Acid tar lagoons : assessment and environmental interaction.

Acid tars are waste residues of obsolete benzole refining, oil re-refining and white oil production processes. They are black, acidic, viscous semi-liquids comprising an extremely complex mixture of water, sulfuric acid and a large range of organic compounds. Acid tars were often dumped into excavations and existing holes in the ground together with various co-disposed materials without any treatment or engineered lining system leaving a legacy of acid tar lagoons. Acid tars may pose potential risks to human health and the environment because of their acidity, volatiles and other hazardous components. The major contamination pathways of acid tar lagoons are considered to be direct contact, gas emission, bulk tar migration offsite, and surface and ground water contamination. The leaching, weathering and migration behaviours of acid tars were investigated by carrying out a series of batch, cascade and flow cell tests. The results demonstrated that acid tars are capable of leaching Significant levels of contaminants if disturbed, while the leaching level is Significantly lower under simulated groundwater flow in a model soil. The time scale to leaching stabilization was of the order of months in the model soil and is expected to significantly exceed that in the natural environment. To support this work a nubmber of analytical methods for determining physical and chemical properties of acid tars had to be adapted from standard techniques. These modified techniques are described in detail together with recommendations for data integration and correlation of all analyses to form a better understanding of acid tars and their potential environmental impact. The overall findings of the research were integrated to generate a conceptual model of acid tar lagoon processes to assist in assessment. Monitored Natural Attenuation was evaluated and is considered to be a potentially viable approach to the environmental management of some acid tar lagoons