An improved comprehensive model for assessing the heavy metals exposure towards waterbirds: A case report from Black-necked cranes (Grus nigricollis) in Caohai Wetland, China.

Heavy metal contamination poses a significant environmental threat to wildlife on global scale, making accurate assessment of exposure risk crucial for conservation efforts, particularly for vulnerable species. Existing risk assessment models have been widely used, but their construction process lacks comprehensive considerations. In this study, we constructed an optimized health risk assessment model based on the well-established "Liu's model" and "ADI model", and applied the pollution allocation factor (AF) to accurately assess the risk of heavy metal exposure to wildlife. Our model was applied to assess exposure risk of heavy metal for the black-necked crane(Grus nigricollis), a flagship species in the alpine wetland ecosystem of Caohai Wetland. Soil, plant and black-necked crane fecal samples were collected from the Caohai Wetland and surrounding areas in Guizhou, China. We revealed varying degrees of As, Cd, Cr, Ni, Pb, and Zn contamination in soil and plants from different habitats, exceeding the background or plant limit values. This indicated that the black-necked crane and other waterbirds living in Caohai Wetland are suffering with the multi-elemental heavy metal contamination, especially in the gutterway and grassland. The exposure dose of As, Cd, Cr, Ni, Cu, and Zn toward black-necked cranes differed significantly in soil and plant pathways (P < 0.05). As, Cd, Cu, and Zn were mainly derived from plants consumption, while Cr and Ni originated from soil. Considering the contribution of soil and plant pathways to heavy metal exposure in black-necked cranes, the exposure doses of each elements calculated via food intake accounted for over half of the exposure calculated via feces (AF>0.5). The risk assessment model identified Cr and Pb were the highest risk elements for black-necked cranes, with exposure risk simulated through feces exceeding those through food. These findings suggested that current Liu's model may underestimate the effects of other pathways and medium. Therefore, we proposed a more comprehensive and accurate model for evaluating the exposure risk of black-necked cranes, incorporating AF to quantify the contribution of risk sources to black-necked cranes and understand their overall health risk. This model can serve as a useful tool for the conservation and habitat quality improvement of the black-necked cranes and other waterbirds.

Coupling stable isotopes to evaluate sources and transformations of nitrate in groundwater and inflowing rivers around the Caohai karst wetland, Southwest China.

Nitrate is one of the most common pollutants in aquatic ecosystems, particularly in highly vulnerable karst aquifers. In Caohai Lake, an important karst wetland in southwestern China, karst surface water and groundwater are important recharge water sources, and nitrates flow into the wetland along with the surface water and groundwater, degrading the wetland water quality. Therefore, identifying the sources of nitrate in the surface water and groundwater in the Caohai catchment is of great significance to the protection of the wetland water environment. In this study, the nitrate concentrations, hydrochemistry and multiple stable isotope ratios (δ18O-H2O, δD-H2O, δ15N-NO3- and δ18O-NO3-) were used to identify the sources and fate of the NO3- in the groundwater and inflowing rivers around the Caohai wetland. The results showed that the NO3- concentrations in the groundwater samples from the southern side exceeded the WHO limit during the wet season, while other samples did not exceed the limit. The mean concentrations of NO3- in groundwater were higher than those in the inflowing river water, and NO3- concentrations decreased in the order of wet season>dry season>normal season in the groundwater and inflowing rivers. The hydrochemistry and multiple isotope ratios suggest that the nitrate transformation was dominated by nitrification processes, while denitrification had an influence on the transformation of NO3- (as evidenced by isotopes) in groundwater during the dry season. According to the analyses based on a stable isotope analysis in R (SIAR), sewage and manure were the main sources of NO3- in the groundwater, while sewage, manure and chemical fertilizer were the major sources of NO3- in the inflowing river water; therefore, the scientific use of farmland fertilizers and the treatment of domestic sewage should be strengthened to safeguard groundwater quality and control the NO3- concentrations in rivers.