Shuang HU

Ba'nan Environmental Monitoring Station，Chongqing 401320，China

Huaxi River is a tributary of main stream of the Yangtze River in Chongqing，and the river valley involves four towns（Nanpeng Town，Jieshi Town，Nanquan Town，Huaxi Town）and one street（Lijiatuo Street）in Ba'nan District and one town（Huangjueya Town）in Nan'an District［1-2］.To ensure that the comprehensive control project of secondary rivers is carried out smoothly and improve the water quality of Huaxi River from the source，based on the dynamic monitoring of main sections in Huaxi River，two monitoring sections of Xiaoquan tributary of Huaxi River are added，located 100 meters down stream of Laolongdongkou intersection in Hongqi Village，and 50 meters up stream of Huaxi River in Hongxing Village，respectively.

1 Current water quality of Xiaoquan tributary of Huaxi River

Through the monitoring and analysis of 25 indicators concerning Hongqi Village section and Hongxing Village section of Xiaoquan tributary of Huaxi River，we make assessment in accordance with class V standard ofSurface Water Quality Standards（GB3838-2002）［3-4］，and it is found that the water in Hongqi Village section is black，stinky and turbid.There are 8monitoring indicators exceeding class V water standard（dissolved oxygen exceeding the standard 1.11mg/L；potassium permanganate index exceeding the standard 0.3 times；five-day BOD exceeding the standard 2.2 times；ammonia nitrogen exceeding the standard 6.5 times；COD exceeding the standard 1.0 time；total nitrogen exceeding the standard 11.8 times；total phosphorus exceeding the standard 3.6 times；fecal coliforms exceeding the standard no less than 5 times），and the water in this section falls within inferior class V water.The water in Hongxing Village section is slightly dark，odorous and turbid.There are 6 monitoring indicators exceeding the class V water standard（five-day BOD exceeding the standard 0.7 times；ammonia nitrogen exceeding the standard 5.8 times；COD exceeding the standard 0.2 times；total nitrogen exceeding the standard 11.1 times；total phosphorus exceeding the standard 2.2 times；fecal coliforms exceeding the standard no less than 5 times），and the water in this section falls within inferior class V water.

2 Water quality assessment

From the water quality analysis of monitoring sections，Hongxing Village section is located in the lower reaches of Hongqi Village section，and the water environment quality in Hongxing Village section has been improved.Through self-purification of water，the dissolved oxygen and potassium permanganate index can meet the class V water standard.The potassium permanganate index，fiveday BOD and COD are greatly reduced，but due to the small water environmental capacity of this tributary，it still can not meet the class V water requirements.In addition，there is a small amount of life pollution sources along the river，which increases the pollution load of ammonia nitrogen，total nitrogen and total phosphorus，so the three indicators are not reduced obviously.Hongqi Village section is an upstream passing section of Xiaoquan tributary，and the water environment deteriorates seriously.The dissolved oxygen concentration is only 0.89mg/L，unable to provide the necessary oxygen content for the aquatic animals to survive.The five-day BOD content is as high as 32.08mg/L，and it can not meet wastewater discharge standards（standard value of 20mg/L）.The ammonia nitrogen and COD concentration can only meet the wastewater discharge standards（ammonia standard value of15mg/L and COD standard value of 100mg/L）.From this perspective，the concentration of some water quality indicators is equivalent to that of sewage indicators，indicating that from the source，Xiaoquan tributary has become a gutter.Based on the above analysis of water quality，the Xiaoquan tributary of Huaxi River is seriously polluted and especially the water of upstream Hongqi Village sec-tion is heavily polluted.

Table 1 The water quality assessment in Hongqi Village section and Hongxing Village section in the Huaxi River valley Unit：mg/L

3 The factors influencing the water quality of Xiaoquan tributary

The water quality of Xiaoquan tributary directly affects the pollution control effect of Huaxi River，and also affects the daily life of residents along the tributary.The factors responsible for the deterioration of water quality in Xiaoquan tributary are as follows：

3.1 Internal pollution sourcesIn addition to external pollution sources，there are also internal pollution influencing the water quality of Xiaoquan tributary.The water environmental capacity of Xiaoquan tributary is small，and the water exchange capacity is not enough.The sewage results in serious river sediment deposition，and the sediment releases large amounts of pollutants in water，reducing the water self-purification capacity.A lot of pollutants are discharged into water，beyond the scope of self-purification capacity of water，leading to the deterioration of the water body.

3.2 Application of fertilizer to breed fishThere are several fish farms in the Huangjueya section of Xiaoquan tributary，and a lot of feces and remnant feed generated during the application of fertilizer to breed fish directly enter into the water，thereby increasing the nutritional content of the water body.The application of chemical fertilizer to breed fish aimed at high yield directly increases the nitrogen and phosphorus content of the water body.Excessive breeding of bighead feeding on zoo plankton causes over production of phytoplankton.

3.3 Domestic garbage pollutionThe domestic garbage is littered and dumped randomly along the Huangjueya section of Xiaoquan tributary，and the garbage collection，removal and disposal work is not perfect.On both sides of the river，there are a lot of rotting leaves，littered peel，discarded plastic bags and food scraps.Part of the garbage is perishable and stinky，and can be a breeding ground for germs.After being washed by rain into the water，the anaerobic fermentation will produce a lot of ammonia and cause eutrophication，and the heavy metal is accumulated in the sediment，thereby affecting the water quality.

3.4 Sewage pollutionThewater from Laolongdong culvert into the runoff forms Xiaoquan tributary，which flows through Huangjueya Town of Nan'an District，and Hongqi Village and Hongxing Village of Ba'nan District，eventually entering Huaxi River.Huangjueya Town up stream of the tributary is densely populated，with frequent social and economic activities，and especially the gourmet street brings environmental pollution while driving economic growth.Due to backward sewage pipe network construction and low collection rate of domestic sewage，the restaurant wastewater rich in COD，animal and vegetable oils，ammonia nitrogen and total phosphorus is directly discharged into the river without biological and chemical treatment，polluting the water quality.The sewage is not completely intercepted，and the upstream sewage converges with the downstream sewage，exacerbating the water pollution.

3.5 Water source pollutionXiaoquan tributary stems from Laolongdong of Mount Nanshan in Nan'an District，and the water source is the culvert water of Laolongdong.The water quality has a direct impact on the water quality of Xiaoquan tributary.However，affected by the pollution of industrial wastewater from small business entrepreneurial base in Nanshan Banbian Street as well as the domestic sewage from residents and schools in Huangjueya Town，the culvert water quality of Laolongdong has seriously deteriorated.The source water flowing into Xiaoquan tributary is mixed with white foam，emitting a pungent odor.Laolongdong has become a sewage ditch.

4 Recommendations

4.1 Continuing to promote secondary river pollution controlIt is necessary to accelerate the construction of second-level and third-level pipe network of Lijiatuo sewage treatment plant，in order to improve the domestic sewage collection rate along Xiaoquan tributary.For the industrial enterprises along the tributary，it is necessary to implement strict supervision，launch a full-scale investigation into the illegal sewage discharge of companies，and implement sediment dredging project to reduce the release of internal contaminants.

4.2 Communicating with the river management departments to improve river water quality from the sourceThe local river management departments should strengthen the poultry and livestock pollution control，and speed up the relocation and pollution control of farms in the forbidden areas；strengthen supervision on industrial enterprises in Huangjueya Town，make a thorough investigation into the illegal sewage discharge of enterprises，and shut down the polluting enterprises along the river；build the sewage treatment plants and trash holding pipes in the upstream section to intercept the domestic sewage.

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