This research set on a former reclaimed opencast coal spoil in Varteg Hills, South Wales, UK explores the phytoremediation potential of Alnus glutinosa (Alder), Betula pendula (Roth) (Birch) and Larix decidua (Larch), which are commonly used in UK land reclamation, to sequester metals: Zn, Cd, Mn, Pb and Cu, from mine spoils. This study also compares the abilities of newly planted trees to sequester metals on a new experimental site, within the landscape and compares these with those from older plantations on adjoining land. This chronosequence is used to evaluate the overall impact of forestation on metals levels in soils over two decades. Metal concentrations in soil were measured over three years and those in tree leaves were measured over two years. The majority of the samples examined lie between the UK defined thresholds for contamination but below levels that trigger immediate action and treatment. Samples within this range are placed within a new 'critical soil' category. Mapping the spatial distribution of contaminants across selected test plots shows the presence of micro-scale contamination hotspots on these sites. These show that while such sites may, on average, have sub-critcial levels for metal contamination, they may still contain sections where contaminants reach very severe levels. Planting trees in mixes is beneficial because different trees selectively remove different metal contaminants while some species benefit the soil in other ways. For example, while Alder leaves showed average concentrations of Cd, Zn and Mn, their roots also aid nitrogen fixation. Of the five metals selected for this study, two are known to be highly mobile (CD, Zn), two iii relatively immobile (Pb, Cu) and one is intermediate (Mn). Foliar analysis of Birch shows consistency in accumulation of Cd and Zn in leaves, while young Larch needles accumulated the highest concentrations of Mn and Pb. Correlation between movement of metals in soil and leaves over time, discovered that the uptake of Mn in Alder and Birch foliar on all plots and Larch on the new experimental plot is positively correlated to the presence of Zn in the soil. Studies of the general effects of forestation on metal concentrations in soil showed that metal levels declined significantly even on the most recently planted test sites. On four year old plots a 14-18% decrease in Mn concentrations and about 8% decrease in Cd and Zn concentrations were observed. Observations of the soils on the full 18 year chronosequence found that metal levels declined very significantly (p= 0.003 to 0.0002) through time. Based on the reduction of metal concentration in soil and uptake in leaves, it is possible to project 40 - 45 years for concentration of Mn to reach normal soil levels and about 20 - 25 years for Cd level to reach normal soil levels, provided the soil is not further disrupted by physical, chemical or biological activities which may recontaminate the spoil. This demonstrates, conclusively, that forestation is an effective means of metal remediation on the moderately contaminated lands produced by opencast coal-mining in Wales. Keywords: Opencast coal mining; Phytoremediation; contaminated land; Heavy metal uptake; Alder; Birch.
Desai, Mansi A.
Faculty of Humanities and Social SciencesDepartment of Social Sciences
Year: 2013
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