Monitoring and measuring the impacts and environmental implications of flood events on contaminated sediment dispersal in the River Swale catchment, North Yorkshire

Abstract

The extraction, processing and smelting of ores has long been responsible for the release of heavy metals into the fluvial environment. As a consequence numerous studies have reported enhanced metal concentrations found in association with the sediments of rivers and in their surrounding floodplains. This thesis investigates the mobilisation, transport and the overbank deposition of sediment-associated heavy metals during flood events on the River Swale, North Yorkshire. This catchment has a prolonged history of mining for lead and zinc which spans approximately 2,000 years.
Contemporary overbank sediment deposits for the fbll length of the river system were examined following three flood events that occurred in 2002. The quantity of sediment deposited and concentrations of associated metals were determined as well as the metal deposition flux and speciation, by using various analytical procedures including a nitric acid digestion and the BCR sequential extraction method. Physical characteristics such as sediment grain size, pH, loss on ignition and carbonate content were also investigated. Floodplain sedimentation rates were relatively high during the floods and the patterns in metal concentrations were both spatially and temporally consistent. The locations of most concern were located within the headwaters of the catchment where lead concentrations exceeded 25,000 mg kg'. Investigations into the mobility of the
sediment-associated metals revealed that up to 88% of cadmium was found within the exchangeable phase of the sediment, however the concentrations of exchangeable lead and zinc were much higher and surpassed UK Government guidelines for crop growth and grazing livestock. Concentrations of exchangeable lead and zinc reached 19,241 mg kg' and 1,457 mg kg' respectively. Locations of 'exchangeable metal hotspots' with high levels of bioavailable metals were repeatedly found within the upper 21 km of the catchnent, around the confluence of tributaries which drain once intensively mined areas.
These very high concentrations potentially pose a risk to flora and fauna that grow or graze on the floodplain surrounding the River Swale if they are taken up or ingested. Remediation measures could be adopted to treat either the source of metals in order to prevent them entering the fluvial system, reduce the concentrations already in the contaminated floodplain, or minimise the area inundated by flood waters. Alternatively, land management strategies could be adopted in order to reduce the potential for metal uptake by plants and animals and subsequently entering into the foodchain.