Pollution potential of the dump has been assessed from existing geological data and knowledge of the local groundwater table. Samples of groundwater have been analysed and calculations made of potential leachate volume and sub-surface flow rates. The following conclusions have been reached: • No evidence of groundwater pollution has been detected adjacent to the rubbish dump. • A thickness of about 20m of volcanic ash with low to moderate permeability beneath the dump should provide efficient treatment of any leachate produced. • Regional groundwater movement is away from Mount Gambier and the Blue Lake and its velocity is low. The site presents no threat to the Blue Lake Water Supply at present. Increased future water abstraction from the Blue Lake however could possibly cause a local reversal of groundwater flow and further pollution checks would be necessary in that event. Any further rubbish dump sites should not be selected without a hydrogeological assessment of their pollution potential.