Study On Regional Risk Assessment And Risk Governance Based On Water Quality Modelling

Both point sources and non-point sources are the main threats to regional environmental health.Pollutants discharged in human activities directly affect the security of the aquatic ecology,and ultimately put hazards on human health and socio-economic development.Risk assessment,which quantitatively explores the potential environmental problems,is one common and effective technique in environmental management.Presently in risk assessment domain on the regional scale,assessment index was solely used.It was difficult to avoid the subjectivity in the index and corresponding weights' screening,resulting in great subjectivity and arbitrariness in conclusions.Therefore,it is significant to construct one reasonable and practical quantitative method in the domain of regional risk assessment.The concept of risk is traditionally described by the two variables:the probability of hazardous events and the adverse consequences of the hazardous events.The risk level is often characterized by the risk curve or the expected values for all possible loss in the hazardous events,which are decided by the two quantities:the probability of the hazardous events and the possible loss of the hazardous events.Regional environmental risk assessment is the procedure to analyze the possible hazard of many environmental risk receptors endangered by multiple environmental risk sources.The level of regional environmental risk is closely related to the characteristics of the migration and diffusion of the pollutants in environmental media.The water quality model is one combined method using mathematical languages to explain and predict the transport and transformation processes of pollutants in environmental media.It can effectively predict the possible concentrations of pollutants' in all pollutants emissions' scenarios,thus it can quantitatively interpreted the relationship among the environmental risk sources,the environmental media and the environmental risk receptors.The water quality models can link these three elements of risk analysis.In the analysis,the risk factors could be analyzed along"risk sources-environmental media-risk receptors",and the hazards of risk receptors brought by multiple pollutants' sources were identified,finally level of regional environmental risk could be quantitatively characterized and assessed.Based on the water quality model,a methodology to assess the regional risk of point source pollution is proposed.The assessment procedure includes five parts:(1)observing hydrological and water quality data of the assessed area,and evaluating the selected water quality model;(2)identifying risk source units and estimating source emissions using Monte Carlo algorithms;(3)using the water quality model introduced in the first step to predict pollutant concentrations for various source emission scenarios and analyzing hazards of risk sources;(4)screening out the assessment endpoints and analyzing receptor vulnerability with the Choquet fuzzy integral algorithm;and(5)finally,using the source hazard values and receptor vulnerability scores to estimate overall regional risk.Using the environmental risk assessment method described above,regional assessment was attempted in multiple typical areas,multiple kinds of risk sources,different pollutants,multiple assessment endpoints and different seasons.The typical regions included the industrial zone of the Wusong River in Kunshan of Jiangsu and the typical drinking water source protection areas of the Taipu River in Wujiang of Jiangsu.The two regions were both in multi-district in the Taihu basin.In recent twenty years,with the rapid development of economy,multi-district water pollution incidents have risen in China,which have aroused strong negative social repercussions.Environmental safety in multi-district regions has been one of the focuses in China and been mentioned many times in the environmental protection law promulgated in 2014.However,environmental risk analysis in these regions was scare.In the inter-provincial boundary region of the Taihu basin,the intensity of economic activities is much greater than other regions and there are a number of national and provincial concentrated industrial zones,which have caused larger pollutants' loads.In addition,this multi-district region is sensitive to environmental pollution and there are several intakes of drinking water suppliers and habitat conservation areas.Environmental safety in the region is not only the security of people's normal production and life,but also the foundation for the coordination of upstream and downstream relationships.Enforceable methodology was still lack on solving the problem of environmental deterioration in multi-district regions.Risk sources evaluated included four types;those were industrial point sources,scwage treatment plants,large-scale livestock farms,non-point source in the studied areas.Pollutants evaluated were screened as chemical oxygen demand(COD)and ammonia nitrogen.The end points assessed contained four aspects,which were aquatic ecological risk,human health risk,social and economic risk and integrated risk.Furthermore,environmental risk in the dry season and rainy season of the study areas was simultaneously assessed.The Water Quality Analysis Simulation Program(WASP),one open water quality analysis program was applied to introduce the water quality models of the study area.The parameters of the models was calibrated and validated using the measured water quality data in 2013-2014 in constructing the water quality models of the Taipu River and the Wusong River in the dry season and rainy season.Using Monte Carlo algorithms,pollution source census data in 2007 and 2011 were used to simulate the emissions of COD and ammonia nitrogen of risk sources.Combining the results of the sensitivity analysis procedure of risk receptors,the risk indexes in different probabilities were calculated.And then according to the three different risk characterization methods,regional aquatic ecological risk,human health risk,economic and social risk and integrated risk were identified.And then risk contributions of sources to receptors were ranked,and the spatial distributions of risk levels were presented.The simulation results showed that the hazards of risk sources in the two seasons were various.In the dry season,the hazards on ammonia nitrogen and chemical oxygen demand of sources posed on each section of the Taipu River were significantly higher than those in the rainy season.In the Wusong River,the hazards on ammonia nitrogen of sources posed on each section of the river in the dry season were 3-4 times of those in the rainy season and the hazards on COD of sources in the dry season were 2-3 times of those in the rainy season.Thus risks in the dry season were all higher than those in the rainy season,characterizing as the higher expectations of risk indexes and higher probability distributions of risk indexes.Because the hazards of risk sources in the Taipu River region were relatively lower,the regional risk indexes were mainly influenced by the sensitivity index values of risk receptors.The differences among the diverse sources on aquatic ecological risk,human health risk,economic and social risk and integrated risk of ammonia nitrogen and COD were small.Among the 15 risk receptor units,the towns of Jinze,Yaozhuang and Liantang are located in the drinking water source protection areas in the downstream of the Taipu River,at the same time;they are all in the inter-provincial boundary buffers of the Taihu basin.The three risk receptors were environmentally sensitive and environmental risks were also high.Among the 23 risk sources,ammonia nitrogen pollution risk of the Taipu river region was mainly contributed by non-point sources and COD pollution risk of the region was mainly contributed by the industrial point sources and non-point sources in the upper reaches of the Taipu River.Risks in the Wusong River region were influenced by the risk sources' hazard and the risk receptors' sensitivities.The environmental risk indexes for each risk receptor posed by various risk sources evaluated appeared large differences and industrial point sources contributed dominantly to the risk level of the region.Among the 20 risk sources units,industrial point sources in Yushan town and Qiandeng town occupied the largest COD and ammonia nitrogen environmental risk contributions to the region.Among the 15 risk receptor units of the Wusong river region,Lujia town and Qiandeng town,located near the downstream of the river,were in the transboundary buffers of the Taihu Basin.The two towns had higher sensitivities to pollutants and at the same time the hazards of sources were high.In the Wusong River region,the two towns occupied the highest environmental risk.By changing the input conditions,risks for a range of scenarios were estimated.Five kinds of regional environmental risk control situations were put forward:closing risk sources of heavily polluting enterprises,30%reduction of the pollutants'emissions in the study area,50%reduction of the pollutants'emissions in the study area,50%increasing of the flow of the river in the dry season,100%increasing of the flow of the river in the dry season,of which three were aimed to control the emission sources and two were aimed to change the conditions of the environmental media.The prediction results show that five kinds of scenarios proposed could reduce the environmental risk of the study area and slight differences existed among various scenarios.Of the two regions,ammonia nitrogen risk in the Taipu River region was specific.Due to the number of risk sources on ammonia nitrogen pollution in the Taipu River region being small,the scenarios aiming to control the emission sources had little effect on reducing the risk of the region and the scenarios increasing the upstream flows could significantly reduce the pollution risk of the region.In the the whole process of risk assessment,the characteristics of the study areas being in the multi-provincial boundary region were considered.In the division of risk assessment units and risk receptors,the spatial heterogeneities of sources and receptors being in the upper or lower,left or right banks of the rivers were fully considered.In the calculation using the water quality model,the distributions of pollutants in rivers were emphasized,which reflected the difference of the concentrations of pollutants between upstreams and downstreams.In the sensitivity analysis of risk receptors,a number of indicators screened out were related to the locations of risk receptors,reflecting the difference of risk receptors being in the vicinity of the provincial boundary or away from the boundaries.According to the evaluation of environmental risk at the regional scale using the proposed procedure,risk sources and vulnerable receptors could be ranked using risk curves,the spatial heterogeneities of regional risk levels could be explored,and the contributions of various sources to each receptor could be examined,furthermore the proposed procedure could be also used by decision makers for long-term dynamic risk prediction.The technical support may be provided to decision makers to help them set priorities for allocating limited resources to manage regional environmental risks.