In a field where scientific and technical knowledge is incomplete and uncertainty is rife, the evaluation of risk is a structured analytical method where data elements are collected, ordered, and evaluated. The aim is to quantify the risk in a transparent manner, enabling the site manager to react in the most decisive way to the situation facing him. The risk assessment must be seen as a method that structures knowledge elements, putting them in perspective when compared to the uncertainties.

A risk evaluation, by integrating the data on hazards and exposure, determines the possibility that toxic, noxious, or physical effects will appear on the different targets taken into account and identified in the study area. It comprises four main steps:

• Hazard identification - what is the possible problem?
• Hazard assessment - how big a problem might be?
• Exposure estimation - what will be the effect?
• Risk characterisation and evaluation - does it matter?

Substances that may cause adverse effects on the identified targets (see chapter 2) have to be identified systematically. This should be done during the Site and Sources characterisation, considering both the existing and the proposed use of the site and its environmental setting.

The objectives of the hazard identification are to: i) produce a qualitative understanding of the potential for the site to present a risk, ii) highlight those sources of risk which will require detailed assessment, and iii) enable the assessor to discount those sources as not requiring further assessment when the source-pathway-target chain may not be complete or plausible.

• the rate of contaminant movement and potential movement,
• the characteristics of the host medium (soils, groundwater, etc),
• the extent to which environmental factors such as dispersion, dilution, degradation, adsorption, etc, could modify contaminant concentrations at point along the pathway,
• the characteristics of the exposure route that determine how much of the contaminant is taken by the target.

Exposure can be estimated either by measurements on the site or by modelling in particular when there is a need to predict future exposure.

2. Risk characterisation and estimation

The incidence and the severity of the effects likely to occur in a human population or environmental compartment due to actual or predicted exposure to a hazardous substance have to be estimated. It is usually done by comparing the exposure estimate and some toxicological limits such as an acceptable intake which differs for each type of target. It may also include a phase of risk estimation, i.e. the quantification of that likelihood.

Risk characterisation needs to take into account the limitations of the data collected, and the assumptions and uncertainties inherent in the data and models used. In most cases, risk characterisation should also take into consideration the effects of remedial actions on receptors as the adverse effect of mitigation actions may negate the benefits of removing the contaminant.

3. Uncertainties as part of the decision - making procedure

There are several types of uncertainty to be dealt within a risk assessment. Four classes of uncertainty are considered most relevant:

• framing uncertainty, related to the translation of policy questions into scientific questions,
• modelling uncertainty, related to the realism and the reliability of the model predictions,
• statistical uncertainty when using this type of tools (chance of rejecting a true hypothesis or of accepting a false hypothesis),
• decision theoretic uncertainty related to the use of the worse case scenario or the more likely scenarios.

Risk Assessment can be applied for:

• the assessment of a group of sites to determine priorities for action or further investigation, i.e. site screening or prioritisation,
• the assessment of the risk presented by a single site,
• the derivation of clean-up criteria or action value for a specific site,
• the derivation of generic guidelines relative to specific media and targets,
• the demonstration that generic criteria provide sufficient protection for a specific site,
• the balancing of risks and benefits,
• the consideration of long-term legal and financial liabilities for current and future landowners.

The following targets can be considered:

• human health,
• ecological health,
• water resources,
• construction materials.

In practice, the targets assessed are those identified during the site characterisation (see the site conceptual model).

The acceptable intakes variously used to quantify the risk are:

• for human health, the acceptable daily intake (ADI), the tolerable daily intake (TDI) or an excess lifetime cancer risk,
• for ecological health, a no observed effect concentration (NOEC) derived from toxicity experiments,
• for water resources, protection levels that vary from country to country,
• construction materials, for which effects such as erosion are taken into account and can be easily protect by preventive options.

Acceptable / Tolerable risk levels for the different targets vary from country to country.

Risk assessment is a process that does not result in a fixed final answer. It is impossible to determine the true magnitude and extend of any actual contamination at a site. Numerous uncertainties are usually encountered:

• variations in individuals and species tolerances to the effects of contaminants,
• environmental conditions and processes affecting the properties of the contaminant such as partitioning, transformation, degradation, etc,
• estimation of average concentrations in soil and water generally done by wide confidence intervals,
• large information gaps about the effects of contaminant mixtures that might have synergistic, magnifying or other effects,
• large information gaps about the specific mechanisms and processes affecting functions and organs within the body, how these interact, and how they might be affected by a contaminant.

However, by making informed, careful and well-documented decisions through a systematic evaluation process, the possibility of underestimating or overlooking adverse effects may be reduced. Anyway, this process will allow to proceed to the best decision-making procedure.

It is generally recognised that it is important to consider other factors along with the science when making decisions about risk management. Some of these factors include:

• economic factors - the costs and benefits of risks and risk mitigation alternatives,
• laws and legal decisions - the framework that prohibits or requires some actions,
• social factors - attributes of individuals or populations that may affect their susceptibility to risks from a particular stressor,
• technological factors - the feasibility, impact and range of risk management options,
• public factors - the attitudes and values of individuals and societies with respect to environmental quality, environmental risk and risk management.

Risk management is the process whereby decisions are made to accept a know or assessed risk and/or the implementation actions to reduce the consequences or probability of its occurrence. It is dependent on the risk assessment as input of the decision. Risk management decision can include remediation (risk reduction), and the monitoring and auditing of the effectiveness of those actions.

At a general level most countries have a common framework for contaminated land risk assessment procedures. In countries where such procedures already exist, risk assessment is often used for registering, classifying, prioritising or reclaiming sites. In those countries, a phased approach is often used.

Differences appear at several levels, such as:

• targets taken into consideration at the different levels of the risk assessment; ecosystems or construction materials are not necessarily considered, in particular at preliminary stages,
• land uses considered leading to obvious differences in human exposure and hence in potential health risks.
• values for exposure parameters, such as lifetime exposure period (nominally 70 years),
• acceptable risks levels for human health: for carcinogenic risks, where the no-dose threshold approach is used, an upper boundary on acceptable life-time risk in the range 10^-4 to 10^-6 chance of mortality from cancer is usually considered acceptable.
• etc.

Extracted from:

CARACAS: Concerted Action on Risk Assessment for Contaminated Sites in the European Union
Risk Assessment for Contaminated Sites in Europe. Volume 1: Scientific Basis. 1998.