Water Pollution Characteristics And Source Apportionment In Two Representative Watersheds In Zhejiang Province

Water quality degradation has caused worldwide concerns especially in developing countries. With the growth of population and economy as well as acceleration of urbanization in recent decades, water pollution has become a severe issue in China; especially when infrastructures such as sewage networks and sewage treatment plants cannot keep up with the pace of economic development. Thus, to understand the pollution status, the spatial and temporal distribution patterns, as well as the sources of pollutants is of great importance for water quality management in a watershed.In the present study, two representative watersheds in Zhejiang Province were chosen as study areas. Wen-Rui-Tang River watershed in southern Zhejiang Province is mainly polluted by municipal and other anthropogenic sources. Hexi Reservoir watershed in northern Zhejiang Province is a water conservation area where the water quality is generally good except for nitrogen (N) from agricultural and other sources. The objectives of this research were to analyze water pollution status of the two watersheds, identify spatial and temporal patterns of water pollutants, and apportion the sources of each pollutant in the watersheds.Water samples were collected and analyzed bimonthly from Wen-Rui-Tang River watershed at 12 monitoring sites from January,2009 to December,2010. In Hexi Reservoir watershed, water samples were collected at 28 sampling sites along the rivers,14 sampling sites of groundwater, and 6 sampling sites in the reservoirs bimonthly from September,2011 to July 2012. The single parameter water quality evaluation, comprehensive water quality evaluation and pollution index (PI) methods were applied for water quality assessment in both watersheds. Besides, spatial and temporal variations of polluted water quality parameters were characterized in both watersheds. In Wen-Rui-Tang River watershed, two receptor models, principal component analysis (PCA) along with APCS-MLR and positive matrix factorization (PMF) were conducted based on the data sets of the three different zones (urban zone, suburban zone, and rural zone) to identify the sources as well as to apportion the contributions of each pollutant source. In Hexi Reservoir watershed, nitrogen and oxygen stable isotopes (δ15N and δ18O) were used for nitrate source identification, and a Bayesian model SIAR (Stable Isotope Analysis in R) was used for nitrate source apportionment. The main results are as follows:1. Both Wen-Rui-Tang River watershed and Hexi Reservoir watershed were seriously polluted, and the water quality was poor or very poor based on the comprehensive water quality evaluationWater pollution status was analyzed with single parameter evaluation, comprehensive evaluation, and pollution index. In Wen-Rui-Tang River watershed, all water samples in the urban zone were classified as Type Ⅵ Water Quality (which is worse than Type Ⅴ), all water samples in the suburban zone were Type Ⅴ and Type Ⅵ, while in the rural zone, Type Ⅵ water quality accounted for 67% and Type Ⅴ accounted for 14% of the total number of samples. Comprehensive water quality evaluation revealed that water quality in the whole watershed was poor or very poor. Using the National Water Quality Type Ⅲ as reference, the pollution indices in the urban zone were larger than 1 for COD、TN and NH3-N, and the out-of-limit ratios were 100% for TN and NH3-N, and 65% for COD. Pollution indices in the suburban zone were larger than 1 for COD、TN and NH3-N, and the out-of-limit ratios were 100% for TN and NH3-N,64.6% for COD,31.3% for CODMn and 18.8% for F-.In the rural zone, the pollution indices for TN and NH3-N were larger than 1, and the out-of-limit ratios were 88.9% and 47.2% for TN and NH3-N respectively,13.9% for CODMn and 19.4% for COD, exceeding the Type Ⅲ standards.In Hexi Reservoir watershed,90% of the river and reservoir water samples were classified as Type Ⅴ or Type Ⅵ, indicating very poor water quality.90% of the groundwater samples were classified as Type Ⅲ Water Quality, indicating good water quality. Using National Surface Water and Groundwater Quality Standard Type Ⅲ as references,100% of the river water samples and 5% of the reservoir water samples had TN concentration higher than the corresponding TN standards, while 4%of the groundwater samples had NH3-N concentration higher than the standard.2. Nitrogen pollution and organic pollution were the main pollution forms of Wen-Rui-Tang River watershedWen-Rui-Tang River watershed was seriously polluted by nitrogen and organic pollutants (parameters) such as TN, NH3-N, DO, CODmn, and COD, among which TN was the most deteriorated one, with 91%of the samples exceeding the National Water Quality Standard Type V of GB3838-2002 (2.0 mg L-1). The highest concentration of TN was 13 times higher than that of the National Water Quality Standard Type V. ANOVA revealed that the spatial distribution of most water quality parameters varied among the three study zones. The pollution of most deteriorated water quality parameters (TN, NH3-N, COD, and CODMn) in the urban zone and suburban zone were severer than that in the rural zone. Through analyzing annual changes of deteriorated water parameters, we found that water quality in 2010 was better than that of 2009 in Wen-Rui-Tang River watershed, but no obvious improvement was found in the bimonthly data.3. Wen-Rui-Tang River watershed was affected by both natural and anthropogenic sources. The major sources of pollution of Wen-Rui-Tang River watershed were domestic sewage, industrial source, transportation source, agricultural non-point source pollution as well as soil sourceBy PCA, four pollution sources in the urban zone, three in the suburban zone and three in the rural zone were identified in Wen-Rui-Tang River watershed. In the urban zone, urban municipal sewage and commercial/service pollution, transportation pollution, industrial pollution, and natural sources such as rock weathering or soil source were the main sources. In the suburban zone, suburban domestic sewage and industrial pollution, transportation pollution, and soil source were the main pollution sources. While in the rural zone, agricultural non-point source pollution and rural domestic sewage pollution, agricultural runoff, and soil source were the main pollution sources. APCS-MLR was then applied to evaluate the contributions of main sources to these pollutants.By PMF, the pollution sources were identified in the three study zones. In the urban zone, natural source such as soil source, industrial source, domestic sewage source, and urban runoff were the main sources. In the suburban zone, industrial pollution, surface runoff, and suburban sewage were the main sources, In the rural zone, rural sewage discharge, soil source, and agricultural non-point source were the main pollution sources. By comparing the results of the two receptor models, both models generated similar results in the urban zone, while in the suburban and rural zones, results were quite different. The commonality of PCA and determination coefficient of PMF revealed that both models generally had good performance for pollution source apportionment in Wen-Rui-Tang River watershed.4. Nitrogen compounds were the main pollutants, and they showed different spatial and temporal distribution patterns of Hexi Reservoir watershedIn Hexi Reservoir watershed, nitrogen was the most serious pollutant. Higher concentrations of TN, NH3-N, and NO3-N were found in the wet seasons. For river water and groundwater, TN and No3-N displayed higher concentrations in the upstream, followed by the midstream and downstream. For the reservoir water, TN and NO3-N displayed lower concentrations in the upstream than in the downstream.5. In Hexi Reservoir watershed, the main pollution sources were manure and sewage in the dry seasons and synthetic fertilizers in the wet seasonsBased on both chemical and isotopic characteristics as well as SIAR, source apportionment was conducted and the results showed that pollution source contribution differed significantly between the dry and wet seasons. Atmospheric deposition (AD) and manure & sewage (M&S) contributed more to nitrate in the three types of waters (river water, reservoir water and groundwater) in December than that in May. In contrast, soil N (SN) and synthetic fertilizer (SF) contributed more to nitrate in May than that in December. Manure & sewage was the major nitrate contributor in December, and it accounted as high as 61%of nitrate in groundwater. Synthetic fertilizer was the major contributor in May, with a mean contribution of 37%in the whole watershed. Atmospheric deposition had the least contribution to nitrate within the watershed.In summary, the sampling data and model analyses were used to identify pollutant sources and apportion their respective contributions in two representative watersheds in Zhejiang Province. The results provide theoretical basis for water quality improvement and protection for the two watersheds, and the methods developed in this research have even wider applications in water conservation.