1.University of Southern California；2.Guangdong Henrida Environmental Technology
Abstract：This paper assesses the water quality of the state-controlled cross-section，upper reaches，and lower reaches of the Danao River by using the single factor assessment method.The assessment results show that the pollution sources of the Danao River are domestic sewage came from residential areas，and industrial sewage came from enterprises along the river.Besides，as the Danao River is a tidal river，the uplift force of the tidal water flows makes the pollutants easy to accumulate.To deal with the pollution problem of the Danao River，the government should construct more sewage pipe networks and sewage treatment plants，build a regional water environment monitoring system，utilize grid management methods to maintain the water quality，and encourage the public participation as social supervision.
Key Words：water pollution；water quality assessment；single factor assessment method
Introduction
Water quality pollution assessment is an essential work in water environment control.Only through the reasonable evaluation of water quality monitoring data，can we formulate an appropriate adjustment plan and take adequate measures to deal with pollution problems.At present，there are various methods for comprehensive water quality assessment of rivers，including single factor assessment method[1]，pollution index method[2-4]，fuzzy mathematical method[5]，grey system evaluation method，analytic hierarchy process[6]，matter-element analysis method，and artificial neural network evaluation method[7].In this paper，the single factor assessment method is applied to assess the water quality of Danao River，as it can directly reflect the relationship between water quality status and evaluation criteria，and provide the standard index of each evaluation factor.
The Danao River is the mainstream in the Daya Bay economic and technological development zone，which directly flows into the Daya Bay in eastern Guangdong Province.With the local development of the economy and the rapid growth of the population，the amount of pollutants produced and emitted to the surrounding environment is increasing，which creates high pressure on water environment security.
The Danao River is a tidal river，including two tributaries of the Xiangshui River and Danao Flood Diversion Channel.The area of the Danao River Basin is about 98km2.The main areas along the Danao River are residential land，public construction land，and industrial land.
The assessment results show that the pollution source of the Danao River includes domestic pollution sources and industrial pollution sources.The water quality of the river cross-section at the junction of Daya Bay District and Huiyang District is in inferior V Class.In Daya Bay District，both sides of the river are mainly built-up urban areas，with communities and old villages.Most sewage is collected into the existing sewage pipe network，but there are still eighteen buildings，and four old villages that have not planned and built sewage pipe networks，consequently the sewage is directly discharged or mixed into the Danao River through rainwater outlets.Besides，the Xiangshui River flows through the Xiangshui Industrial Park，which has a large amount of industrial sewage discharge.These two pollution sources lead to the increment of pollution degree of some pollution factors in the Danao River，and the water quality of the cross-section at the mouth of the Danao river is still in inferior V Class.
Assessment Model
The single factor evaluation method is to compare the representative values of each parameter concentration with the evaluation criteria，which is the most widely used method for water pollution evaluation.
Standard index of water quality parameter i at point j：
Si，j = Ci，j / Cs，i
In the above formula，Si，j is the pollution index of pollutant i at point j；Ci，j means the measured concentration of pollutant i at point j（mg/L）；Cs，i means the evaluation standard of pollutant i（mg/L）.
If the standard index of water quality parameters is greater than 1，the water quality parameters exceed the prescribed water quality standards.The bigger the standard index is，the more serious the pollution is.On the contrary，the water body is less polluted.
According to the "13th Five-Year Plan for Environmental Protection"，three evaluation indicators are selected in this paper，namely chemical oxygen demand（CODCr），ammonia nitrogen（NH3-N），and total phosphorus（TP）.
The Danao River is a V-type water environment functional zone，so the water quality of the Danao River is under the V-type standard in Surface Water Environmental Quality Standard（GB3838-2002）shown in Table 1.
Table 1.Standards for Surface Water Environmental Quality
Results and Analysis
In this paper，the Huzhao cross-section，upstream and downstream of the Danao River are monitored respectively.The Huzhao cross-section of the Danao River is a state-controlled cross-section，which has been listed in the Notice of the Huizhou Government on the Quantitative Accountability Work Plan for the Water Quality of the Environment Assessment Cross-sections（No.35，2017，Huifu Office）.As the Danao River is a tidal and sea-entering river，through monitoring and analyzing the abnormal fluctuation of NH3-N at the Huzhao cross-section，it can be identified whether the source of pollution is from the upstream river or the estuary.Monitoring the upstream and downstream cross-sections of the river can contribute to the analysis of pollution distribution and inflow points.
1.Water Quality Assessment at State-controlled Cross-section
The monitoring was carried out from May 16 to May 25，2018.Each day the Huzhao cross-section was monitored at high tide and low tide for ten consecutive days.Indicators monitored included CODCr，NH3-N，and TP.The monitoring and analysis methods are shown in Table 2，and the testing results of the Huzhao cross-section are shown in Table S1.
Table 2.Monitoring and Analysis Methods of Indicators
The trend charts of main factors in the high tide and low tide periods of the Huzhao state-controlled cross-section are shown in Fig.1 and Fig.2.
The trends of CODCr，NH3-N，and TP in both high tide and low tide periods were similar，indicating that these three indicators were related.As the Danao River is a tidal river，it is difficult for pollutants to diffuse because of the uplift force of tidal water flow.During the tidal process twice a day，the uplift force of water makes the pollutants diffuse，dilute and degrade slowly，and the pollutants are easy to accumulate.
Fig.1 Tendency of Main Factors in High Tide Period
The individual trends of CODCr，NH3-N，and TP are shown in Fig.3 to Fig.5.According to the Surface Water Environmental Quality Standard（GB3838-2002），the standard indexes of CODCr，NH3-N，and TP are 0.53-1.35，8.4-27.15，and 1.4-2.38 respectively，so the NH3-N exceeded the standard more than the CODCr and TP.
The pollution of CODCr，NH3-N，and TP in the low tide period was generally higher than that in the high tide period，which indicated that the pollution mainly came from the enterprises upstream of the state-controlled cross-section rather than from the seawater recharge.
Fig.3 Tendency of CODCr at State-controlled Cross-section
2.Water Quality Assessment at the Upstream River
The monitoring was carried out from May 21 to May 30，2018，once a day for ten consecutive days.The upstream cross-sections of the Danao River were monitored at intervals of 100-200 meters，with 128 monitoring cross-sections.The monitoring locations are shown in Table 4.The indicator monitored was NH3-N，and the monitoring and analysis method is shown in Table 5.
Table 4.Monitoring Sites
The tendency of NH3-N at the upstream river is shown in Fig.6，and the testing results are shown in Table S2.From the state-controlled cross-section to the cross-section under Zhongxing Road Bridge（1-24#），the monitoring results of NH3-N were higher after the Xiangshui River flows into the Danao River and gradually increased with distance.The monitoring result of NH3-N at cross-cross-section 26# was relatively low，only 10.6-14.1 mg/L，while the monitoring result at cross-section 25#，the Xiangshui River confluence，was as high as 89mg/L（average）.It showed that the influx of the Xiangshui River caused the abnormally high NH3-N monitoring results at the state-controlled cross-section of the Danao River.
From the Xiangshui River confluence to the cross-section under the Xinqiao Bridge（27-53#），the NH3-N was stable at 92 mg/L（average）.The pollution of this river segment came from the influx of domestic sewage.
At the cross-section under the Xinqiao Bridge（53-54#），the NH3-N had a peak value of 135 mg/L.A drain outlet（coordinates：22°44'35.71 "N，114°29'27.00" E）was found at 50m upstream of the cross-section.Investigation showed that this outlet belongs to the Xiangshui River Industrial Zone，and the sewage flowed directly into the Xiangshui River without closure.Also，the sewage pipes of the real estates in residential areas along the river were not included in the sewage pipe networks，resulting in sewage discharged into the Xiangshui River without treatment by the city sewage plant.
From the cross-section under Xinqiao Bridge to the Zhuli River confluence（55-83#），the NH3-N monitoring results were maintained around 72 mg/L（average），relatively lower than last river segment（53-54#），indicating that there was no new pollution influx.
Fig.6 Tendency of NH3-N at Upstream Cross-sections of the Danao River
At the cross-section of the Zhuli River confluence（84-85#），the monitoring results of NH3-N reached another peak，and the highest data even reached 147 mg/L.It showed that one of the primary sources of NH3-N pollution in the Danao River was the influx of the Zhuli River，and this was a suspected phenomenon of illegal discharging by enterprises.
The monitoring results of NH3-N at the upstream cross-sections of the Zhuli River（86-101#）were relatively high.The pollutants around this water segment were mainly from industrial enterprises.There was the possibility that the sewage from industrial enterprises was illegally discharged into the river without treatment，which led to higher NH3-N values in this segment of the river.
The monitoring results of NH3-N at the upstream cross-sections of the Xiangshui River（102-128#）were relatively low，basically stable at 20 mg/L.
3.Water Quality Assessment at the Downstream River
The monitoring was carried out from May 21 to May 30，2018，and the upstream river was tested synchronously.The monitoring was conducted once a day for ten consecutive days.The lower reaches of the Danao River（from the Huzhao State-controlled cross-section to the Baishou Bay estuary）were monitored at intervals of 100-200 meters，with 37 monitoring cross-sections.The monitoring locations are shown in Table 6.The indicator monitored was NH3-N，and the monitoring and analysis method is shown in Table 7.
Table 6.Monitoring Sites
The tendency of NH3-N at the upstream river is shown in Fig.7，and the testing results are shown in Table S3.
From the state-controlled cross-section to the Baishou Bay estuary，NH3-N showed a downward trend，indicating that NH3-N in the Danao River was reduced naturally with the self-purification of the river.
At the Mamiao River confluence，the monitoring results of NH3-N had a peak with an average value of 43 mg/L.The main source of pollution was domestic sewage produced by nearby residents.Because the sewage pipe network had not been built，the domestic sewage in this area was discharged directly into the river without treatment.
From the Mamiao River confluence to the Baishou Bay，the NH3-N in the Danao River declined steadily，which indicated that there was no other pollution source influx.
Fig.7 Tendency of NH3-N at Downstream Cross-sections in the Danao River
Conclusion
According to the assessment results of water quality，the CODCr，NH3-N，and TP of the Danao River exceeded the standard，and the standard index of NH3-N was larger than CODCr and TP.The water quality of the Danao River was still in the inferior V Class.With the rapid development of urban integration and industrialization，lots of pollutants are produced and discharged，and the water environment of the Danao River is under enormous pressure.The leading causes of water environment problems in the Danao River are as follows：
1.The Danao River flows through two administrative districts.It was affected by domestic sewage discharged by residents and industrial sewage discharged by industrial enterprises along the river.
2.Within the rainwater catchment area of the Danao River，some residential districts did not have sewage pipes，or the sewage pipes had not been integrated in the sewage pipe networks.In these residential districts，the domestic sewage was discharged into the Danao River without treatment，leading to excess water pollution.
3.The Xiangshui River，the Zhuli River，and the Mamiao River converge in Daya Bay，and their water quality were all in inferior V Class.As there were lots of industrial enterprises along the Xiangshui River，abnormal pollution might be caused by illegal industrial sewage discharge.
4.The Danao River is a tidal river.With two rising tides a day，the uplift force of tidal water flow makes the pollutants diffuse，dilute degrade slowly，but accumulates easily.
Recommendations
1.For domestic sewage pollution，it is crucial to strengthen the construction of sewage pipe networks and sewage treatment plants，improve the collection rate of domestic sewage，and reduce the pollution of domestic sewage to the water environment of the Danao River.To deal with the temporary lack of domestic sewage collection networks，the government can lease or purchase some sewage treatment facilities for short-term use.
2.Improving the regional water environment monitoring system can provide criteria for the planning，construction，and operation management of water pollution control projects and evaluation.The monitoring system can also effectively monitor the industrial pollution caused by enterprises and any other potential pollution accidents，so that the government can take necessary emergency measures in time to ensure the regional water environment safety.
3.Grid Management is appropriate for the Danao River Basin environmental protection.The government can build its working objectives as "grading responsibility，complete coverage，and responsibility to people"，comprehensively grasp the information of key environmental monitoring targets，constructively divide the grid，and establish an environmental monitoring system that has "three clear and three in place"（i.e.clear region，clear baseline，clear responsibility；supervision in place，communication in place，and service in place）.The government should also strengthen the environmental supervision，prevent and punish environmental violations in time，protect the environmental safety，and safeguard citizens’ environmental rights.
4.Public participation is an important factor for the implementation of environmental protection regulations，and an integrated social supervision system is an essential way for effective public participation.The environmental protection department can employ public representatives as environmental protection supervisors who give feedback to the environmental protection department，while building a rewarding and reporting system for the public.Fully utilizing public participation and exposing environmental violations can help to prevent potential pollution.
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论文作者:He，Sirui1,Tan，Manyu2
论文发表刊物:《基层建设》2019年第24期
论文发表时间:2019/12/17
标签：mg论文; parameters论文; means论文; measured论文; prescribed论文; standards论文; greater论文; 《基层建设》2019年第24期论文;