An overview is presented of a field-based research program that is examining the significance of chemical reactions in heavy-metal transport in ground water. Both natural-gradient tracer tests and laboratory experiments with subsurface materials are being used to evaluate the relative importance of hydrologic and geochemical processes affecting metal transport. The experiments are being conducted in an uncontaminated recharge zone and in sewage-contaminated zones of the sand and gravel aquifer at the Cape Cod Toxic-Substance Hydrology Research Site. The results of tracer tests with zinc, chromium, and selenium showed that various chemical processes can have a dominant effect on metal transport; these processes include oxidation-reduction, adsorption-desorption, and aqueous complexation reactions. In some cases, significant speciation changes occurred as the injected tracers moved downgradient, and the changes generally had marked effects on the reactivity of the tracers with the porous medium. The experiments revealed that equilibrium geochemical models will be difficult to apply in modeling the transport of some redox-sensitive contaminants, such as chromium (VI) and selenium (VI), because of the specific nature of interactions of each element with reductants. A practical modeling approach for describing metal transport in ground water may require laboratory experiments designed specifically to identify and minimize the number of chemical reactions considered in a hydrogeochemical transport model.