Colloid-associated export of arsenic in stream water during stormflow events

• The export of NOM, Fe, and As from a forested catchment area is dependent on hydrological conditions.
• The formation of NOM and Fe hydroxide colloids is dependent on hydraulically active soil layers.
• As is both dissolved in the stream water, and is associated with NOM.
• The logKD values of the As-NOM association increase with increasing pH.
• The formation of ternary complexes with Fe may enhance the binding of As to NOM.

Significant correlations between arsenic concentrations and those of iron and natural organic matter (NOM) have been found in run-off from wetlands. This has been suggested to be a result of mobilization of arsenic-NOM colloids. The aim of this study was therefore to elucidate the possible association of iron and arsenic with colloids in surface water from a small, forested catchment area. The impacts that groundwater levels prior to stormflow events and the chemistry of the hydraulically active soil layers have on the release and formation of colloids, such as NOM and iron (oxy)hydroxide colloids, were also investigated.At baseflow, the NOM, iron, and arsenic concentrations in the stream water were relatively low (< 650 μmol∙L− 1, < 5.5 μmol∙L− 1, and 8–16 nmol∙L− 1, respectively), and the pH was close to deep groundwater (4.6–5.5). At low groundwater levels prior to stormflow events, the discharging stream water was fed by anoxic groundwater from the deeper layers of the peat, and by deep, oxic groundwater. The iron/DOC ratio in the stream water was high, and iron was present as iron-NOM colloids and precipitated as iron (oxy)hydroxide colloids. Arsenic was dissolved and associated with NOM, and the conditional distribution coefficients of arsenic binding to NOM (logKD values) were relatively high (around 3 L∙mol− 1).When initial groundwater table levels were high before stormflow events, the stream was fed by shallow peat layers rich in NOM, iron, and arsenic during the event. The iron/DOC ratios were low and most of the iron was present in iron-NOM colloids in the stream water. The pH of the stream water was also lower under these conditions, and the logKD values of As-NOM associations in the stream water were accordingly lower (< 3 L∙mol− 1). Large quantities of dissolved arsenic (< 1000 g∙mol− 1) were exported under these conditions.Our data reveal that the logKD values of As-NOM associations decreased with increasing discharge as a consequence of decreasing pH. The logKD values for arsenic-NOM associations in this study are higher than those reported elsewhere in published literature, which had been derived from laboratory tests with NOM and arsenic. The formation of ternary complexes with ferric iron may therefore have enhanced the binding of arsenic to NOM within the studied stream.

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