Identification And Control Of Nonpoint Source Pollution In Longhongjian Watershed

As point source pollution is controlled,nonpoint source pollution becomes the dominant pollution of the water environment,which causes severe ecological damage and becomes the leading threat to the surface and underground water.Therefore,nonpoint source pollution research plays a very important role in pollution control and management of water resources.In the present study,a case study was conducted in Longhongjian watershed where the point source pollution was controlled.The nitrogen in the stream keeps in a high level under the impact of the nonpoint source pollution.Thus,a combination of stream water quality,runoff analysis,15N and 18O analysis,remote image and arcGIS was conduct to elucidate water quality,pollution sources and transformation process,assess the ecological restoration,and optimize parameters of wetland design and strategies of pollution control.15 water sampling sites were sampled and analyzed monthly for two years,and 11 sediment sites were sampled and analyzed seasonally for one year.21 water and sediment parameters were determined.The arcGIS were used to analyse the temporal and spatial variations of the water quality.For water,the results from descriptive analysis illustrate that Longhongjian Stream was eutrophied,the water quality was charaterised with "low carbon,low phosphorate,high nitrogen,high dissolved oxygen,and weak alkalinity".Nitrogen were the main pollution,and far exceeded the water quality standard(Type ? in China).Spatial analysis shows that TN had a higher concentration upstream than downstream.TP had a higher concentration at the sites near the tea plantation.NH4+-N and BOD5 had higher concentratons at the sites near residential area.Higher concentraton of CODMn was observed.Temporal analysis concludes that pollution was more severe in wet season than dry season,which reveals that Longhongjian watershed was severely polluted by nonpoint source pollution.Descriptive analysis,Spearman's correlation analysis,cluster analysis and principal component analysis were used to analyse water and sediment quality and identify the latent pollution factors in Longhonjian watershed.Spearman's correlation shows that TDS and SAL had a positive correlation with TN,NO3--N,NO2--N and TP,which demonstrates that clay soil and groundwater might be the sources of high nitrogen in water.HCA devides fifteen sampling sites into three groups,which represents three different water quality in the watershed.PCA interprets that water quality variation were mainly caused by agricultural runoff and natural environment.For sediment,the results suggest that sediment had no harmful effects on water quality and sediment-dwelling organisms.Runoff was analysed for six times in the four land uses in the watershed.The results show that TSS,CODcr,TN,NO3--N,and TP were the main pollution parameters in runoff,and higher than the concentrations of parameters in stream water,which indicates that runoff was the main source of N,P,and organic matter of stream water.Runoff in the tea plantation contained the highest concentrations of TN and NO3--N,and runoff in the forest followed,which indicates that tea plantation and forest were the main sources of N pollution.The possible pollution sources were fertilizer,soil,and litter.Most of P in runoff was particle,and fertilizer is considered as the main source.CODcr mainly existed as particle,and was influenced by rainfall intensity and dry period.Residential and transportation area caused little influence on the stream water.The pollution concentration of rainfall was low,which means rainfall was not the source of the water pollution,and the main pollution sources were from ground.Identifying nonpoint sources of nitrate has been a long-term challenge in mixed land-use watershed.In the present study,dual nitrate isotope,runoff and stream water monitoring are combined to elucidate the nonpoint nitrate sources across land use,and determine the relative importance of biogeochemical processes for nitrate export in Longhongjian watershed.There were large changes in nitrate sources in response' to season and land use.NO3--N had higher concentration in spring and summer,and lower concentrion in autumn and winter.?15N-NO3-and ?180-NO3-were enriched in summer and downstream.?15N-NO3-and ?18O-NO3-analysis indicates that NH4+ fertilizer,soil NH4+,litter fall and groundwater were the main nitrate sources in Longhongjian Stream.Precipitation,NO3-fertilizer and manure had litter influence on the water pollution.Nitrification and plant uptake were the significant biogeochemical processes contributing to the nitrogen transformation,and denitrification hardly occurred in the stream.In this study,performance of two wetland systems(comprising of nine wetlands)installed downstream of Longhongjian Stream are assessed.The wetland systems were monitored and evaluated for their effects on nutrients removal,particularly TN,NO3--N,TP,and DP.Based on wetland input-output metrics,removal efficiency(RE)is used to quantify wetland system.Results show that both wetland systems effectively removed nutrients,with RE as high as 45%of TN,57%of NO3--N,78%of TP and 86%of DP.In general,nutrient removal efficiency is seasonally dependent,with better removal efficiency occurring during warmer seasons than others.Macrophyte uptake was a primary removal process in these wetlands.More wetlands working concurrently can provide a greater level of control on nutrients in lotic environments that are vulnerable to water quality impairment.Wetland design parameters like volume,area,flow path,and perimeter play an important in removal of nutrients.The strategies to control nonpoint source pollution in Longhongjian watershed are recommended to use BMPs such as source control,transfer control and hydrological remediation.