Objectives
The distribution of groundwater age in aquifers is strongly affected by facies heterogeneities. We propose to explore this relationship between heterogeneities and groundwater age in hypothetical fluvial aquifers built up by different meandering histories.
Methodology
We use the fluvial and stratigraphic modules of the Channel-Hillslope Integrated Landscape Development Model (CHILD) [1, 2] to simulate the evolution and deposits of a meandering river using a simplified physical model. Varying input parameters, especially aggradation or incision rate, lead to varying meandering histories. This results in 3D models with variable sand proportions. We assume constant porosity and permeability for sandy and muddy deposits. Then, we use PFLOTRAN [3] to simulate subsurface flow and groundwater age based on a flow direction parallel to the channel belt.
Results
In our models, sandy deposits from the rivers form the only permeable medium. Thus, the channel belt in each model stands out as a major preferential pathway for the groundwater flow, with lower ages than lateral floodplain deposits and the basement below. As they migrate within the channel belt, meanders rework previous deposits. As a result, the channel belt itself becomes heterogeneous, with varying sand proportions creating baffles. These baffles decrease the connectivity within the aquifer and increase groundwater age. Low aggradation rates and periods of incision favour such reworking, leading to more heterogeneous channel belts.
Conclusion
Varying river migration and aggradation or incision rates lead to heterogeneous channel belts with varying proportion of sandy deposits. If the channel belt itself funnels the flow, its heterogeneities lead to heterogeneous groundwater ages, even at short distances. This work paves the way for detailed sensitivity analyses linking parameters controlling river evolution to subsurface heterogeneities and groundwater age.