| Lying along the Wen Rui Plain, Wen Ruitang River is crowned as the mother river by the people of Wen Zhou due to its contributions such as flood control, water supply, shipping, irrigation, and sightseeing. After the1980s, the water quality of Wen Ruitang River in Wenzhou was inferior to Standard V because of a combination of factors. For example, large quantities of domestic sewage were directly poured into the river. In addition, the irrational exploitation and development of agriculture and industry as well as urban villages caused by urban development were among the main reasons. Consequently, a wide variety of organic pollutants, nitrogen phosphorus nutrients and heavy metals flew into the river, causing the seasonal or even perennial water blackening and smell. Since the Wen Ruitang River basin was filled with hardware, electroplating, leather and shoemaking industry, bottom sediments were seriously polluted by heavy metals. Under certain condition, the pollutants in the bottom sediments would move to the overlying water part, thus causing secondary pollution. The worsening water pollution of Wen Ruitang River put a damper on the sustainable development of Wen Zhou’s society and economy.In the21st century, governments of all levels are attaching great importance to the water environmental treatment of Wen Ruitang River basin. They have formulated rules and regulations to improve regional water system management. A series of effective measures have been taken, such as factory reconstruction, ecological water diversion, management of major pollution sources and ecological restoration of rivers. Ecological water diversion has been widely adopted considering its effectiveness and minimal side effect.The present paper takes the overlying water body, typical polluted river section surface and sediment core samples as research subjects. Based on the data collected through several field samplings, fixed observation and experimental analysis, the paper probes into the pollution status and the temporal and spatial variations character of such pollutants as nitrogen and phosphorus and oxygen-demanding organic matters contained in the overlying water of Wen Ruitang River in Wenzhou urban district before and after the ecological water diversion. The laboratory has conducted a simulation study and analyzed water sediment’s capacity for absorbing nitrogen and phosphorus, and the dynamic re-suspension releasing rule of pollutants in surface sediments. The index of geo-accumulation method and ecological factor enrichment method have been applied to assess ecological risks of typical heavy metals in surface sediments. The paper mainly discusses the effects of ecological water diversion on the environmental improvement of Wen Ruitang River in Wenzhou urban district. The conclusions are as follows:(1) During the winter and summer period of ecological water diversion, water in most samples of Wenrui Tang River in Wenzhou urban district is nearly below the V standard, especially TP and TN. The index of geo-accumulation method and ecological factor enrichment method applied to evaluate heavy metal pollution (Cu、Zn、Pb、Cr Fe、Mn、Ni) of surface sediment. The two evaluation methods drew consistent conclusions that Zn、Cu、Pb show a strong contamination in most of samples, which are apparently affected by human activities. Ni、Mn、Fe、Cr nearly remain non-pollution.(2) Ecological water diversion has some positive effects on the improvement of quality of the overlying water in the short term, especially for the broad main river of the middle and upper reaches, and the river near the water-gate of Oujiang River. When the diversion pauses, the water quality will get worse in a short period of time. Gate operation schemes should be tailored via analogue computation of the entire river network diversion depending on different water qualities and inflows from the upper reaches(3) Sediments only exerts a weak effect on the adsorption of nitrogen and phosphorus, probably because of the dilution during the process of diversion, as well as the saturation of nitrogen and phosphorus in sediments. While settling, the sediments can carry a certain amount of phosphorus, but it does not apply to nitrogen. This is because phosphorus has a stronger affinity of the sediment in the water compared to nitrogen. Phosphorus exists mainly in the form of particle in water while nitrogen is water soluble. The settlement of sediments greatly increases the content of nitrogen and phosphorus, which plays an important role in controlling algae growth.(4) When the Ecological water diversion stop, the wash of water flow and movements of ships can also cause the perturbation of sediments, which will accelerate different kinds of inner pollutants releasing from the bottom sediments, and the water quality will get worse gradually. The content of such pollutants as TP、TN、CODcr increases with water movement. The releasing rate of the pollutants decreases with the increase of perturbation time. Within a certain period of time, the releasing rate roughly presents exponential decline and gradually becomes stable. It is not only influenced by perturbation, but also is related to various environmental factors such as physicochemical property in the bottom sediments and the respective concentration content of the pollutants. |
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