DeniGW – Location- and climate-dependent factors of denitrification in groundwater
Country / Region: Worldwide
Begin of project: July 1, 2024
End of project: June 30, 2027
Status of project: November 11, 2024
N₂-argon sampling in Chad
Source: BGR
Project overview
Denitrification refers to several, mainly natural, microbiological processes in soil and groundwater that ensure the breakdown of nitrate. High and, in some cases, increasing publication numbers, show continued great interest in the topic. With the amendment of the groundwater regulation of July 22, 2022, the federal states are obliged to determine and report the denitrification that has occurred in aquifers by the end of 2025. The best available technique for this is the N₂-argon method, which is also implemented in the Geocenter, in Hanover, Germany. Pure nitrate measurements in groundwater only provide limited information about input and degradation. On the other hand, these determine the nitrogen balance in groundwater.
This is where the “DeniGW” project comes in. Our aim is to investigate nitrate degradation capacities at several locations in different climate zones, thereby gaining important scientific knowledge about the different processes. It is known that bacterial activity during nitrate degradation doubles with an increase in temperature of around 10°C. On the other hand, ground frost plays an important role because denitrification is virtually suspended in the winter months and high, pulse-like nitrate levels can occur in spring. The climate dependency of denitrification will be explored using different locations already studied by the BGR (e.g. Chad, Jordan, Paraguay and Germany). This means that future changes due to climate change should be included in the forecast of denitrification rates.
Nitrate build-up and variability
Precise knowledge of the input is necessary to predict nitrate degradation in groundwater. In addition to the amount of seepage water, nitrate production through nitrification in the soil is the crucial factor. Nitrification in agricultural soils depends on many factors, but above all on nitrogen fertilization and management measures. Therefore, it varies from field to field, but it is also highly variable on a small scale (cm scale). It also varies over time due to the dependence on temperature and soil moisture. We will examine and describe the variability in the project so that we can use it in models for the entry of nitrate into groundwater.
Irrigation system at the Fuhrberger Feld forest site near Hanover
Source: BGR
An interdisciplinary approach to denitrification
Only with a precise description of the input can legacy effects be analysed, which could cause nitrate concentrations to continue to rise in the future, even if the input remains the same or even declines. On the other hand, the activity of the nitrate-degrading bacteria also depends on the availability of reaction partners, in the case of autotrophic denitrification by sulfide oxidation, e.g. by pyrite. However, the absolute amount of pyrite in the aquifer does not always play a role; under certain circumstances; the formation of coatings on grain surfaces can lead to only partial conversion, and thus to an earlier end to denitrification. A second central aspect of the project is therefore the investigation of the finite degradation capacity of nitrate.
The investigations in Germany are based on long-term time series from the BGR in the Fuhrberger Feld near Hanover and from the Bourtanger Moor in Emsland. There, the observation series of over 20 years will be continued. On the other hand, the geophysical method of surface nuclear magnetic resonance (NMR) will be used to evaluate its suitability for a non-invasive estimation of the depth of denitrification fronts. Previous experiments have shown that the distribution of the NMR relaxation time in the subsurface can provide an indication of the distribution of pyrite. Together with planned microscopic and geochemical analyzes of pyrites on various drill cores from northern Germany, conversion rates and displacements of fronts can be quantified even better and different denitrification-determining parameters can be identified.
Correlation of pyrite content and surface NMR relaxation time, as well as water content with depth
Source: BGR
Nitrate discharges
In addition to these activities, the nitrate discharge of groundwater into surface water such as the Steinhuder Meer in Lower Saxony will be researched. Depending on how nitrate concentrations develop, increased eutrophication rates must be expected. For this purpose, in addition to lance sampling in the beach area of the Steinhuder Meer, a permanent multilevel measuring point will also be installed in order to be able to repeatedly sample exactly the same points and thus draw conclusions about the small-scale changes in hydrochemistry.
Conclusion
The topic of denitrification is part of the BGR Strategy 2025+, in which the interactions between groundwater and surrounding rock is explicitly investigated in order to determine and evaluate spatial and temporal developments in groundwater constituents such as nitrate. Through the close networking between disciplines of groundwater, soil science and geophysics, the interdisciplinary approach of this BGR project will contribute to a global understanding of the nitrogen cycle. The knowledge acquired not only serves the federal states and water suppliers, but also aims to improve forecasting skills in order to be able to make statements about changes in nitrate concentrations in groundwater.
Literature:
Peer-reviewed papers
- COSTABEL, S., HILLER, T. & HOUBEN, G.J. (2023): Nuclear magnetic resonance at the laboratory and field scale as a tool for detecting redox fronts in aquifers. - Geophysics, 88(2): KS13-KS25. DOI: 10.1190/GEO2022-0127.1.
- HOUBEN, G.J., POST, V.E.A., GRÖGER-TRAMPE, J., PESCI, M. & SÜLTENFUß, J. (2021): On the propagation of reaction fronts in a sandy aquifer over 20+ years: lessons from a test site in northwestern Germany. - Water Resources Research, 57(8), e2020WR028706. DOI: 10.1029/2020WR028706.
- HOUBEN, G.J., SITNIKOVA, M.A. & POST, V.E.A. (2017): Terrestrial sedimentary pyrites as a potential source of trace metal release to groundwater – A case study from the Emsland, Germany. - Applied Geochemistry 76, 99-111. DOI: 10.1016/j.apgeochem.2016.11.019.
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