A conceptual site model (CSM) represents a synthesis of available information regarding the physical, chemical, and biological conditions at a site. It is maintained through an iterative process that accommodates revisions as new information becomes available. This paper describes the utility and critical aspects of updating a CSM, and includes a review of the Waterdog Recreational Annex CSM in Arizona by the Air Force Center for Engineering and the Environment (AFCEE) Environmental Restoration Program Optimization (E-RPO) Team. The site is on the shoreline of Apache Lake, and had been under investigation and active remediation for releases of petroleum hydrocarbons since 1991. The majority of contaminants had been removed, but further remedial action involving blast-enhanced fracturing of the bedrock to remove hydrocarbons had been proposed. A review of the CSM for the site by the E-RPO Team indicated that the extent and concentrations of hydrocarbons were in compliance for the vadose zone, and that no smear zone was present as a continuing source to groundwater in the bedrock. Dissolved hydrocarbon concentrations in groundwater had been declining, and the exposure risk to human and ecological receptors was low. These conditions contrasted with the historical CSM which included a significant smear zone and the existence of free product, and had not incorporated recent data reflecting source removal/control and improving groundwater quality. The updated CSM prepared by the E-RPO Team satisfied the majority of criteria for No Further Action, with several conditions that could be resolved by additional data collection and monitoring for a minimum period of five years. The proposed use of blast-enhanced fracturing and remediation was therefore considered unnecessary for closure of the release.