main tributaries in both seasons (the slope of the linear regression between d15N-NO3 and d18O-NO3 was 0.23 (R2 = 0.14) in SC1 and 1.3 (R2 = 0.25) in SC2). Further, the overall increase of d15N-NO3 and d18O-NO3 from upstream to downstream in SC1 and SC2 (Table 1), suggests that de-nitrification had not predominantly occurred in the well oxygenated Thur River during both the sampling days.