Iron reduction in the sediments of a hydrocarbon-contaminated aquifer

Sediments sampled at a hydrocarbon-contaminated, glacial-outwash, sandy aqu ifer near Bemidji, Minnesota, were analyzed for sediment-associated Fe with several techniques. Extraction with 0.5 M HCl dissolved poorly crystalline Fe oxides and small amounts of Fe in crystalline Fe oxides, and extracted Fe from phyllosilicates. Use of Ti-citrate-EDTA-bicarbonate results in more complete removal of crystalline Fe oxides. The average HCl-extractable Fe( III) concentration in the sediments closest to the crude-oil contamination (16.2 mu mol/g) has been reduced by up to 30% from background values (23.8 mu mol/g) as a result of Fe(III) reduction in contaminated anoxic groundwat er. Iron(II) concentrations are elevated in sediments within an anoxic plum e in the aquifer. Iron(II) values under the oil body (19.2 mu mol/g) are as much as 4 times those in the background sediments (4.6 mu mol/g), indicati ng incorporation of reduced Fe in the contaminated sediments. A 70% increas e in total extractable Fe at the anoxic/oxic transition zone indicates: reo xidation and precipitation of Fe mobilized from sediment in the anoxic plum e. Scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant Fe(III) oxyhydroxides at the anoxic /oxic boundary. The redox biogeochemistry of Fe in this system is coupled t o contaminant degradation and is important in predicting processes of hydro carbon degradation. (C) 1999 Elsevier Science Ltd. All rights reserved.