Leaching of legacy paper mill sludge induces lithification by cementation of fluvial sediment

This study investigates the formation of anthropogenic conglomeratic rock caused by the lithification of natural fluvial sediment due to calcium leaching from adjacent paper mill sludge (PMS) deposits. The research focuses on a site near Penicuik, Scotland, where historical paper mill activities resulted in calcium-rich waste accumulation. Field observations identified lithified sediment on the stream bed beneath the PMS heaps, with natural clasts cemented by calcium carbonate (CaCO₃). Microstructural analysis using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) confirmed that the cementing material is predominantly calcite. X-ray Diffraction (XRD) analysis further validated the mineralogical composition, revealing calcite-rich cementation. Stable isotope analysis (δ¹³C and δ¹⁸O) indicated a mixed carbon source, with approximately 40% derived from atmospheric CO₂ and 60% from lithogenic origins. These findings demonstrate that Ca 2+ leached from PMS dissolved into pore water which migrated down into the fluvial sediment, facilitating calcite precipitation and binding sediment particles into a cohesive structure. This anthropogenic conglomerate challenges traditional classifications of sedimentary rocks and has implications for waste management, pollutant immobilization, and localized carbon sequestration. This anthropogenic lithification process parallels natural clastic sedimentary rock formation but occurs at the Earth's surface and on much shorter timescales. The results highlight the geochemical interactions between industrial waste and natural sediments, emphasizing human influence on sedimentary systems, bridging gaps between natural and anthropogenic geology.