ARSENIC RELEASE DUE TO DISSIMILATORY REDUCTION OF IRON OXIDES IN PETROLEUM-CONTAMINATED AQUIFERS

Elevated arsenic concentrations in aquifers are commonly attributed to reducing conditions. Naturally-occurring arsenic sorbs strongly to mineral surfaces such as Fe(III) oxides. Under anaerobic conditions, Fe(III)-reducing microorganisms can couple the reduction of solid phase Fe(III) with the oxidation of organic matter, potentially releasing arsenic to groundwater. Although this process has been documented in pristine environments where the carbon source is natural organic matter, minimal information exists on the potential for arsenic release fueled by the introduction of anthropogenic sources of carbon, such as petroleum hydrocarbon compounds. Elevated arsenic concentrations have been detected at many petroleum contaminated field sites, underscoring the need for increased understanding of this arsenic release mechanism.

The objective of this research is to quantify the arsenic release rates due to Fe(III) reduction in petroleum-contaminated aquifers. These rates will be determined by conducting controlled experiments using a series of staged batch reactors and mixed flow reactors containing an arsenic-bearing Fe(III) oxide, 2-line ferrihydrite, and an Fe(III)-reducing microorganism, Geobacter metallireducens. The experiments are designed to quantify the release rate of arsenic from iron oxides under biotic and abiotic conditions. Results will be used to develop a mathematical model for arsenic release in association with the degradation of hydrocarbon. The conceptual model will be used in a follow-up project to construct an arsenic module for the numerical biodegradation model, SEAM3D (Waddill and Widdowson, 1998). The model will then be compared to field conditions where arsenic release due to iron oxide reduction coupled to petroleum biodegradation appears to be occurring. This coupled laboratory-modeling study will yield critical information for improving understanding of this challenging environmental problem.