Chromium contamination in ground water is of importance due to its potential to cause health effects. Chromium(III, VI) is stable in aqueous systems and its speciation is dependent on oxidation-reduction potential, pH, and dissolved concentration of chromium. Chromium(III) forms hydrolysis species that adsorb onto ferric (oxy)hydroxide and other adsorbents under reducing and circumneutral pH conditions. Chromium precipitates from solution as chromium(III) hydroxide and as a solid solution consisting of iron(III)-chromium(III) hydroxide. Chromium(VI) forms bichromate and chromate under oxidizing conditions. Adsorption of these two oxyanions onto ferric (oxy)hydroxide decreases with increasing pH, as surface charge becomes increasingly negative. Beneath the Pajarito Plateau, New Mexico, dissolved concentrations of background chromium(VI) range from 1 to 6 micrograms per liter in deep groundwater. Equilibrium between chromium(III) hydroxide and dissolved chromium(VI) species is consistent with both observed background concentrations in groundwater and simulations using the computer program MINTEQA2. Dichromate and chromate have been released to the environment from the early 1950s through the mid-1970s, in the form of liquid discharges from cooling towers at a steam plant, at Los Alamos National Laboratory. Downstream of these releases, elevated concentrations of chromium(III) associated with solids occurring in wetlands, buried alluvium, and Bandelier Tuff partly limit the mobility of this trace metal through adsorption and precipitation. Ferrous iron in the Cerros del Rio basalt is predicted to attenuate chromium through reduction processes. A fraction of anthropogenic chromium(VI), however, migrates considerable depths to different aquifer systems beneath the Pajarito Plateau. Concentrations of chromium(VI) have been measured up to 400 micrograms per liter in deep groundwater. Other contaminants including phosphate, total carbonate alkalinity, and sulfate present in waste streams compete for adsorption sites on ferric (oxy)hydroxide, which decrease the amount of chromium(VI) adsorption.