This publication has been produced by the Remediation Working Group of the BIOMASS project and is intended for use by experts in environmental remediation assessment. The main aim of this group was to test the accuracy of predictions of environmental assessment models that form part of the assessment of the radiological impact of remediation decisions. Two scenarios were constructed and applied based on the contamination around the site of a former radium extraction plant in Olen, Belgium, which arose due to the discharge of liquid effluents into a local brook — waste disposal practices and the use of waste material as a road surfacing material. These scenarios were designed to allow modellers the ability to consider the impact of possible future remediation actions, based on input data for a real site. Differences between model predictions were mainly due to differences in user interpretation of the scenario description. The main sources of uncertainty were the radium distribution in the root zone before deep ploughing and the effectiveness of deep ploughing.