Identification and understanding of the chemical, physical, and biological processes controlling subsurface contaminant migration is essential for making accurate predictions on the fate and transport of these constituents. Remediation assessment requires these predictions where pollution from municipal and industrial activities has occurred, and for the responsible siting of waste isolation and storage facilities. Geochemical processes include ion-exchange, precipitation, organic partitioning, chemisorption, aqueous complexation, and colloidal stability and transport. Current approaches to quantify the effect of these processes on transport in a ground water system primarily involve laboratory techniques. These include the use of closed static systems (batch experiments) and dynamic systems (column experiments) where a larger segment of the aquifer is investigated by analyzing the breakthrough profiles of reactive and non-reactive species. The latter approach may be more representative of in situ conditions than the former, however, when compared to large-scale field experiments both are still constrained by: differences in scale, alteration of media during sample collection and use, and spatial variability. More field reactivity studies are needed to complement established laboratory approaches for the determination of retardation factors, scaling factors for laboratory versus field data, corroboration or confirmation of batch and column results, and for validation of sampling techniques.