Imaging Legacy Mine Wastes: Geoelectrical Characterisation of Tailings and Waste Rocks in the Philippines and the UK

Legacy mine wastes have accumulated over centuries in regions such as Benguet, Philippines, and Cornwall, United Kingdom. Inefficient historical processing left valuable metals in these materials, supporting their potential role in a circular economy through secondary resource recovery. However, elevated concentrations of toxic metals such as arsenic also pose long-term environmental risks. This dual character highlights the need to evaluate legacy mine wastes not only as potential secondary metal resources but also as sources of environmental liability.

Studying legacy mine wastes presents several challenges. These materials are often highly heterogeneous, vary significantly across short spatial scales, and are often poorly documented in terms of their composition, emplacement, and subsequent alteration. Furthermore, intrusive investigations can disturb legacy wastes and mobilise toxic metals, while conventional methods such as drilling are costly, invasive, and limited in coverage. In contrast, geoelectrical techniques provide a non-invasive and cost-effective way to characterise the internal structure and hydrogeological behaviour of mine wastes over larger areas, and are therefore applied in this study to investigate legacy tailings at the Padcal–Philex mine in Benguet, Philippines, and at Wheal Maid (WM), as well as legacy waste rock deposits at Binner Downs (BD) in Cornwall, UK.

At the Padcal–Philex site, several arrays of PRIME electrical resistivity monitoring equipment were installed along and across Benches 4 and 5 of Tailings Storage Facility 1 (TSF1) to characterise the hydrogeological behaviour of the tailings. Daily measurements collected between April 2023 and May 2024 demonstrate that electrical resistivity tomography (ERT) can effectively monitor moisture dynamics within the tailings during both monsoon and dry seasons, a key factor in assessing tailings stability and in planning future secondary resource recovery. Further data processing shows that ERT can distinguish between muddy and sandy tailings units, which have different average copper content, validated by auger sampling.

At Wheal Maid, ERT surveys were conducted along multiple profiles in July 2024, May 2025, and September 2025 to investigate acid mine drainage (AMD) processes in the lower lagoon area. The resistivity models reveal zones of persistently low resistivity, interpreted as areas influenced by AMD within the lagoon. These low-resistivity zones may indicate seepage of AMD-rich water beneath the tailings and tailings dam, suggesting pathways for contaminant transport to the downstream River Carnon. At Binner Downs, ERT imaging successfully delineates the boundary between legacy waste rock and the original ground surface, improving understanding of waste distribution and thickness.

Overall, these results demonstrate the value of geoelectrical techniques for investigating legacy mine wastes. By enhancing understanding of subsurface heterogeneity, moisture behaviour, and contaminant pathways, geoelectrical techniques provide a non-intrusive framework to support environmental risk assessment, remediation planning, and the evaluation of targeted reprocessing opportunities in legacy mining landscapes.