Effect of episodic rainfall on aqueous metal mobility from historical mine sites

Abstract

Environmental contextEpisodic extreme rainfall events may affect metal dynamics in rivers flowing within historical metal mining areas. This study provides an analysis of the water chemistry and geochemical processes associated with mobilisation of metals during episodic rainfall events. Findings could be used to assess the environmental quality of streams draining spoil waste areas with similar geochemical conditions, and thereby be used to guide future management strategies. AbstractThe increasing frequency and magnitude of episodic rainfall events may affect historical metal mining areas by remobilisation and deposition of metal-rich sediments and enhancing metal-rich run off, impacting river water quality. This study assesses the effects of episodic rainfall in a Carboniferous headwater catchment contaminated by historical Pb and Zn mining. Comprehensive hourly water chemistry measurements combined with modelling using PHREEQC, WHAM/Model VII and WHAM-FTOX were used in this assessment. For the episodic event, we measured flow increases from a baseline of 0.05 to 2.12 m3 s−1 at peak flow. Changes in metal concentration were most marked for ephemeral tributary, with Pb increasing from a baseline concentration of 55 μg L−1 to a peak of 576 μg L−1. Behaviour for Pb showed great affinity to form organic complexes or bind to colloidal Al and Fe oxides, whereas for Zn and the tributary flowing subsurface a more complex behaviour was observed. For example, the dissolution of secondary metal carbonate minerals (e.g. smithsonite (ZnCO3)) is likely constrained by higher concentrations of carbonate and bicarbonate derived from increased bedrock weathering under flow conditions induced by episodic rainfall. The abundance of secondary mineral sources and circumneutral pH present during episodic rainfall are important factors controlling the mobilisation of Pb and Zn. Furthermore, episodic rainfall events could enhance metal toxicity but there are aggravating and mitigating factors that depend on site-specific chemical changes. Overall, this study highlighted the complexity of metal mobility and toxicity during these events.

Hydrotransport-Oriented Zn, Cu, and Pb Behavior Assessment and Source Identification in the River Network of a Historically Mined Area in the Hokuroku Basin, Northeast Japan

“…This is consistent with the results of Gozzard et al, who reported that sub-watershed nonpoint sources were of little importance in the low-water season, with point source human water discharge dominating instream concentration and load [55]. This is an effective way to arrange industrial water discharge during the precipitation period, reducing the effect on the aquatic environment since the large water quantity can effectively dilute the pollutants [56].…”

Seasonal quality variation and environmental risks associated with the consumption of surface water: implication from the Landzun Stream, Bida Nigeria

“…for Zn, LWS14, LWS17 and LWS 18 for Mn, LWS15 and LWS16 for Pb and LWS02 – LWS18 for Fe. This is suggestive of the influence of diffusion on the concentrations of these metals, and in consonance with the view that episodic rainfall mobilises aqueous metals from regions of higher concentrations to lower concentrations (Valencia-Avellan et al., 2017). This observation is in contrast with that of Yao et al.…”

Metal Mobility in a Mine-Affected Floodplain

“…This decrease in water pH may be compensated by the loss of calcium from the loams due to leaching by rainwater, usually having a slightly acid reaction (pH = 5.4) [63], as suggested by the high content of this element in the surface sediment stratum. The leaching of Ca may be accompanied by an acceleration of metal mobilization and, after the exhaustion of the mobile part of metals from the surface strata, the process may be of a lower intensity, which will mainly be related to local rainfall, as observed in similar post-mining areas [64][65][66]. Overall, the processes of metal mobilization in the Biała valley seems to be hampered by the thick loams of low permeability strata covered by organic matter which is able to preclude access to oxygen by lower sediment strata [42].…”