Morphological controls on groundwater residence times of shallow aquifers

In shallow aquifers, including weathered zones characteristic of crystalline geologic basements, subsurface flows strongly depend on the geomorphological evolution of landscapes as well as on the geological heterogeneity structures. Yet, it remains largely unknown how geomorphology and geology shape the residence times in the aquifers and the transit times  in the receiving stream water bodies.

We investigate this issue with 3D synthetic models of free aquifers. Aquifer models represent hillslopes from the river to the catchment divide with constant slopes, evolving widths and depths. They are submitted to uniform and constant recharge. All flows end up in the river either through the aquifer or through the surface as return flows and saturation excess overland flows. Steady-state flows and transit times to the river are simulated with Modflow and Modpath (Niswonger et al., 2011; Pollock, 2016). The mean and standard deviation of the transit time distribution are systematically determined as functions of the hillslope shapes (convergent or divergent to the river, thinning or thickening to the river) and the ratio of recharge to hydraulic conductivity.

We show that the mean transit time distribution is a function of the geology through the volume of the aquifer divided by the recharge rate even in the presence of seepage areas. The standard deviation of the transit time distribution is a function of the geomorphology through the bulk organization of the groundwater body from the river to the catchment divide. Without seepage, the organization of the groundwater body is efficiently characterized by its barycenter. When seepage occurs, the standard deviation becomes also sensitive to the extent of the seepage zone.

We conclude that mean of the transit time distribution is primarily determined by geology through the accessible aquifer volume while the ratio of the standard deviation to the mean (coefficient of variation) is rather determined by geomorphology through the profile of the aquifer from the river to the catchment divide. We discuss how geophysical data might help to determine the groundwater body and assess the transit time distribution. We illustrate these findings on natural aquifers in the crystalline basements of Brittany-Normandy (France).

References

Niswonger, R.G., Panday, S., Ibaraki, M., 2011. MODFLOW-NWT, A Newton formulation for MODFLOW-2005.

Pollock, D.W., 2016. User guide for MODPATH Version 7—A particle-tracking model for MODFLOW (Report No. 2016–1086), Open-File Report. Reston, VA. https://doi.org/10.3133/ofr20161086