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Arsenic mobility and transport in the environment are strongly influenced by associations with solid phases. This dissertation investigates the mechanisms affecting arsenic retention in contaminated wetland and riverbed sediments. A sequential extraction procedure was designed and tested to differentiate solid phase arsenic speciation, including adsorbed As and As coprecipitated with amorphous and crystalline oxides and sulfides. The sequential extraction was performed on Wells G & H wetland (Woburn, MA) sediments, and the inferred As associations were compared to XANES analyses. Geochemical modeling was used to predict redox conditions and As associations. We found that most As in the wetland was adsorbed onto amorphous Fe (hydr)oxide phases. Riverbed sediments differed from wetland sediments in that redox conditions were more reducing, and As was associated with more reducing and crystalline phases, including sulfides. As(lll) and As(V) oxidation states coexist in the wetland and riverbed sediments, with more oxidized As in the wetland. We tested the hypothesis that As associations with more oxidized phases in the wetland ma