Watershed scale spatiotemporal nitrogen transport and source tracing using dual isotopes among surface water, sediments and groundwater in the Yiluo River Watershed, Middle of China

Nitrogen pollution has been shown to have strong potential threaten to the human drinking water and agriculture. However, identifying the nitrogen and spatial-temporal variation and nitrogen pollution sources among surface water, sediments and groundwater at the watershed scale is still of insufficient understanding. In this study, multi-methods (dual isotopes, hydraulic, hydrogeochemical methods) have been used and 400 samplings (40 sediments, 20 shallow groundwater and 40 surface waters in four periods in dry and wet seasons) were collected from 2018 to 2020. The results showed that the concentration of NO3--N, NH4+-N, NO2--N and total nitrogen (TN) had variable spatial and temporal changes in whole watershed. The concentration of TN, NO3--N, NH4+-N and NO2--N in downstream was higher than midstream and upstream both in dry and wet seasons. The concentration of TN, NO3--N, NH4+-N and NO2--N of the whole watershed in wet season was higher than dry season. The dual isotope values indicate that nitrogen sources were mainly derived from manure and sewage waste input (MSI), agriculture chemical fertilizers (ACFI) and sediments nitrogen input (SNI). Those nitrogen sources have different proportion in downstream, midstream and upstream in dry and wet seasons (the largest proportion: MSI 95.24% in downstream and ACFI 86.26% in upstream both in dry season, SNI 31.75% in midstream in wet season). Water exchange has positive correlation with the nitrogen concentration. High level of nitrogen in river also can be a diver in different location and seasons. Those results can be useful for developing regional management strategies and plans for water pollution control and treatment at watershed-scale.