Effect of nitrification in the river: In theory, the d18O values of nitrate produced by microbial nitrification would have approximately one-third of the oxygen in NO3 derived from oxygen in the air (d18O-O2) that has a value of +23.5‰ (Kroopnick and Craig, 1972), while two-thirds should be derived from ambient water oxygen (d18O-H2O) (Andersson and Hooper, 1983). Therefore, based on this assumption, the calculated d18O-NO3 in the Thur River from nitrification is with a mean value of 1.58‰ across all stations in SC1 and is with a mean value of 0.23‰ across all stations in SC2 (Table 1).