Arsenic-Rich Iron Floc Deposits in Seeps Downgradient of Solid Waste Landfills

Iron flocculate or “floc” deposits are commonly observed in groundwater discharge zones downgradient of unlined solid waste landfills. Bright orange in color, and composed predominantly of amorphous iron oxyhydroxides, these deposits generally have been regarded as aesthetically undesirable but environmentally benign. In recent years, there has been increased awareness of the widespread occurrence of elevated arsenic in reducing groundwaters. Research carried out at municipal landfills in New England indicates that naturally occurring arsenic exhibits redox-mediated mobility and is frequently associated with reduced iron as a dissolved constituent in leachate-impacted groundwaters. If iron precipitates in discharge zones where reduced groundwaters are exposed to atmospheric oxygen, it follows that arsenic may co-precipitate with iron in these areas. To assess the prevalence of arsenic as a constituent of iron floc deposits, samples were collected at seven landfills and at one natural mineral spring in the lower Hudson Valley of southeastern New York State. At six of seven landfill sites, arsenic concentrations exceeded 33 mg/kg, which represents the “severe effects level” for aquatic life as identified in New York State regulatory guidance for screening contaminated sediments. These results indicate that arsenic contamination is of potential concern for downgradient of landfills wherever iron-stained leachate discharges are observed. Sampling and analysis of iron flocs associated with such leachates could also provide a means of identifying landfills that may present risks of arsenic contamination to downgradient water supply wells, especially in cases where groundwater monitoring wells are not available for sampling.