Advective groundwater fl ow and aqueous-phase diffusion combine to facilitate contact between dissolved chemical compounds entrained in fl owing groundwater and a large fraction of the aquifer’s matrix material. Aquifer matrices comprise a wide array of materials, including igneous, metamorphic and sedimentary rock and mineral grains, fragments of plant and animal matter and colloidal masses of partially-decomposed organic matter. Each of these classes of matrix material can interact with some, or all, of the dissolved chemical species entrained in groundwater fl ow. Sorptive processes may slow the solute mass transport, relative to groundwater movement, and chemical reactions that occur when entrained solutes contact the aquifer matrix may consume or precipitate signifi cant amounts of the mass in transit, or may add dissolved aquifer matrix mass to the transport through dissolution. We provide a brief overview of these processes and direct the reader to Suthersan and Payne (2005) for a more complete description of reactions and sorptive processes associated with in situ remediation engineering.