their potential hazardousness to human health. However, the environmental fate and effect of only a few metals (As, B, Cd, Cr, Cu, Mo, Ni, Pb, and Zn) have been studied extensively (Rechcigl 1995). For a given metal the potential to cause harm depends on the identifiable risk pathway, which is different for different metals. One pathway usually provides the highest probability of adverse affects to some receptor and is, therefore, the limiting pathway (Ryan Bryndzia 1997). The most toxic elements to humans are Hg, Pb, Cd, Ni, and Co. Some of the principal limiting pathways for various metals are the direct ingestion of Pb-contaminated soil by children plant phytotoxicity from Cu, Zn, Ni food-chain concentration and transfer of Cd and Hg to humans and food-chain transfer of Se and Mo to livestock (Ryan Bryndzia 1997). 

Characterize possible mechanisms of exposure to hazardous substances. The pathways by which hazardous substances released from a disposal facility can be transported through the biosphere and the resulting routes of human exposure are specified, often along with their respective probabilities. To estimate exposures of humans at assumed receptor locations, dilution of contaminants by transport in air or water as well as concentration by various means, such as precipitation and uptake by intermediate biological organisms consumed by humans, must be considered. An example of the potentially complex web of exposure pathways is shown in Figure 3.3. 

A conceptual site model is useful in helping to determine the type of environmental samples that is required. A conceptual model emphasizes the type and extent of the contamination, defines the pathways for contaminant migration, and identifies potential receptors (e.g., well users, surface water bodies, and food and feed material) (US EPA, 2002). 

The source is defined by the amount and nature of a potentially hazardous contaminant. The degree to which a source poses a risk depends on the presence of a means of transport (the pathway) for the contaminants to the receptor (the plants, animals and/or humans and even buildings that may be adversely affected by the contamination). Contaminants can move from the source to the receptor via food, soil, air and water. For humans, the main ways that contaminants can enter our bodies are by ingestion, inhalation and direct contact, for example, by absorption through the skin. 

It is important to note that groundwater and surface water may act both as pathways (e.g., through percolation through the unsaturated zone, saturated ground-water flow and surface water flow) and as receptors (e.g., vulnerable water abstractions, resources or ecological systems). Evaluation of surface water and groundwater as part of contaminated site investigation studies is, therefore, a major concern. 

The mere presence of any single chemical or chemical mixture in the environment does not indicate that a health threat exists. An important step of mixture risk assessment is the evaluation of completed exposure pathways. Completed exposure pathways link together the source of contamination, environmental medium, point of exposure, route of exposure, and a receptor population. It means that without the potential for chemicals actually entering (or contacting) the human body, no threat is present. 

Exposure pathway The physical route by which a contaminant moves from a source to a biological receptor. A pathway may involve exchange among multiple media and may include transformation of the contaminant. 

Hazardous waste problems are frequently generated by mixmres of complex wastes that have been disposed of on land and that have migrated through the subsurface. One approach to assessing the risks of contaminated sites has been to divide the problem into three elements sources, pathways, and receptors (Watts, 1998) as noted in Table 2. The first step in assessing the risk at a hazardous waste site is to identify the waste components at the source, including their concentrations and physical properties such as density, water solubility, and flash point. After the source has been characterized, the pathways of the hazardous chemicals are analyzed by quantifying the rates at which the... 

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