Tracing N-Nutrient and Organic Carbon Sources in Surface Water of Nhue River Sub-Catchment in Vietnam Using Stable Isotope Fingerprinting and Related Techniques

Concentrations of NH₄⁺, NO₃⁻, PO₄³⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺, F⁻, Cl⁻, and SO₄²⁻ and the isotopic compositions (δ²H and δ¹⁸O) in surface water within a sub-catchment of the Nhue River in Hanoi city (Vietnam) were quantified. Based on the concentrations of the constituents, sources of N- and P-nutrients released into the aquatic environment of the region were grouped by their relationships. Additionally, the sources of nutrients in the environment were discussed based on the isotopic signatures of nitrogen in ammonium and nitrate (δ¹⁵ N-NH₄ and δ¹⁵ N-NO₃) as well as oxygen in nitrate (δ¹⁸O-NO₃). The sources of organic carbon in the aquatic environment were discussed using the elemental C to N ratios in organic matter in the surface sediment. Our results showed that the concentrations of ammonium were 25 times and phosphate 3 times higher than the National Standard for quality of surface water that was set up by the Ministry of Natural Resources and Environment of Vietnam, but nitrate concentration in water was about 400 times lower than the Standard. The δ¹⁸O-NO₃ to δ¹⁵ N-NO₃ ratio was found to be 1.38 and [NO₃⁻] negatively correlated (R = − 0.81) with [SO₄²⁻] suggesting that denitrification was the reason for the nitrate in surface water of the region to be very low. The results of the study suggest that the sources of nutrients in aquatic environment of the region were from the domestic water waste and influx from inorganic N-P-K fertilizers, but the former source seems to be more important than the latter. The C:N ratio in the organic matter present in surface sediment indicates that domestic water waste was the main source of dissolved organic carbon (DOC) in the water of the region. The brown water waste contributed up to 30% of the stream in the region as estimated by the two-end-member mixing model based on the oxygen 18 signatures of the brown and irrigation water. It is recommended that the Government invests material and financial resources to construct centralized facilities to treat brown water waste to reduce eutrophication in canals of the inner Hanoi city because of intensive urbanizing.