Understanding the potential for disturbance-induced contaminant release from degraded peatlands: a global review of heavy metals in peatlands

Human industry has contaminated peatlands through the atmospheric deposition of pollutants released by industrial processes over many centuries. Relative to other ecosystems, peatlands sequester a far greater proportion of toxic metals than their areal extent. This is especially true in industry-impacted landscapes, where toxic metals in surficial peat can be elevated well above natural concentrations. Despite peatlands acting as contaminant sinks that can maintain their carbon storage functionality under low metal concentrations, high rates of metal pollution can lead to the degradation of peatland processes that sustain carbon sequestration. For example, the loss of keystone peatland species, such as Sphagnum mosses, limits peat accumulation and long-term carbon accumulation. Therefore, in these degraded peatlands, peat forming processes are often suppressed even decades after the source of contamination has reduced or ceased.

Once degraded, peatlands become susceptible to additional disturbances such as fire or erosion, which can release their toxic legacy into the environment and drinking water. Predicted future warmer and drier conditions are expected to increase wildfire prevalence on the landscape and may be further compounded by land use change.  The release of previously sequestered metals arguably represents one of the largest contemporary global environmental disasters and greatest future global environmental challenge.

In this paper, we present an in-depth review of existing literature on ombrotrophic peatland metal contamination from a range of disciplines. After a detailed search and screening process, data were extracted from 97 studies. 500 individual points covering 26 countries were extracted from these studies, which were published between 1973 to 2022. For each study, the depth at which maximum heavy metal concentration (C_max) occurred was recorded, along with surface concentration and concentration at depth. Using Kernel Density Estimates, the distribution of C_max was typically within the top 0.2m of the peat surface across all studies, though with variation in the mean depth profiles between different metals. For example, C_max for Cd and Zn typically peaked at 0.1m below the surface with few studies showing C_max below 0.2m. As, Cu and Pb also had mean C_max values <0.2m depth but showed a tail of C_max values extending to at least 0.5m depth.

The review provides much needed understanding of the spatial extent of peatland contamination as an essential first step in tackling contaminant release from peatland fires. By quantifying the extent of heavy metals at depth in the peat profile, this work can link with peat fire modellers, ecohydrologists and climate scientists to better predict the impact of severe fires under future climate and land use change.