Distribution, sources, and ecological risk assessment of heavy metals in soil of Caizi Lake Wetland

Heavy metal contamination has emerged as a critical threat to global wetland ecosystems, posing significant risks to biodiversity, water quality, and overall ecological security. This study focuses on the Caizi Lake Wetland to elucidate the current status, sources, and associated ecological risks of heavy metal accumulation in its soils. Specifically, we investigated the spatial distribution of heavy metals (Co, Cr, Cu, Hg, Ni, Pb, Zn) in surface soil layers (0-20 cm). To accurately apportion the contributions from various sources, the Positive Matrix Factorization (PMF) receptor model was employed. Subsequently, a comprehensive ecological risk assessment was conducted to evaluate the potential environmental hazards. The findings revealed distinct patterns in heavy metal accumulation. The measured average concentrations were 19.04 mg/kg for Co, 75.27 mg/kg for Cr, 27.26 mg/kg for Cu, 0.05 mg/kg for Hg, 28.91 mg/kg for Ni, 30.55 mg/kg for Pb, and 77.01 mg/kg for Zn. Analysis of variability showed coefficients of variation (CV) ranging from 29.07% to 42.62%, with Ni exhibiting the highest spatial variability (CV=42.62%). This moderate to high variability strongly suggests differential influences from anthropogenic activities across the wetland area. Spatial distribution analysis further indicated that during the dry season, mudflat soils contained elevated levels of all studied heavy metals except for Hg. In contrast, Hg concentrations in farmland soils remained consistently and relatively high throughout all three monitored hydrological periods (dry, normal, and wet), highlighting a distinct contamination pattern for this element. Source apportionment using the PMF model identified four primary contributors to the soil heavy metal burden. These sources were quantified as agricultural activities, predominantly associated with the use of fertilizers, pesticides, and irrigation; a combined industrial and traffic emission source, likely related to nearby manufacturing, energy production, and vehicle exhaust; mineral mining and associated weathering processes; and natural pedogenic origins, deriving from the parent geological material. Regarding pollution and risk levels, the overall degree of heavy metal contamination in the Caizi Lake Wetland soils was assessed to be relatively low. Among the elements, Cobalt (Co) presented the highest level of pollution, with approximately 41.82% of the sampled sites reaching a rating of mild pollution or higher. Temporally, the dry season was identified as the period with the most pronounced pollution status for most metals. Crucially, the calculated comprehensive potential ecological risk indices for all samples fell consistently within the slight risk category. This indicates that, under current conditions, the combined ecological risk posed by the mixture of heavy metals in the Caizi Lake Wetland is minimal. However, the identified anthropogenic influences, particularly in specific zones like farmlands for Hg and mudflats during dry periods for other metals, warrant continuous monitoring to prevent future risk escalation. This study provides a scientific basis for targeted soil management and pollution control strategies in this ecologically sensitive wetland area.