Why does the revised guidance no longer use probabilistic modelling when it is increasingly being recommended for managing uncertainty?
The original CLEA model introduced a partially probabilistic or stochastic modelling approach for regulatory risk assessment in the UK. However, we have taken the view, and also listened to the views of others, that such an approach made deriving generic assessment criteria for screening contaminated sites too complex. A probabilistic or stochastic approach has a number of advantages, such as the ability to analyse the shape and distribution of uncertainty for a wide range of parameters. But, the primary purpose of the CLEA model was to derive Soil Guideline Values (SGV) and this did not make full use of these advantages. We still support the appropriate use of probabilistic or stochastic modelling as part of a detailed quantitative risk assessment. This is consistent with the view taken by experts in other countries, including the United States Environmental Protection Agency.
