When a light, immiscible oil leaks above an unconfined aquifer, it spreads and forms a floating mound on the water table. The oil mound migrates in the direction of ambient groundwater how. In this study we present a governing equation for the migrating mound thickness by averaging the oil phase mass balance equation. Analytical and numerical solutions to an advective-dispersive type equation are presented to estimate the temporal and spatial distribution of the migrating oil mound thickness for two problems of practical importance: formation, spreading, and migration of an oil mound on the water table and spreading and migration of an established layer of oil with ambient groundwater flow. The model results compare favorably with test data obtained by laboratory flume experiments. Although the model has some simplifying assumptions such as the absence of capillary pressure gradients, sharp saturation changes across the phase interfaces, and single mobile phase (i.e., oil flow only), it can be useful as a screening or site assessment tool because of its relative simplicity.