Mobility of Pb and Zn in a neutral mine drainage setting, Gunnerside Gill, Yorkshire

Tame, Catherine Diana McLaren (2020) Mobility of Pb and Zn in a neutral mine drainage setting, Gunnerside Gill, Yorkshire. PhD thesis, Birkbeck, University of London.

Abstract

Numerous areas throughout the world are affected by circum-neutral pH, low iron (Fe) drainage with high concentrations of lead (Pb) and zinc (Zn) arising from discharges from, and weathering of, mine wastes. Gunnerside Gill, a small upland tributary in the headwaters of the River Swale in Yorkshire, is such a site affected by historic Pb mining. The aim of the study is to assess the controls on Pb and Zn mobilisation from the mine tailings and alluvial sediments to the river water through geochemical characterisation (including total digestion, x-ray diffraction and sequential extraction), column leaching experiments and aqueous field data. Previous studies have largely focused on the metallic element concentrations within the sediments and there has been little or no investigation into the concentrations within the mine wastes. Total concentrations of Pb ranged from 1,840 to 70,000 mg/kg and Zn concentrations ranged from 2,000 to 64,300 mg/kg. The column leachate experiment showed that pH is the main control on the dissolution of ore minerals within Gunnerside Gill. The control of Zn mobility is largely governed by the dissolution of primary (sphalerite) and secondary minerals (for example smithsonite) within the mine waste samples and from the dissolution of secondary minerals within the alluvial sediments. In the mine waste samples Pb mobility was largely controlled by the dissolution of secondary minerals, with weathering rims were often noted on the primary ore grains. The mobility of Pb within the alluvial sediment sample appeared to be controlled by the dissolution of primary and secondary ore minerals, but also Mn oxyhydroxides that act as a temporary sink for Pb. The alluvial sediment sample was found to leach between 10 to 17 times more Pb than the mine waste samples. Similarly a higher concentration of Zn was mobilised from the sediment sample than the mine wastes. This is considered to be a function of age of the mine wastes and the location of the alluvial sample downstream of both the main mining and processing areas. Potential management options include localised reactive barriers at the mine adits, phytostabilisation of mine wastes and sediments, and small scale civil engineering barriers to reduce the direct mobilisation of mine wastes and sediments into Gunnerside Gill.

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