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.