The integrated use of stable isotopic and CSSI fingerprinting techniques to identify the sources of N pollutant in surface water and sediment in an agricultural catchment, north China

The intensive farmland in north China accounts for more than 17% of China's arable land, which is main wheat and corn production area in China. Consequently, excessive use of chemical fertilizers and pesticides, as well as rural household waste, are the environmental problems in this region. How the surface water quality responses to large scale of intensive agricultural activities in north China remains unclear. The study aims to identify the sources of nitrogen (N) pollutant in the surface water and to understand the contributions of different land use types to sediment at catchment scale. The study site is in a representative agricultural catchment (Jiangou catchment), which located in north China intensive farming areas. The main stream in the catchment, the Hai river system, is one of China's seven big rivers. The topography, land use and agricultural management practices and precipitation of the catchment were investigated using field observations and remote sensing. Fertilizer types applied in the farmland in the upstream area was investigated, and water samples along the tributary of Hai river (Ju river) from upstream at different sites representing different sources and samples of water entering Ju river were collected. The compound specific stable isotope (CSSI) analysis for soil samples collected under different land use types was done. Stable isotopic (δ¹⁸O, δ²H, δ¹⁵N-NO₃ and δ¹⁸O-NO₃) techniques to identify the N pollutant sources in the catchment surface water, and CSSI fingerprinting techniques were used to identify the contribution of different land use types to catchment sediment production. The δ¹⁸O and δ²H values of collected water samples suggested that the water source of these samples dominantly came from the local precipitation. Source contribution of N pollutant in water body of the Jiangou catchment identified by δ¹⁵N-NO₃ and δ¹⁸O-NO₃ data using IsoSource model and chemical fertilization from maize farmland (38±2%) and excrements of livestock from dairy (32±7%) were the dominant nitrate pollutant sources in the water body. Approximately 20±11% and 11±7% were attributed to discharge of manure from vegetable farmland and rural domestic sewage from village, respectively. Based on the CSSI data, the sediment source was mainly derived from maize farmland which contributed by 71%, the followed were bean and vegetable farmlands which accounted for 14% and 10%, respectively, and least contribution from forest land (5%). The combined use of CSSI fingerprinting and stable isotopic techniques could quantitatively identify the source contribution of N pollutant in surface water and sediment in the catchment, which is critical to the assessment and implementation of optimised agricultural and land management practices.