Research On Nitrogen And Phosphate Transport And Transformation At The Dagu Estuary

With the rapid development of society and economy of Qingdao, its industry, agriculture and urbanization process is accelerating. A large amount of industrial, agricultural wastewater and domestic sewage was discharged into the Jiaozhou Bay through river channels every year, resulting in the water quality deterioration of the Jiaozhou Bay. The Dagu River is the longest among those rivers discharging into the Jiaozhou Bay. It is rich in water resources, with high suspended sediment concentration and large amounts of pollutants, and subsequently, it is the most significant source of inorganic nitrogen and phosphate for the Jiaozhou Bay. It is necessary to investigate the transport and transformation behavior of the inorganic nitrogen and phosphate with the coupled effect of runoff and tide at the Dagu estuary, which can provide a technical support and reference for environmental protection and water quality improvement in the Jiaozhou Bay. This research has not only a theoretical significance, but also practical application value in future.In this dissertation, field data collection, theoretical research and numerical simulation method were adopted to analyze the behavior of the hydraulic characteristics and water quality of the Dagu estuary and Jiaozhou Bay. Considering the couple effect of runoff and tidal current, a 2-D hydrodynamic mathematical model, an inorganic nitrogen, phosphate transport and transformation water quality mathematical model and a eutrophication water quality model were establishment, and they were validated with the field data. The computational results were analyzed to find the trend of the spatial and temporal distribution of COD, NH4+, NO2-, NO3-, PO43-, phytoplankton nitrogen(PN), phytoplankton phosphorus (PP), chlorophyll-a, zooplankton carbon, detritus carbon, detritus nitrogen(DN), detritus phosphorus(DP), inorganic nitrogen(IN) and inorganic phosphorus (IP) with special runoff, current process and wastewater discharge standards. The major research contents, significant results and useful conclusions have been summarized as follow:(1) The pollutants discharge situation of the Jiaozhou Bay were investigated based on the field data collection and analysis taking into account the influence of the runoff and current at the Dagu estuarine and the Jiaozhou Bay. The parameters, such as water temperature, salinity, pH, DO, COD, NH4+, NO2-, NO3- and PO43- are analyzed at 11 stations from 2009 June to 2012 December, and the trends of distribution was preliminarily discussed It was found that the values of COD, NH4+, NO2-, NO3- and PO43-are high at the northeast Jiaozhou Bay, indicating the water quality is seriously polluted. Phosphate is the main factor to limit the phytoplankton growth, which may result in red tide. Eutrophication occurs at the northeast of the Jiaozhou Bay, while the eutrophication index at the western of the Jiaozhou Bay is relatively low.(2) Considering the coupled effect of runoff and current, a 2-D hydrodynamic model and a mathematical model for nutrients transportation and transformation were established respectively. The latter includes 7 major variables for DO, NH4+, NO3-, PO43- and chlorophyll a etc.,9 auxiliary variables and 39 constants. Tidal level, current velocity, current direction and water quality parameters at typical periods were employed to validate the accuracy and reliability of the models.(3) The computational results show that DO concentration is lower at the Dagu estuary after 2 hours discharge at the median water period or wet season, while the concentration of NH4+, NO3- and PO43- is much high. With the increase of flow distance, DO concentration increases, and NH4+, NO3-; and PO43- decreases gradually due to the turbulence and diffusion with local salt water.Regarding the influencing area of the runoff, wet season is 6.3 times of that at median water season, in terms of DO concentration lowering than 5.2mg/L, and 7.2 times in terms of NH4+ lowering than 1 mg/L,5.25 times in terms of NO3- lowering than 1 mg/L,5.8 times in terms of PO43-lowering than 0.16 mg/LIn pictures of the maximum influencing area of the runoff concerning different water quality parameters, the distributions of DO, NH4+, NO3 and PO43- releasing from the Dagu River deflects to right side of the sea shore due to the current effect at median water period. The flow rate at wet season is much higher than that of the median water period, leading to the increase of mixing area and turbulence strength. The concentration distribution of DO, NH4+, NO3- and PO43- finally tends to be uniform relatively. And it appears an approximate symmetry, whose symmetrical axis lies at the major trough for longitudinal direction at the Dagu estuary.(4) The local water quality parameters were evaluated to meet the first and second grade conditions of Integrated Wastewater Discharge Standard after wastewater discharge. The results show that DO concentration increases from north to south near the Dagu estuary, but concentrations of the NH4+, NO3- and PO43- are just opposite.15 days later, DO concentration meets the first grade of the Sea Water Quality Standard, but inorganic nitrogen (NH4+ +NO3-) and PO43- concentrations exceed the fourth grade of the Sea Water Quality Standard near the Dagu estuary. The increase of industrial wastewater quality or improvement of domestic sewage discharging into the Jiaozhou Bay will exert little effect on the water quality improvement of the Dagu estuary in a short period. To control the pollutants in the runoff of the Dagu River and raise the water quality standard that discharging into the Jiaozhou Bay, will be the favorite method to protect the Jiaozhou Bay water environment in the future.(5) A eutrophication water quality model was established and validated with the field data, including the variables for phytoplankton carbon, PN, PP, chlorophyll-a, zooplankton carbon, detritus carbon, DN, DP, IN, IP, DO and benthic vegetation carbon. It was found that:At median water period, the concentrations of PN、PP、IN、IP at Dagu estuary are high. Wih increasing time, the maximum values of PN< PP> DN and DP increase, but the maximum values of IN and IP vary little. In terms of planar distribution, the maximum values aformensioned distributes symmetrically for the symmtry axis of longitudinal major trough at first, then the maximum values area tend to the right side at the Dagu estuary as time goes by.At wet season, their distribution area of PN, PP, DN, DP, IN and IP greatly increases. With increasing time, the maximum value area tends to the left side at the Dagu estuary from the symmetical distribution for the symmtry axis of longitudinal major trough. The front line of IN and IP maximum values area appears irregular ellipse, bow shape and semi circle in turn, IN and IP maximum values area distributes symmetrically for the symmtry axis of longitudinal major trough at first. After a relative long period, the front line tends to be parallel with the coastal line at Dagu estuary, and it pushes to the mouth of the Jiaozhou Bay. Finally, the symmetrical distribution vanishes gradually.In a short period, IN and IP from the Dagu River plays a significant role in total nitrogen (TN) and total phosphorus (TP) at the Dagu Estuary. PN, PP, DN and DP in the Dagu River are not the sensitive factors to influence TN and TP.