Land Contamination: Technical Guidance on Special Sites: Acid Tar ...

The chemicals listed in Table B.1, can be grouped into several categories, as follows:• aliphatic hydrocarbons;• aromatic hydrocarbons;• phenols;• metals;• organic acids;• sulphonated hydrocarbons; and• gases such as hydrogen sulphide, sulphur dioxide and methane.However, the chemicals tend to form a dense mixture in which pure compound physicochemicalproperties do not apply (due to matrix effects).The identity of non-tar related chemicals can vary greatly, although polychlorinated biphenyls(PCBs) are a common additional contaminant (presumably co-disposal of oily wastes).Where the acid tar disposal area was located at the source of production, the other materialsmay be expected to be process or plant-related (other oils, demolition wastes, etc.). However,a wider range of wastes may be found where tars were disposed of at general tips. These maybenefit the containment of the acid tars by providing a degree of retardation of solublecomponents due to absorption processes.3.4 Breakdown ProductsSome breakdown by natural biodegradation will occur and can result in production, amongother compounds, of hydrogen sulphide, sulphur dioxide and methane. Generally, rates ofbiodegradation within the main disposal area will be limited by toxic effects on bacteria (fromhigh concentrations of contaminants and low pH). However, at the edges, more microbialaction is likely. Although weathering is likely to release sulphur dioxide and reduce aciditysomewhat (release of sulphur dioxide (as “puffs”) is an issue for remediation), there appearsto have been little study of other breakdown products. However the higher molecular weightpolycyclic aromatic hydrocarbons (PAHs) are likely to be relatively resistant to degradation.3.5 Mobile CompoundsThe chemicals liable to migration in or associated with groundwater are principally those inthe aqueous phase (phenols and derivatives, dissolved sulphuric acid, BTEX compounds,organic acids) and free oils. The high molecular weight asphaltenes (tarry substances) have alow solubility and remain relatively immobile. High acidity levels surrounding the lagoonswill also tend to mobilise metals such as iron from naturally occurring mineral deposits.R&D <strong>Technical</strong> Report P5-042/TR/04 19