Analysis Of Eutrophication Characteristics And Preliminary In-situ Remediation Study Of Urban Lakes, Beijing

Urban lakes are important part of urban ecological system and the landscape diversity. Under the influences of excessive pollutant carrying, subcritical flow and structural transformation, most of our urban lakes were listed in eutrophication or heavy eutrophication grades.Compare to the large eutrophication lakes such as the Taihu and Caohu lakes, the research foundation of urban lakes are relatively weak. Therefore, several urban lakes in Beijing city were selected for research object in our study, and we mainly carried out the investigation as follows:1) seasonal water quality variation of Shi Chahai lake in a year, polluted characters of water and surface sediment in six urban lakes; 2) In-situ biological oxidation experimental investigation of the heavy polluted urban lakes sediments.3) Lab simulation and field experiments about the effection of long-term intermittent aeration on phosphorus migration and transformation in water-sediment interface. Results of our research work were aim to provide basic datas and technical references for the prevention and control of pollution in Beijing urban lakes, as well as provide references for ecological management and restoration of urban lake eutrophication in our country.Monitoring and analysis work of 10 water environment and ecology indicators from the Shi Chahai lakes (including Xihai, Houhai and Qianhai lake) were carried out in year of 2009. The results showed that water quality of Xihai was worst and which of Qianhai was best. Analysised by the TLI index, water quality of Xihai was ranked as medium eutrophication level while Qihai was in mild eutrophication level. Varations of index in Xihai were more obvious than the other two lakes. The changes of water temperature, chlorophyll-a (Chla), total phosphorus (TP), ammonia nitrogen (NH3-N) were in similar regulations, while the change regulation of NO3-N was in opposite. In Xihai lake water, NO3-N was the primarily nitrogen nutrient which absorped by algae, and high water temperature (about 30℃) is the main factor which caused proliferation of algae in summer.Analysis and evaluation by the TLI index, eutrophication grades of Qianhai lake(QH), Kunming lake (KM), Qingnian lake (QN), Zi Zhuyuan lake (ZZ), Tao Ranting lake (TR), and Hong Linjing lake (HLJ) were obviously different. Among them the eutrophication grades of KM was lower while which of ZZ, TR, and HLJ were higher. Five Beijing uban lakes out of the six were in eutrophication, which demonstrate that water quality of Beijing urban lakes were in grim situation. Therefore, relevant measures for water pollution control and ecological restoration were needed in urgency.The surface (0-10cm) sediment of the six lakes were also severely polluted. Organic index of the six lake surface sediment were all exceeded the pollution grade, the lowest of which was the twice as much as the value of the pollution marginal value. With the standard of Canada, the ecological toxic of sediments TN from two lakes were both above serious toxicity standards, which of other three lakes were all near to the serious toxicity standards. Average TP content of surface sediment from the six lakes were about 1100mg·kg-1, which was higher than that from the large lakes in middle and lower reaches of the Yangtze River.In surface sediment of the six lakes, transformation of nitrogen and phosphorus (P) fractions were closely related to microbial activity, and the microbial activity were mainly affected by biomass of benthic algae.The generation of NH3-N and the transformation of NH3-N to NO3-N were both mainly controlle by the bio-activity of benthic algae in the surface sediment. Ca-P (P bounded to Calcium) was the dominant P fractions in surface sediment of the six lakes, while it was not the case for lakes in middle and lower reaches of the Yangtze River. Variations of Fe-P (P bounded to iron) and OP (organic phosphorus) content among the six lakes were more obvious than the other P fraction, and Fe-P and OP content from TR,QN lakes were higher than the other four lakes. OP and alkaline phosphatase activity (APA) behaved extremely significant positive correlation (p<0.01; n=36), correlation between APA and Chla content was also extremely significant (p<0.01; n=36).These results showed that the most part of inorganic P in the six lakes surface sediment were absorbed by benthic algae, only less part of which were released to the overlying water.Correlation analysis between water and sediment index from the six Beijing urban lakes shows that the transfer and diffusion of P from surface to water was not obvious, which may decided by the high ratios of Ca-P in the surface sediment. Correlation between TN and TOC content in surface sediment was extremely significant (p<0.01; n=18), and the correlation between water TN and the surface sediment TN was also extremely significant (p<0.01; n=18). In conclusion, the control and reduce of TN and TOC content in surface sediment is of great importance to the work of nitrogen pollution control in the unban city lakes.Treatment the heavy pollution sediments with in-situ biological oxidation technology has good application prospect in China. The surface sediment of QN lake were selected as experimental materials, in which Ca (NO3)2 and nutrients were added to promote indigenous microbial oxidation of pollutants in sediment. The tests aim to improve the ecological environment of surface sediment, which could lay the foundation of ecological restoration with large submerged plant and benthonic animal in the urban lakes. The results shows that (1) The added of Ca(NO3)2 and microbial nutrients could both promoted the rise of microbial activity in sediment, however, the efficiency of Ca(NO3)2 was better than microbial nutrients in the same economic costs. (2) Ca(NO3)2 could significantly reduce the release of P from sediment to overlying water, the combination of Ca(NO3)2 and microbial nutrients could reduce the release of NO3-N and NH3-N from sediment. (3) 298gN/m2 Ca(NO3)2,0.42L/m2 MT and 0.35L/m2BE were mixed and injected to sediment at depth of 15-18 cm may be the suitable agentia launch way and dose. (4) Compared to the controls, odours of the sediment after treatment was not strong and its color becomes shallow obviously. (5) The dosing of Ca(NO3)2 and microbial nutrition could both promote the increase of sediment alkaline phosphatase activity (APA), however the more obvious increase of APA were observed when Ca(NO3)2 was added in sediment. Aeration is the engineering technology which usually applicated in water purification and ecological restoration measures. The effect of areation in the overlying water on P fraction transfermation at water-sediment interface, and the efficiency of the microorganism during the course were investigated in our study. The experiments were aimed to provides technical reference and datas for in-situ repairment of the water eutrophication. Results showed that (1) The long-term intermittent areation lead to the increase of microbial activity in surface sediment, which mainly caused the decline of dissolved oxygen (DO) and TP as well as the increase of pH in overlying water. (2) long-term intermittent areation lead to the increase of APA in surface sediment, during the cause the Fe-P were obsorbed and transformed to OP by the microorganism. In a word, overlying water aeration affected the microbial activity in the surface sediment, which eventually caused the transformation of P fractions in the surface sediment.The urban lake eutrophication control tests with solar cycle flow outfit apparatus (NO.1 and NO.2) showed that (1) The apparatus could long-term intermittent operate quietly by solar energy, with the advantages of energy-saving and environmental friendly. (2) During the experiment, TN and NO3-N content in water were obviously declined, while the NH3-N content was not changed. The apparatus (NO.2) could mixed the water with a radius of 11 meters under condition of 30-40 r/min operating speed. (3) With operating of the apparatus (NO.2), moisture content of surface sediment have significantly increased, and the NH3-N diffusion effect from sediment pore water to overlying water were enhanced in the tested field. However, the apparatus (NO.1 and NO.2) removal efficiency of phosphorus and turbidity were not obvious. Therefore, when the apparatus were applicated in urban lakes purification work, the auxiliary engineering measures should be considered to remove turbidity and phosphorus in water.