Processes controlling the nickel concentration in groundwater have been studied in a well field of a sandy aquifer capped by clayey till. The water table in the aquifer was lowered due to groundwater abstraction, and in association with pyrite oxidation in the unsaturated zone, nickel is released to the groundwater in concentrations of up to 4000 nM. The nickel concentration in pyrite was determined to be 40-140 x 10(-5) mol of Ni/mol of pyrite. Groundwater nickel concentrations are particularly high in the unsaturated zone and in the recently resubmerged uppermost saturated zone. The resubmerged zone is furthermore characterized by enhanced Mn2+ concentrations. Apparently nickel accumulates on manganese oxides during pyrite oxidation. When the water table rises again, partially oxidized pyritic layers are resubmerged, and due to an insufficient supply of oxygen, the oxidation of Fe2+ released during pyrite oxidation becomes incomplete. The mobilized Fe2+ may reduce manganese oxides and thereby release large amounts of Ni2+ to the groundwater. Calculations using a surface complexation model indicate retardation of nickel to be strongly affected by bulk water composition. At the background groundwater composition, nickel retardation is high. However, intensive pyrite oxidation in combination with carbonate dissolution results in lower pH and high Ca2+ and dissolved carbonate concentrations, factors that may significantly mobilize nickel.
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