Fraction Characteristics And Risk Assessment Of Heavy Metals In Wetland Soil Of The Yellow River Delta

As the largest river estuarine wetland in the North of China, the Yellow River Delta (YRD) spreads broad riverbed, tidal flat and shallow coast. Due to multivariate land and hydrology conditions as well as complex natural environment, wetland ecosystem in this area is abundant and varied, which has extremely important value of ecosystem service function. However, owing to a serious lack of fresh water supply, as well as anthropogenic interference from oilfield construction, agricultural reclamation, and regional urban development, wetland soil quality has been affected in different degree, which is facing with a potential pollution threat from heavy metals.This study selected natural wetlands in tidal zone (T), petroleum exploitation zone (P) and ecological restoration zone (R) in the YRD as study area. In the three zones, different vegetation covers- Phragmites australis (p) and Suaeda salsa (s) and bare land (b) were also selected, respectively. Different depths soil samples were collected in each sampling site. Heavy metal total and fractions concentration and soil properties of each soil sample were also measured and analyzed. Results showed that for different vegetation covers, the order of mean concentrations of all metals was b>s>p in zone P, while it shifted to p>b>s in zone T and p>s>b in zone R, respectively. For different kinds of heavy metal, chemical fractions of heavy metal in soil were also different. Cu, Ni, Pb and Zn were mainly in the form of residual fraction existing in soil, while Cd was mainly in forms of exchangeable fraction and bound to carbonates fraction existing in soil. In the oil polluted area, heavy metals accumulated in the layer of 20-30 cm. Geoaccumulation index (Igeo), potential ecological risk index (RI) and risk assessment code (RAC) were selected to evaluate heavy metal ecological risk of wetland soil. Results showed that mean values of potential ecological risk index of heavy metal kept the order of P>T>R. And pollution level of Cd was the highest among the five studied heavy metals. Principal component analysis inferred that Cu and Ni were derived from natural alluviation and sedimentation, while Cd and Pb were derived from allochthonous inputs (e.g. wastewater discharge, atmospheric precipitation) and air emissions of oil extraction. Correlation analysis indicated that soil organic matter, moisture, large aggregates as well as electric conductivity were key factors influencing metal distributions in these zones. And soil organic matter and moisture were key factors influencing metal fractions distribution.