Risk management and communicating results + to interested parties
FIGURE 2.1 Framework for environmental risk assessment (after U.S. EPA guidelines ).
(23). A further British reference on risk assessment for contaminated land is the work of Cairney (5). Late in 1999, the United Kingdom Environment Agency published its own methodologies (36).
With the principles of risk assessment thus understood, the methods by which these principles can be applied to assessment of contaminated land may now be considered. There are three broad categories of assessment technique of increasing complexity:
• General statements
• Qualitative assessment
• Quantitative assessment
General Statements. General statements concerning risk usually are suitable for use only at an early stage of site assessment and are based on desk study information rather than actual physical data. They usually comprise simple statements such as the nature of contaminants expected to be present given the historical use of the site. The type and location of sensitive receptors and resources that may be sensitive to the site as well as the general significance of the identified hazards to these targets may be considered. General statements usually highlight the need for further investigation into potential hazards, pathways, and receptors.
Qualitative Risk Assessment. When applied to potentially contaminated sites* the purpose of a qualitative risk assessment is to express the significance or degree of real risk (as opposed to perceived risk) by use of a systematic and structured approach. Qualitative assessment is applicable to both desk study and intrusive investigations and is based on a systematic assessment of site-specific critical factors using professional judgment and expertise in addition to guidelines and standards. The causal chain of hazard-pathway-target is again the basis for qualitative risk assessment.
The degree of risk is dependent on both the nature of the impact on the receptor and the probability of this impact occurring. The underlying principle of site remediation is to eliminate or modify one or more of the above factors so that the risk is reduced to meet site-specific requirements. Formulation of the remedial objectives and strategy will essentially identify whether the source and/or pathway should be the focus of remedial objectives or whether protection of the receptor is a more viable option.
The risk assessment must be based on a dynamic assessment of the site for not just the existing, measured conditions but also foreseeable changes in any of the critical factors. For example, gas generation, groundwater level fluctuations, flooding, removal of surface pavement, or development could radically change conditions from those at the time of investigation. The long-term leaching potential for soil contaminants and gas generation from organic materials are two common causes for remedial work in advance of development even where existing water quality and soil gas concentrations do not necessarily pose a risk until development has taken place.
Quantitative Risk Assessment. As opposed to qualitative risk evaluation, the aim of a quantified risk assessment is to assign values for the existing and future deleterious effects associated with exposure. These values can be compared with acceptable health risks, which tend to vary from country to country.
Quantitative risk assessment requires high-quality data or a predetermined factor of safety and is often applied where a site is suspected to pose excessive human health risks. Risk assessments of this kind can be worked backwards by using an acceptable health risk as a starting point. The risk assessment procedure is then inverted until acceptable soil cleanup levels are obtained.
One of the reasons that quantified risk assessments are so data-intensive is that not only direct pathways need to be considered. Indirect contact can occur when contaminants are transported through soil, groundwater, surface water, uptake or adsorption by plants, dusts, or aerosols. Current understanding of the complex interactions between chemicals in the subsurface is poor. Also, most contaminated ground has previously been used for industrial or chemical works, and the presence of made ground and foundations usually causes a large degree of uncertainty in the various fate, attenuation, and transport processes that affect the movement of contaminants (58).
Quantitative risk assessment is a site-specific process, and the results and decisions drawn from a risk assessment are often dictated by the end use of a site. Many possible receptors exist, although in the majority of cases currently in the United Kingdom the primary driver for local authorities has tended to be the protection of human health.
The risk to human health posed by contaminants on a site is dependent on the concentration of the contaminant and the means of exposure, e.g., skin contact, inhalation, or ingestion. Essentially, the exposure from a certain contaminant can be quantified from the following equation or permutations of it (21):
(soil intake rate) (exposure time) (resorption rate) (contaminant concentration)
body weight where exposure or absorbed dose is the daily mass of contaminant absorbed per day, divided by the body weight of the receptor (in milligrams per kilogram of body weight per day); soil intake rate is the daily amount of soil a receptor is exposed to (in grams); exposure time is the number of days of exposure to the contaminant; resorption rate is the toxicoki-netics-based empirical value quantifying the daily transfer of contaminants from the intake medium into the systemic circulation; contaminant concentration is the concentration of contaminant in the uptake medium (in milligrams per gram of soil); and body weight is the mass of receptor (in kilograms).
An understanding of the fate and transport of contaminants is crucial if a meaningful risk assessment is to be obtained. This analysis can be very complicated, since the number and types of processes affecting contaminants during transport are governed by both inherent contaminant characteristics and environmental conditions. Understanding of these complex dynamic processes requires the best approximation of the environmental chemistry of contaminants (e.g., biodegradability and hydro-phobicity) and the environment at the site (e.g., geology and geochemistry).
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