Recognition Method Research On Nitrogen And Phosphorus Sensitive Spots In Closed Water Of The Mountain Cities

The eutrophication is one of the hot issue concerned by the world, and the content of nitrogen and phosphorus in water are the main factors influencing the degree of eutrophication. Compared with the plain lake, besides illiquid, simple water ecosystem, small water environment capacity,the closed water in mountain city has deep water, smaller wind flow disturbances, the existence of thermocline and the density current and so on. Factors such as water depth, temperature, and flow condition have an important effect on the distribution of nitrogen and phosphorus in water. This paper takes Long Jing lake which located in the mountain city of Chongqing as the research object. The regional general situation investigated and the water quality and eutrophication status evaluated. This paper explored the relationship between the water depth and the concentration of total nitrogen and total phosphorus in closed water of mountain city. And using EFDC model to simulate the characteristics of total nitrogen and total phosphorus distribution in Long Jing lake. Nitrogen and phosphorus sensitive spots of Long Jing lake are found. And the recognition methods are established. The main conclusions are as follows:①According to the hydrodynamic conditions of Long Jing lake, A, B, C, D, E five areas have been divided. Based on measured water quality data of the lake for many year, the current situation of water quality and eutrophication status have analyzed。The total phosphorus concentration of A, B, C areas is in 0.05 mg/L⁰.23 mg/L. From October to the following year in February, the total phosphorus concentration is above 0.1 mg/L. For D and E areas, it surpassed the standard of 0.1 mg/L in the autumn and winter. The total nitrogen content is relatively stable, only appear excessive phenomenon in February. The total nitrogen content of Area D is overweight in October to February. The total nitrogen content of area E is very high, in half of the year is higher than 1.5 mg/L. The Long Jing lake is in Light eutrophication state all year, the average TLI index is 57.69. Especially in June, July, September and February, eutrophication of the lake is in a moderate condition, the average TLI index is 63.68.② Study the response characteristic of the concentration of nitrogen and phosphorus and the water depth in A,B,C,D,E areas of Long Jing lake. Different partitions, different sampling time and different water quality index, the response relationship have differences. On August 15 and September 4 the water depth and total phosphorus content in area B have the highest value of the fitting curve of goodness of fit, the R2 reached 0.997 and 0.992 respectively, have the most obvious response relationship. This may be due to different partitions has different water environment, different lake sediment types, temperature changes, wind speed and direction changes, the main existence form of pollutants in the water, different diffusion coefficient and other factors.③On the foundation of fully understand the principle of diffusion, based on the Fick’s law, the Mike-Chang equations and other scientific laws and experience formula, establishing the relationship equation between the pollutant concentration and the water temperature and depth, the equation is C_A=y₂e^z/（y₁T）+x₂ Then using MATLAB（matrix laboratory） for binary nonlinear fitting, calculate the unknown coefficients in this equation, and get the specific equation form of total nitrogen and total phosphorus respectively. Equation of total nitrogen goodness-of-fit is 0.714, the average relative error is 12.96%, and total phosphorus is 0.662, the average relative error is 23.16%.The change of total nitrogen and total phosphorus concentration with the water depth have calculated, and the result show that total nitrogen and total phosphorus content rises with the increase of the depth of the water. And, in the natural environment, vertical temperature change influence on the total nitrogen and total phosphorus concentration is very small.④Through the EFDC（Environmental Fluid Dynamics Code） model of water environment, Long Jing lake hydrodynamic and water quality model have been built. In the absence of external pollution sources into the lake, to study the concentration distribution characteristics of total nitrogen and total phosphorus influenced only by the lake’s own conditions such as hydrodynamic conditions and sediment suction release, pollutant input of the model is set to 0. In this condition, model results show that total phosphorus most easily agglomeration area is area D, total nitrogen accumulation area is area D in autumn and winter, and it’s area E in spring and summer.⑤According to the vertical distribution of nitrogen and phosphorus and the spatial distribution of the simulation in Long Jing lake, analyzed the most sensitive spots of nitrogen and phosphorus concentrations in the Long Jing lake. In terms of total nitrogen, it has certain seasonal. The most sensitive spot is A10 sampling point in area D in autumn and winter, and it’s A08 sampling point in area E in spring and summer. Total phosphorus distribution throughout the year is stable, the most sensitive points are A09 and A10 points in the area D. Sampling points vertical concentration of total nitrogen and total phosphorus sensitivity are based on the depth of the water, the deeper the more to gather.