A finite element flow and transport code, highly optimized for a Cray Y-MP, is used to investigate the complex character of multispecies contaminant plumes that can develop in variable density flow systems. The reference data set for the simulation is based on a study of dense leachate plumes developed at Babylon, New York. A shallow unconfined aquifer receives leachates generated by annual recharge throughout the unlined landfill. The reference simulation generally reproduces the distribution of a density-determining species in the leachate for both intermittent and continuous sources. A sensitivity analysis illustrates how the initial source concentrations for both a density-determining species and a trace organic compound, the style of loading, and the reactive character of the organic compound can profoundly influence concentration distributions. For relatively large density contrasts between the leachate and the ambient groundwater, pockets of denser water sink to or near the bottom of the aquifer. A trace organic compound that is chromatographically separating from a dense plume due to sorption may follow different pathways as compared to the dense species.