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Environmental contamination comprises a complex mixture of both organic and inorganic contaminants. Understanding their distribution, behaviour and chemical interactions provides the evidence necessary to make informed decision and implement robust remediation strategies. However most of the current risk assessment frameworks, used to manage land contamination, are based on the total contaminant concentration rather than the concentration likely to pose significant risk, the bioavailable concentration. Further to this, the exposure assessments embedded within the frameworks do not explicitly address the partitioning and bioavailability of chemical mixtures. This inability may contribute to an overestimation of both the eco-toxicological effects of the fractions and their mobility in air and water; leading to an overestimation of health and environmental effects. In turn, this may limit the efficacy of the risk assessment frameworks to inform targeted and proportionate remediation strategies. The aim of this PhD study was to address this gap by delivering an integrated risk assessment framework for sites contaminated with complex chemical mixtures. Specifically, this PhD study investigated the fate and behaviour of complex mixtures of petroleum hydrocarbons, metals and metalloids in soils and its implication for partitioning, bioavailability and risk assessment through a 12 month mesocosms study. Further to this, an integrated approach, where contaminants bioavailability and distribution changes along with a range of microbiological indicators and ecotoxicological bioassays, was used to provide multiple lines of evidence to support the risk characterisation and assess the remediation end-point over a 6 month study. From the empirical data obtained from the two mesocosm studies, two Machine Leaning (ML) approaches have been developed to provide a quick and reliable tool to assess multi-contaminated sites with Visible and Near-Infrared Spectroscopy (Vis-NIRS), and to predict bioavailability and toxicity changes occurring during bioremediation. Overall this PhD study shed light on the behaviour of bioavailability, and toxicity of complex chemical mixtures in soils genuinely contaminated. This was supported through a comprehensive and integrated analytical framework providing the necessary lines of evidence to evaluate the implications for risk assessment and identify the end point remediation. The developed framework can significantly help to identify optimal remediation strategies and contribute to change the over-conservative nature of the current risk assessments.

The implementation of bioavailability-based water quality standards for metals in the United Kingdom, Europe and the United States is likely to challenge water utilities to reduce the concentrations of metals in effluent discharges. Municipal wastewater treatment works (WWTWs) are not, however, specifically designed to remove metals, with current treatment technologies providing a relatively low and inconsistent level of removal. New technologies will therefore be required to achieve removal rates to meet potentially more stringent discharge criteria. Improvements in the understanding of the form of metals in effluent is crucial for the development of technologies to reduce metal concentrations in effluent discharges. In the present study, effluent samples were separated into three parts; a particulate fraction (> 0.45 μm), a 'colloidal' fraction (0.45 μm - 1 kDa) and a 'truly dissolved' (< 1 kDa) size fraction. Soluble (i.e. < 0.45 μm) copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn) was found to be distributed predominantly in the colloidal fraction, and therefore technologies capable of removing particles from the colloidal fraction have good potential to reduce the concentration of these metals in effluent. Bench-scale studies tested the coagulation-flocculation (CF) treatment process and found this treatment approach to be effective for the removal of Cu, Pb and Zn from effluent, but was not effective at removing Ni. Two different CF treatment technologies were assessed, namely, the biopolymer, Floculan, and a conventional coagulant, ferric chloride (FeCl₃). Floculan and FeCl₃ demonstrated similar removal rates for metals (except for Cu, for which Floculan offered a better level of removal), however, Floculan maintained the concentration of dissolved organic carbon (DOC), which offers an important control on metal bioavailability. Floculan therefore simultaneously reduces metal concentrations and bioavailability whereas FeCl₃ reduces metal concentrations and DOC in proportion and therefore bioavailability is unaffected. Floculan offers the opportunity to control the concentrations of trace metals in effluent in an environmentally and financially sustainable manner. This thesis reports research funded by the Natural Environment Research Council and Severn Trent Water.