Impacts of Human Activities on the Occurrence of Groundwater Nitrate in an Alluvial Plain: A Multiple Isotopic Tracers Approach

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 (¹⁸O, ³H, and ¹⁵N). 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 recharge area, where nitrate enters into groundwater through vertical infiltration, corresponding to high ³H and enriched ¹⁸O 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 transects 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.