We measure and model the concentration of separate phase hydrocarbons downgradient of a 19 year old aviation gasoline spill at the US Coast Guard Air Station in Traverse City, Michigan. The separate phase aviation gasoline is presumed to exist in mobile and residual partitions whose transport is modeled as a simple one dimensional balance of storage, advection, volatilization, and linear sorption. Field calibration suggests a retardation factor of 13.1, while volatilization accounts for about 30% of the originally spilled product, underscoring the importance of these two mechanisms in immiscible gasoline fate and transport studies.