Dissolved and solid phase organic carbon influences on the sorption/desorption of hydrophobic organic contaminants in clay barriers

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Landfill leachates contain a wide range of toxic organic contaminants, including a number of listed substances which must be prevented from reaching groundwater. To this end, the Landfill Regulations require that new landfills are designed with low permeability liner systems, comprising engineered clay layers or a composite of synthetic geomembranes and clay soils, and are located in areas underlain by low permeability geological formations. In addition to reducing the flow of leachate from the landfill, natural and engineered clays must reduce the concentration of contaminants reaching groundwater by natural attenuation processes such as sorption and biodegradation. The potential for geological layers (clay liner barriers) to retain or retard contaminants may be incorporated into landfill risk assessments. A landfill risk assessment must demonstrate that there will be no unacceptable discharge of listed substances to groundwater over the operational and post closure life of the site, a period of time that could last for hundreds if not thousands of year. Failure to demonstrate this in a robust way will provide cause for the refusal of an operational permit. Predictive models are used in risk assessments to demonstrate the discharge (or otherwise) of listed substances to groundwater, but it is the premise of this research proposal that the models currently in use are seriously flawed and do not take into account recent developments in the science (the evidence for this is outlined below). As predictions need to be carried so far ahead into the future, it is imperative that the most accurate model description of the process is used. The consequences of incorrect predictions are twofold: (1) landfill sites may be permitted that will actually cause pollution at some time in the future or (2) landfill sites may be refused a permit, when the type of clay being used would have prevented discharge of listed substances, There is only limited information about attenuation of different compounds on UK clay materials. Some work has been carried out but further research is required to provide more reliable data for use in models such as Landsim. For hydrophobic organic contaminants (HOCs) such as aromatic or aliphatic hydrocarbons, models which relate sorption to soil or sediment organic carbon (OC) content are often used, but these models do not take account of the diverse chemical nature of OC. Studies have shown important deviations between the sorption capacity predicted by existing correlations and that determined experimentally for a range of soils and sediments. Recent research has demonstrated that sorption is controlled not just by the quantity of organic carbon but also by its composition (e.g. a combination of organic matter (OM), source, age, and diagenetic alteration history). The risk assessment models currently in use do not take this into account and are therefore flawed. In addition, dissolved organic carbon (DOC) can have an impact on the sorption of HOCs. The presence of DOC in synthetic leachate has been shown to decrease the sorption of HOCs onto mineral liner materials. This phenomenon is particularly important in view of the implementation of the Landfill Directive (1999/31/EC); the new leachates formed in post-Landfill Directive landfills will have different concentrations of dissolved organic and inorganic compounds than leachates from current landfills, and will contain different concentrations of priority pollutants. Information currently available on natural attenuation of HOCs in landfill liners may therefore have limited application to the new types of leachates. The aims of this research are therefore to increase understanding of the role and mechanisms of both dissolved phase and solid phase organic carbon in sorption/desorption of listed HOCs in landfill liner clays, and, to provide more reliable data and improved models for regulatory risk assessment of landfills.