Beyond acid-base accounting: Integrated characterisation of coal tailings for AMD reactivity ranking and closure planning.

Standardised acid-base accounting (ABA) classifies coal tailings as potentially acid-forming (PAF), non-acid-forming (NAF), or uncertain, and represents the most widely applied first-tier approach for acid mine drainage (AMD) hazard assessment. However, ABA does not resolve differences in near-term oxidation susceptibility among PAF-classified materials with similar bulk sulfide content, because the processes governing oxidation initiation, including mineral liberation state, hydrological controls on oxygen ingress, and microbial community structure, are not captured by static measurements alone. This limitation constrains closure planning by providing insufficient resolution to differentiate AMD reactivity or prioritise kinetic testing among PAF tailings. This study evaluates coal tailings from the Bowen Basin, Australia, using integrated mineralogical, physical, hydrological, and microbiological characterisation to assess whether parameters beyond bulk sulfide content and acid neutralising capacity improve the basis for inferring chemical stability. Amorphous content correlates strongly with total organic carbon (R2 = 0.89), confirming carbonaceous maceral dominance and introducing systematic uncertainty into mineral inventories derived from quantitative X-ray diffraction alone. Pyrite liberation varies from 14.4% to 66% of sulfide mass exceeding 40% surface exposure across a bulk sulfide range of 0.6 to 3.3 wt%, with marcasite-to-pyrite ratios providing additional discrimination of intrinsic oxidation kinetics. Saturated hydraulic conductivity spans more than one order of magnitude (8.64× 10-7 to 5.83 × 10-6 m/s), while air-entry values of 26 to 200 hPa distinguish materials with rapid post-drainage desaturation from those sustaining moisture retention that restricts oxygen ingress to sulfide-bearing depths. Microbial communities in saturated tailings zones are dominated by methanogenic archaea and sulfate-reducing bacteria under persistently reducing, circumneutral to alkaline conditions, with no evidence of acidophilic iron- and sulfur-oxidising populations at the sampled depths and under the prevailing porewater chemistry. A material with moderate bulk sulfide (1.6 wt%) but high pyrite liberation (66%) and low air-entry value (26 hPa) exhibits greater inferred near-term oxidation susceptibility than a higher-sulfide material (2.8 wt%) with limited liberation and sustained moisture retention, consistent with liberation state and hydrological behaviour exerting independent control on reactivity beyond bulk sulfide content. These results indicate that integrated characterisation provides a mechanistic basis for ranking PAF coal tailings by inferred oxidation susceptibility, supporting targeted allocation of kinetic testing and more informed closure and rehabilitation planning. Validation against kinetic test outputs and field porewater monitoring is required to confirm these rankings under field conditions.