|M.Sc Thesis||Department of Civil and Environmental Engineering|
|Supervisors:||Assoc. Prof. Shavit Uri|
|Prof. Shaviv Abraham|
The Isotope Pairing Technique (IPT) is a tracer method for measuring denitrification rates in sediments. Unique to the IPT is the capacity to differentiate between denitrification of nitrate which enters the sediments via the water column (Dw) and the denitrification of nitrate from coupled nitrification-denitrification in the sediments (Dn).
In this work, a numerical transport model of the nitrogen processes in sediments was constructed to investigate the validity of the key assumptions of the IPT. The numerical model points to large potential errors in the calculation of Dw and Dn under certain experimental conditions and particularly when there is a high ambient concentration of nitrate in the water column. The errors are related to an incorrect representation of the diffusion flux of the nitrate from the water column and the assumption about the binomial formation of nitrogen gas isotopes.
With error in the estimation of the denitrification rates inevitable, the numerical model was used to determine the most applicable range of the IPT and to determine how the method can be extended to include processes that had previously been ignored in calculations, such as the inclusion of nitrous oxide in the modified isotope pairing technique (MIPT). It was found that for systems with low ambient nitrate in the overlying water and high added concentrations of the heavier nitrate isotope, the IPT was most accurate. Overall this work emphasises the importance of numerical simulations in the understanding of processes in order to avoid potentially large errors and provides a workable range for which the IPT is applicable.