AUTHORS: OLIVER S. SCHILLING, DYLAN J. IRVINE, HARRIE-JAN HENDRICKS FRANSSEN, AND PHILIP BRUNNER
The presence of unsaturated zones at the river-aquifer interface has large implications on numerous hydraulic and chemical processes. However, the hydrological and geological controls that influence the development of unsaturated zones have so far only been analyzed with simplified conceptualizations of flow processes, or homogeneous conceptualizations of the hydraulic conductivity in either the aquifer or the riverbed. We systematically investigated the influence of heterogeneous structures in both the riverbed and the aquifer on the development of unsaturated zones. The three fundamentally different states of connection resulting from the different degrees of saturation underneath the riverbed are conceptually illustrated below, including examples of the probability distributions of hydraulic conductivity of the riverbed and the aquifer that may lead to these states of connection (the saturated parts of the aquifer underneath the riverbed are illustrated in blue):
The investigations were based on a large number of numerical flow experiments using HydroGeoSphere. One of the goals of the study was to develop a simple method to predict the spatial extent of the unsaturated zone underneath a riverbed for heterogeneous river-aquifer systems, without the need to undertake complex numerical simulations. For this purpose, simulations of the following degrees of complexity were carried out:
Based on the results of the numerical simulations with HydroGeoSphere, a stochastic 1-D criterion that takes both riverbed and aquifer heterogeneity into account was developed using a Monte Carlo sampling technique. The approach allows the reliable estimation of the upper bound of the spatial extent of unsaturated areas underneath a riverbed. Through systematic numerical modeling experiments, we furthermore show that horizontal capillary forces can reduce the spatial extent of unsaturated zones under clogged areas. An example of these simualtions is provided below:
This analysis shows how the spatial structure of clogging layers and aquifers influence the propensity for unsaturated zones to develop: In riverbeds where clogged areas are made up of many small, spatially disconnected patches with a diameter in the order of 1 m, unsaturated areas are less likely to develop compared to riverbeds where large clogged areas exist adjacent to unclogged areas. A combination of the stochastic 1-D criterion with an analysis of the spatial structure of the clogging layers and the potential for resaturation can help develop an appropriate conceptual model and inform the choice of a suitable numerical simulator for river-aquifer systems.