Numerical Study On Water Quality And Environmental Capacity Of Maowei Sea

Maowei Sea is the most important bay of Guangxi province and even china’s southern coast as the enclosed sea of Qinzhou Bay located at the top of Beibu Gulf, and playing a significant role in economic growth and trade contacts between china and ASEAN in the area around the Beibu Gulf. Maowei Sea is rich in fishery resources, it also has huge tidal prism which make it possible to build big tonnage berths which above 100,000 tons and large expanse mangrove surrounded which gives it high ecological resources. In recent years, with the rapid growth of economy and the increasing of disordered breeding and other human behavior, many environmental problems had been caused, such as sediment siltation, area declining and water contamination. The development of local economic and social met some negative effects. In order to solve this problem and enable Maowei Sea and Qinzhou Bay serve our social development better, comprehensive and thorough simulation study on water quality and environmental capacity in here appears especially essential. In this study we hope to provide a certain theoretical reference and technical support for improving the environment quality and the sustainable development of Maowei Sea.This paper researched the features of current field in this area through establishing a 2-dimension hydrodynamic model of the Maowei Sea, and a water quality model was established on this basis. We using COD and DIN as the index to study the water quality, and calculated the environmental capacity of COD and DIN, the share ratio of each pollution sources in Maowei Sea was calculated by combining the water quality model and the Marine Functional Zoning of the Guangxi Zhuang autonomous region(2011-2020).1. Combining the chart data of coastline and measured data of water depth, a 2-dimension hydrodynamic model of the Maowei Sea was established using MIKE 21 FM and verified with the measured data. Result showed that the model can be used to simulate the current field of Maowei Sea. The conclusions were shown below:(1)The tidal current of Maowei Sea was irregular diurnal tidal current. There are two days of semi-diurnal tidal current in neap tide of each month. While in rest time of neap tide and the whole spring tide it was obvious diurnal tidal current. The maximum of tidal range was 5.17m.(2)The tide of Maowei Sea and the neck region of Qinzhou Bay are reciprocating current of NNW-SSE direction. In the flood tide, the maximal velocity (1.3m/s) appears at the central shipping lanes of Maowei Sea and Longmen waterway which located at the neck region of Qinzhou Bay. The average velocity in Maowei Sea is 0.38m/s in flood tide. The current direction of ebb tide is contrary to flood tide. The maximal velocity of Longmen waterway is up to 1.5m/s, and the average velocity of Maowei Sea at ebb tide is 0.45m/s, it is obvious that the current speed of ebb tide is much greater than it of flood tide. The current outside the Qinzhou Bay presented a certain feature of rotational current and has large velocity, the velocity of both ebb tide and flood tide can reach 1m/s.(3) The average tidal prism of this sea area is 1.99×109m3 and account for 84.7% of sea water volume.2. Building a water quality prediction model of Maowei Sea by coupling transport module with the 2-dimension hydrodynamic model which was the basic current field. We simulated the concentration distribution of COD and DIN in this area at spring tide and neap tide, respectively. Comparing with the measured data to verify the model, the result proved that this water quality model can reflect the diffusion law of contaminants in water precisely and be used to predict the water quality of Maowei Sea.3. Combining the water quality model of Maowei Sea with the Marine Functional Zoning of the Guangxi Zhuang autonomous region (2011-2020), we calculated the environmental capacity of COD and DIN, respectively. Meanwhile we calculated the share ratio of each pollution sources through the simulation of the contaminant density field formed by pollution sources discharged alone.(1)In satisfies the water quality standard of the Marine Functional Zoning of the Guangxi Zhuang autonomous region (2011-2020), the standard environmental capacity of COD and DIN in Maowei Sea is 10207.3t and 1020.7t, respectively. The background environmental capacity of COD and DIN in Maowei Sea is 2875.74t and 641.13t, respectively. The remainder environmental capacity of COD and DIN in Maowei Sea is 7331.56t and 379.57t, respectively.(2)The share ratio of COD and DIN in Qin River is 33.33% and 24.49%, the value in Maoling River is 38.63% and 26.06%, the value in the Qinzhou port industrial park west sewage outlet is 19.86% and 33.41%. We can found that pollution from other sources is low and the area limited, which showed that the main pollutant of Maowei Sea is from Qin River, Maoling River and the Qinzhou port industrial park west sewage outlet.(3)The residual rate of COD and DIN in the Maowei Sea is 16.9% and 16.6%.