HGS RESEARCH HIGHLIGHT – Groundwater flow reversal between small water bodies and their adjoining aquifers: A numerical experiment
Steidl, J., Gliege, S., Semiromi, M. T., & Lischeid, G. (2023). Groundwater flow reversal between small water bodies and their adjoining aquifers: A numerical experiment. In Hydrological Processes (Vol. 37, Issue 5). Wiley. https://doi.org/10.1002/hyp.14890
“Golden et al. (2014) and Amado et al. (2018) assert, for instance, that [HydroGeoSphere] could provide a useful insight into the key hydrological processes involved in the land surface and the unsaturated and saturated domain, which control the hydrologic connectivity between potholes and the proximate shallow groundwater system.”
Fig. 2. Cross section along the vertical plane showing the model domain dimensions of the kettle hole between the adjacent overlying (upstream) and underlying (downstream) kettle holes with sediment layers, boundary conditions and model internal observation wells (A, B see Figure 1). Note different scaling of the x- and y-axes.
This recent study uses HydroGeoSphere to investigate groundwater-surface water interactions within “kettle holes” - post glacial landscape features prevalent across Northern Europe and Northern America. These kettle holes are quite similar to lakes, but their smaller size makes them prone to drying out, which results in very dynamic and variable groundwater-surface water interactions, sometimes being subject to groundwater flow reversal between a kettle hole and the adjacent aquifer. These kettle holes are then seen as very helpful indicators of the larger regional hydrologic system.
To investigate the intricate flow system between these kettle holes and the surrounding shallow aquifers, the researchers have developed a systematic approach to test the impact that changes to conceptual model elements (e.g. boundary conditions, soil parameterization, layer thickness) exerted on the overall flow system.
Using a 2-D cross sectional conceptual model, a total of 24 different model variations were tested, each of which factored in slight changes to the following model design components:
Changing the nature of lateral flow boundary conditions (constant vs variable defined head)
Thickness of the top sediment layer
Hydraulic conductivity of the lower aquitard
Hydraulic properties of the top sediment layer
The results of this research conclude that “kettle holes and shallow groundwater can be much more closely and dynamically linked than previously thought”, and that changes to the flow system can not be attributed only to season influences, with some groundwater flow reversals lasting for several years and extending more than 140m downstream of the kettle holes. In some cases (e.g. with very high evapotranspiration rates, and impact of heavy storm events) can cause repeated groundwater reversals, resulting in carbon, nutrients and pollutants turnover within the kettle holes, with significant impacts on shallow groundwater quality.
Fig. 8. Spatial distribution of the kettle hole water in the down-gradient aquifer at two different points in time and for the three selected model variants ddsS, ddsL and ddsU.
Abstract:
The countless kettle holes in the Late Pleistocene landscapes of Northern Europe are hotspots for biodiversity and biogeochemical processes. As a rule, they are hydraulically connected to the shallow groundwater system. The rapid, intensive turnover of carbon, nutrients and pollutants in the kettle holes therefore has a major impact on the quality of the shallow groundwater downstream. As a result of high-evapotranspiration rates from their riparian vegetation or strong storm events, the process of downstream groundwater flow may stagnate and reverse back towards the kettle hole, making interactions between the groundwater and kettle hole more complex. Furthermore, the highly heterogeneous soil landscape in the catchment contributes to this complexity. Therefore, the present study aims to enhance our understanding of this complicated interaction. To this end, 24 model variants were integrated into HydroGeoSphere, capturing a wide range of uncertainties in quantifying the extent and timing of groundwater flow reversal between a kettle hole and the adjacent aquifer. The findings revealed that the groundwater flow reversal lasted between 1 month and 19 years at most and occurred in a distance of more than 140 m downstream of the kettle hole. Our results demonstrated that the groundwater flow reversal arises especially often in areas where the shallow aquifer possesses low-hydraulic conductivity. There may also be a recurrent circulating flow between the groundwater and kettle hole, resulting in solute turnover within the kettle hole. This holds particularly true in dry periods with medium to low-water levels within the kettle hole and a negative water balance. However, shallow groundwater flow reversals are not necessarily a consequence of seasonal effects. In this respect, the properties of the local shallow aquifer by far outweigh the effect of the kettle hole location in the regional flow regime.
Fig. 5. Water level fluctuation within the kettle hole (middle) and in the up- and down-gradient fringe (left, right) for all model variants (box plots) in comparison to the bathymetry of the kettle hole (dashed lines).
Want to see an example of fully-integrated solute transport at the catchment scale? Check out this old research highlight: An exploration of coupled surface-subsurface solute transport in a fully integrated catchment model