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Abstract

A legacy from mid-19th Century mining near Newtownards is lead-rich tailings and derived soils containing typically 10% Pb. Lead occurs dominantly as very fine-grained cerussite (PbCO₃) and the tailings are also rich in calcite and dolomite. Rainwash and overland flow has transferred tailings material from the original impoundments into a meadow, forming a depositional ‘plume’ characterised by contrasting soil colour, vegetation species and vigour of growth. Cerussite is dispersed farther than calcite and dolomite. In areas of the plume proximal to the tailings source, the Pb content (dry weight) of dock and plantain is up to 6540μg/g in roots and 1140μg/g Pb in leaves. Plume soils and plants are also enriched in Fe, Mn, Cu, Zn, Cd and Sb. Bracken foliage contains up to 1500μg/g Pb in non-tailings soil and 2800μg/g Pb in tailings-contaminated soil. For most elements analysed, there is a low contrast between bracken plants growing on tailings-contaminated and normal soil. This reflects the low bioavailability of these elements within the carbonate-rich tailings together with absorption thresholds imposed by plant metabolism. Plants growing on tailings-dominated substrates appear to be stressed by low bio-availability of trace elements (e.g. Mn) and nutrients (e.g. P, K), rather than metal toxicity. Metal uptake is higher in healthy, vigorously growing plants that are not rooted in tailings-dominated soil. The bio-availability of lead and other metals increases when: (i) tailings material is mechanically transported and mixed with soil; and (ii) relatively acidic groundwater dissolves tailings and transfers metals in solution into adjacent soil. Remedial action is required to limit further tailings erosion and acidification.

Keywords

CERUSSITE MINE TAILINGS PLANT UPTAKE HEAVY METALS HISTORICAL LEAD MINES

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Dispersion of cerussite-rich tailings and plant uptake of heavy metals at historical lead mines near Newtownards,Northern Ireland

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