HGS RESEARCH HIGHLIGHT – Transit-Time and Temperature Control the Spatial Patterns of Aerobic Respiration and Denitrification in the Riparian Zone
Nogueira, G. E. H., Schmidt, C., Brunner, P., Graeber, D., & Fleckenstein, J. H. (2021). Transit‐Time and Temperature Control the Spatial Patterns of Aerobic Respiration and Denitrification in the Riparian Zone. In Water Resources Research (Vol. 57, Issue 12). American Geophysical Union (AGU). https://doi.org/10.1029/2021wr030117
Figure 1: (a) Mesh of the model domain and applied BC's; the inset plot shows the region where well-to-well tracer-tests were performed; (b) Lateral view of model domain colored with hydrogeological units, and detail to layer thickness on the top cells near the stream.
At this time, HydroGeoSphere only accepts 0th or 1st order reaction-rates, which can not be linked to temperature variations. This makes it difficult to assess temperature-sensitive processes. Since most of bacteria activities and solute transformations facilitated by them are sensitive to temperature variations, the lack of temperature dependency is a limitation to HydroGeoSphere’s reactivate transport capabilities. Even more so at SW-GW interfaces where the infiltration of stream water can alter the ambient groundwater temperature and in turn affect such biotic processes. The paper highlighted this week introduces a novel method of implementing temperature-dependent reactions in a HydroGeoSphere solute transport model by pairing a Lagrangian flow path-reaction model to the results of a 2nd order Runge-Kutta particle tracking analysis.
One of the biggest advantages of the approach used in this paper for HGS users is the relatively straightforward simulation of temperature-dependent reactions during the transport (therefore reactive-transport) of solutes (in this case dissolved oxygen and nitrate) through individual groundwater flowpaths. While this implementation overlooks dispersion and diffusion – the authors expect they do not significantly influence the results at this scale and in the type of system under analysis.
In this study the authors have assessed the interplay of temperature and flow variations, as well as on the change of solute concentrations on aerobic respiration and denitrification in a riparian zone with the loosely-coupled flow and temperature-dependent reactive transport simulations. This Lagrangian approach used to solve the particle displacements, as well as the reactions is an advantage as it is much easier than solving all the processes in each cell of the domain. Moreover, at the scale of this analysis, this approach seemed to work very well as the authors could capture most of flow and solute dynamics in the riparian zone. As a side note: the transient model was calibrated based on stream discharge values, groundwater levels in different wells, as well as based on different tracer-tests' breakthrough curves, which further reduces the equifinality problem intrinsic to numerical models and provide extra information to the calibration process.
Figure 7: (a) DOz for different constant TGW and varying Q scenarios. The black crosses denote the resulting DOz volumes for the simulated period; (b and c) 3D views of DOz volumes for (b) High and for (c) Low) Q scenarios and two different TGW values. Note the vertical exaggeration of the 3D plots (20x).
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Abstract:
During the flow of stream water from losing reaches through aquifer sediments, aerobic and anaerobic respiration (denitrification) can deplete dissolved oxygen and nitrate (NO3−), impacting water quality in the floodplain and downstream gaining reaches. Such processes, which vary in time with short and long-term changes in stream flow and temperature, need to be assessed at the stream corridor scale to fully capture their effects on net turnover, but this has rarely been done. To address this gap, we combine a fully-integrated 3D transient numerical flow model with temperature-dependent reactive transport along advective subsurface flow paths to assess aerobic and anaerobic respiration dynamics at the stream corridor scale in a predominantly losing stream. Our results suggest that given carbon availability (as an electron donor), complete NO3− removal occurred further away from the stream after complete oxygen depletion and was relatively insensitive to variations in temperature and transit-times. Conversely, transit-times and oxygen concentrations constrained nitrate removal along short hyporheic flow paths. Even under limited carbon availability and low-temperatures, NO3− removal fractions (R-NO3) will be greater at locations further from the stream than along shorter hyporheic flow paths (R-NO3 = 0.4 and R-NO3 = 0.1, respectively). With increasing temperature, the relative effects of stream flow and solute concentrations on biogeochemical turnover and the redox zonation around the stream decreased. The study highlights the importance of seasonal variations of stream flow and temperature for water quality at the stream-corridor scale. It also provides an adaptive framework to assess and quantify reach-scale biogeochemical turnover around dynamic streams.
Plain Language Summary:
Nitrate pollution is a widespread problem in many catchments with intense agricultural activities. Denitrification is a redox process that removes nitrate from the aquatic system via its transformation to nitrogen gas. Denitrification is difficult to assess at larger scales since it depends on multiple factors, such as solute concentrations, temperature variations, and also the time that water resides in the subsurface, where reactions can take place. To evaluate how these factors can influence denitrification, we employed a coupled modeling approach representing the riparian zone of a 4th order stream in central Germany. We found that temperature variations strongly regulate the process and that during the winter the aerobic (oxygen rich) zone around the stream expands, which further inhibits denitrification in the near stream groundwater. However, even in the winter denitrification occurs, but at larger distance from the stream where oxygen has been depleted sufficiently. With increasing temperature, the influence of other factors on denitrification and on the redox zonation around the stream decreases. Coupled numerical models can provide further insights into the occurrence and interrelations of the multiple processes controlling water quality patterns in river corridors.