UNDERSTANDING PARTITIONING OF FE, CU, AND ZN, IN CONTAMINATED OXIC AND ANOXIC SOILS

Leather tannery metals waste discharges have contaminated a wetland that ranges from sandy-oxic soils to peaty-anoxic sediments and varies in degree of water saturation, organic matter content, and metal concentrations. A sequential chemical extraction (SCE) procedure (Tessier et al., 1979; Belzile et al., 1989) was used to determine metals partitioning among solid phases. The data set comprises three vertical samples at 80 soil sampling sites that capture the diversity of the wetland soils. SCE results were studied in light of porewater chemistry, geochemical modeling, and acid volatile sulfide extractions/simultaneously extracted metals (AVS/SEM). SCEs were also performed on pure minerals representative of those identified in anoxic soils. Fe, Cu and Zn were predominantly sequestered by the easily reducible (ER - reactive Fe- and Mn-oxides), moderately reducible (MR - Fe-oxides) and basic oxidizable (OX1 - organic matter) operationally defined phases regardless of soil redox conditions. Variations in patterns of metal partitioning were correlated to the presence of soil organic carbon and redox conditions. Fe was associated mostly with the MR fraction for all soil textures and types, but the relative importance of the other fractions (ER, weakly acid soluble [WAS -carbonate], OX1, and acid oxidizable (OX2 - sulfide) increased with decreasing redox. Cu was found to be associated predominantly with the OX1 phase in samples with significant amounts of organic matter. Zn was found to be associated primarily with the MR followed by ER phases with the importance of the ER increasing over the MR with decreasing redox or increasing organic matter. The association of Fe with ER and MR phases is expected, but under anoxic conditions, oxides might not be stable, yet Fe was still extracted by the ER and MR leaches. From the results of SCE analysis on pure minerals we have found that FeS is primarily dissolved by the ER and MR extractions, ZnS and CuS by the OX1 extraction and FeS₂ by the OX2 extraction. Mass balance calculations show that sulfide minerals cannot account for all Fe removed by the OX2 extraction. It is possible that meta-stable oxyhydroxides are present. These results indicate for robust interpretations, SCE results need to be considered in light of other assessment methods particularly in anoxic environments.