| Arsenic(As) is a typical noxious element and regarded as one of the most serious contaminants, which toxicity is growing widely concerned by scientists and governments all over the world. As the State Nature Reserve Area, Caohai is one of the largest freshwater lakes in Guizhou Province which has suffered drain project many times since 1958. Studies have reported that arsenic in Caohai has influenced by human activities, so Caohai which represents particular wetland is choosed as the research subject. For making clear contaminated status, the samples were collected from September in 2014 to April in 2015. The samples included rain, river water(flowing samples and flow-into samples), wetland water(surface water, middle water, interface water), sediment and plants.The conventional parameters of water(such as p H, EC, Eh, DO, etc.), As plasma levels, sediment p H, organic matter, and different forms of As were tested and analyzed and one-dimensional pore water diffusion model was utilized to estimate the arsenical diffusion flux across the sediment-water interface and study the migration characteristics of As in Caohai.The conclusions are as follows.1. There are some differences of water chemistry parameters in Caohai between flood period and lower water period. The results show that the p H value is alkaline and the content of DO is high, the EC value is relatively low and Eh value is overall positive in the submerged plant, indicating that the oxidation environment exsists in Caohai. However, the trend shows opposite in the water plant area. The p H value shows weak acid and the content of DO is low, EC value is relatively high and the Eh value is generally negative. In vertical range of water, DO, Eh values gradually reduce with the increase of depth of water, while EC and p H varies little in the different depth range.2. The results indicate that the arsenical concentration at export in Caohai is 1.59 ?g/L, evidently higher than that at entrance which average value is 0.32 ?g/L as well as in rain(0.37 ?g/L). The arsenic contents in overlying water are 0.40~3.32 ?g/L in surface layer water and 0.49 ~ 3.48 ?g/L in middle water, 0.68 ~ 3.35 ?g/L in interface water in the flood period, respectively. However, the arsenic contents in overlying water show 1.20~4.98 ?g/L in surface layer water and 1.30~4.73 ?g/L in middle water and 1.37~5.71 ?g/L in interface water in the lower water period, respectively. The concentration of arsenical in emerged plant areas is larger than that in submerged plant areas, and the arsenic contents change little on the vertical distribution at most of the sampling points. The arsenic content shows slightly higher in the submerged plant area during the flood period than during the lower water speriod while presents the opposite thrend in the emerged plant area. The arsenic probably exsists as As(III) H3 As O3 during the flood period and lower water period in the reductive environment. The overlying water of the submerged plant area shows oxidized and the arsenic probably exsists as As(V) HAs O4 in this area, while the arsenic probably exsists as As(V) HAs O42- that is relatively less toxic in the water which flows out of Caohai.3. The average concentration of arsenic is 26.59 mg/kg and arsenic contents range from 17.72 ~ 40.25 mg/kg in surface sediments during the flood period, while the average concentration of arsenic is 25.02 mg/kg and arsenic contents range from 11.44 ~ 40.72 mg/kg in surface sediments during the lower water period in Caohai. The arsenic contents show emerged plant area > submerged plant area which trend is similar to overlying water. The arsenic probably exsits in residual state in surface sediments during the flood period and lower water period. The proportions of arsenic in the three extractable forms rank as oxidizable form> reducible form> weak acid extractable form. The organic matters show a significantly positive correlation with F2, F3 state while show a significantly negative correlation with F4 state, p H and F1 state, F3 state correlated positively and significantly correlated positively with organic matter during the flood period, respectively. However, F4 state, p H are significantly negatively correlated with organic matter. The p H shows a certain extent of correlation with the arsenic in different forms during the flood period, while showing a significantly negative correlation with the bioavailable form of arsenic during the lower water period. However, the p H shows a significant positive correlation with F4 state. The potential ecological risk index, index of geoaccumulation and sediment quality criteria for heavy metals are utilized to evaluate the contamination of arsenic in surface sediments in Caohai. The results show that the arsenic in surface sediments in Caohai has small potential risk and will not cause a serious threat to the surrounding biological environment in the short term.4. Regarding the total arsenical concentration, there exist large fluctuation in vertical direction in sediments and pore water and the arsenical concentration in sediments in emerged plant areas is larger than that in submerged plant areas. The arsenical concentration in sediments in emerged plant areas is between 19.86 mg/kg and 36.45 mg/kg and the average value is 27.84 mg/kg.The arsenical concentrations range from 13.05 mg/kg to 32.32 mg/kg and the average value is 19.79 mg/kg in sediments in the submerged plant areas. The diffusion flux across the sediment-water interface in the emerged plant areas and submerged plant areas are 73.84 ?g·m-2·d-1 and 18.99 ?g·m-2·d-1, 11.45 ?g·m-2·d-1, respectively, showing that arsenic storing in pore water in sediments release towards the overlying water in the flood period. The diffusion flux across the sediment-water interface of C19, C36, T4 and T5 are-1.42 ?g·m-2·d-1,-0.30 ?g·m-2·d-1,-10.49 ?g·m-2·d-1 and-9.98 ?g·m-2·d-1, respectively, showing that arsenic storing in the overlying water release towards pore water in sediments. The other sampling points show that arsenic storing in pore water in sediments release towards the overlying water in the lower water period and therefore demonstrate that sediments might be the significant arsenical input source in Caohai water.5. The concentrations of arsenic are 8.31 mg / kg and 8.87 mg / kg in potamogeton illinoensis morong and myriophyllum verticillatum in the flood period, respectively, while arsenic contents decrease in the lower water period which show 3.37 mg / kg and 4.62 mg / kg, respectively. The total concentrations of arsenic in six different submerged plants rank as myriophyllum verticillatum > potamogeton illinoensis morong > ceratophyllum demersum > potamogeton perfoliatus > charophytes > najas marina. Arsenic content is higher in the submerged plants than emergent plants above ground... |
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