| Water-level fluctuation zones of Three Gorges Reservoir Region has been become an important ecological system because of its unique characteristics of alternation of wetting and drying,and it has been widely concerned.Water-level fluctuation zones is amphibious cohesive zones whose soil is periodicly flooded and exposed because of unseasonal water level fluctuation caused by regulating reservoir. The vertical drop in the Three Gorges Reservoir Region is 30m in one year.Water-level fluctuation zones of Three Gorges Reservoir Region is an active region of energy, material and information transfer and transformation of water,land ecological system,which has the function of buffer and vegetation. Furthermore,it is sensitive to the change of external environment. Dissolved organic matter (DOM) plays an important role in the migration of pollutants, which can react with nutrient elements (N, P)and trace heavy metal ions from water and soil,and influence their migration transformation, toxicity and biological availability in the land and water interface. Water level of water-level fluctuation zones rises when Three Gorges Reservoir retains water,and then DOM from soils will be released into waters.Furthermore,it will affect geochemical characteristics of DOM and environmental chemical behavior of pollutants. Therefore, understanding the formation and source of DOM in the alternation of wetting and drying environment is important basis of understanding the role of natural organic matter (DOM) in the process of contaminant migration.Therefore,ultraviolet-visible(UV-Vis) absorption spectra, three-dimensional fluorescence spectroscopy and dissolved organic carbon analyzer, combined with fluorescence regional integration and fluorescence quenching titration were conducted to investigate the geochemical characteristics of DOM extracted from waters and soils,and complexation ability between mercury (Hg2+) and DOM in typical water-level fluctuation zones of Three Gorges Reservoir Region. The results showed that:(1) Average colored dissolved organic matter(CDOM) concentrations[a(355)] of waters in water-level fluctuation zones of Three Gorges Reservoir Region were in order of Kaixian-hanfenghu(KX-HF)>Zhongxian-tujing(ZX-TJ)>Zhongxian-shibaozhai(ZX-SB)>Fuling-wangjiagou(FL-WJG) in April and July,and KX-HF>ZX-SB>FL-WJG> ZX-TJ in September.It presents degressive trend from upstream to downstream.CDOM concentrations [a(355)] in different sampling time were in order of April>September> July,a(355) in July and a(355) in April,September showed significant correlation (P<0.05). Four fluorophores were observed in all water DOM samples, including three humic-like fluorescence peaks (A, C and M respectively) and one tryptophan-like fluorescence peak (T). The total integrated fluorescence intensity(TOT) of three-dimensional excitation emission matrix fluorescence spectroscopy(EEMs) in different sampling time were in order of September>April>July(P<0.05).TOT and a(355) showed significant correlation (P<0.01) in April and September. The a(355) in July was lowest,it and chlorophylla(Chla) showed significant correlation (P<0.05).At the same time,humification index(HIX) (10.53±4.41) and autochthonous index(BIX) (0.92±0.09) were all hightest. It may be related to strong light, high water temperature, microbial activity and photobleaching. Furthermore, "man-made interference" features of water DOM samples in water-level fluctuation zones of Three Gorges Reservoir Region were not obvious[r(T/c)<2.0].(2)Obvious photochemical degradation of CDOM concentrations [a(280)] and all fluorescence peaks intensity(A,C,M and T) occurred under natural solar radiation, and the degradation degree was in order of JLJ>FJ-JLJ>FJ. Photobleaching properties of DOM samples from different locations showed significantly differences, which could be partially explained by the sampling sites surroundings including various landuse types, and dilution effect of river confluence. Light-induced bleaching activity of JLJ samples, which was mainly terrestrial input from forest system, was the highest as compared to the lowest activity of FJ samples, which was predominated by urban inputs. Samples from confluence were in middle. Additionally, the spectrum slope(S) increased, but humification index(HIX) decreased with increasing irradiation time, which can be used as important indicators of photobleaching properties changes during the process. More importantly, the predominantly allochthonous (terrigenous) characteristics of DOM almost showed a tendency of transferring to autochthonous (authigenic) characteristics due to photobleaching. In addition, mechanisms of light-induced DOM degradation process consistently showed by absorption and fluorescence spectrum parameters suggests that the validation of analyzing DOM geochemical characteristics by the two important spectra tools.(3)Average CDOM concentrations in soils were in order of Zhongxian>Fengdu> Fuling>Wanzhou>Wushan>Yunyang>Fengjie>Kaixian. Additionally, in Zhongxian, Fengdu and Fuling, CDOM concentration [a(355)], aromaticity(SUVA254) and hydrophobicity(SUVA260) were all much higher than other sampling sites, but humification index (HIX) was lower. Four fluorophores were observed in all soil DOM samples, including three humic-like fluorescence peaks (A, C and M respectively) and one tryptophan-like fluorescence peak (T). Proportion of fluorescence regional integration of ultraviolet region humic-like A fluorophore was the highest as compared with others. More importantly, tryptophan-like fluorophore(T) and a(355) showed significant correlation (r=0.674,P<0.01), indicating variance of CDOM concentration possibly depended on T fluorophore. Meanwhile, the total integrated fluorescence intensity(TOT) of EEMs was a appropriate parameter to characterize the total contributions of fluorophores in DOM. Furthermore, humification degree of DOM in soils was low in comparison with higher biological availability. Conclusively it seems that geochemical characteristics of DOM from different elevation soil were not significant as expectation. It may be related to local agricultural activity, littoral plant growth and DOM mineralization process.(4) The fluorescence peaks of DOM contained in soils of water-level fluctuation zones of Three Gorges Reservoir Region was similar to the DOM contained in waters. Average a(355) of soils (66.48 m-1±150.92 m-1) were greater than waters(April,4.60 m-1±2.50 m-1; July,3.11 m-1±0.96 m-1; September,4.33 m-1±2.02 m-1).Soil was the main carbon library of organic carbon compared with water. Furthermore, humification index(HIX) and autochthonous index (BIX) of DOM in waters were greater than DOM in soils.(5) Fluorescence intensity of all fluorophores in surface waters and soils of water-level fluctuation zones of Three Gorges Reservoir Region declined with the increase of concentration of Hg2+. DOM in water and soil reacted with Hg2+,and the proportion of fluorescent groups complexed by Hg2+ was higher. Complexation ability between mercury (Hg2+) and protein-like fluorescence(T) was greater than Hg2+ and humic-like fluorescences(A,C and M).Complexation constants between Hg2+ and DOM in soils were greater than that was in waters. Conclusively it seems that complexation ability between Hg2+ and DOM (DOM-Hg2+) of different elevation soils were not significant.(6)The influence of CDOM concentrations, molecular weight, aromaticity, hydrophobic components, source, composition, humification and autochthonous characteristics of water and soil to DOM-Hg2+ complexation stability constants has no specific rules. The influence of geochemical circulation characteristics of DOM in water and soil of water-level fluctuation zones of Three Gorges Reservoir Region to DOM-Hg2+ complexation stability constants needs long-term monitoring works in future. |
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