Groundwater contamination by dense brines is addressed from a theoretical and experimental perspective. Numerous human activities have released denser-than-water aqueous solutions into the subsurface, including acid mine drainage, landfill leachates, nuclear wastes, and solid rocket fuel wastes. In addition, a number of subsurface remedial approaches propose the injection of dense, viscous aqueous solutions into the subsurface. The literature on brine characteristics that arise from the release and recovery of these solutions. The importantce of gravitational and viscous instabilities was recognized, but the criterion developed by Hill in 1952 has not been fully utilized. An experimental program to measure brine dispersion in one-dimensional sand columns has verified the Hill stability criterion for two brines over a range in concentrations and medium permeabilities. In addition, the experimental results have quantified the dispersion coefficients for flows where gravity and viscocity have opposite effects on the displacement mechanisms. These results support the concept that dense brines can become long-term sources of groundwater contamination that will be difficult to detect and remediate.