Spatial distribution, retention and transport of hexachlorobutadiene in Arctic permafrost soils

Hexachlorobutadiene (HCBD) is a type of persistent organic pollutants (POPs) listed under the Stockholm Convention. POPs are usually considered to distribute globally and accumulate in cold environments through the "global distillation effect". However, the behavior of HCBD in Arctic permafrost soil have not been illustrated yet. In this study, we quantified HCBD concentrations in Alaskan Arctic permafrost soils and discussed the related environmental factors impacting HCBD retention and migration. The results indicated that HCBD concentrations in permafrost soils ranged from 0.0551 to 2.08 ng/g-dw, with average of 0.43 ± 0.40 ng/g-dw. HCBD levels in permafrost were comparable to or slightly higher than those of traditional POPs such as hexachlorobenzene (HCB) and polychlorinated biphenyls (PCBs), from Arctic and Tibetan Plateau soils. A first-order mass balance estimate indicates a total burden of approximately 160 metric tons of HCBD sequestered within the top 30 cm of Alaskan permafrost, providing a conservative baseline for its regional inventory. The random forest model identified dissolved organic carbon as the dominant factor controlling HCBD distribution (>42%), highlighting the key role of dissolved organic matter-mediated transport regulated by soil pH and electrical conductivity. Meanwhile Fickian diffusion analysis revealed bidirectional migration pattern upward in shallow layers and deeper in downward zones. Potential sources of HCBD may include atmospheric transport, local pollution inputs, and secondary emissions resulting from historical deposition within permafrost. It demonstrated that permafrost can act as both a long-term reservoir and a secondary release source of HCBD under the continuous degradation of permafrost. This study provides the first quantitative baseline for understanding HCBD dynamics in Alaskan permafrost soils, which will enhance our understanding of the environmental behavior of POPs cycling in permafrost regions.