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Groundwater samples with high concentrations of Fe(II) and Mn(II), obtained from an anaerobic pollution plume, were speciated under anaerobic conditions in terms of colloidal (screen filtration and cross-flow ultrafiltration), complexed (ion-exchange resins), and free divalent species of Fe(II) and Mn(II). Most of the metal content in a nonfiltered sample passed through a 0.001 µm membrane and was considered truly dissolved [Fe(II), 78-100%, average of 90%; Mn(II), 87-100%, average of 91%]. Although free metal ions were the dominant species in the dissolved fraction, speciation modeling using MINTEQA2 suggested an average of 19% Fe(II) and 20% Mn(II) were present as bicarbonate complexes, which are here operationally defined as labile. Organic complexes were not significant, although the groundwater contained up to 67 mg of C L -1 . Calculations with MINTEQA2 indicated that sample solutions representative of the truly dissolved fraction (<0.001 µm) were strongly supersaturated with respect to siderite (FeCO 3 ) and moderately supersaturated with respect to rhodochrosite (MnCO 3 ). SEM-EDS (Scanning Electron Microscopy combined with Energy Dispersive X-ray Spectroscopy) analysis indicated that the colloidal fractions most likely consisted of clay type particles and iron sulfide precipitates. The amount of Mn(II) in the colloidal fractions was too low for determination by SEM-EDS. These results show that roughly 65% of the iron and manganese in anaerobic pollution plumes is present as free divalent ions and, according to the current thermodynamic database in MINTEQA2, the solutions are supersaturated with respect to carbonate.

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