Understanding the impact of flooding events on contaminated sediments in rivers and estuaries
Mineral exploration in the UK has contaminated broad areas since the Bronze Age and reached its peak during the Industrial Revolution leaving a legacy of sediment contamination by metals in rivers and streams. This legacy comprises one of the most significant sources of diffuse pollution to surface waters in the UK.
The impacts of climate change including increasing frequency of precipitation and flooding magnitude have the potential to release the legacy of metals trapped in sediments. Flooding events induce the re-suspension and transport of sediments resulting at the mobilisation of sediment-bound metals. In addition, in small streams with high gradients, contaminants may be remobilised in freshwater environments, but deposited downstream in brackish and fully saline estuaries.
The re-suspension of contaminated sediments can influence the partitioning of metals bound to sediment particles through changes in chemical and physical factors such as pH, dissolved oxygen (DO), sediment acid volatile/metal sulphides (AVS), organic carbon (OC), grain size, salinity and redox potential. These factors can influence the rise of dissolved metal concentrations and increase the bioavailability of metals to aquatic organisms. Whilst, the presence of sodium in brackish waters can compete with the bond between sediments and metals, thus inducing the release of metals. These interactions, added to oxic conditions can weaken even more the bond between metals and sediments increasing the bioavailability.
The aim of this research is understand the impacts of re-suspension and transport of contaminated sediments on ecological integrity. The Cotehele stream, located in SW England, was chosen as the research site due to the metal mining legacy located in tributary rivers.