Heavy metal contamination of river water : sources, behaviour and remediation

Pollution from metal mining has led to severe environmental damage. The assessment of metals is very complex as they interact with a broad spectrum of biotic and abiotic components depending on physicochemical conditions. Worldwide, discharges from ancient mines are considered one of the major causes of point and diffuse pollution. This thesis investigated the sources and mobility of metal pollution associated with historical mining in a carboniferous upland catchment, located in the Northern Pennines in the UK. From chemical analysis and geochemical modelling I identified metal sulphates and metal carbonates as the main mineral sources of metals. I also demonstrated that metal carbonates are controlling metal mobility, while seasonality is also producing changes in flow and pH conditions, affecting metal concentrations and behaviour. By using speciation modelling, ecotoxicological assessment tools and in situ macroinvertebrate survey I highlighted the dynamics of metals occurring in neutral mine drainage; and suggested the application of environmental quality standards based on bioavailability data within a realistic context relating response of aquatic organisms to river water chemistry and metals. I also evaluated the effects of episodic rainfall on aqueous metal mobility and toxicity to address some knowledge gaps. I found that rainfall conditions did not alter the circumneutral conditions of the catchment, although metal mobility and speciation were affected by the abundance of carbonate and bicarbonate minerals derived from bedrock weathering. I showed that metal toxicity occurred at circumneutral pH, and mainly attributable to zinc. Moreover, I assessed the effects of episodic rainfall in metal toxicity and calculations revealed that short-term fluctuations of metal concentrations are not reflected in the predicted acute toxicity risk to aquatic organisms, underlining the complexity of chemical speciation especially during episodic events. Likewise, I provided a baseline for future mitigation strategies for catchments under risk of metal pollution. Finally, I stressed the importance of the public perception and community involvement in a holistic management of catchments for protecting riverine ecosystems and improving their water quality. Overall, this thesis provides the evidence that a comprehensive metal assessment requires a great understanding of processes and reactions occurring from metal sources to potential endpoint environments (e.g. water, sediments, and biota). For regulatory purposes, technical knowledge needs to be sensibly transferred to the community for achieving an effective integrated catchment management. Findings from this thesis are suitable for the assessment of streams draining spoil waste areas with similar geochemical conditions and inform future management strategies.