Critical control of flooding and draining sequences on the environmental risk of Zn-contaminated riverbank sediments

Purpose: Diffuse pollution emanating from metal mining impacted sediment could serve as a barrier to achieving European Union-Water Framework Directive and United States-Clean Water Act requirements. United Kingdom climate projections (UKCP09) predict increases in rainfall and aridity that will influence river stage alternately exposing and submersing contaminated riverbank sediment. Research focuses on environmental contaminant dissolved Zn, investigating patterns of release, key geochemical mechanisms controlling Zn mobilization and the environmental risk of sediment subjected to these perturbations.
Materials and methods: Using two laboratory mesocosm experiments, metal mining-contaminated sediment was subjected to alternate wet and dry sequences of different duration and frequency. The first experiment was run to determine the influence of submersion and exposure of contaminated sediment on releases of Zn and to establish environmental risk. The second experiment utilised diffusional equilibration in thin film (DET) to observe the patterns of Zn release, with depth, in the sediment. A combination of pore water and sediment chemical analysis enabled elucidation of key geochemical mechanisms of control of Zn mobilization.
Results and discussion: Patterns of Zn release were found to be different depending on the length of wet and dry period. High concentrations of dissolved Zn were released at the start of a flood for runs with longer dry periods. A build-up of soluble Zn sulphate minerals over long dry periods followed by dissolution on first flood wetting was a key geochemical mechanism controlling Zn release. For longer wet runs, increases in dissolved Mn and Zn were observed over the flood period. Key geochemical mechanisms controlling Zn mobilization for these runs were (i) reductive dissolution of Mn (hydr)oxides and release of partitioned Zn over prolonged flood periods followed by (ii) oxidation and precipitation of Mn (hydr)oxides and sorption of Zn on exposure to atmospheric conditions.
Conclusions: Mesocosm experiments were a first step in understanding the effects of UK climate projections on the riverbank environment. Contaminated sediment was found to pose a significant environmental risk in response to hydrological perturbations. The ‘transient’ nature of dissolved Zn release could make identifying the exact sources of pollution a challenge, therefore further field studies are advised to monitor contaminant releases under a range of hydrological conditions and account for complex hydrology at mining sites.