Coastal Contamination by Mining Tailings: Unusual Minor Role of Humic Substances to Soluble and Dissolved Iron Distribution

Anthropogenic activities, such as iron ore mining have been suggested to change the iron chemical speciation, bioavailability and ultimately ecological aspects in coastal sea. In order to evaluate the effects of iron ore tailings on the physical and chemical distribution of iron species, dissolved iron (dFe) concentrations in three size-fractionated phases and their relation with humic substances (HS) were determined in the continental shelf affected by the one of the worst global mining disaster (Fundão dam collapse, Southeast Brazil). The surveys were conducted in the acute phase of the impact (November 2015 and February 2016) and 2 years and 9 months post dam failure (August 2018). dFe (0.45µm) was analysed by ICP-MS whereas dFe (0.22µm), sFe and HS were quantified by Adsorptive Cathodic Stripping Voltammetry (AdCSV).

dFe concentrations ranged from 0.78 to 2.8µM for dFe (0.45µm), from 30 to 700nM for dFe (0.22µm) and from 9 to 38nM for sFe. These concentrations were higher when compared to other places vulnerable to mining impacts even two years and nine months after the disaster showing the stability and solubility of iron in the water column. HS concentrations varied from 15 to 350 µg SRFA.L^-1. The binding capacity of HS accounted for 0.2 to 5.8 nmol of Fe-HS bound which explains only 0.2 to 12.9% of dFe (0.22µm) and from 0.2 to 43.0% of the sFe concentrations. Alike to other coastal areas, HS is playing a minor role in dFe and sFe distribution in that study area indicating that other complexing organic compounds are present and/or that iron is present under small (below 20nm) colloidal species. The massive loading from iron ore tailings and continuing input by remobilisation/resuspension of this material have changed the iron forms and complexation with HS in the continental shelf (Figure 1). The relation between dFe and other organic ligands should be investigated in further studies.