Abstract: Treatment is described for acidity caused by water seeping down through a seam of acid-generating mineral such as pyrite. The pyrite oxidises through exposure to the atmosphere (as a result of mine workings). The treatment system creates an galvanic cell, using the submerged portion of the pyrite seam as the cathode, the water in the aquifer as the electrolyte, and a body of scrap iron immersed in the water as the (sacrificial) anode. Contact rods are inserted into the pyrite seam, and are connected, via a cable, to the body of scrap iron, which is placed in a nearby pond. Hydrogen ions migrate to the cathode and bubble off as hydrogen gas, raising the pH. The acid generating reactions in the pyrite seam are inhibited, and acid concentrations in the already contaminated water in the pond and in the aquifer are reduced. In an alternative, the exposed beaches of pyrite in a tailings pond are provided with grids of metal mesh or graphite in place of the inserted rods.