Abstract
Metal mining activities have resulted in the widespread metal pollution of soils and sediments and are a worldwide health concern. Pb is often prolific in metal-mining impacted systems and has acute and chronic toxic effects. Environmental factors controlling diffuse pollution from contaminated riverbank sediment are currently seen as a “black box” from a process perspective. This limits our ability to accurately predict and model releases of dissolved Pb. Previous work by the authors uncovered key mechanisms responsible for the mobilisation of dissolved Zn. The current study identifies key mechanisms controlling the mobilisation of dissolved Pb, and the environmental risk these releases pose, in response to various sequences of “riverbank” inundation/drainage. Mesocosm experiments designed to mimic the riverbank environment were run using sediment severely contaminated with Pb, from a mining-impacted site. Results indicated that, although Pb is generally reported as less mobile than Zn, high concentrations of dissolved Pb are released in response to longer or more frequent flood events. Furthermore, the geochemical mechanisms of release for Zn and Pb were different. For Zn, mechanisms were related to reductive dissolution of Mn (hydr)oxides with higher concentrations released, at depth, over prolonged flood periods. For Pb, key mechanisms of release were related to the solubility of anglesite and the oxidation of primary mineral galena, where periodic drainage events serve to keep sediments oxic, particularly at the surface. The results are concerning because climate projections for the UK indicate a rise in the occurrence of localized heavy rainfall events that could increase flood frequency and/or duration. This study is unique in that it is the first to uncover key mechanisms responsible for dissolved Pb mobilisation from riverbank sediments. The mineralogy at the mining-impacted site is common to many sites worldwide and it is likely the mechanisms identified in this study are widespread.
Highlights
Metal mining activities including mineral extraction, processing and dumping of contaminated waste alongside river channels has resulted in the widespread metal pollution of soils and sediments and is a worldwide health concern (Foulds et al, 2014; Frau et al, 2018; Zhang et al, 2012; Zadnik, 2010)
PHREEQC (Ph-Redox-Equilibrium in “C”) The geochemical computer program PHREEQC was used for speciation and saturation index (SI) calculations using the WATEQ4F.dat database distributed with the PHREEQC program
A key objective of the current study was to determine whether mining contaminated sediments became a source of dissolved Pb in response to flooding and draining sequences
Summary
Metal mining activities including mineral extraction, processing and dumping of contaminated waste alongside river channels has resulted in the widespread metal pollution of soils and sediments and is a worldwide health concern (Foulds et al, 2014; Frau et al, 2018; Zhang et al, 2012; Zadnik, 2010). Key hydrogeochemical mechanisms may include: (i) Pb coprecipitation with and sorption to Fe/Mn (hydr)oxides under oxidised (drained) periods followed by reductive dissolution and release of dissolved Pb due to a fall in redox potential conditions over prolonged flood periods; (ii) the oxidation of the primary mineral galena and release of dissolved Pb and sulphate where previously reduced sediment is exposed to oxic conditions (Wragg and Palumbo-Roe, 2011) (iii) the precipitation of insoluble Pb sulphides due to a fall in redox potential conditions over prolonged flood periods (Lynch et al, 2014); (iv) control of dissolved Pb concentrations to low levels through saturation with respect to anglesite (PbSO4) over flooded periods (Palumbo-Roe et al, 2013; Appelo and Postma, 2010); (v) hydrological saturation and precipitation of soluble sulphate salts over long dry periods followed by dissolution of these salts, and a “flush” of dissolved Pb and sulphate, in response to flood wetting (Byrne et al, 2013). The objectives were to: (i) investigate if flooding and draining sequences influence the patterns of dissolved Pb release from severely contaminated river bank sediment; (ii) identify key hydrogeochemical processes responsible for controlling the mobilisation of dissolved Pb and if they differ from the mechanisms of control for dissolved Zn; (iii) establish if Pb contaminated riverbank sediment poses an environmental risk when exposed to alternate flooding and draining sequences
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