After the flames: Post-wildfire heavy metal mobilisation in a contaminated temperate peatland

Intact peatlands are key resources for freshwater that contribute to multiple hydrological ecosystem services. They retain rainwater and regulate water quality downstream by storing contaminants in the peat profile. The release of heavy metals and nutrients, through burning or erosion of near-surface deposits, has the potential to provide a persistent source of legacy contamination, namely metal contamination, to downstream drinking water supplies. With future climate change increasing the frequency and severity of wildfires in summer and heavy rainfall in winter, the risk of contaminant release from high-latitude peat regions and downstream impact is uncertain.

In June 2018, a major wildfire affected an area of upland moorland (Saddleworth Moor, UK), which contains peat deposits contaminated with atmospherically derived metal deposits. We assessed potential water quality impacts from hillslope contaminant source to the fluvial system by monitoring of heavy metals in the catchment, namely lead (Pb), zinc (Zn), copper (Cu) and nickel (Ni). Specifically, we quantified the (1) metal concentrations in ash deposits resulting from contrasting burn severities; (2) dissolution and erosion of ash and peat deposits under intense rainstorm events; and (3) their transport via the stream network to the receiving reservoir. Ash and peat samples obtained following the wildfire were analysed for total elemental concentration and leaching potential. We calculated ash loads at different burn severities and hillslope erosion was monitored through a series of sediment fences. Heavy metal concentrations in five rainstorm runoff events were measured at the stream outlet of a small catchment within the burn perimeter in the year following the wildfire.

Both ash and peat samples had elevated total heavy metal concentrations, which varied spatially across the study site. The spatial variability was partly associated with different burn severities and ash loads. In extreme burn severity areas, ash loads reached nearly 40 t ha^-1 and Pb concentrations in ash, for example, were as high as 2650 µg g^-1, indicating particularly high potential for contamination of water sources. Conversely, the maximum concentration of dissolved heavy metals in the stream-flow were much lower during the initial post-wildfire storm events (Pb 0.77 µg g^-1; Zn 38.67 µg g^-1; Cu 5.05 µg g^-1; Ni 0.26 µg g^-1).

The low solubility of heavy metals in both ash and peat samples likely constrains mobilisation by dissolution during storm events, suggesting low acute risk to drinking water quality post-wildfire. Instead, we hypothesise that metals likely remain bound to peat and ash particles, and are subsequently transported downstream in particulate form. Further quantification of heavy metals in sediment cores from sink zones will test if the metal contaminants pose a future chronic threat to drinking water quality.