Apparatus (10) for treating a waterway having low pH and containing metals such as aluminium in solution, as a result of runoff from acid sulphate soils, is capable of dosing the waterway with a variety of reagents, to bring the metals out of solution and to achieve a neutral pH. The apparatus (10) includes a rotating drum tumbler (14) having a cylindrical drum (52) which, as it rotates, causes limestone rock to abrade to produce particles of calcium carbonate with which the waterway may be dosed. The inner surface of the drum (52) is provided with an abrasive surface (68) which contacts rock pieces (74) and transports them within the drum (52) as the drum (52) rotates, and with guide vanes (72) which contact and deflect the rock pieces (74), causing the rock pieces (74) to abrade each other. Other reagents in liquid and/or powder form may also be dispensed.