Moisture on underground walls in abandoned mines as source of acid drainage

The occurrence of secondary sulphates over mine walls and their dissolution-precipitation cycles were investigated by micrometeorological techniques. Continuous measurements of the underground temperature and relative humidity were compared with data of the external atmosphere. The sulphate minerals occurrence were detected by monthly field-work during winter that coincides with the time of relative humidity decrease. On the other hands, these salt efflorescence disappeared since the late spring, during the whole summer, and until November. For this period the underground atmosphere is over saturated in air water vapour and water droplets were observed on the mine walls. This rock moisture is related to vapour condensation brought by external air masses entering into the shallow part of the mine. As the interaction between condensation water and sulphide materials generates acid drainages that potentially increase the solubility of toxic elements, these results contribute to understand contamination pathways occurring at abandoned mining sites.