LAUSR

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