Twelve month surveys of acid-soluble and dissolved trace metal concentrations in the lower Waikato River (in 1998/9 and 2005/6) showed abnormally low particulate Fe, Mn, Cu, Pb and Zn concentrations and mass flux in autumn, when the suspended particulate material (SPM) had a relatively high diatom and organic carbon content, and low Fe and Al content. Dissolved Mn, Cu and Zn concentrations also decreased in autumn, while dissolved Fe and Pb concentrations were unaffected. While SPM settlement under the low river flow conditions present in autumn can explain the removal of particulate metals, it does not explain dissolved metal removal. SPM-metal interaction was therefore investigated using seasonal monitoring data, experimental adsorption studies, sequential extraction and geochemical modelling. Pb binding to SPM occurred predominantly via Fe-oxide surfaces, and could be reliably predicted using surface complexation adsorption modelling. Dissolved Mn concentrations were controlled by the solubility of Mn oxide, but enhanced removal during autumn could be attributed to uptake by diatoms. Zn and Cu were also adsorbed on Fe-oxide in the SPM, but removal from the water column in autumn appeared augmented by Zn adsorption onto Mn-oxide, and Cu adsorption onto the organic extracellular surfaces of the diatoms.

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