| Dissolved oxygen(DO)is an important indicator of water quality and water ecological health.It is also an important condition for self-purification of water body.Dissolved oxygen had received increasing attention since the last century,and its hazards and causes of hypoxia are taken seriously.Based on the Shanghai scientific research project of environmental protection,the spatial and temporal variation characteristics of dissolved oxygen,the characteristic of typical dissolved oxygen areas and the abnormal phenomena of dissolved oxygen in surface water in Shanghai were discussed with GIS processing tools and mathematical statistics tools.It can provide the corresponding basis for environmental assessment and fine management,and also provides a reference for further improving water quality.The main research works and conclusions of this paper are as follows.1.Using GIS spatial analysis tools and the data of 259 monitoring points to characterize and calculate the annual and seasonal spatial distribution of dissolved oxygen in Shanghai from 2016 to 2018.And the correlation between dissolved oxygen and other water quality factors was also discussed.The results showed that:(1)The dissolved oxygen gradually increased from 2016 to 2018,winter >spring> autumn > summer.On the whole,the dissolved oxygen increased gradually from the downtown area to the suburbs.The low-DO area,mainly in the central urban areas with high urbanization,decreased year by year.It basically disappeared in winter and spring,and the response degree of dissolved oxygen in summer and autumn was relatively low under the impact of non-point source runoff pollution.The high-DO mainly appeared in the suburbs with low urbanization and developed water system in spring and winter.Flood season should focus on flood control and source pollution control.(2)The evaluation of dissolved oxygen showed that the dissolved oxygen increased as a whole,and the improvement was obvious in the low temperature season compared with the high temperature season.Different seasons and standards had different upgrade level.Based on the evaluation of water quality grade,the dissolved oxygen increased significantly in winter,of which class II water increased from 18.84% to 91.66%.There was also a significant increase in summer,and class V water decreased by 91.19%.From the perspective of water ecology,the increase of DO > 6mg/L in winter was obvious(116.49%),but there was no significant change of DO < 4mg/L in summer.(3)There was a significant positive correlation between dissolved oxygen and pH,and a significant negative correlation with other factors such as water temperature,ammonia nitrogen,and total phosphorus.2.Based on the spatial distribution characteristics of dissolved oxygen,seasonal low-DO areas,low-DO areas in the combined sewer urban area,and high-DO areas were analyzed,The results showed that:(1)Taking FengXian District,with seasonal change of dissolved oxygen,as an example.The dissolved oxygen in winter was obviously higher than other seasons,and the non-flood season was obviously higher than the flood season.In addition to the influence of water temperature,the proportion of farmland in the region was relatively high and was significantly affected by nitrogen and phosphorus.The total phosphorus was the main pollution factor during the flood season,with the highest concentration in the early stage and the loss rate gradually decreasing in the later stage.The control of farmland surface runoff should be strengthened in flood season and agricultural green production should be realized.(2)Selecting Suzhou River as an example for analyzing the characteristics of low-DO in central city.Anoxic duration accounted for 55.6%.It was polluted by upstream ammonia nitrogen and tributaries nitrogen and phosphorus.There was a negative correlation between the overflow discharge in the combined pumping station and dissolved oxygen.The annual and flood season water quality pollution factor was ammonia nitrogen,indicating that the highly urbanized area should mainly control domestic and industrial pollution.(3)Taking ChongMing district as an example to explore the causes of high-DO.Mainly due to the developed water systems,the low degree of urbanization,and there is no obvious pollution.The main pollution source is agricultural runoff pollution,ammonia nitrogen has a significant impact,and the main water quality influencing factor was total phosphorus in the flood season.Attention should be paid to the control of agricultural runoff pollution to prevent the impact of sudden pollution on dissolved oxygen.3.Based on the spatial and temporal characteristics of dissolved oxygen,the abnormal phenomena of dissolved oxygen in surface water environment in Shanghai were discussed from the perspective of detection data,so as to provide theoretical support for the fine control in the monitoring process.It mainly includes three aspects:(1)The phenomenon of dissolved oxygen supersaturation which is affected by the factors such as algal overabundance.(2)The phenomenon of "two ends" of dissolved oxygen and ammonia nitrogen,which both are too high or too low,affected by nitrifying bacteria.(3)Different results of three dissolved oxygen detection methods(iodine content method,electrochemical method and fluorescence method). |
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