Baseline characterization of Pb-Zn-Cu-Cd-As mine soils using pXRF for phyto-stabilisation trials using Phalaris arundinacea

The former Queensbury tailings pond at Wanlockhead has been identified by the Scottish Environmental Protection Agency (SEPA) as a major source of Pb, Zn, Cu & Cd entering Wanlock Water (a tributary of the River Nith) contributing to sediment loadings and its “less than good” status for water chemistry under the Water Framework Directive. Phytoremediation trials are being conducted as part of the CERESiS project (www.ceresis.eu) as a nature-based solution to reduce the erosion of mine tailings under flood events, sheet wash or infiltration. This involves planting a novel native energy crop species (Phalaris arundinacea) capable of physically stabilising contaminated soils without significant uptake.In 2022 a Niton XL3t980 pXRF was used to define the extent of potentially toxic elements as part of the site investigation and baseline survey The feasibility survey focused directly on the 10 x 10 m plant trial site where a regular grid of 25 surface samples (n=25) were taken using a hand trowel (0-10cm), with 5 gouge auger continuous samples (0-30cm) at the grid corners and centre. These were oven dried and sieved < 2mm in the laboratory before being analysed remotely at BRGM. In the follow-up survey, random surface samples (n=32) were taken across the full extent of the former tailings area (0.75 ha), bagged and analysed wet and un sieved in the field. Results confirmed the elevated concentrations, extent and spatial variability of PTE concentrations in the phytostabilisation site soils, including with depth. Sieving samples increased concentrations of some PTE indicating preferential fine-grain contamination. In general, using the pXRF allowed for fast and effective mapping of contaminant distribution and could be used to guide the assessment of sites for energy crops on heterogeneous non-agricultural sites and the design of nature-based approaches to mitigate future impacts of metal mining legacy sites.