Variation Mechanism Of Heavy Metal(Pb, Cd) In Hyporheic Zone For The Weihe River

Heavy metal plays an important role in distinguishing river water environmental quality due to it is characterized by high toxicity, degradation-resistant, and easy to accumulate, thus becomes the important factor of interfering the river ecological function. Hyporheic zone is an ecotone in the riverine system, and has the special hydrological effects and functions including hydrology regulating, environmental buffering and ecological protection. Therefore the clarification of temporal and spatial variation and drive mechanism of heavy metals in sediments in hyperheic zone will be achieved an integrated management and evaluation for the river ecosystem. The Weihe River, as a typical river in northern China, is facing severe problem of water ecology and water environment. The study of heavy metals magnitude, temporal changes and driving mechanism of hyporheic sediments in the Weihe River will provide the ideas in improving water ecology and water environment for northern rivers, and provide a scientific basis for the comprehensive management of the river, and promote socio-economic sustainability in the Weihe River Basin. Consideration of the benthic range, seasonal changes and effect of clogging and scouring in the Weihe River, the measurements of field study was conducted in spring, summer and winter,, the characteristics of water exchange, physi-chemical properties of sediments and Pb, Cd concentrations in sediments, river water and pore water in the hyporheic zone are analyzed and evaluated. The conclusions are summarized as follows:(1) There are apparent seasonal changes of the average concentration of Pb, Cd, its vertical stratification results at the same depth have the differences in five test sites of the Weihe River. All in all, the concentration of Pb, Cd from high to low is winter, spring and summer, furthermore, the change range of the heavy metal concentration tend to be distinctive in the different sites. There are great differences of the heavy metal concentrations in the different sites and even at different depth in the same site. Overall, the maximum and minimum values of heavy metals don’t have the significant correlation in the vertical depth in the hyporheic sediments.(2) The polluted degree and level of the heavy metals in the hyporheic sediments are assessed based on enrichment coefficient method, in five test sites in the Weihe River. All in all, the concentration of Pb is obviously less than Cd. For the enrichment coefficient, the pollution levels and degree of Pb, Cd is greater in winter and spring than in summer. The contamination degree of Pb, Cd is different in the same season and different test sites. There is apparent distinction in winter and spring than in summer. In general, the enrichment coefficients of Pb, Cd have difference at the same season and different depths in all the test sites. For the pollution degree, there is a not significant variation except for certain test points. The bentonic organism, the amount of ahake mosquito larvae and tubifex take for more than 94.7% in the hyporheic zone in the Weihe River, the active range from Ocm to -10cm and from -10cm to -20cm, the depth can reach at 30cm in several test sites, and the biomass is decreasing with the increase of the depth. Evaluation with the quality standard shows that the contamination of Cd is widespread in spring, summer and winter, reach more than moderate pollution and most of the test sites suffer the serious pollution. There’s inevitable toxic effect to organism. The contamination status of Pb is lower than TEL in most river sections in summer, as non-pollution or mild contamination. The contamination of Pb is higher in spring and winter than in summer, as moderate or severe contamination. And the contamination of seasonal change is obvious.(3)The content of Cd and Pb in the hyporheic sediments in seasonal change is related to the concentration of Cd, Pb in different season in river, pore water and groundwater, and also related to pH, ORP and NTU of river. Different seasons and different hydraulic conditions have different scouring and silting frequency and degrees to sediments. And it is the significant reason for resulting in seasonal changes of the hyporheic sediments. The source of sediments of riverbed is different in different seasons, the obvious difference of discharge way, type and quantity of pollutant in different seasons is also the significant reason for seasonal changes of heavy metal in the hyporheic sediments in the Weihe River.(4)Sediment composition and grading curve have obvious changes in different seasons and depth, the vertical stratification of sediment is clear, the different source of sediment in different depth result in different content of heavy metals in different depth. And the water exchange capacity of sediments, exchange patterns and the degree of river pollution also lead to the different concentration of Cd and Pb in vertical sediments.(5)Correlation analysis show that Cd and Pb are not homologous, Cd and OM has certain relevance, but cannot reach significant level, OM has a certain effect on the content of Cd in sediment, but not the main controlling factors. The particle size is not the main controlling factors for the content change of Cd and Pb in the sediment.