Long-term fate of a pulse arsenic input to a eutrophic lake

The long-term fate of a 30-year-old pulse arsenic input to a eutrophic lake was studied to determine if As has become effectively trapped in sediments or remains in active exchange with the water column. Legacy As was readily mobilized from sediments of Spy Pond (Arlington, MA), a eutrophic kettle-hole lake that was treated with 1000s kg As in the 1960s to manage excessive aquatic macrophyte growth. Arsenic was mobilized from hypolimnetic sediments during bottom-water anoxia in spring, summer, and fall, and As accumulated to maximum concentrations of 2100 nM. Mobilization of As from epilimnetic sediments was the largest source of As to the water column on a mass basis (145 mol), despite the fact that the epilimnion remains oxic year-round. Sediment cores revealed that surficial sediments contained As at 30-50 times background levels and suggested that there is contemporary As loading to hypolimnetic sediments (590 mol y(-1)). Mass balance estimates indicate that <5% of the contemporary As load comes from external inputs and that the remainder can be explained by mobilization and redistribution of legacy As, both through the water column and by vertical migration of dissolved As within sediments. These findings demonstrate that, decades after As inputs cease, As in contaminated sediments may remain labile and be mobilized to both anoxic and oxic water columns and accumulate to levels near the sediment surface and in the water column that may pose ongoing risks to ecological health.