Soil contamination in urban Tyneside : a chemical and biological risk assessment
The extent of soil contamination with organic and inorganic pollutants in most urban areas in the UK is largely unknown but due to past and present industrial activity it is likely that pollutant levels are high. Such contamination could have a serious impact on human health. Therefore this thesis set out to examine the extent of soil contamination within Newcastle upon Tyne with a focus on the contribution of an incinerator (Byker) to contamination levels of urban soils. The Byker incinerator is situated in central Newcastle and has been the subject of much media controversy due to the disposal of incinerator ash on local allotments. The current work extended past investigations to see if the incinerator had contributed to general urban soil pollution (heavy metals and dioxins) by aerial deposition and allowed a useful investigation into levels of urban soil contamination in Newcastle. In addition to examining metal and dioxin levels the bioaccessibility (human and bacterial) of pollutants in selected soil samples was estimated and an attempt to develop a human cell based soil toxicity assay made. These measurements permit a preliminary assessment of risk to human health from soil contamination. A total of 163 soil samples were collected based on predicted aerial deposition from the Byker incinerator and analysed for dioxins and heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb & Zn) content (mg pollutant /kg soil). A high proportion of samples (90/163) had contamination levels above soil guideline values (as proscribed in current UK regulations; CLEA). The highest dioxin levels were South West of the incinerator (1911 ng I-TEQ/kg) and the congener profiles coupled with a detailed historical survey demonstrated that the source of contamination was likely to be an old alkali works and not the incinerator. Overall it was found that the incinerator did not contribute significantly to dioxins found in the urban soils except those in the incinerator plant grounds. Soil metal contamination levels varied but were not related to incinerator deposition. Many samples contained levels of heavy metals well over soil guideline values with the highest values being found for Cu (12,108 mg/kg), Pb (4,134 mg/kg) and Zn (4,625 mg/kg) To determine the potential human health risk associated with heavy metal contaminated soils, selected samples (16) were subjected to two in vitro digestion techniques which simulate the bioaccessibility of metals (Cu, Ni, Pb & Zn) to humans in case of soil ingestion. Only three samples had high levels of metal availability (over SGV's) indicating that these soils should be subjected to further risk assessment. All other samples tested had low metal availability most likely due to a combination of metal speciation, and complexation to soil particles over time. An attempt was made to develop a human cell based system to determine the toxicity of contaminated soil. Using an in vitro system, human liver cells (HepG2's) were exposed to extracts from soils; cytotoxic effects (membrane integrity, metabolic capability and oxidative stress status) and genotoxicity potential (DNA damage) of Cu and Zn were first investigated in order to standardise the biological assays used. Between 0.1 and 10 mg/L Cu caused DNA damage and higher concentrations caused cytotoxicity. Zn was also proven to cause genotoxic effects from O. lmg/L. From 10 mg/L cytotoxic responses occurred and DNA damage could be attributed to cell death. Due to difficulties in sterilising soil extracts and physical damage caused to HepG2 cells by the abrasive nature of soil, it was not possible to elucidate whether metal contaminated soil extracts were capable of causing a cytotoxic or genotoxic response in human liver cells. Finally, the same soil samples were then subjected to a bacterial (lux) biosensor technique to examine soil toxicity. Interestingly, despite the high levels of contamination found, none of the soil samples were found to be toxic to the two Pseudomonas strains used which again indicates a low level of ecosystem risk and suggests that most of the contaminants present are either in a form that is unavailable to living microbes or are complexed to soil particles. In summary, this research has shown that the high level of soil contamination of urban areas in Newcastle is due to past industrial activity and a similar situation is likely in most other urban areas of the UK and internationally. The high cost of remediation means that if contaminated sites are shown to be a potential risk then bioaccessibility of contaminants should be examined in order to provide a more realistic assessment of the need for remediation. This work demonstrates that only a small proportion of urban contaminated sites are likely to require remediation based on bioaccessibility determination measurements.