Flow patterns and aquifer recharge controls under Amazon rainforest influence: The case of the Alter do Chão aquifer system

The present study clarifies the role of rainforests in the recharge processes of the Alter do Chão Aquifer System in the outcropping region based on a dataset collected in reference areas. Different methods and techniques commonly applied to hydrogeology studies were integrated to achieve this objective, including soil profile description (in field and laboratory), potentiometric analysis, hydrochemistry data, and stable isotopic evaluation. The Alter do Chão Aquifer System occurs as an unconfined and semiconfined groundwater reservoir in the Amazonian plateau regions, with the following profile sequence: topsoil (higher porosity and hydraulic conductivity), yellow clay (mean porosity of 18% with small pore throats, resulting in lower hydraulic conductivity), aluminous iron crusts, and solid bauxite (highest hydraulic conductivity values resulting from porosity values from 30 to 45%), and at the bottom, the mottled stratified argillite (acting as an aquitard layer, with leaky confining attributes). The relief pattern with high plateau areas cut by shallow and deeper valleys allows regional and local hydrogeological flow systems. The dense forest cover acts as a vital recharge control due to bioturbation by the roots, observed up to 15 m deep. This bioturbation causes preferential flow zones and is responsible for increasing the hydraulic conductivity in the pelitic layers. Oxygen and deuterium stable isotope data demonstrate that recharge is due to rainwater infiltration because the local groundwater line is plotted next to or over the regional meteoric water line. Tritium data indicate that the aquifer is supplied by relatively recent rainfall or at least by mixtures containing recent rainwater. The conceptual flow model for the semiconfined and unconfined portions of the Alter do Chão Aquifer System is represented by the following processes: infiltration of large volumes of rainwater, high loss by forestry evapotranspiration, loss by interflow at the interfaces of contrasting hydraulic conductivity layers, and local loss due to regional lateral flow to the discharge areas. Considering a 1 km2 polygon, it is possible to determine a recharge amount of at least 236,400 and 350,000 m3/year for the semiconfined and unconfined portions of the aquifer, respectively. •Study of aquifer recharge and circulation patterns by hydrochemistry, piezometry and isotopic data in the Amazonian.•Determination of groundwater recharge in semiconfined and unconfined portions of