Comprehensive Assessment Of Water Environment And Environmental Capacity Study On Shenzhen Bay

As the rapid development of economy, the scale and intensity of ocean development increase continuously. Coastal waters have being suffered more and more serious pollution, and the marine environment quality is deteriorating. Marine environment quality assessment and environmental capacity study is a basic task for environmental protection, and is also an important means for environmental management. According to study on marine environment quality assessment method, an accurate, effective and workable environment quality assessment method can be established, which can make the evaluation process more simple, the evaluation result more reasonable. Based on a comprehensive understanding of marine pollution status and a correct evaluation of marine environment quality, environmental capacity study on the main pollutants is great important for designing a emission reduction scheme of regional pollutants, reducing the pressure on marine pollution and improving the marine environment quality. Shenzhen Bay is a boundary bay between Shenzhen and Hong Kong, and the major pollutants of the two areas are inpoured into the sea. A large contrast between the serious water pollution problems in the bay and the rapid urban development of Shenzhen and Hong Kong has been formed and has become a main factor which restricts the harmonious development of the two cities. In this paper, supported by the research project of "Environmental Capacity and Pollution Total Amount Control of Shenzhen Bay", comprehensive assessment of water environment and environmental capacity of main pollutants are researched based on the monitoring data of Shenzhen Bay in 2008. Main contents and conclusions are as follows:1.According to the statistical results of Shenzhen Bay in 2008, ten indicators are selected to analyze their spatial and temporal variations. The results show that:the water temperature in summer is higher than that in winter, and the salinity in summer is lower than that in winter. The CODMn in rainy season is higher than that in dry season. The spatial distribution patterns of temperature, salinity and CODMn contours are consilient that the distribution is from the inner bay to the outer bay in winter and from the west coastline to the east coastline in other seasons. The non-point sources pollution in rainy season is the main reason for the increased values of CODMn. The DIN in rainy season is higher than that in dry season and is adverse for the DIP and silicon. The spatial distribution of nutrients is varied, but the highest concentration always appears at the inner bay. This is mainly caused by the high nitrogen, low phosphorus and low silicon sea water from the Pearl River in summer besides the terrestrial runoff and sediment nutrient exchanges. The highest concentration of oil always appears at the estuary of Sham Chun River, Dashahe and snake port sea area which shows that the sewage from land-based sources and port are the main source of oil in the bay. The Pb and Hg in dry season are lower than that in rainy season, and is adverse for the Zn. There are no disciplinary changes to follow for the temporal distribution of heavy metals including Pb, Zn and Hg. These are mainly caused by the land-based sources of sewage, the two-way input from the atmosphere-sea and the concentration changes of the heavy metals in external sea area.2.According to the third class standard, a single-factor evaluation method is used to evaluate the water quality. To restrict the extremum with the water quality standard, a further comprehensive evaluation is done by constructing a grey correlation evaluation model based on the value initialized method by using a linear membership function. The results show that:the water quality in Shenzhen Bay is in the fourth class standard and the overall pollution trends indicate that the inner bay is more serious than the middle of the bay, the middle of the bay is more serious than the mouth of the bay, and the estuaries and piers are the most serious. Compared with the common grey correlation evaluation model, the model in this paper combines with the advantages of grey and fuzzy evaluations, and can truly reflects the comprehensive characteristics of water quality in the actual evaluation.3.A three-dimensional, baroclinic wetting and drying scheme is introduced into POM model to simulate the three-dimensional flow field of Shenzhen Bay. The simulation results show that:the tide in Shenzhen Bay is an irregular semidiurnal tide, the horizontal tidal current is a reciprocating current and mainly in southwest-northeast direction by the restriction of the terrain. The tidal current velocity of ebb tide is slightly larger than that of flood tide. A similar structure has been shown in the flow fields of surface, middle and bottom layers. Influenced by the bottom friction, the tidal current velocity in the surface layer is slightly larger than that in the middle layer, and velocity in middle layer larger than that in the bottom layer. At the tidal current turning moment, a counter-clockwise circulation current always appears in the mouth of the bay. The vertical velocity of the whole flow field is slower. There exists a circulation current process which is up in the inner bay and down in the outer bay in Shenzhen Bay.4.A material transport equation with an attenuation item is coupled into the POM model. The sharing rate method is used to calculate the environmental capacity of COD, DIN and DIP and a distance formula of mixing zone based on the response coefficient and the sharing rate attained from the hydrodynamic model is proposed. The simulation results show that the Sham Chun River and Dashahe are the main pollution sources that influence the whole sea area. There are discharge capacities of COD in all pollution sources in the whole year except that there is no discharge capacity of COD in Dashahe in rainy season. The DIN and DIP throughout the sea area are far excessive and there is no remaining capacity both in rainy season and in dry season. A strict emission reduction scheme should be taken in total amount control.5.Three different emission reduction schemes for COD, DIN and DIP are proposed according to their different contamination status. The simulation results indicate that in order to agree with the third class standard, a little reduction for COD and a large reduction for DIP should be performed based on the status quo, and a large reduction for DIN is also needed based on the precondition that the DIN concentration at the open boundary meets the third class standard.