Movement And Transformation Of Heavy Metals In Water-Level-Fluctuating Zone Of Three Gorges Reservoir Area

The Three Gorges Dam (TGD) is the world's largest dam, measuring 2335 m long and 185 m high, and the reservoir created by it has an area of 1080 km2. The TGD adopts the scheme of storing fresh water and excluding the muddy water, that is, low-water in summer and high-water in winter, which will form water-level-fluctuating zone(WLFZ) with a drop of 30 meters in maximum water level in the areas along the river. WLFZ is an extraordinary and the most fragile zone with aquatic and land ecosystems. Accompanied with periodic flooding and alternation of wet and dry condition, WLFZ will be an important pool and source of heavy metals. The aquatic environmental pollution by heavy metals has become a question of considerable public and scientific concern. Thus, in this study, comparative studies of concentration, speciation, and ecological risk of heavy metals (Cu, Pb, Cd and Cr) in water, soils, and sediments from WLFZ were systematically conducted, and the characteristic of movement and transformation of heavy metals in WLFZ was investigated. The research results were showed as follows:?Water environment parameters were different in various periods. In flood season, DO, Eh and Ph were lower, and SS was higher. According to the EQSSW of China (State Environmental Protection Administration, 2002), the concentrations of four heavy metals in all stations complied with Class I of EQSSW. Thus, heavy metal concentration in river water was low, there's no apparent pollutions were observed. Spatial distributions of heavy metals in water showed the decreases in Cu, Pb, Cd, and Cr from upstream to downstream. Heavy metal concentrations decreased after flood season.?The average concentrations of Cu, Pb, Cd and Cr in sediment from WLFZ were 85.7, 56.0, 0.62, 95.3mg/kg, respectively. Variation coefficients decreased as following sequence: Cd>Cu>Pb>Cr. Spatial distributions of heavy metals in sediments showed the decreases in Cu, Pb, Cd, and Cr from upstream to downstream. The average I_geoindicated that the pollution degree of four heavy metals decreased as following sequence: Cd>Cu>Pb>Cr. Risk assessment code (RAC) analysis indicated that the ecological risk followed: Cd>Pb>Cu>Cr. In flood season, the exposure of sediment from river bottom to atmosphere caused the decrease of heavy metals. Heavy metals had the tendency to release to water body. Cu exist in Organic/sulphidic fraction, Pb and Cd exist in carbonates decreased apparently.?The concentrations of Cu, Pb, Cd and Cr in sediments were higher than that in soils. The average concentrations of Cu, Pb, Cd and Cr in soils were 29.3, 37.2, 0.26, 63.3mg/kg, respectively. Variation coefficients of Cu and Cd were higher than that of Pb and Cr. The average I_geo indicated that the pollution degree of four heavy metals decreased as following sequence: Cd>Pb>Cu>Cr. Cu and Cr mainly exist in residual fraction. The residual fraction of Cu and Cr reached 86.95% and 96.54% respectively. Pb mainly exist in Fe–Mn oxides and residual fraction; Cd mainly exist in exchangeable, carbonates and Fe–Mn oxides fraction. The extractable Cd reached 88.46%. The RAC values were 0.49–4.90 for Cu, 0.29–2.01 for Pb, 35.96–52.90 for Cd, and 0.10–0.27 for Cr. This suggests that Cd posed a high ecological risk to local environment, Cu, Pb, and Cr showed no to low risk level in soils.?In 147m water level, the exposure of soils from river bottom to atmosphere caused no obvious changes of pH, CEC content, organic matter, and Grain-size of soils, but the content of Fe(II) decreased apparently, and the concentrations of four heavy metals decreased. In 172m water level, there are no apparent changes of physical- chemicial indexes and concentrations of four heavy metals after soils were submerged. The concentrations of Cu and Pb in soil of 147m water level were apparently different from that in soil of 172m water level, and there were no apparent differences for Cd and Cr. In sum, the concentrations of four heavy metals in soils of water-level-fluctuating zone have the tendency to increase. The degree of four heavy metals increased as following sequence: Pb>Cu>Cr>Cd.?Purple soil, yellow soil, and yellow-brown soil of WLFZ were submerged and incubated under a nitrogen atmosphere. With the transition of soil redox condition, the content of Fe(?) increases significantly, and the soluble heavy metal concentrations were always lower. In the process of redox processes, there was little change for chemical form of Cu and Cr in soils, and part of Pb and Cd transform to Fe–Mn oxides fraction.?Control experiments showed that there was no obviously accumulation of Fe(II) in the system in the absence of microorganism, which suggested that dissimilatory iron reduction is a microbiological process, typically. Both Fe(II)accumulation amount and rate constant were increased at higher temperature. The dissimilatory iron reduction was inhibited when heavy metals(Cu, Pb, Cd and Cr) were added to soils, and accelerated by the increasing concentration. With glucose as carbon source, both Fe(II) accumulation amount and rate constant were increased, and accelerated by the increasing concentration.?The adsorption capacity of Pb2+ and Cd2+ in tested soils (purple soil, yellow soil, and yellow-brown soil) was increased with its increasing initial concentration, and the sorption capacity of reduced soil to Pb²⁺ and Cd²⁺ was higher than that of oxic soil. The sorption capacity of soil to Pb²⁺ and Cd²⁺ followed: yellow-brown soil > yellow soil > purple soil. The desorptive rates of oxidizing soil to Pb²⁺ and Cd²⁺ was higher than that of reducing soil. In the process of redox processes, part of Pb existed in exchanged fraction and carbonated fraction transformed to Fe–Mn oxides fraction. In purple soil, part of Cd existed in exchanged fraction transformed to carbonated fraction. However, part of Cd existed in exchanged fraction and carbonated fraction transform to Fe–Mn oxides fraction for yellow soil and yellow-brown soil. The stability of Cd in yellow soil and yellow-brown soil was higher than that in purple soil. The stability of Pb and Cd in soil was significant correlated with the process of redox processes of soil.