Combined effects of recharge and hydrogeochemical conditions on nitrate in groundwater of a highland agricultural basin based on multiple environmental tracers

Abstract

δ18O, δD, 3H, chlorofluorocarbon (CFC), and hydrogeochemical parameters were investigated to evaluate factors controlling nitrate levels in groundwater in an agricultural basin with steep topographic gradients. Groundwater is predominantly recharged from mountainous terrain toward the bottom of the basin. Cultivation of Chinese cabbage and radish as highland summer crops is widespread in the vegetable fields, supported by heavy application of chemical fertilizers. The nitrate concentration was low (0.02–6.8 mg/L as NO3-N) in rice paddies of the northern region and high (2.0–15.2 mg/L as NO3-N) in vegetable fields of the southeastern region. The δD and δ18O values of water samples ranged from −79.9 to −60.5‰ and from −10.6 to −7.7‰, respectively in the study area. Water stable isotopes showed that recharge occurs mainly from local precipitation, and evaporation signatures were observed in the rice paddies. The apparent groundwater ages determined from CFCs ranged from <20 to 54 years. Older and more mineralized groundwater was observed in the discharge zone of the downgradient area, while young groundwater was found in the recharge zone of the northern region, indicating greater vulnerability to contamination. Elevated nitrate concentrations (6.8–8.5 mg/L as NO3-N) in some old groundwater in the vegetable fields of the southeastern region may be due to binary mixing of old stagnant water and newly recharged water, as identified by comparing 3H and CFC-113 with CFC-12 levels. Groundwater in the rice paddies had lower nitrate concentration and higher concentrations of HCO3, Fe, and Mn, indicating denitrification under reducing conditions. Principal component analysis revealed four major processes controlling groundwater flow and chemistry: agricultural contamination under reducing conditions in the vegetable fields, occurrence of old groundwater under reducing conditions, and denitrification in the rice paddies. These results suggest that management of nitrate sources is needed to alleviate nitrate contamination, particularly in the vegetable fields of both upgradient and downgradient areas with fast infiltration, high fertilizer usage, and low natural attenuation of nitrate.