Inverse geochemical modelling was used to investigate rock–water interactions in flooded underground coal mines in northern Appalachia, USA. In early flooding, Pittsburgh seam mine waters are usually acidic (c. pH 3), with dissolved metals Fe and Al ranging from 10 to >100 mg l−1. Within a few decades, however, waters in fully flooded mines usually have pH of about 7 S.U., and alkalinity >300 mg l−1 CaCO3 Eq. Eh shifts from oxidizing (c. 500 to 700 mv) to reduced (−100 to −200 mv) conditions. Sodium concentrations may increase an order of magnitude; sulphate and iron concentrations may also increase. Water samples were collected from several mine-pools in West Virginia and Pennsylvania. A conceptual model was developed based on quantitative hydrology, mine-pool chemistry, mining conditions and mineralogy. The model was tested with the geochemical code PHREEQC. Simulations included mixing recharge and acid mine waters, precipitation–dissolution reactions involving carbonates, sulphates, oxy-hydroxides and sulphides, and ion adsorption and exchange. Na exchange was a dominant process in all models. Carbonates are orders of magnitude undersaturated in the juvenile mine-pool, but near saturation in the mature mine-pool, suggesting they are a primary source of acid neutralization and alkalinity. The mature mine-pool is simultaneously near equilibrium with iron sulphide, iron carbonate and iron oxy-hydroxide mineral phases. The rapid change in mine-pool water quality has substantial implications for management of these systems. Corresponding author [email protected]

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