This calculated mean nitrate value matches very well with the mean measured d18O-NO3 value of 1.53‰ in SC1 but is different from the mean of 0.08‰ in SC2 in the Thur River and its tributaries. This shift of measured value from the calculated d18O-NO3 in SC2, can result from mixing processes during infiltration through soil. Thus, microbial nitrification of ammonium from precipitation and soil nitrogen is identified as the main transformation process aiding additional nitrate production in the river (other than direct input from treated waste water) in both the seasons. Additionally, a positive linear correlation was observed between chloride (a conservative tracer) and nitrate concentrations (r2 = 0.95) in both the seasons, indicating that both are derived from the same source (e.g. treated waste water) (Yue et al., 2014) (Fig. S1). There is a very small positive correlation between the isotopes of nitrate in all the samples obtained from the Thur River and its