NICKEL MOBILIZATION IN A GROUNDWATER WELL FIELD - RELEASE BY PYRITE OXIDATION AND DESORPTION FROM MANGANESE OXIDES

Processes controlling the nickel concentration in groundwater have bee n studied in a well field of a sandy aquifer capped by clayey till. Th e water table in the aquifer was lowered due to groundwater abstractio n, and in association with pyrite oxidation in the unsaturated zone, n ickel is released to the groundwater in concentrations of up to 4000 n M. The nickel concentration in pyrite was determined to be 40-140 x 10 (-5) mol of Ni/mol of pyrite. Groundwater nickel concentrations are pa rticularly high in the unsaturated zone and in the recently resubmerge d uppermost saturated zone. The resubmerged zone is furthermore charac terized by enhanced Mn2+ concentrations. Apparently nickel accumulates on manganese oxides during pyrite oxidation. When the water table ris es again, partially oxidized pyritic layers are resubmerged, and due t o an insufficient supply of oxygen, the oxidation of Fe2+ released dur ing pyrite oxidation becomes incomplete. The mobilized Fe2+ may reduce manganese oxides and thereby release large amounts of Ni2+ to the gro undwater. Calculations using a surface complexation model indicate ret ardation 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 dis solution results in lower pH and high Ca2+ and dissolved carbonate con centrations, factors that may significantly mobilize nickel.