Nitrate Sources Apportionment In Surface Water And Shallow Groundwater In Tianjin (China) Using A Dual Isotope Approach And A Bayesian Isotope Mixing Model

Tianjin is located at the downstream of Haihe River Basin in China North Plain. Extensive agriculture, increasing population and the rapid development of urban brought nitrate contamination in the surface-and groundwater. Thus, it is essential to specify predominant NO3- sources and assess potential NO3- source apportionment, which is of great significance to control water contamination and protect coastal ecological system.In this study, six rivers, corresponding estuaries and the shallow groundwater have been investigated, the study rivers distributed from north to south are Jiyun River, Chaobaixin River, Haihe River, Dagu sewage River, Duliujian River and Ziyaxin River, and the estuaries are Haihe Estuary and the co-estuary of Jiyun River and Chaobaixin River.A combined use of dual isotope approach (δ15N-NO3- and δO-NO3-) and the Bayesian mixing model SIAR with the surrounding land use type and the physical and chemical characteristics of waters has been applied to identify the nitrate souces. The results show that NH4+ and NO3- are the dominant inorganic nitrogenous compounds in rivers and estuaries, and severe NH4+ contamination has been detected in both Dagu sewage River and Ziyaxin River. For shallow groundwater, the dominant inorganic nitrogenous compound is NO3-. Manure and sewage (M&S), soil organic nitrogen (Soil) and NH4+ in fertilizer and rain (NF&R) are the dominant nitrate source for rivers and estuaries. The nitrate of Chaobaixin River, co-estuary of Jiyun River and Chaobaixin River, Dagu sewage River and Ziyaxin River are all mainly derived from M&S, but the Chaobaixin River is mainly influenced by manure, the co-estuary of Jiyun River and Chaobaixin River is mainly influenced by domestic sewage, and the Ziyaxin River is mainly influenced by industrial sewage. The nitrate of Jiyun River and the upstream of Haihe River both mainly derived from Soil, and the nitrate of Duliujian River is mainly derived from M&S, Soil and NF&R. denitrification occurred in the upstream of Haihe River, Ziyaxin River and the shallow groundwater, while there is no denitrification in other rivers or estuaries. The shallow groundwater of Tianjin is influenced by multi-nitrate sources, the NO3-concentration is decreasing along the flow because of the denitrification.Based on the results above, the mixing model SIAR has been applied to estimate the proportional contributions from the nitrate sources of M&S, Soil, NF&R, NF and NP. The majority of the NO3- in rivers and estuaries was shown to derive from M&S (17-56%) and Soil (21-37%), which occupied more than 60% contribution in total. The contributions of the residual three NO3- sources, NF&R (14-25%), NF (6-20%), and NP (2-11%) occupied less than 40% contribution. The majority of the NO3- in shallow groundwater was shown to derive from M&S (27%), Soil (23%) and NF&R (21%), which occupied about 70% contribution in total. The contributions of the residual two NO3- sources, NF (19%), and NP (10%) occupied about 30% contribution. Proportional contribution of nitrate sources to shallow groundwater present a little bit different between the different sampling seasons (May and November), in general, the proportions arranged from big to small is M&S>Soil>NF&R> NF>NP.