Sustainable remediation of abandoned lead mine tailings
Innes, Andrew J.
The extraction of lead (Pb) has occurred throughout Europe for over 3,000 years, bringing substantial benefits to economies, technological advances and the development of infrastructure. The relative ease of extraction and refinement of Pb, coupled with its usefulness in a range of applications resulted in extensive, and often unmanaged, exploitation of this element. The environmental legacy of historic Pb mining activities are evident throughout Europe, with hundreds of abandoned mine tailings documented throughout the EU and in the UK. These mining sites often present a risk of point and diffuse source pollution, with the main issues occurring from the introduction of potentially toxic elements (PTE) into the surrounding environment. Abandoned (Pb) mines are often inherited by new landowners who are consequently liable for their management, generating a burden of responsibility that may involve restorative action. Unfortunately, the cost of conventional remedial strategies is frequently prohibitive for small landowners to undertake, leading to the possible continued dereliction of these sites. Identifying economically feasible and effective solutions to this issue is therefore of importance. Critically, reducing the risk of PTE impacts to the environment is pertinent, as are endeavours to convert these derelict mines into more productive landscapes. Therefore, the overarching aim of this thesis was to further knowledge on ecologically driven remedial approaches to metalliferous mine tailings to lessen the economic issues faced in environmental protection. In particular, the thesis aimed to ascertain if it was feasible to use native plant species and locally available waste materials to promote the revegetation of abandoned Pb mines, and consequently provide a sustainable vegetative barrier that could limit PTE release into the surrounding environment. The investigation was centred on two abandoned Pb mines, located on the West coast of Scotland (Tyndrum) and the South of France (Le Bleymard). Geo-statistical investigations identified elevated concentrations of total Cd, Pb and Zn at both sites combined with a lack of organic matter, plant nutrients and poor physical structure conditions for vegetation. Contrasting findings were identified, with the Tyndrum tailings exhibiting a higher potential mobility of PTE than Le Bleymard and a particular risk of hydrological movement of PTE enriched material from the mine to the wider environment. In opposition, the calcareous nature of the Le Bleymard tailings minimised PTE mobility via surface absorption and precipitation mechanisms, with the greatest risk to the surrounding environment likely caused by the aeolian transportation of fine particulates and inhalation of PTE enriched material. The use of inverse distance weighting and empirical Bayesian kriging highlighted the heterogeneous distribution of PTE enriched material at Tyndrum and the movement of small (<100 μm), Pb enriched particulates at Le Bleymard. A combination of laboratory and field studies highlighted the use of common bent (Agrostis capillaris) as a useful phytostabilisation species, which was observed to be tolerant of up to 10,000 mg kg-1 Pb, 12,000 mg kg-1 Zn, and 180 mg kg-1 Cd in plant tissues. Furthermore, this species was generally effective in reducing the transference of PTE from the roots to aerial compartments of the plant, therefore minimising possible PTE transference to the food web. It was found that laboratory studies involving the spiking of PTE into synthetic growth media substantially underestimated plant uptake of Pb, Zn and Cd in the field and so should be used with caution when informing field trials. The addition of organic amendments into the tailings produced contrary results depending on the source of the organic amendment and the underlying tailing characteristics. Broadly, the introduction of organic amendments into the slightly acidic tailings at Tyndrum can elicit the mobility of Pb by complexation with soluble Fe compounds and organic ligands, whilst concurrently promoting the immobilisation of Cd and Zn. Over a longer period, the separate incorporation of spent coffee grounds (SCG) and green waste compost (GWC) reduced the concentration of weakly associated Pb by complexation with stable organic matter and Fe/Mn oxy-hydroxides by 36.9 and 28.9 % respectively. Furthermore, improvements in plant available nitrogen (N), phosphorous (P), microbial activity and organic matter were observed in both field and laboratory settings, with the inclusion of SCG observed to have the largest effect across all properties. Contrasting results were found in the calcareous tailings of Le Bleymard, where the potential mobilisation effects generated by organic amendments to PTE were mitigated by the elevated pH and occurrence of adsorption and precipitation events, leading to initial net gains of increased tailing fertility and minimal PTE mobilisation. However, a loss of plant available and total N coupled with the immobilisation of soluble P was observed, which was partially attributable for the stunted growth of A.capillaris during field trials when comparing the study sites. The use of A.capillaris and the incorporation of organic amendments in the potential remediation of two contrasting abandoned Pb tailings were found to be substantially influenced by the underlying geo-chemical characteristics of the mine tailings. In conclusion it was suggested that the re-vegetation of the Tyndrum tailings could be achieved using SCG or a similar material which would provide a source of plant and microbial support combined with a high capacity for stable complexation of PTEs with C compounds. For Le Bleymard, suggesting a suitable material is more difficult due to conflicting needs to balance the risk of PTE release via the acidification of the tailings and enhancing the availability of plant nutrients. Overall, it was found that the use of A.capillaris combined with SCG or a comparable organic material could be highly effective in revegetation efforts at the Tyndrum site. However, wider investigations are suggested to be undertaken to identify a suitable material in the promotion of vegetation at the Le Bleymard tailings. It can be concluded that this work has furthered our understanding of ecologically driven rehabilitation of abandoned mine tailings and has highlighted the need for more work to be completed on calcareous mine tailings.