SUPERVISOR: Christine STUMPP

PROJECT ASSIGNED TO: Monis Nolitha GCAKASI

Nitrate (NO3-) has become a global source of concern in agricultural and urban areas due to pervasive nitrate pollution in both surface and shallow groundwaters (Singh & Craswell, 2021). Nitrate is commonly sourced from nitrogen fertilizers present in agricultural runoff and from animal and human waste (Singh, et al., 2022). One prominent concern is the deterioration in drinking water quality due to high NO3- levels in water supply (Zhang, et al., 2014). There is general global recognition of the need to constrain and quantify the sources and biogeochemical cycling of nitrate in aquatic systems. Analysis of nitrate stable isotopes (15NNO3 and 18ONO3) has so far been an effective tool in identifying the sources of nitrate contamination as nitrate sources usually have a distinct N and O isotopic signature (Singh & Craswell, 2021). Together with geospatial models and isotopic time series, stable isotopes can also be used to trace the movement of nitrate contamination in agricultural and urban land. This information can then be used to constrain areas of high contamination where use of N-fertilizers can be optimized and to identify areas of natural bioremediation from processes such as denitrification (Zhang, et al., 2014). At the University of Natural Resources and Life Sciences (BOKU), we have established the Ti(III) reduction method which uses Ti(III) chloride to reduce NO3- to N2O gas under a one-step chemical conversion (Altabet, et al., 2019). The converted N2O gas is then used for the stable isotope analysis of 15N/14N and 18O/16O using Purge and Trap Continuous Flow Isotope Ratio Mass Spectrometry (PT-CF-IRMS). The advantages of the Ti(III) reduction method are that it is: 1.Cost effective and no need for toxic chemicals or anaerobic bacterial cultures, 2. Easy to establish in laboratories currently using N2O headspace gas sampling instrumentation, and 3. Requires only minor corrections for sample nitrate concentration variance and potential chemical interferences. The Ti(III) reduction method is the most recently established method for nitrate isotope analysis in aqueous samples. As such, much is still unknown about the chemistry of the method and how it is affected by changes in chemical conditions. In addition, there is no in-depth examination of how this method compares to existing contemporary methods for nitrate isotope analysis and its applicability on varying environmental samples.

Therefore, the aim of this PhD is to (i) compare the Ti(III) reduction method to other contemporary methods used for analysis of nitrate isotopes via an interlaboratory comparison (ILC) study, (ii) test the method on different environmental samples, and (iii) investigate the applicability of the method to studies investigating nitrate dynamics and biogeochemical cycling. We will apply the method in studies investigating the denitrification potential and the dynamic biogeochemical cycling of nitrate in floodplain aquifers. The latter study aims to quantify the extent to which denitrification potential/rates are controlled by electron-acceptor turnover in aquifer sediments and the depositional environment. Six floodplain aquifers in Germany and Austria have been identified for sampling. Groundwater and sediment core sampling along with laboratory flow through column experiments have already commenced at the Limnology Group, University of Vienna. Pore-water samples taken from laboratory flow-through column experiments will be analysed for nitrate isotopes using the Ti(III) reduction method at BOKU. A temporal study will also be undertaken to investigate the biogeochemical cycling of nitrate within the floodplain aquifer at the Lower Lobau in Lower Austria. Monthly groundwater sampling has commenced at four wells and will run from November 2023 until October 2024. Amongst other parameters, samples will be analysed for nitrate concentrations at the Limnology Group, University of Vienna. Nitrate isotopes will be analysed at BOKU using the Ti(III) reduction method. With this study, we aim to improve the understanding of the N-cycle and the dynamic interplay of different nitrate sources and redox processes within the Lower Lobau.

References

Altabet, M. A., Wassenaar, L. I., Douence, C. & Roy, R., 2019. Ti(III) reduction method for one‐step conversion of seawater and freshwater nitrate into N2O for stable isotopic analysis of 15N/14N, 18O/16O and 17O/16O. Rapid Communications in Mass Spectrometry, Volume 33, p. 1227–1239.

Singh, B. & Craswell, E., 2021. Fertilizers and nitrate pollution of surface and ground water: an increasingly pervasive global problem. SN Applied Sciences, Volume 3, p. 518.

Singh, S. et al., 2022. Nitrates in the environment: A critical review of their distribution, sensing techniques, ecological effects, and remediation. Chemosphere, Volume 287, p. 131996.

Vitousek, P. M. et al., 1997. Human alteration of the Global Nitrogen Cycle: sources and consequences. Ecological Applications, 7(3), pp. 737-750.

Wick, K., Heumesser, C. & Schmid, E., 2012. Groundwater nitrate contamination: factors and indicators. Journal of Environmental Management, 111(2012), pp. 178-186.

Zhang, Y. et al., 2014. Tracing nitrate pollution sources and transformation in surface- and ground-waters using environmental isotopes. Science of the Total Environment, 490(2014), pp.

213-212.