The Navy used a dynamic field activity (i.e., a project that combines on-site data generation with on-site decision making) for a CERCLA remedial investigation (RI) at the Marine Corps Air Station (MCAS) Tustin between 1995 and 1996. Field-based analytical methods (FAMs) provided defensible data that met project objectives for delineating the nature and extent of contamination in the base-wide soil, surface water, and groundwater investigations. The FAMs were also used to choose monitoring well locations and select a subset of risk assessment samples for off-site analysis. At one location on the base, the dynamic field activity saved the Navy over 15 percent of the total site cost of the investigation and helped to compress the investigation schedule by an estimated 60 percent.