Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (ΣS−II), total zero-valent sulfur (ΣS0), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, 210Pb and 137Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As ( ). The model defines depth intervals where As is either released to (positive ) or removed from (negative ) the porewaters.