Tracking trace element pollution in Wallonia through lichen biomonitoring: past trends and spatial contrast

Air pollution by trace elements (TE) remains insufficiently characterized at regional scales, particularly in urbanized and industrialized contexts. Wallonia, a densely populated region of Belgium with a strong history of steel and mining industries, provides a relevant case study for that purpose. In this context, biomonitoring with lichens offers an integrated and cost-effective approach to assess background concentrations of TE and to identify areas where populations may face higher exposure, complementing conventional physico-chemical sensors. This study aimed to (1) assess long-term changes in TE pollution in Wallonia and (2) identify the main sources of trace elements contamination and areas at risk. To evaluate temporal trends in background pollution, we analyzed 42 herbarium specimens of Xanthoria parietina collected between 1960 and 1970 in rural areas across Wallonia, which serve as reliable indicators of diffuse atmospheric deposition. These samples were compared with new collections made at the same locations in 2023. To complement this dataset, additional samples were collected in underrepresented rural areas as well as in major residential and industrial cities. In total, 14 TE were quantified in Xanthoria parietina from 132 sampling sites. Concentrations of Pb, Zn, As, and Cd have declined markedly since the 1970s, following the reduction of Walloon heavy industry and the ban on leaded gasoline in the early 2000s. Nevertheless, background contamination by Zn and Cd remains elevated (mean enrichment factors >10), and hotspots persist in industrial cities such as Charleroi and Liège, where local influences—including metallurgical activities and road traffic—drive enrichment factors up to 100. These results demonstrate the effectiveness of lichen biomonitoring for assessing both temporal trends and spatial patterns of TE contamination at the regional scale.