The issue of soil heavy metal pollution has garnered significant attention due to its increasing severity in the wake of industrial and mining development (Burbacher et al., 2015; Kasuya,2000). Approximately 14.3% of soil survey sites across Europe have been identified as requiring remediation for heavy metal contamination (Mahar et al., 2016). Cadmium, in particular, poses a considerable threat as a primary pollutant due to its easy accumulation, high mobility, and high toxicity (Gobe and Crane, 2010; Jarup et al., 1998). In China, the majority of soil affected by heavy metal pollution is contaminated with cadmium (China soil pollution investigation bulletin, 2014).
Cadmium contamination can be classified into two categories based on the influence of human activities: geological backgrounds pollution and anthropogenic pollution. The former is intimately linked to the natural weathering of strata with high cadmium concentrations (Mndrescu et al.,2022; Islam et al.,2018). Cadmium within these strata accumulate in metal sulfides such as pyrite and sphalerite in an isostructural manner (Liu et al.,2017; Cullen and Maldonado,2013). As weathering continues, rocks containing cadmium disintegrate and fragment, exposing their metal sulfides to air and water where they undergo oxidation. Ultimately, metallic elements such as cadmium and iron are released into water in their ionic forms (Yang et al.,2018). In contrast to anthropogenic pollution sources, cadmium contamination from geological backgrounds is characterized by a slower rate of release and a longer duration.
Globally, black shale is generally rich in Cd (Liu et al.,2017;Liu et al.,2020; Lavergren et al., 2009). The Low Cambrian Hetang Formation in the eastern China represents a quintessential region with high geological background levels of cadmium (Xin and Sui,2022༛Zhao et al.,2018). Concurrently, abundant stone coal resources are present at the base of the Hetang Formation (Song,2009). Historically, haphazard extraction of stone coal resulted in a substantial expansion of Hetang Formation strata exposed to water and air. This exposure produced copious amounts of acid mine drainage (AMD) enriched with iron and cadmium that persistently contaminated proximate water systems and soil. Consequently, to devise a scientifically sound strategy for managing cadmium pollution, it is imperative to elucidate the migration patterns of cadmium originating from geological background sources.
Currently, the assessment of mixed cadmium pollution from both geological background and anthropogenic sources primarily employs the geo-accumulation index (Williams and Antoine,2020; Liu et al.,2021) and multi-element geochemical valuation methods (Han et al.,2022; Hamon et al.,2004). However, the methodology for determining the constant K value within Muller’s cumulative index formula, which accounts for fluctuations in background values induced by diagenesis, and its associated conditions of use are absent. Consequently, it is no longer capable of fulfilling the requirements for a scientific evaluation of heavy metal contamination across regions with varying geological backgrounds. Conversely, due to the shared origin of anthropogenic and geological background pollution, it is challenging to discern their respective contributions using geochemical analysis techniques such as cadmium isotopes. As such, there is an exigent need for meticulous investigation into the processes underlying cadmium pollution from high cadmium level geological backgrounds regions. Such research would elucidate the migration patterns of cadmium and furnish a foundation for scientifically evaluating the proportion of cadmium pollution attributable to geological background sources within areas affected by complex pollution.
The geological background source of cadmium pollution can be attributed to two distinct processes: the leaching of cadmium-rich parent rocks on slopes exposed to the surface environment, and the erosion of cadmium-rich strata by hydrodynamic force (Chen et al., 2014).
The weathering of parent rocks can be further classified into chemical and physical weathering (Riebe et al.,2003; Qin et al.,2006). Notably, due to direct exposure to water erosion, the rate of erosion for riverbed bedrock is comparatively rapid (Tarun et al.,2012). This is particularly evident in hilly terrain where there is a significant elevation difference between the upstream and downstream sections of a river and where water flow is relatively swift. Under these conditions, chemical weathering and hydraulic action result in a more rapid rate of weathering for cadmium-rich parent rocks in riverbeds compared to those on slopes (Inoue et al., 2017; Dosseto et al., 2014). Consequently, this represents the most significant geological source for cadmium pollution at present.
The process of weathering involves numerous factors that contribute to cadmium pollution caused by geological background. These factors include rock collapse, mineral dissolution and element migration within water systems. The release and migration of cadmium is influenced by various environmental conditions such as temperature, pH levels and ion strength (Yang et al., 2021; Lasaga et al.,1994). Compared to cadmium pollution caused by anthropogenic activities, the process caused by geological background is considerably more complex. However, these differences also provide an opportunity to distinguish between geological background causes and anthropogenic cadmium pollution from the same source. This study focuses on the Hetang formation in Kaihua County, Zhejiang Province. Given that approximately 90% of cadmium is adsorbed by iron-sediment during migration (Zhou et al.,2020), this research utilizes the migration characteristics of iron elements to reconstruct the migration and enrichment patterns of geological background source cadmium resulting from natural weathering of Hetang formation.