The transboundary Amazonian Aquifer System extends over a vast territory of around 2.7x106 km2 in the western Amazon. Its major portion is located in Brazil, although comprising areas in Peru, Ecuador and Colombia.
The Amazon Aquifer System is composed of three major hidrogeologic subunits: the outcropping Alter do Chão and Solimões aquifers, and Tikuna Aquifer, a confined extension of Alter do Chão aquifer. From these subunits, the Tikuna Aquifer was the last one to be defined. The other two subunits have historically been used as water supply sources in the Amazon plain. Tikuna Aquifer has a limited water use, both due to its location, involving the great depths and remoteness of the western Amazon region, and its quality, that is predominantly composed by brackish water, saltwater or even brines, sometimes with a high temperature, except near recharge areas or in the farthest discharging areas.
Sampling campaigns were focused on aquifer recharge and discharge areas with relatively easy access, due to high field trip costs in the region as a consequence of remoteness and lack of transport means. Study areas comprised Ecuadorian provinces of Napo and Sucumbios, Peruvian Contamana city in Ucayali province, Serra do Divisor highlands in Acre State, Brazil, and the Eastern border of the aquifer system, in Iranduba, Manacapurú, Careiro, Manaquiri and Codajás cities, in Amazonas State, Brazil. The integration of hydrochemical data, groundwater head measurements, and other geological data, such as geophysics and stratigraphic data, obtained both during field campaigns, cooperation with research groups in Brazil, Ecuador and Peru and literature review, allowed developing the Tikuna Aquifer flow conceptual model and a regional 3D mathematical flow model considering a variable density water flow, using a modified version of the finite-element CODE_BRIGHT (COupledDEformation, BRIne, Gas and Heat Transport) code, originally developed by the Geotechnical Engineering Department at Politechnical University of Cataluña, Spain.
From the field work, some new Tikuna Aquifer discharging areas have been identified. These discharging areas have peculiarities, such as the discharge of very hot water, forming cascades and high temperature rivers in the Peruvian territory. Hydrochemical facies comprise HCO3(Cl)-Na-Ca waters with Cl-Na waters in salty water bodies. Isotopically, 14C age dating and 18O-2H results showed a well-behaved aquifer system in agreement with the conceptual model initially proposed by Rosário et al. (2016). One regional numerical flow model helped to identify the groundwater connection between Ecuadorian and Peruvian aquifer basins with the Brazilian Amazonian aquifers and its influence over the groundwater quality and flow pattern.