Contribution of Nitrogen Sources in Water Sources by Combining Nitrogen and Oxygen Isotopes and SIAR

Abstract

It is very important to identify nitrate sources in reservoirs that serve as high quality water sources to control its eutrophication. Stable isotopes (δ15 N and δ18O) and a Bayesian model (stable isotope analysis in R, SIAR) were applied to identify nitrate sources and estimate the proportional contributions of multiple nitrate sources in four reservoirs (Qingshan reservoir, Duihekou reservoir, Siling reservoir, and Lifan reservoir) that serve as sources of drinking water in the Hangjiahu area, one of the most densely populated and most quickly developing areas in East China. It was shown that nitrogen pollution, which was dominated by nitrate (NO3-), existed in the four reservoirs. Greater human activities caused more nitrogen pollution (average NO3- concentration 0.21 mmol ·L-1) in the Qingshan reservoir. A significant positive correlation (P<0.01) was observed between Cl- and NO3-. The analysis of the water in the Duihekou reservoir, Siling reservoir, and Lifan reservoir, with lower Cl- concentrations and higher NO3-/Cl- ratios, suggested that chemical fertilizer was the main source, while the analysis of the water in the Qingshan reservoir, with medium Cl- concentrations and NO3-/Cl- ratios, indicated a mixture of NO3- sources. The δ15 N ranged from 0.9‰ to 7.2‰, and the δ18O ranged from 2.8‰ to 14.1‰ in the four reservoirs. The δ18O values in more than 86% of the water samples were less than 10‰, and the δ15 N/δ18O values in 93% of the water samples were less than 1.3. It was identified that nitrification rather than denitrification acted as the primary N cycling process in the four reservoirs. SIAR was used to estimate the proportional contribution of five NO3- sources (industrial wastewater, sewage/manure, chemical fertilizer, soil nitrogen, and precipitation) in the Qingshan reservoir and of three NO3- sources (chemical fertilizer, soil nitrogen, and precipitation) in the Duihekou reservoir, Siling reservoir, and Lifan reservoir. The source apportionment results showed that chemical fertilizers and soil nitrogen were the dominant nitrate sources and their contributions were 75%-82%. It was revealed that nitrogen pollution in the water source reservoir caused by cropping non-point source pollution was very serious. Nitrate source contributions in Qingshan reservoir also included sewage/manure (25%), soil nitrogen (7%), and precipitation (6%), indicating that nitrogen pollution by sewage/manure should not be ignored in the higher human activity areas. The nitrate source in the Duihekou reservoir, Siling reservoir, and Lifan reservoir also included precipitation, with the nitrate contribution from precipitation at 21%, 24%, and 15%, respectively. It was suggested that precipitation contributed more nitrate to the water in areas with less human activity.