| Longjing Lake is a typical artificial deep-water lake of the Expo Garden, in Chongqing, China. It plays an important role in buliding the garden landscape and adjusting the microclimate in the lake district. In summers and autumns, H2 S leaded black-odor problem occurs in Longjing Lake which has severely hindered the development of the Expo Garden. The six sampling points were identified according to the land use types, the construction of the fresh and old water, the location of the drainage outlet and the landscape factors. In all four seasons, the inorganic sulfur in sediment and overlying water were studied in order to determine the spatial and seasonal characteristics and find out the potential location of black-odor problems. The NO3-, NO2-, NH4+, TOC and CH4 along the depth direction in sediments and overlying water were determined in order to study their effects on SO42- reduction process; The simultaneously extracted metals and total heavy metals were determined to evaluate their biological effectiveness and ecological toxicity in sediments; Ca(NO3)2 was Added into the sediments to study it’s influence on the process of inorganic sulfur transformation.(1) In overlying and pore water, SO42- showed an increasing trend along the depth direction and it showed a decreasing trend in the sequence of winter, spring, summer and autumn under the influence of the season changes(mainly the temperature changes). It’s favorable for SO42- diffuse from the overlying water to sediments with the low temperatures and high dissolved oxygen concentration in winters, and that is opposite when it is summer. In winter, the SO42- diffusion flux was 0.059 mg/m2·d at the original fish ponds and it were 0.338-12.925 mg/m2·d at other points, there is no clear correlation between S2- diffusion flux and SO42- diffusion flux affected by the high dissolved oxygen concentration. In summer, the SO42- diffusion flux in the bay near outlet flux was 0.672 mg/m2·d and it was 0.725-1.349 mg/m2·d at other points. SO42-accumulation in the bay near outlet were detected that was caused by drag force of artificial drainage, it could lead SO42- endogenous release due to the increased SO42-reduction strength. In the points where SO42- diffusion flux was negative or at a low point,he H2 S concentration might be at a high level. The amount of SO42- diffusion from water to the sediment in the whole lake was 0.709t·a-1 in winter and 0.116t·a-1 in summer under the influence of the season changes and artificial intervention(drainage, aeration, aquatic animal and plant breeding, and so on). The soluble sulfide in overlying water was gradually increasing along the depth direction and the growth rate was accelerating, and it presented an increasing trend at most of points in sequence of winter, spring, autumn and summer except the bay near outlet and the original fish ponds which intervened by artificial drainage and aeration. Longjing lake was polluted seriously, the bay near outlet and Lingyunqiao were the key points to control sulfur pollution as they frequently occur "black-odor" problems in the summer and fall.(2) The acid volatile sulfide(AVS) was 100.90-316.38mg/kg, it reached a peak between 8 to 15 cm from the surface of layer sediment and then gradually reduced; The chromium reductive sulfur(CRS) and elemental sulphur(ES) were 106.01-471.53mg/kg,57.87~351.63mg/kg,respectively; Both CRS and ES increased firstly and then decreased. The relationship of the three types of sulfur was CRS >AVS > ES. At present, the process of inorganic sulfur conversion was seriously intervened by human factor, and the inversion quantity of AVS to pyrite wasn’t high in sediments in Longjing lake.(3) The variation trend of CH4 and SO42- in overlying water and pore water revealed the exist of anaerobic oxidation of CH4(AOM), SMT was located in-5 ~ +5cm from the sediments and overlying water interface. As NO3- content in sediment interface was limited, the process of denitrification and dissimilatory reduction to ammonia had no significant inhibitory effect on SO42- reduction. The TOC content was sufficient and could be used easily by SRB in overlying water, hence sulfate was a controlling factor for sulfate reduction; The TOC content was relatively low and difficult to be used by SRB in pore water, at the same time, other heterotrophic microorganisms also competed for organic matter, so the TOC content and its bio-availability became the controlling factor for sulfate reduction. The TOC in overlying water and pore water played an important role in the process of SO42-reduction, denitrification and methanation,the TOC consumption of SO42- reduction and methanation were 2.23%-12.76%,29.32%-33.36%,respectively. More than fifty percent of TOC was remained.(4)The results showed that there existed both horizontal and vertical difference in the contents of simultaneously extracted metals(SEM). Both the ratios of SEM/AVS and the differentials of SEM-AVS in the sediments indicated that acute toxicity for benthic organisms among the original fish ponds, the original Long Jing lake reservoir and lake was occurred. The correlation coefficient analysis showed that there were a significant correlation between AVS and SEM. And the significantly positive correlation among SEMZn, SEMNi, EMCr, SEMPb and SEMCu demonstrated that metal contaminant may have the same pollution source. Compared with lakes of China and the three gorges reservoir, the heavy metals contents in sediments of Longjing Lake were obviously higher. The assessment of the potential ecological risk of such pollutants to the environment was showed that each point was at a low degree of ecological risk. The potential ecological risk coefficient of Hg is the largest, which is at medium ecological risk level; the result of single factor ecological risk of heavy metals was in order of Hg > Cd > Ni > Pb > Cu > Zn > Cr.(5)Reduced sulfide in sediments and overlying water can be oxidized to SO42- by adding Ca(NO3)2, thus inhibiting the H2 S overflow. The removal rates of AVS were 89% at 12°C which increased to 89.4% at the temperature of 20°C. It indicated that the temperature had little influence on the removal rate of AVS, but temperature can accelerate the removal rate. In order to avoid incomplete oxidation of sulfide or exogenous nitrogen pollution, concentrations of pollutants that consuming NO3-(AVS, TOC et al.) and other physical indicators(temperature) should be considered. |
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