Recent Deposition Environmental Quality Evolution Of The Anning River In Western Sichuan Province

Rivers are the main way to transport materials in the supergene environment. The composition of materials transported is closely related to the geological environment, rock components and geochemical backgrounds of area flowed through. The sediments of river systems not only reflect the characteristics of present sedimentary environment, but also preserve the environmental evolution history of the river. In the scale of hundreds of years, phased development of mining activities is one of the important factors for the environmental quality of river sediment. Mining activities in various stages have different environmental impacts in the river sediments. By combining the changes of elements in the sedimentation profile with mining activities history, it is able to invert the evolution history of river system sedimentary environment’s quality and provide scientific basis for tracing the sedimentary matter source in different stages and discussing the evolution process of sedimentary environment’s quality in the study area.The Anning River basin is the second largest granary of Sichuan, and is also an area distributed with concentrated mining activities. The development of these mining activities has very prominent characteristics in different stages. Available studies indicated that mining activities in recent years had great influence on the ecological environment of the Anning River basin, which not only changed the chemical composition characteristic of river sediments, but also had influenced the agricultural soil obviously along the river. The contradictory between the environmental pollution caused by mining activities in the river basin and the foundation of ecological civilization have become unnegligible.This thesis is based on the item “Eco-geochemical survey on the lake and mining environment of the Anning River Basin in Sichuan Province”. Using elements geochemical methods and isotope geochemical methods, by systematically collecting surface sediments and profile sediments along the Anning River, this research combines the dating of modern of river sedimentation with geochemical features of sediments from the profiles to discuss the modern environmental quality evolution of sediments in the Anning River. The research finds that:The content of elements in surface sediment of the Anning River appears a fluctuation change and elements Mo, Pb, As, Cd, F, Mn, U, Ca enrich apparently.In sedimentary profiles, the spatial distribution characteristics of elements in different layers were mainly influenced by geological background and human activities apparently. The mining of Maoniuping rare earth deposit made the concentration of Pb, F, Mo, Tl, U, Th and K rise in the sediments of the Anining River from the Maoniuping to the Lugu Village; the mining of Lugu iron deposit made the concentration of As, Cd, Cu, Pb, Zn, Mn, Se, Fe, Ca, Mg and P rise in the sediments of the Anning River from the Lugu Village to the Huanglianguan Town; the mining of vanadium titano-magnetite in Taihe and Baima made the concentration of Ti, Fe, Cu, Ni, Cr, Co, Se, P, Ca and Mg rise in the sediments of the Anning River from the Taihe Town to the Huanglianguan Town, and from Binggu Village to Deshi Town, respectively; The industry drainage and living drainage in Xichang City and Dechang City made the concentration of Hg, Cd rise in the sediments in the Anning River till Leyue Town.In sediments of the Anning River, the element Cd has great risks on the environment, because its average exchangeable fraction takes up about 20%. The exchangeable and reducible fractions of Cd and Pb take up a high percentage, so that they have strong bioactivity and toxicity. The exchangeable and reducible fractions of Cu and Zn take up a lower percentage comparing with Cd and Pb. Moreover, elements Ni and Cr mainly exist in the state of residue, which have no bioactivity and toxicity.Flood plain is a semi-closed system, when using 210 Pb dating, the emanation coefficient of 222 Rn should be considered to amend the calculation of excess 210 Pb. 137 Cs and 210 Pb combining with the change of geochemical element content in sedimentary profile is an effective method to verify the results of dating. The deposition rate of the Anning River is influenced by both human beings and the nature, and the deposition rate of the upstream(0.54cm/y) is bigger than that of the downstream(0.41cm/y). After the 1960 s, human activities lead the Aning River into a period of rapid deposition with large fluctuating changes.The early mining exploitation and the later extending mining activities in the Anning river basin supplied more sources of heavy metals for the sedimentation. The rectification and governance in the mining areas received evident environmental effects, and the content of heavy metals in sediments decreased greatly. Influenced by geological background and human activities, the potential ecological hazard degree of heavy metals in the sediments of the Anning River is from slight to strong with fluctuating change. In addition, pollution in the upstream is more serious than that in the downsteam.