Spatial and temporal variations in potentially toxic elemental (Sb, Pb, Cu and Zn) and PAH concentrations and associations in run-off from urban and rural areas of Scotland

Field: Environmental Science (Physical Sciences)

Since the UK industrial revolution, coal combustion, ore smelting and other industrial \nactivities have resulted in a marked increase in emissions of potentially toxic elements \n(PTEs) such as antimony (Sb), lead (Pb), arsenic (As), copper (Cu) and zinc (Zn), along with \npolycyclic aromatic hydrocarbons (PAHs), to the atmosphere. Although stricter \nenvironmental regulation and improved technology has led to a notable decline for some \ncontaminant emissions in recent decades, this has not been observed for all elements, e.g. Sb, \nwhere only a modest reduction in emissions have been recorded. Once emitted, Sb along \nwith Pb, As, Cu, Zn and PAHs may persist in the environment for considerable periods of \ntime after their release; although their chemical associations may change, elements are not \nbroken down over time and organic contaminants may break down slowly. Above all, PTEs \nand PAHs are detrimental to human and environmental health, with chemical forms of Sb, \nPb, As and PAHs categorised as carcinogenic. Understanding their behaviour and fate in the \nenvironment is therefore an important step towards evaluating their likely impact on both \necosystem and human health. Consequently, this study focuses on the release, behaviour and \nfate of contaminants from current and past anthropogenic sources in the urban and rural \nenvironment, with a specific interest in Sb and PAHs, where emissions originate from \nsimilar anthropogenic sources, with Pb, As, Cu and Zn also included for comparison \npurposes. \n \nCurrent and past industrial activity was identified as the dominant source of PTEs and PAHs \nto the urban environment, with emissions from vehicle, coal combustion and metal smelting \nidentified as main contributors to total contaminant concentrations. Using road dust collected \nfrom Edinburgh at five high- and low-traffic roads at a distance of 10 and 50 m from the \nclosest road junction, concentrations of Sb, Pb, Cu, Zn, PAHs and Pb isotope ratios were \ndetermined, with road dust undergoing further characterisation using chemical (sequential \nextraction) and spectroscopic (X-ray diffraction, SEM-EDX) techniques. No consistent trend \nfor the element concentrations released from vehicles braking at 10 and 50 m from the \nclosest road junction was observed. Mean concentrations for Sb, Cu and Zn were 5.3 ± 2.8 \nmg kg-1, 91.4 ± 48 mg kg-1 and 237 ± 144 mg kg-1, respectively, and were similar to road \ndust sampled from five high- and five low-traffic locations in Glasgow (Sb 4.5 ± 2.1 mg kg-1; \nCu 117 ± 71.9 mg kg-1; Zn: 283 ± 146 mg kg-1). This was in contrast to mean concentrations \nfor Pb and Σ16PAHs obtained from Glasgow (Pb 250 ± 283 mg kg-1, Σ16PAH 7.7 ± 4.3 \nmg kg-1) where values were approximately double and two-thirds greater than those found in \nEdinburgh (Pb 135 ± 129 mg kg-1, Σ16PAH 4.7 ± 2.9 mg kg-1), respectively. Lead isotopic \nanalysis of Glasgow road dust (206Pb/207Pb range of 1.140-1.174) showed a strong influence \nof past emissions from coal combustion and metal smelting, and was in agreement with \nGlasgow's industrial history. For Edinburgh, the isotopic signature was considerably lower \n(206Pb/207Pb range of 1.116-1.151), and was influenced moreso by emissions of Australian \nsourced Pb in leaded fuel. Isotopic signatures in Edinburgh were lowest at easterly locations \nwithin 5 km of Edinburgh airport (206Pb/207Pb ~ 1.12), and corresponded with past vehicle \nemissions from leaded petrol use, and to a lesser degree, emissions from avgas, which was \nconsistent with the mean annual wind direction for Edinburgh. The mobility of elements \nfrom the road dust to the aqueous phase were assessed by sequential extraction, and by using \nroad surface water samples which showed mobility decreased in the order of Zn>Cu>Pb>Sb. \nRoad dust characterised by XRD and SEM-EDX had a high proportion of quartz present \n(~55%), whilst the presence of less abundant minerals such as calcite were found to increase \nPb mobility through ease of dissolution into the aqueous phase. \n \nFor the rural environment, the behaviour and fate of elemental pollution originating from \ntwo former mining sites, an Sb mine at Glendinning, SW Scotland, and a Pb mine at \nTyndrum in central Scotland was examined. Under specific environmental conditions, Sb \nwas found to be both mobile and immobile in the environment. The chemical weathering of \nstibnite found in spoil heaps at Glendinning Sb mine demonstrated that ~3% of total Sb can \nbe mobilised during the chemical weathering process, while hydrous Fe oxides and organic \nmatter in the surrounding soil favoured its retention. The retention of Sb, along with Pb, was \nsimilarly observed in Loch Tay sediment downstream of Tyndrum Pb mine, where upon \ndeposition, Sb and Pb remained immobile in sediment and allowed the construction of \ndeposition chronologies for two sediment cores to be established. Excellent agreement \nbetween the sediment core deposition chronologies was observed, with both chronologies \nidentifying atmospheric deposition as the primary source of Sb to Loch Tay sediment, whilst \nthe dominant source of Pb was from Tyndrum Pb mine ~25 km upstream of Loch Tay. \nRelative to Sb and Pb, As had the greatest mobility, with its geochemical behaviour and \npartial retention by the solid phase influenced by the presence of Fe. This was evident in the \nsurrounding soil at Glendinning Sb mine, where As was associated with hydrous Fe oxides \npresent in the solid phase, while at Loch Tay, the redox cycling of Fe resulted in the post-depositional \nmobility of As in sediment. \n \nThe use of ombrotrophic peat bogs for this study provided an effective means to assess \natmospheric deposition of contaminants over past centuries; they continually accumulate and \nreceive all their nutrients and contaminants exclusively by deposition from the atmosphere. \nThe deposition archives of Sb and Pb from two Scottish peat cores sampled from Great \nMoss, Cairngorms Mountains, and, Auchencorth Moss, Midlothian, were used to construct \nchronologies for historic and contemporary emissions, particularly in relation to current and \nhistoric anthropogenic activities observed in urban and rural environments. At Great Moss, \nthe deposition of Sb and Pb during the 19th century increased by a factor of 10 and 4, \nrespectively, as a result of the industrial revolution and emissions from the combustion of \ncoal and metal smelting. The trend continued into the 20th century where Sb and Pb \ndeposition peaked ~1950, followed by a decline towards the early 21st century by a factor of \n5 and 11, respectively. Over this period of time, the contribution from coal combustion and \nmetal smelting towards total anthropogenic emissions was on the decline, while emissions \nfrom the combustion of leaded fuel increased until the ~1980s. Although deposition \nchronologies before 1970 for Sb and Pb at Auchencorth Moss were generally in agreement \nwith those from Great Moss, several differences were observed after 1970, or more \nspecifically, in the top ~10 cm of the peat core. This was a result of sub-surface perturbations \nfor Ti, Sb, Pb and 210Pb concentrations, and indicated once deposited, elements were \nsusceptible to post-depositional mobility brought about from a change in environmental \nconditions. The thicker acrotelm layer present at Auchencorth Moss, and the vertical \nmovement of the peat water-table within this layer, resulted in a change in redox conditions \nand led to the redox cycling of Mn and Fe, which in turn, influenced vertical concentrations \nof Ti, Sb, Pb and 210Pb. While Sb and Pb are usually found immobile in peat systems, the \npost-deposition mobility of Sb and Pb at Auchencorth Moss was comparable to a peat core \nsampled from Flanders Moss, and indicated that under specific environmental conditions, \nboth elements can become mobile in ombrotrophic peat bogs. It is worth bearing in mind \nhowever, that these results are the exception, and in all other cases ombrotrophic peat bogs \nremain a reliable archival material to use.