Abandoned and improperly sealed boreholes, monitoring wells, and water supply wells are common features at many contaminated sites. These features can act as conduits that transmit contaminants between aquifers separated by otherwise continuous aquitards. In this work the leaky boreholes are represented as highly conductive one-dimensional line elements superimposed onto a mesh of three-dimensional finite elements representing the porous medium. Simulation results are presented for a series of scenarios involving a simple hydrogeologic setting composed of an upper unconfined aquifer, a middle aquitard, and a lower confined aquifer. The simulations examine the effect of varying the borehole properties, the vertical hydraulic gradient across the aquitard, and the borehole location. The results show that a contaminant can rapidly migrate downward along a leaky borehole and create an extensive plume in the lower aquifer, even if the borehole is filled with aquifer sediments. If the borehole is an open feature across the aquitard, the entire plume, or a significant portion of it, that is migrating into the upper aquifer can be diverted into the lower one if the vertical hydraulic gradient across the aquitard is sufficiently strong. The peak concentration arriving at a pumping well located in the lower aquifer and the time of arrival are functions of the proximity of the leaky borehole to the pumping well and its angular offset from the central flow line passing through the surficial source. Overpressurization of the lower aquifer due to injection can overcome downward preexisting hydraulic gradients across the aquitard such that contaminants can rapidly migrate upward along the leaky borehole and cause contamination of the otherwise protected upper aquifer. ¿ American Geophysical Union 1995