Impact of Sediment-Bound Iron on Redox Buffering in a Landfill Leachate Polluted Aquifer (Vejen, Denmark)

Sediments sampled along a central flow line of the leachate pollution plume at the Vejen Landfill, Denmark, were characterized in detail with respect to the forms and pools of Fe(ll) and Fe(lll). After 15 yr of leaching, redox reactions had diminished the pool of iron(ll1) oxides and hydroxides in the reduced zones close to the landfill, and the aquifer oxidation capacity (OXC) related to iron oxides was depleted. Less than 2% of the total Fe(ll) was recovered as dissolved Fe(ll), whereas 1-20% was ion-exchangeable on the sediments. The majority of the Fe(ll) was in the solid state either as pyrite or in the ill-defined fraction extractable by 5 M HCI. The total reduction capacities (TRC) of these anaerobic sediments were significantly elevated relative to the unpolluted sediments. Samples from the oxidized, weakly polluted part of the plume contained Fe(lll) minerals and insignificant amounts of Fe(ll). This study presents evidence of substantial iron reduction buffering the reducing power of landfill leachate entering a shallow aquifer. It is also proposed that reduced sedimentbound iron species form in the plume, thereby increasing the need for oxygen if the aquifer was to be remediated.