Study On Pollution Characteristics Of High Risk Tributary In Dongjiang River And Effect Of Constructed Wetland Treatment

Dongjiang River,a drinking water source,has been supplying production and living water for about 40 million people of Guangdong Province and Hong Kong.Thus the safety of water quality is especially important.Gaopu River is a high-risk tributary in the upper reaches of the Dongjiang River Basin.The rapid development,intensive industry,staggered farmland and village,and incomplete sewage pipe network,make part of production and living sewage directly discharge into the river.In addition,rainfall runoff as a non-point source pollution,carries a large amount of surface pollutants and farmland pesticides into the water body,which will cause serious pollution to Gaopu River and affect the safety of water quality of Dongjiang River.This study is based on Gaopu River.Conventional nitrate and phosphorous pollutant indicators,and trace organic pollutant indicators including atrazine,ethoxyquin,metolachlor,pirimicarb,tebuconazole,caffeine and DEET are monitored in one year.The temporal and spatial distribution characteristics of pollutants are analyzed according to the monitoring results.Water quality and ecological risk are also evaluated and assessed in Gaopu River water.Rainfall runoff is the main non-point source in the river basin.So rainfall runoff of commercial and residential mixed zone,agricultural zone and industrial zone are typically selected to monitor pollutants.The characteristics of pollutant distribution is analyzed and the pollution loads of rainfall runoff into the river are estimated to study the contribution of pollutants in rainfall runoff.The combination process of "combined packing bed and wavy-subsurface flow constructed wetland" is used as ectopic ecological restoration technology for the heavily polluted tributary water of Gaopu River,meaningwhile the pollutants removal effect is studied.The main conclusions are as follows:(1)The concentration of conventional pollutants in the Gaopu River is greatly influenced by the seasonal and hydrological periods.Indicators such as CODCr,total nitrate(TN)and total phosphorous(TP)are high in dry season,low in wet period;Dissolved oxygen concentration is high in winter,but low in summer.The concentration of conventional pollutants is increasing along the river flow.Conventional pollutants sources are from upstream farmland runoff,tributary flow and effluent of sewage treatment plant in downstream.TN and TP are exceedingly beyond the class III of the surface water environmental quality standard(GB3838-2002)in the monitoring year except June,and some CODCr concentrations exceed the Class III standard.The results show that the tributary of Gaopu River is black-odor water in the evaluation of the single factor contaminant index method and multi-factor weighted index model.The trace organic pollutants concentration is much higher before and after April and August in crop planted circle,and the concentration in dry season is higher than that in the wet period.The wastewater from farmland runoff in the upstream and tributary in the middle leads to the gradually increase of pollutants concentration,the degradation of trace organic pollutants by natural microorganism is not obvious;the trace organic pollutant concentration decrease in the downstream,probably due to the dilution of the high standard drainage of the sewage treatment plant.The ecological risk assessment of trace organic pollutants is evaluated by the Risk Quotients(RQ)method.The results show that seven kinds of trace organic pollutants are in the range of acceptable environmental risk(RQ<1),of which atrazine,pirimicarb and tebuconazole are in middle risk(0.1<RQ<1),ethoxyquine and metolachlor are in low risk(0.01<RQ<0.1)and Caffeine and DEET are no risk(RQ<0.01).(2)The concentration of conventional pollutants in rainfall runoff is high.The concentration of TN in the commercial and residential mixed zone is the highest,and the concentrations of CODCr,TP and SS in the agricultural zone are the highest.Conventional pollutant concentrations are all relatively low in the industrial zone because of perfect pipeline infrastructures and ground timely cleaning.In addition to caffeine in the agricultural area is not detected,other trace organic pollutants in the three functional areas of rainfall runoff are detected,and the concentration range is wide.The distribution characteristics are affected by the using of pesticides,underlying surface and rainfall parameters.The total amount of trace organic pollutants in the agricultural zone is the largest,followed by the industrial zone.The application of pesticides in the greening land of the industrial zone is the main pollution source.The concentration of caffeine in the rainfall runoff is the highest.(3)Estimation of annual pollution load in Gaopu River shows that the pollution load of CODCr is the highest,reaching 401448.42 kg.TN and TP are 41466.53 kg and 5011.23 kg respectively.Among the seven kinds of trace organic pollutants,the annual loads of ethoxyquine and metolachlor are the highest,444.38 g and 534.93 g respectively,which have a greatly adverse influence on the river water quality and the load of the DEET is minimum with 14.12 g.As a typical non-point source pollution,the rainfall runoff contribution of pollutants to Gaopu River is high,which should be payed enough attention.Relevant departments have to take control measures in time,improve the pipeline facilities,reduce the use of chemical pesticides,switch to non-toxic organisms pesticides to replace toxic chemical pesticides and reduce the risk of river pollution from the source.(4)The pollutants removal effect of combination process is good.TP removal rates are as high as 79.66% and 79.65% in two groups of constructed wetlands(CWs),and the removal rates of CODCr are 66.74% and 72.50% respectively.The CODCr and TP of the effluent in two groups of CWs can achieve the goal of elevating the river water quality from the inferior class V to the class IV according to the surface water quality standard.TN removal rates reach at 52.04% and 45.21% respectively in two groups of CWs,with relatively high average concentration of effluent.During the monitoring period,the temperature is low,the activity of microorganisms and plants is restricted,and the removal effects could not reach the best condition.This study analyzes the pollution characteristics of water and rainfall runoff in Gaopu River and apportions the source of pollution,providing scientific evidence for river management and ecological restoration.The effect of "combined packed bed and wavy-subsurface flow constructed wetland" process on the treatment of heavily polluted river water is studied,which provides some references for the ecological restoration of black-odor water body in China.