The speciation of arsenic in the environment is controlled by reduction, methylation, and oxidation processes and is therefore influenced by redox conditions. However, in the lakes studied, speciation was found to be far from thermodynamic equilibrium. A Superfund site is a major source of As to the watershed, with the Aberjona River being the main conduit for As found in the Mystic Lakes. Total As concentrations in the water column decrease downstream, from >lo0 nM in the Hall's Brook Storage Area (HBSA), where As(II1) enters via reducing groundwater, to <20 nM in the Lower Mystic Lake (LML); the sediments of all the lakes are sinks for As. Biologically mediated reduction, at rates of 0.2-0.5 % of the total As day', and methylation, at rates of 0.4-0.6 % of the total As day', occur in the mixed layers of these lakes. However, these processes are slow or absent in the hypolimnion, allowing As(V) to accumulate in seasonally anoxic hypolimnetic waters. High micromolar concentrations of As, predominantly As(III), persist in the saline, sulfidic monimolimnion of the LML. The production of As(II1) and methylated As species in the water column of these lakes inhibits the removal of As to the sediments, thereby increasing the mobility of As in this watershed.