Spatial variability of metal(loid) leaching from coastal colliery wastes under freshwater and saline water conditions

Historical disposal of coal mine wastes in the coastal zone has left a significant environmental pollution legacy. Climate change is increasing the likelihood that erosion of these wastes will lead to release of metal(loid)s to coastal environments. Whilst previous research has focussed on the generation of acidic, metal-rich waters from coal mine wastes in freshwater environments, a comprehensive investigation of metal(loid) leaching from such wastes in the coastal zone has not been undertaken. This study investigated the leaching behaviour of coal mine wastes under freshwater and saline conditions and determined the impacts of spatial heterogeneity of waste composition on such behaviour. The degree of leaching varied considerably within and between sites due to the heterogenous nature of the waste. Leachate pH varied from 1.80 to 6.99 with acidic leachates particularly enriched in Fe (≤17,000 mg/kg dry waste) and sulfate (≤48,000 mg/kg dry waste) due to dissolution of acid sulfate phases. Dissolution of Fe and Mn oxides, hydroxides and oxyhydroxides also led to release of surface adsorbed metal(loid)s such as As (≤21 mg/kg dry waste), Zn (≤86 mg/kg dry waste) and Cu (≤14 mg/kg dry waste). Adsorption of As to high surface area minerals was confirmed by X-ray Absorption Near Edge Spectroscopy (XANES) analysis. Metal(loid) release was typically lower in the presence of seawater than deionised water due to the greater pH buffering capacity of seawater. This research provides an insight into the considerable challenges faced by coastal managers globally as they seek to mitigate the risks from such legacy pollution.