Using the dual isotopes approach to identify the nitrate sources of karst groundwater, Guiyang, Southwest China

Abstract

180 values can provide significant 815N and 8 information to understand variation of nitrate sources of groundwater. Groundwater samples were collected during summer and winter in the karstic city, XsO-NO3- -615N-NO3- at Guiyang, southwest China and analyzed for major ions, 180 values were determined on Finnigan MAT 252 mass spectrometer by 815N and ~ combustion tube method. NO3-/CI ratios of most groundwater were higher in summer than those in winter, which indicates that nitrate contents increased due -15 to atmospheric or anthropogenic input in most cases into groundwater and/or C1 N-NO3 in groundwater is +7.0%0. The mean values in winter, and +4.1%0 in summer, showing that nitrate in winter was more enriched in 180 -NO3- in groundwater. The mean values of +12.59/0o in winter, and +10.7%o in summer, showing that nitrate in winter was slightly enriched in lSO relative to that in summer. The mean values of 180-NO3" in rainwater were +1.5%o (n=21) and +34.2%0 (n=14), which means that the nitrogen deposition just contributes a little nitrate to groundwater due to the isotopes and low contents. It is likely that chemical fertilizer and nitrification contributed nitrate to suburb groundwater, 180 values of nitrate. It is considered that ~15N suggested by the high input of nitrate (chemical fertilizer and septic effluents) into the groundwater of Guiyang in summer, as evidenced by the variations of chemicals and isotopes with fast response of groundwater to rain in karst environment.