[en] A Remedial Investigation (RI) Plan was developed for the Oak Ridge National Laboratory (ORNL) Waste Area Grouping (WAG) 5 in March 1988 and submitted to the Environmental Protection Agency (EPA) Region IV and the State of Tennessee for review and approval. The observational approach was evaluated by the ORNL Environmental Restoration (ER) Program and accepted by EPA as an accelerated and cost-effective approach to the RI/Feasibility Study (FS) process for remediation of WAG 5. The traditional approach used in preparing the 1988 RI Plan focused on data completeness, included a typical range of RI data-gathering activities for determining the nature and extent of contamination at WAG 5, and used multiple iterations of sampling activities to deal with uncertainties without consideration of potential deviations. In fall 1991, a revised Field Sampling Plan (FSP) was developed that used the observational approach to integrate site characterization with site remediation needs. This approach recognized the uncertainties of site characterization/remedial planning and developed contingency plans for dealing with them. The observational approach emphasizes data sufficiency to support remedial planning decisions for WAG 5