Spatio-temporal Variation And Pollution Source Apportionment Of Water Quality At Watershed Scale

Water pollution has become a worldwide environmental concern.Protection of drinking water source is an urgent issue since it is closely related to human health.Therefore,comprehensive understanding of the riverine environmental status,the spatio-temporal variation of pollution distribution,and the main sources of pollution of drinking water is critical for the sustainable development of a region.In this research,Hexi Reservoir watershed located in Changxing County,Zhejiang Province,China was chosen as the study site.Based on the comprehensive investigation and hydrochemical characteristic analyses,the status of water pollution in Hexi Reservoir watershed was evaluated using single factor evaluation and Nemero comprehensive pollution index method.The spatiotemporal variation of water pollution for each tributary was characterized by geographic information spatial analysis combined with different land use types.Furthermore,principal component analysis(PCA)and positive matrix factorization(PMF)model were employed to identify and apportion the sources of water pollution.The main results are as follows:(1)Water quality was polluted to a certain extent at Hexi Reservoir watershed,especially nitrogen pollution was very serious.Water samples were collected bimonthly between November 2015 and November 2016 at 32 monitoring sites in the three main tributaries of Hexi Reservoir watershed.Based on the Class Ⅲ water quality standards of the National Environment Quality Standards for surface water(GB 3838-2002),100%,72.9%,18.7%,81.8%and 0.98%,respectively,of the samples exceeded the standard limits for total nitrogen(TN),dissolved oxygen(DO),chemical oxygen demand(COD),fluoride and chlorides In addition,96.6%of the samples exceeded the Class V water quality standards for TN.According to the Class Ⅲ water quality standards of the National Environmental Quality Standards for groundwater(GB/T 14848-93),33.3%of the 21 groundwater samples exceeded the standard limit for ammonia nitrogen.In terms of different types of water body,the riverine water was considered as extremely heavy-polluted,while reservoir water was moderately polluted,and the groundwater was slightly polluted mainly by ammonia nitrogen.(2)The spatiotemporal variation of water quality showed significant distribution characteristics and patterns.Based on the temporal variations of the water quality in Hexi Reservoir watershed,the pollution index for the river water quality was the highest in January and the lowest in July,but the pollution index for the Reservoir water and groundwater showed significant time lags.It was observed that the TN concentration of the river water in the dry season(January)was significantly higher than that in the wet season(July).Total nitrogen concentrations in the watershed showed certain spatial patterns.The TN concentrations in the upstream of Yangmei tributary were generally high and they exceeded the Class Ⅲ water quality standard.The urban area in midstream of Hexi South tributary and Hexi North tributary had high TN concentrations,and the agricultural region in the downstream of Hexi North tributary also had high TN concentration.The spatial distribution of TN was closely associated with the land use types.The average TN concentrations in the areas around cultivated lands were generally high,and those in the areas around woodlands were low.For the riverine water in the watershed,the degree of water pollution followed the patter of(according to land use classification):cultivated land>household area>woodland>industrial and mining area,while for the three tributaries:Yangmei tributary(most polluted)>Hexi South tributary>Hexi North tributary(least polluted).(3)Based on the discharge of pollutants in Hexi Reservoir watershed,the main pollution sources and pollution contribution rates of each tributaries were identified and analyzed using PCA and PMF,respectively.According to the analysis of the discharge of pollutants in the whole Hexi Reservoir watershed,agricultural activities were considered as the major pollution source in Hexi Reservoir watershed.While industrial sewage accounted for relatively small portion of the total pollution.For TN,agricultural production including crop production and livestock were the main source,while domestic sewage was the second largest contributor to TN.Industrial sewage accounted for only a small portion of TN emission.For ammonia nitrogen emission in the water,livestock and poultry sewage were the main sources,while domestic sewage was the second largest contributor.The order of main sources for COD:livestock and poultry breeding>domestic sewage>industrial wastewater.Based on the analysis of PCA and PMF model as well as the previous study,four main pollution sources were observed:agricultural pollution source,industrial pollution source,domestic sewage,and urban surface runoff.The contribution of agricultural pollution source to TN and NH3-N was 75.26%and 59.42%,respectively.The industrial pollution sources were closely related to F concentration,and it’s contribution to F-in water was 71.87%.The contribution of domestic sewage for COD and ammonia nitrogen was 41.2%and 34.33%,respectively.The urban runoff was main sources for TOC and COD,and the contributions were 75.19%and 56.8%,respectively.Based on the PCA and the PMF model,the main pollution sources of Hexi South tributary were identified as:urban sewage,industrial sewage,agricultural and manure wastewater.The main pollution sources in Hexi North tributary were identified as:domestic sewage,surface runoff and agricultural pollution.The main pollution sources in Yangmei tributary were urban domestic sewage,traffic pollution,agricultural pollution and surface runoff.Similar conclusions from both the commonality of the PCA method and the fitting of the PMF model indicates that the pollution source analysis is rational.Our findings provide useful information for developing better pollution control strategies for the Hexi Reservoir watershed.