Nitrogen in soil, manure and sewage has become a major challenge in controlling nitrate pollution in groundwater around plateau lakes, Southwest China

As an important recharge source for lakes, groundwater has become one of the dominant routes of nitrogen (N) input into lakes. Understanding the source and behaviour of N around plateau lakes is essential for improving groundwater and lake water quality. This study used NO3− stable isotopes (δ15N-NO3− and δ18O-NO3−) combined with hydrochemical data, environmental parameters and a Bayesian stable isotope mixing model (SIAR) to elaborate the NO3− sources and contributions and N concentrations and cycling processes in groundwater (n = 476) from 8 plateau lake basins in regard to land uses and seasonal changes. We found that multiple N form concentrations in the rainy season (RS) and cropland (CA) were higher than those in the dry season (DS) and residential area (RA); NO3−-N was the most dominant N form, and 45% of groundwater samples exceeded the WHO NO3− drinking water threshold. The NO3− isotopes and SIAR model revealed that soil N (SN), manure and sewage (M&S) and N fertilizer (NF) were the main NO3− sources in groundwater, contributing 47.5%, 30.5% and 19.5% based on land uses and 39.0%, 37.5% and 20.0% based on seasonal changes, respectively. Moreover, denitrification was considered the primary N transformation process in the groundwater in the RS, CA and RA, but the N transformation process in the DS was dominated by nitrification. These results indicated that N concentration and source varied with land use and seasonal changes, and the cross distribution of CA and RA and difference in seasonal rainfall resulted in the difference in multiple N sources and source contributions in groundwater. Despite changes in land uses and seasons, SN and M&S remain the largest NO3− contributors to groundwater, which poses new challenges to control groundwater NO3− pollution in the area around plateau lakes with a cross distribution of CA and RA. Therefore, improving septic tanks and sewage networks and scientific soil N management are essential for controlling groundwater pollution.