| Long Hong Jian, one of the main streams of West Lake, is located in the southwest of West Lake in Hangzhou. The point source pollution in Long Hong Jian watershed has been controlled, but its nitrogen level is very high. This study relies on the subject "Urban lake eutrophication Nutrient removal and control technology research" (2013ZX07310-001). Taking mainstream and tributary in Long Hong Jian as research object, the sediments and water quality were sampled and analyzed. The characteristics and interaction of sediments and overlying water were analyzed, and impact of dissolved organic matter (DOM) on sediments accumulation rate was tested to demonstrate the transportation and transformation processes of the main pollutants, which provides scientific basis for pollution control and management in Long Hong Jian watershed. The research contents are as follows:(1) assess sediments pollution according to distribution characteristics of sediments, organic index and organic nitrogen assessment method; (2) explore impact of sediments on nitrogen and phosphorus pollution of overlying water based on spatial distribution characteristics of total nitrogen and total phosphorus and its correlation with sediments composition; (3) analyze impact of exogenous DOM on nitrification rate of sediments by adding different concentrations of exogenous DOM into sediments.The main research conclusions are as follows:(1) The order of the mean quantity of total organic matter in sediments from high to low was M2, M6, Z2, M3, Z5, M5, M4, Z4, Z1, M7, Z6, Z3 and M1. The mean value of total organic matter of sediments in four seasons was all M-shape distribution. The order of the mean quantity of humus from high to low was HM, FA, HA and DOM. The quantity of FA, HA and HM in sediments was significantly and positively correlated with organic matter content. The PQ value of sediments ranged from 0.11 to 0.60, and the mean value was 0.36. The degree of humification was far lower than the severely polluted urban river.(2) C/N of sediments was between 8.8 and 19.1, with mean value of 13.6, which indicates that organic matter and nutrients in sediments are mainly from higher plants. Organic matter and nutrients of sediments in upstream are mainly from litters of terrestrial higher plant, and that in downstream are from terrestrial higher plants and residues of authigenic aquatic plants, in which residues of authigenic aquatic plants have bigger contribution.(3)The evaluation of organic index shows small fluctuations in organic index. The sediments are clean and belong to class I in the sediments quality standard. The annual mean value of organic nitrogen is 0.0321%, and it belong to class I as well.(4) DO concentration of overlying water was positively correlated with nitrate concentration, but the correlation was not significant, which indicates that nitrate of overlying water is partly from the nitration reaction which is transformed by ammonia nitrogen. Although the point source pollution in Long Hong Jian watershed has been controlled, tea cultivation, runoff pollution and other factors contribute large amounts of nitrogen pollutants to the water. Nitrogen pollutants produced by this may have been already transformed into nitrate nitrogen before they flow into stream. The mean concentration of total phosphorus in overlying water in the four seasons was 0.015, 0.049,0.062 and 0.033mg/L, and dissolved phosphorus was the main part of total phosphorus. Phosphorus concentration from overlying water in the source of the main stream and tributaries were significantly lower than other points, which shows that phosphorus from soil erosion and runoff in the forest is limited.(5) There was no correlation between the total nitrogen content of the sediments and the total nitrogen concentration of overlying water, which indicates that nitrogen in overlying water is mainly from exogenous pollutants, and the direct release or nitrification transformation of ammonia nitrogen is not the main reason for high concentration of nitrate nitrogen in overlying water. The total phosphorus concentration of overlying water and total phosphorus content of sediments were positively related, but the correlation was not significant.(6) Under the test condition, adding DOM had a certain effect on the transformation of ammonia nitrogen to nitrate in sediments, but it was not obvious and the effect varied with concentration of exogenous DOM, which indicates that low concentration of exogenous DOM is not the cause of high concentration of nitrate nitrogen in overlying water. |
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