| 其他摘要 | It is often found that some trace metals tend to be enriched in the top centimeters of sediment, which were commonly regarded as the obvious evidence of the increasing anthropic pollution since the industrial revolution. However, recent studies have shown that the strong early diagenesis can cause the remobilization of some trace metals along the sediment depth, to some extent, leading to the change of environmental records, and hence the distribution records of trace metals in sediment may not be considered as the direct input indications of drainage basin. In some cases, some metals in sediments could be released into the overlying water, causing second pollution, and become a "chemical bomb" hiding under lake water. Accordingly, more and more attentions have been paid to the "sink/ source" transformation of trace metals due to various biogeochemical processes near the sediment-water interface, for the sake of water resource management. Recently, it has been often reported that the water quality of the Hongfeng Lake and Aha Lake has the tendency to become worse seasonally. In view of the scientific significance and the environmental problems mentioned above, this Ph.D work has been focused on the geochemical process of trace metals near the water-sediment interface of this two lakes. The Hongfeng and Aha lakes are the man-made reservoirs in the center of Guizhou Province, and have the similarities of Karst lakes, and obvious differences as well. The aquatic chemistry of Hongfeng Lake water has the typical characteristics of karst waters, with moderate eutrophication , while total salinity of Aha Lake reaches to over 600 mg/L, which is caused by the long-term input of acidic mining drainage (AMD) of coal mining in the upper reaches of the input rivers, and also by the adding of lime to neutralize the AMD in recent years. We have carried out systematic researches on the water geochemistry, especially on the redistribution of trace metals at the water-sediment interface (WSI), and its controlling factors. The main results we have obtained are as follows: It has been realized that the continuous input of organic matter into the sediment and the seasonal change of redox condition are the important factors affecting the preferential reduction sequence of primary oxidants due to organic matter degradation in lake sediment. We found, by means of the counting of SRB and DIRB in sediments, that in the two lakes, preferential reduction sequence of primary oxidants were found as sulfate reduction (Mn reduction), followed by iron reduction, which is in a good accordance with the concentration profiles of sulfate,dissolved iron and manganese in pore water. The simulation experiments of sediment cores show that the dissolved oxygen (DO) at the WSI is the basic external factor leading to worsening of the water quality, and microbial activities are the most important mechanism causing various WSI processes. Furthermore, the cycling mechanisms of iron, sulfur and manganese in seasonally anoxic lakes were demonstrated as well. Microbes counting and biological macromolecule analysis indicate that the top sediments (0-10 cm) are the most active area of biological activities, where most labile organics are decomposed. The persisting depths of organics decomposition reach to 10 cm and 4 cm respectively in the Hongfeng Lake and Aha Lake. This is mainly because the differences in concentration of electronic accepters for organics anoxic decomposition. In the Aha Lake, due to the AMD input, sulfate, iron and manganese are excessively enriched in water and sediment, which provide enough accepters for organics anoxic decomposition and lead to the rapid degradation of organic matters in the surficial sediments. 3. The 5 nCDic compositions and DIC concentrations of the pore waters of the Aha Lake indicate that the methane-producing process is not obvious or very weak because of the restraint of sulfate reduction. Furthermore, from the 5 34S-sa,r-SO42" relationship and the distribution characteristics of SRB, we think that there is another sulfur source under the sediment of our sampling site and two reduction process can be divided as well: (I) slow reduction phase (6 cm below WSI), where the sulfur isotope fractionation is great, with maximum fractionation reaching to 13.7196o, and fractionation factor lie between 1.024 to 1.026; (II) rapid reduction/oxidation phase (0-6 cm below SWI), where sulfur has obvious isotopic fractionation, with maximum fractionation reaching to 38‰ The iron, manganese and sulfate reduction in both lake sediments have the tendency to shift vertically along sediment depth in different seasons, which can cause the seasonal change of the diffusion flux of some trace metals. The iron and manganese cycling near WSI in the Hongfeng Lake are relatively weak, which is dependent on the iron and manganese contents in the sediments, while the Aha Lake, with high manganese content in the sediment, has strong manganese cycling near its WSI, which caused the excessive enrichment of some trace metals in upside sediment to various extents. According to our researches of the Hongfeng Lake sediment profiles, we found that the remobilization degrees of various trace metals are different, due to their different geochemical characteristics: (1) The enrichment of Manganese in upper sediment is very obvious, while no remarkable remobilization was found with Iron; (2) Co and Ni almost have the same remobilization degrees as that of iron; (3) Due to their strong chalcophile affinity, Cu and Zn appear to enrich in the middle - upper part of sediment, gradually. Mo, with obvious remobilization effect, has the "dual content peaks", which indicates the dual controls of Mn cycling and sulfate reduction. As compared with those of the Hongfeng Lake, trace metals in the Aha Lake sediment have different distribution characteristics. (1) The contents of trace metals in each of the geochemical phase are mainly controlled by their total contents in sediment. (2) The REE distribution patterns of the sediments can clearly identify the location of the original bottom before this lake was formed, that is 18-19 cm below the WSI. (3) The changes in concentrations of Ti, Zr ands REE+Y through the whole sediment core also confirm the location of the original bottom, and indicate the gradual decrease of terrestrial input after the lake area widening. (4) The fractionation or separation betv/een Fe and Mn exists along the depth. (5) Fe has good relationship with Cu and U, while Mn with Pb, Co, Ni, Cd, Zn, indicating that these metals have similar geochemical behaviors to those: of iron and manganese, respectively. Basically, the distribution patterns of most trace metals can be divided into iron style and manganese style, both of which were controlled not only by their different remobilization degrees, but the different input from coal mining drainage, mainly influenced by pH. |
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