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DIVISION S-10—WETLAND SOILS

Seasonal Transformations of Manganese in a Palustrine Emergent Wetland

^a Dep. of Geosciences, San Francisco State Univ., San Francisco, CA 94132-4163
^b Dep. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305-2115

* Corresponding author (laforce{at}sfsu.edu)

Naturally occurring wetlands need to be investigated to assess their role in contaminant sequestration. Accordingly, aqueous- and solid-phase analyses were conducted to monitor Mn response to changing physicochemical conditions within a Palustrine Emergent Wetland. A rhodochrosite-like phase (MnCO₃) was the dominant Mn bearing phase (by mass) within the solids throughout the year. Aqueous-phase Mn concentrations were highly variable, but only two sampling sites in the interior of the wetland had pore-waters supersaturated with respect to rhodochrosite. Despite that a rhodochrosite-like material was the dominant quantity of Mn in the solids through the year, the reactivity of Mn solids, as measured by selective sequential extractions, did change seasonally. In spring, Mn was preferentially associated with the hydrofluoric acid-extractable pool, comprising 35% of the total extractable Mn. The sodium acetate (SA)/acetic acid (AA) fraction appears elevated from spring through fall. Amorphous sulfide phases denoted by the difference between hydrochloric- and oxalic-acid extractable Mn increased during the summer months and then decreased in early fall as the site dried. Furthermore, as the site dried, Mn associations with the MnCl₂ (water soluble and exchangeable) extractable phase increased significantly (P < 0.05), comprising between 26 and 43% of the total extractable Mn. Manganese removed using hydroxylamine-hydrochloride/AA (crystalline oxide) increased significantly (P < 0.05) in the summer to 40% of total extractable Mn. It is therefore apparent that seasonal changes in temperature and water level, with associated redox status, drive changes in surficial coatings of Mn phases and thus its reactivity.

Abbreviations: AA, acetic acid • AOD, ammonium oxalate in the dark • AVS, acid volatile sulfides • CdAR, Coeur d'Arlene • EXAFS, extended x-ray absorption fine structure spectroscopy • HA, hydroxylamine-hydrochloride • ICP, inductively, coupled plasma • PVC, polyvinyl chloride • RSD, relative standard deviation • SA, sodium acetate • SSE, selective sequential extraction • XAFS, x-ray absorption fine structure • XAS, x-ray absorption spectroscopy