BIOTIC GENERATION OF ARSENIC(III) IN METAL(LOID)-CONTAMINATED FRESH-WATER LAKE-SEDIMENTS

Sediments of Coeur d'Alene Lake, ID, are heavily contaminated with min e tailings that contain high levels of arsenic, iron, lead, and other trace elements. Maximal abundance of redox-active elements such as As and Fe is generally found close to the sediment-water interface, where as peak abundance of less redox-active elements such as Pb is found at >25 cm. The suggestion that As is mobile within reduced sediments led us to characterize the sediment microbiota with regard to organisms w hose activities favor As mobilization. Most probable number (MPN) esti mates reveal that the densities of cultivable sulfate-, iron-, and ars enate-reducing bacteria approach 10(6), 10(5), and 10(4) cells g(-1) w et weight sediment, respectively. Because As is considered more mobile in environments that produce As(lll), we measured aqueous As(lll) gen eration within As(V)-amended sediment microcosms. In organic acid-stim ulated microcosms, > 50% of a 10mM As(V) amendment is transformed to A s(III), compared to 30% and 5% in unstimulated microcosms and abiotic controls, respectively. in microcosms amended with an inhibitor of SRB metabolism (molybdate), As(V) reduction was in some cases diminished, suggesting that SRB may contribute to As(V) reduction. The capacity f or biotic As(V) reduction clearly exists in CDAL sediments, and the pr ofile of As abundance may be partly attributed to metal(loid)-transfor ming bacteria.