Magnetic and interpolation techniques in the identification and analysis of metal contaminants In urban soils
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Soil pollution has been identified as the third most important threat to soil quality in Europe. There are an estimated ~2,000 potentially contaminated sites in Ireland alone. Metal contamination has been identified as one of the most common types of soil contamination. Urban soils are particularly susceptible to metal contamination as they are subjected to constant inputs of anthropogenic origin. This thesis describes cost and time saving methodological approaches involving the use of magnetic susceptibility (MS) measurements and spatial analysis techniques which can be applied in the preliminary and exploratory phases of site assessment of suspected metal contamination in urban topsoils. The first study describes a methodological approach to the investigation of metal contamination of bonfire affected soils in a residential green space. Measurements of low frequency mass specific magnetic susceptibility (χlf) were recorded from the soil samples collected from a high density 1 x 1 m2 grid. Local Moran’s I hotspot analysis maps were produced for χlf and elemental concentrations of the soil samples. At this density, localized variations in susceptibility signals were distinguishable using the hotspot analysis technique to reveal statistically significant high value clusters where the most recent bonfire presided. Potential locations of historic bonfires were also identified. This application was shown to be effective, even when physical evidence on site was not obvious. At a second site, two methodological-based studies were conducted on the use of volume magnetic susceptibility measurements in the identification of metal contamination in topsoil. The study area was a former unregulated landfill site and is currently used as a local amenity park. The geostatistical analysis technique of ordinary cokriging (CK) was applied to the lead and volume magnetic susceptibility (κ) data collected to explore the use of this method in efficiently estimating the spatial distribution of lead present at the site. The objective was to improve spatial interpolation for lead using the auxiliary information of κ which could be more easily collected. The results were compared to ordinary kriging (OK) estimates of Pb concentrations. Root mean square errors (RMSE) and coefficients of determination (R2) signified a marked improvement in estimation capability of the CK procedure, demonstrating the efficiency of this method in an exploratory site investigation of this nature. In addition to site assessments, practical experiments were conducted to aid others in investigations of this kind. The effects of grass cover on in-situ volume MS (κ) measurements of urban soils was also investigated at the former unregulated landfill site. The results suggested that the removal of grass coverage prior to obtaining in-situ κ measurements of urban soil is unnecessary. This layer does not impede the MS sensor from accurately measuring elevated κ in soils (in typical urban green spaces with low vegetation cover <10 cm), and therefore κ measurements recorded with grass coverage present can be reliably used to identify areas of urban soil metal contamination. During the examination of bonfire soils a small experiment was carried out to isolate what temperature the soils directly below the base of the bonfire ash likely reached and it was determined they likely reached a maximum of 300°C and at this temperature had minimal impact on the MS signal of the soils. The methods of investigation detailed in this thesis can easily be adapted and applied in the inspection of suspected illegal burning or municipal waste disposal sites by local governing bodies charged with containing this issue.
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