[HTT18-P06] Basic kinetic study on stable isotope ratios of nitrogen compounds: a case study for groundwater in the Kathmandu Valley, Nepal
Keywords:groundwater , nitrogen and oxygen stable isotope ratios, nitrification, denitrification, mixing, fractionation
The data used for the analysis was obtained from the groundwater survey of the Kathmandu Valley conducted from September 2014 to August 2018. Based on the stable isotope ratios of nitrogen and oxygen of nitrate, the contamination patterns were roughly categorized. The mixing line and the denitrification line were prepared based on the dynamic fractionation for the stable isotope ratios of nitrate nitrogen, and the spatial distribution of denitrification-related samples was examined on the map. In addition, assuming that the reactants exist sufficiently and homogeneously in the system, an equilibrium fractionation model was formulated by combining nitrification and denitrification for oxygen stable isotope ratios of nitrate and water. The range of the model parameters were determined by using the measured data from the filed survey, the influence of each process on the stable oxygen isotope ratio of nitrate was quantified.
The result showed that the nitrate in shallow groundwater partially originated from the atmospheric source through infiltration of rainwater. A characteristic of denitrification was observed, which was initiated and proceeded over the valley. Additionally, the correspondent isotopic fractionation differed at the location and the season, particularly controlled by ammonia oxidation, nitrite reduction and oxygen exchange between nitrite and water.