Natural production of nitrate by groundwater nitrification in New Zealand karst springs
- 1GNS Science, Lower Hutt, New Zealand (m.stewart@gns.cri.nz)
- 2RMA Science, Hamilton, New Zealand
- 3GNS Science, Wairakei, New Zealand
- 4Tasman District Council, Richmond, New Zealand
- 5Cawthron Institute, Nelson, New Zealand
The objective of this work is to understand the sources of nitrate in Te Waikoropupū Springs (‘the springs’) in New Zealand, and thereby contribute to their preservation. Previous work has shed light on the recharge sources of water to the springs (Stewart and Thomas, 2008), and nitrate mass balance based on this recharge model reveals the nitrate values in the recharge waters when they reach the springs. The major recharge source (from pristine forest on karst uplands) delivers much more nitrate to the springs than expected from measurements on water in its recharge area. This excess nitrate is attributed to nitrification (following mineralisation of organic-N) within the oxic karst groundwater system as the water flows to the springs.
Nitrification (bacterial conversion of ammonium to nitrate) is widespread in soils, unsaturated zones and oxic groundwater systems. Evidence showing natural production of nitrate by nitrification in the current system is given by: (1) Total nitrogen measurements: If organic-N is converted to nitrate-N along the flowpath from the Karst Uplands to the springs, there must be input of 5 kg/ha/yr of organic-N from the recharge area of 170 km2. This is within the range found by McGroddy et al. (2008) for first order streams from pristine forests in NZ. (2) 15N and 18O measurements: Nitrification produces nitrate with low values of the isotope ratios as observed for the springs, whereas denitrification would cause high values which are not observed. (3) Scientific literature: Recent papers have reported nitrification as a previously unrecognised source of nitrate in oxic karst systems. For example: Musgrove et al. (2016) showed that groundwater nitrate concentrations in the Edwards Aquifer were higher than those in the surface water recharge. They concluded that nitrification within the aquifer is the source of the extra nitrate in the groundwater.
References
McGroddy, M.E., Baisden, W.T., Hedin, L.O. 2008. Global Biogeochemical Cycles 22, GB1026.
Musgrove, M., Opsahl, S.P., Mahler, B.J., Herrington, C., Sample, L.L., Banta, J.R. 2016. Science of the Total Environment 568, 457–469.
Stewart, M.K., Thomas, J.T. 2008. Hydrology and Earth System Sciences 12(1), 1-19.
How to cite: Stewart, M., Hickey, C., Moreau, M., Thomas, J., and Young, R.: Natural production of nitrate by groundwater nitrification in New Zealand karst springs, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2979, https://doi.org/10.5194/egusphere-egu23-2979, 2023.
