Stimulated Release of Natural Arsenic from Organic Matter Decomposition in Unlined Landfills

Arsenic contamination is a ubiquitous and persistent problem at many current and former landfills. Natural arsenic from overburden or adjacent geological formations may be an important source of aqueous As in some of these sites, many of which have little or no known anthropogenic As. Here we examine the reductive processes that modulate As concentrations in the Coakley Landfill Superfund site near Portsmouth, NH. Elevated aqueous As levels are heterogeneously distributed throughout the site, and are linked to high dissolved Fe levels. These data indicate that reduction of As-bearing Fe(III) oxides such as ferrihydrite is the major process releasing As into solution. In this site, considerable As(V) is associated primarily with ferrihydrite within overburden and an clay-rich aquitard underlying the site, and probably represents the source of contributing arsenic to groundwater. Dissolved benzene concentrations have decreased in the ~10 y since capping the landfill, consistent with the natural attenuation of organic contaminants. Within specific wells, the change in benzene concentrations (presumably representative of the broad suite of organic contaminants and sources) is proportional to the change in dissolved Fe and As levels. These data indicate that microbial degradation of benzene (and presumably other organic compounds within the landfill) is linked to the dissimilatory reduction of Fe(III), dissolving ferrihydrite, and releasing Fe(II) and As(V) into solution. Data from a broad range of Superfund sites suggests that Fe reduction by organic contaminants may represent an important source of groundwater As levels in many systems and disparate locations.