Insights into heavy metal accumulation and chemical speciation in Huixian Karst Wetland, China: Depositional history, mercury isotope tracing and environmental risk assessment.

Abstract

Heavy metal (HM) contamination in wetland sediments has attracted increasing attention because of its persistence and potential ecological impacts. In this study, HMs including Cd, Pb, Zn, Ni, As, and Hg in the surface and core sediments of the Huixian Karst Wetland were investigated in terms of the contamination characteristics, ecological risks, depositional history and possible sources for the first time. The mean concentrations of six metals exceeded their respective background values, with Cd, Pb, and Hg showing high enrichment in Zone 2. Based on the Nemerow Integrated Pollution Index (averaging 3.55), the sediments exhibit a severe level of overall contamination. Sequential extraction revealed that while most HMs predominantly remained in the residual fraction, anthropogenic disturbances increased their reducible and acid-exchangeable forms. Notably, according to the risk assessment code, Pb and Hg exhibited high bioavailability-with highly mobile fractions accounting for approximately 67.34% and 59.95%, respectively-thereby posing significant ecological risks. The depositional history (1925-2020) revealed distinct temporal variations in HM accumulation corresponding to historical and socioeconomic developments, including wartime activities, economic reforms, and industrial expansion. Furthermore, mercury isotope compositions (negative δ202Hg values and a significant Δ199Hg-Δ201Hg relationship) suggested possible anthropogenic contributions to sediment Hg, potentially linked to industrial activities.