| Lake is an important part of human living environment. With the acceleration of urbanization and industrialization, lake water pollution becomes one of the most prominent water environmental problems. To monitor lake pollution Fast and effectively, and make an accurate assessment of it, is the base to make programs to use water resources effectively and orderly, and then achieve sustainable development.Bosten Lake is the largest fresh lake in inland of China. It is also the important water resource which influence the ecological environment and human survival and development. Because of its unique water features, problems in development and utilization, and effects by climate, Bosten Lake exists serious water environmental problems. Study of the lake can reveals influences to lake water environment by human and natural activities, and provides monitoring indicators and information on lake evolution.Taking Bosten Lake, an arid areas shallow lake, as the study object, from the perspective of environmental geochemistry, we use the n-alkanes distribution and stable isotope composition of surface sediments, to analyse lake organic pollution and eutrophication problems. Hope to find a fast and efficient monitoring method to make up the deficiency of existing monitoring tools.We sampled 47 water samples and 41 surface sediment samples, did corresponding analysis, and obtained some conclusions as follow:(1) Because Bosten Lake is a semi-closed lake, the inlet and outlet are on the same side and in close proximity, the hydrodynamic condition is poor. Water from Kaidu River can not mix up with lake water fully. Moreover, sewage outfalls mainly distribute in Huangshui bay area. All these conditions make hydrochemistry environment diversely in different regions. It is reflected in conductivity contour and pH contour:The values are little in Kaidu River estuary, big in Huangshui region. middle and steady in lake center, shallow-water circulation by winds is another important driving force. Its also work upon fragmental products transport and deposition.(2) Choosing peak carbon, CPI, OEP, Paq,∑nC23-/∑nC24+ to describe the distribution of n-alkanes and do source analysis, the results tell us that terrestrial organic matter occupies certain proportion'in the whole lake. BST-47 is near the Kaidu River inlet, and its organic matter is almost from terrestrial plants. Except this, other samples are all showed a double-peak distribution, indicating terrestrial and aquatic input together. The∑nC23-/∑nC24+ values of BST-40,31,7,9 are slightly larger than 1, displaying that aquatic input is predominate compared with terrestrial input. BST-17,24,25 are near the end of the lake, the percentage of terrestrial matter is higher than the center of lake due to the decline of aquatic. Aquatic content has significant difference in different regions, but terrestrial input comparatively steady, inferring that terrestrial input in the center and end of lake is controlled by wind, however aquatic is relating to water media and the depth. BST-22 has no comparability with other samples for the different depositional environment.(3) According to n-alkanes distribution, We submit three parameters: nC17/(∑nC14:nC34), nC23/(∑nC14:nC34) and nC17/nC23, which can reflect the reproductive status of Submersed Macrophyte and Phytoplankton, to discuss the relative level of lake eutrophication. If both of the nC17/(∑nC14:nC34) and nC23/(∑nC14:nC34) values are small, it indicates that water is dystrophic and lower hydrobiont is not developed or seldom. If the content of nC23 is high, but nC17 is low, it is explained as large population of submerged vegetation and high lake productivity. Water is polluted in a certain degree and corresponds to moderate nutrient level. If the content of nC17 is high, but nC23 is low, it reflects that submerged vegetation degrades and Phytoplankton takes dominance. Pollution in the region is serious and it shows eutrophication. If both of nC17 and nC23 have certain contents, nC17/nC23 approach to 1, it indicates a intermediate state of the above two status.The results of parameters used in the research of Bosten Lake sediments are: Water quality in Kaidu River inlet is good because of fresh water input (nC17/(∑ nC14:nC34)=1.2%; nC23/(∑nC14:nC34)=1.68%). Huangshui Bay, Heishui Bay, and southward extending to Kaidu River inlet nearby, water, pollution is more serious (nC17/(∑nC14:nC34)≥22.8%; nC17/nC23≥2.66). To the lake heart direction region, it shows a intermediate state (nC17/nC23=0.74). The center and end of lake is medium-polluted (nC17/nC23≤0.49).(4) Theδ13Corg are between-28.23‰and-23.07‰, shows a significant difference in organic source composition in whole lake. It is agreed with the results of n-alkanes distribution.(5) Carbonates in surface sediments are mainly anthigenic, its carbon and oxygen isotope composition shows positive correlation. Except the samples very near the estuary and sewage outfall, other samples'δ13C andδI8O are comparatively stable. The variation relation about stable carbon and oxygen isotope composition can not reflect the degree of contamination well.
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