A permeable reactive barrier (PRB) was in- stalled in Aznalcóllar (Spain) in order to rehabilitate the Agrio aquifer groundwater severely contaminated with acid mine drainage after a serious mining accident. The filling material of the PRB consisted of a mixture of calcite, vegetal compost and, locally, Fe 0 and sewage sludge. Among the successes of the PRB are the con- tinuous neutralisation of pH and the removal of metals from groundwater within the PRB (removals of >95 %). Among the shortcomings are the improper PRB design due to the complexity of the internal structure of the Agrioalluvialdeposits(whichresultedinaninefficient capture of the contaminated plume), the poor degrad- ability of the compost used an d the short residence time within the PRB (which hindered a complete sulphate reduction), the clogging of a section of the PRB and the heterogeneities of the filling material (which resulted in preferential flows within the PRB). Undoubtedly, it is only through accumulated expe rience at field-scale sys- tems that the potentials and limits of the PRB technology can be determined.

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