Tracing legacy coal mine waste using sulfur isotopes

Sulfur stable isotope ratios (δ 34 S) provide a robust tool for tracing sulfur sources, and, with appropriate caution, associated processes and pathways in soils, plants, coal and water. This study measured δ 34 S across a remediated coal mining landscape at South Hetton Colliery and Hawthorn Combine Mine and Cokeworks, East Durham, England. Post-mining remediation, including artificial infilling (the placement of imported materials to restore ground levels) and backstowing (the filling of mined voids with waste material), disrupted natural soil horizons, generating heterogeneous layers that could influence pollution transport pathways. 56 shallow cores were collected along a systematic spatial grid to assess the sulfur distribution within a remediated post-coal mining artificial ground site. Topsoil δ 34 S (0–28 cm) ranged from − 8.7‰ to + 14.9‰. Plant δ 34 S varied between − 4.1‰ and + 18.4‰, coal fragments within cores between − 0.2‰ to + 13.6‰ and water between + 6.2‰ to + 12.5‰. Soil δ 34 S exhibited a consistent southwest-to-northeast trend to lower values across all depths. Plant δ 34 S tracked soil with an average offset of 1.5‰. Sulfur isotopes indicate multiple potential sulfur sources and processes in the artificial soils, including coal-mining derived materials, groundwater inputs, and local variability linked to remediation and land use. Irrespective of the heterogenous nature of the artificial ground, sulfur isoscape patterns are consistent with past groundwater flow investigations. These findings help demonstrate the use of soil δ 34 S for source attribution and inferences related to sulfur movement in reclaimed coal mining landscapes. Further investigations of this type would provide additional evidence on management strategies in reclaimed coal mining landscapes—abundant throughout England and Wales.