Summary

The disposal of liquid wastes containing heavy metals and organic solvents into lagoons excavated to beneath the water table in a shallow, unconsolidated sand aquifer has resulted in local groundwater pollution. The development of the pollution plume appears to be controlled by the morphology of the aquifer, the distribution of permeability within it and the head distribution in the vicinity of the lagoons. On the basis of redox reactions three geochemical zones have been identified down hydraulic gradient in the transition from strongly reducing conditions near the lagoon and at the base of the aquifer to oxidizing conditions in natural groundwater. Heavy metals are attenuated within a short distance of the lagoons, probably as a result of precipitation as sulphides and carbonates, but organic wastes have been found in excess of 300 m from the site. With increasing distance from the lagoons changes in bacterial populations and the character of organic compounds present at the base of the aquifer suggest that organic transformations are occurring despite little change in the concentration of total organic carbon, although ultimately biodegradation to methane and carbon dioxide takes place. Studies are continuing in order to develop a three dimensional mathematical model integrating chemical reactions with groundwater flow.

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