Nitrate pollution is a severe problem in areas with intensive agricultural activities. This study focuses on nitrate occurrence and its constraints in a selected alluvial fan using chemical data combined with environmental isotopic tracers （180, 3H, and lSN）. Results show that groundwater nitrate in the study area is as high as 258.0 mg/L （hereafter NO3） with an average of 86.8 mg/L against national drinking water limit of 45 mg/L and a regional baseline value of 14.4 mg/L. Outside of the riparian zone, nitrate occurrence is closely related to groundwater circulation and application of chemical fertilizer.High groundwater nitrate is found in the re- charge area, where nitrate enters into ground- water through vertical infiltration, correspond- ing to high 3H and enriched 1SO in the water. In the riparian zone, on the contrary, the fate of groundwater nitrate is strongly affected by groundwater level. Based on two sampling tran- sects perpendicular to the riverbank, we found that the high level of nitrate corresponds to the deeper water table （25 m） near the urban center, where groundwater is heavily extracted.Groundwater nitrate is much lower （〈12.4 mg/L） at localities with a shallow water table （5 m）, which is likely caused by denitrification in the aquifer.
