After all remediation activities have been completed, clearance environmental sampling and analysis should be performed. This clearance sampling may include activities such as aggressive sampling, using blowers that may potentially aerosolize any residual hazards, and sampling in any area where the agent might possibly remain unaffected by the decontamination activities. Sampling objectives and quality assurance procedures should be established prior to initializing sampling.
Clearance sampling and analysis are carried out to determine whether the cleanup methods were successful. The objective of clearance sampling is not to provide a risk-free environment, but to provide the best available scientific evidence for the potential for any residual risk to human health or the environment. Clearance criteria (based on a weight-of-evidence approach using a combination of quantitative measures such as sampling results and other more qualitative factors) are generally determined before cleanup steps are taken; this allows the overall process for judging the success of the cleanup to be clear and unbiased.
The strategy for conducting post-cleanup environmental sampling depends on the nature and extent of the contamination, as determined by characterization sampling that was conducted prior to remediation. For example, if characterization sampling indicates heavy contamination in one area, some contamination in the surrounding area, and none in remaining areas, the strategy can implement targeted surface sampling for the first area (i.e., taking clearance samples at exactly the same locations where positive samples occurred), biased surface sampling in the second area (i.e., taking samples at locations close to areas found during characterization to be contaminated or expected to have considerable contact by people), and random surface sampling in the remaining areas. The plan also must specify what kinds of samples will be taken and in which locations.