Impacts of climate-driven wetting and drying on mobilization of trace metals in legacy coastal wastes in the United Kingdom

Climate change is expected to significantly affect the release of toxic elements from legacy waste landfills across coastal areas because it will enhance severe flooding, erosion, and extreme wetting-drying cycles. The UK’s industrial past resulted in a wide range of legacy wastes deposited in estuarine and coastal settings, for a total of 2550 sites. In this perspective, a NERC project is ongoing to provide a national-scale assessment of geochemistry and mobility of these wastes, which is necessary to establish environmental and ecological risks and to prioritize sites for management interventions. In the project framework, a mesocosm experiment was carried out to investigate the leaching processes that can occur in different types of legacy wastes (LW) disposed across the UK coasts. The experiment was designed to study how wetting and drying cycles affects the release of pollutants in the coastal areas. Four sites were selected, each one for a different type of LW: colliery (L), landfill municipal wastes (CB), metal smelter (CP), and mining-impacted sediments (HE). These four sites represent the range of LW geochemistry encountered across the UK. Each wetting/drying cycle (WDC) lasted 2 weeks. Artificial rainwater was used to inundate the columns. Water samples were collected weekly for Dissolved Organic Carbon (DOC), major ions and metals. EC, Eh, DO, and DOC showed a great variability both at inter- and intra-sites: the L samples were the most acid ones (pH = 3.3 – 5.1), whereas the other sites showed pH values ranging from 6.0 to 8.7. HE samples showed the largest variation ranges for EC and DOC (409 – 109000 µS/cm and 8.3 – 405.8 mg/l, respectively), whereas the CP ones showed the narrowest ranges for both parameters (176 – 592 µS/cm and 0.2 – 248.1 mg/l, respectively). The maximum variation for EC and pH occurred after the first WDC for all the sites but CP, which showed a progressive decrease in both parameters throughout the experiment. DO, Eh, and DOC varied more irregularly for all sites. For each site, Control samples showed wider ranges of variation than those that experienced the WDC but the trends were similar for both control and non-control ones. These preliminary data, together with trace metal data, suggest that leaching processes are more likely to be efficient at the end of prolonged dry periods when heavy storms occur. Extreme climate conditions are expected to become more frequent in the near future, therefore a proper management is mandatory to reduce the risk posed by the coastal legacy wastes.