CHARACTERIZATION OF PREDOMINANT REDUCTANTS IN AN ANAEROBIC LEACHATE-CONTAMINATED AQUIFER BY NITROAROMATIC PROBE COMPOUNDS

The biogeochemical processes controlling the reductive transformation of contaminants in an anaerobic aquifer were inferred from the relativ e reactivity patterns of redox-sensitive probe compounds. The fate of five nitroaromatic compounds (NACs) was monitored under different redo x conditions in a landfill leachate plume of a sandy aquifer. Results of field experiments (continuous injection and in situ microcosms) wer e compared to the findings of laboratory batch and column experiments (using aquifer matrix and model systems for sulfate-and iron-reducing conditions). NACs were transformed within 2-70 days in the leachate pl ume as well as in microbially active and in microbially deactivated ex periments. Generally, aromatic amines were the predominant reduction p roducts, and these compounds were stable within the time frame and und er the conditions of our experiments. Despite the presence of various potential reductants (e.g., H2S/HS-, Fe(II)(aq), reduced organic matte r, microorganisms), the patterns of relative reactivity of the probe c ompounds indicated that ferrous iron associated with iron(lll) (hydr)o xide surfaces was the dominant reductant throughout the anaerobic regi on of the plume. Our results suggest that Fe(ll) associated with ferri c iron minerals is a highly reactive reductant in anaerobic aquifers, which may also determine the fate of other classes of reducible contam inants such as halogenated solvents, azo compounds, sulfoxides, chroma te, or arsenate.