Abstract Vertical profiles of total dissolved As, Fe, Mn, phosphate, nitrate, and sulfate, Eh, pH, and total solid As, hydroxylamine/acetic acid-extractable As and Fe, and extractable pyrite were determined in seven cores from the estuary and the Gulf of the Laurentian Trough. The surface enrichment of solid As and the increase of dissolved As with depth at many stations suggests that a mobile portion of As is associated with Fe-oxyhydroxides and follows a redox-pattern of dissolution in the suboxic zone, upwards diffusion, and precipitation near the sediment-water interface under non-steady state conditions. Conditions existing in the Gulf, promote the formation of pyrite in the reducing zone, the presence of which strongly affects the distribution of arsenic. Porewater arsenic concentrations are increasing along with the concentrations of solid As associated with pyrite, and As is captured by pyrite according to a constant Fe As ratio of 1000. The behavior of As is presented in a qualitative model.