The writers investigated the relative performance of three different numerical techniques available in the SEAWAT/MT3DMS code by simulating two new variable-density-flow and transport experimental data sets. The experiments were designed to represent two distinctly different variable-density configurations that involve transport of a sinking groundwater plume and a rising groundwater plume. The numerical techniques used for simulating these experiments included the method of characteristics approach (MOC), total-variationdiminishing scheme (TVD), and finite-difference scheme (FD). Both homogeneous and heterogeneous hydraulic conductivity fields were employed in the numerical simulations to explore the effects of heterogeneities. The results indicate that all three numerical methods have limitations and were not able to satisfactorily reproduce the instabilities observed in the experimental data sets. The results show the need for improving the accuracy of numerical techniques that are currently being used for solving variable-density groundwater flow problems.