Passive treatment of acidic mine waters in subsurface-flow systems: exploring RAPS and permeable reactive barriers

It is now clear that the most effective passive treatment pathways for acidic mine waters require anoxic conditions, in which carbonate dissolution can occur in the absence of Fe3+ and/or bacterial sulphate reduction can occur, removing pollutant metals as sulphides, generating bicarbonate alkalinity and raising pH. By mimicking nature, it is most appropriate to achieve and maintain suitably anoxic conditions by designing treatment system hydraulics such that they are based on subsurface flow through porous media. Some of the challenges involved in this activity are discussed here, such as: predicting and prescribing the permeability of manure-based composts; matching sufficiently high permeability with a high enough organic matter content to ensure microbialactivity; geometrical considerations in systems design. The ways in which such challenges have been overcome in recent designs are discussed in relation to the Bowden Close RAPS system in County Durham and theShilbottle PRB (permeable reactive barrier) in Northumberland. These two systems form nuclei of the new National Mine Site Remediation Research Facility (NMSRF) sponsored by CL: AIRE, and are study sites for the new DTi Bioremediation LINK project "ASURE", which concerns bioremediation of acidic mine waters by sulphate reduction in novel, compost-based, field-scale bioreactors. (Original abstract)