Uncovering UK coastal legacy wastes and their potential contaminant release risks through mineralogy and geochemistry

For centuries, coastal zones have been used as disposal sites for industrial and municipal wastes, often without adequate records of their composition, raising long-term concerns regarding the potential release of contaminants. The absence of detailed mineralogical and geochemical data has limited the ability to evaluate the mobility of metal(loid)s and to design effective protection strategies. In this study, 83 samples of legacy wastes were collected from coastal sites in England, Wales, and Scotland to investigate the influence of mineralogy on the release of potentially toxic elements (PTEs). Samples were collected from 18 priority sites, selected to be representative of the major legacy waste types found along the UK coastline. Non-ferrous slags displayed variability in mineralogy and PTE concentrations, including As, Cd, Cr, Cu, Ni, Pb, Sb, V, and Zn. These elements were predominantly hosted in stable silicates (e.g., sorosilicates, olivine) and oxides (e.g., cassiterite, spinel), with minor associations in sulfides, sulfates, and carbonates. Coal and metal mining wastes contained PTEs in redox-sensitive sulfides and secondary phases such as sulfates and iron oxides, representing the highest potential for contaminant release under seawater inundation and erosion. Bauxite waste exhibited the greatest median concentrations of Cr and V, whereas municipal solid wastes were enriched in Pb and Ni compared to ferrous slags and colliery wastes, with Ni median concentrations being the highest among all waste types. These insights emphasise the importance of mineralogical characterisation in assessing risks and managing coastal legacy wastes under climate change scenarios.