3.3 Revealing the integration of soil-aquifer in wetland combining UAV-ERT tools
Based on a joint analysis of the surface geoprocessing and on our understanding of the groundwater dynamics, we were able to ascertain the occurrence of a lateral water flow from compartment 1 that is, theoretically, responsible for prolonged flooding in compartment 3. Hillslope with saturated flow dominates lateral flow and reflects the relative contribution to the lateral hydraulic potential gradient of the relief of the lower-most impermeable boundary (Harman & Kim, 2019). During wet periods, for instance, when water tables are close to the surface, shallow subsurface flow can occur (Brannen, Spence, & Ireson, 2015) due to the formation of an effective transmission zone (van der Kamp & Hayashi, 2009) (Figure 10).
In the field, we observed that compartment 3 remains flooded for the longest period. This observation, in addition to the analysis of the electrical resistivity tomography, justifies the hypothesis that direct aquifer recharge occurs in this compartment.
In the area, the soil water has been supplied by rainfall concentrated during the summer seasons (October-March). In the end of that period, the water level remains over the land surface conducting to long-term soil saturation characterizing a typical wetlands hydrology. That conditions are accept when the water table is within 30 cm of soil surface or above the surface for period of 14 days or more in a year (Vepraskas and Lindbo, 2012). The studied wetland is not connected with any river by surficial channel, imposing the vertical flows towards the shallow aquifer. That dynamic characterizes the hydrology in the topographic depression where surface (water that enter in soil by rain) and subsurface (shallow aquifer) waters are mixed. The soils in catena allow the direct recharge of aquifer although the occurrence of subsurface massive layers with higher bulk density could diminish the velocity of vertical flow. That layer has light grey color, in the ERT sections, due to iron-depletion and is continuous in the entire extension of the catena.
Combining soil morphology and ERT images made it possible to propose a broader hydrologic interpretation on wetland on the plateau. On the plateau, the topographic depression that characterize wetlands with aquic soils are zones of aquifer recharge characterized by preferential downward movement of water from periodically unsaturated soil surface to saturated groundwater. The vertical flows occurred by the opening of leakage point in the center of the depression (compartment 3) which was formed by geochemical erosion, particularly the iron-depletion from soil matrices. The contemporary hydrology in the wetland is a result of soil landscape changes closed linked to its past geological history. The flat plateau is a remnant of South American Surface and the landform was preserved by the presence of continuous ferricrete. Regional environmental changes to more humid climatic conditions (Salgado-Laboriau et al., 1998) enhanced the leaching of iron from the oxidized phase in the ferricrete opening soft zones that act as preferential flows path to water. The chemical process (deferrugination) is followed by loss of pedality resulting in topographic lowering (Lucas and Chauvel, (1992), Antonellini et al., 2019). The topographic depression developed in flat surfaces retains water inducing to endosaturation of soils (Vepraskas and Lindbo, 2012), which induces the rejuvenation of the old lateritic materials. Ferricrete developed in lateritic soils is an impermeable layer that can retains rainwater over in a pershed watertable on the highest positions of the landscape (Tardy, 1993). The excess of water in top soil intensify ferricrete dissolution opening permeable points through which the soil water percolates deep in the profile reaching the shallow aquifer characterizing a recharge zone by leaching and redox depletions (Evans and Freeland, 2000). That soil-water process corresponds to the contemporaneously hydrology in the wetland developed in the plateau.