Hydrogeology of the Western Amazon Aquifer System (WAAS)

•A new hydrostratigraphic approach is provided for the Amazon Aquifer System, defining its aquifer components and boundaries.•The area comprised by the Amazon Aquifer System highlights it as one of the world’s greatest transboundary aquifer systems.•A new hydrostratigraphic unit, the Tikuna Aquifer System, is proposed to the Amazon Aquifer System.•The existence of Tikuna Aquifer System modifies the belief about Amazon Aquifer System as an exclusive fresh water reservoir.

The Western Amazon Aquifer System (WAAS), as defined and proposed in the present work, encompasses an area of about 2.0·106 km2 located in the northwestern portion of South America. Published and unpublished data were used to define WAAS boundaries and main hydrogeologic characteristics. Petroleum industry data, environmental data, and other diverse thematic data were compiled for this study according to the data's origin. The analysis, treatment and integration of available data allowed us to define the WAAS as a multilayered aquifer system comprised of the Tertiary Solimões Aquifer System (SAS) and the Cretaceous Tikuna Aquifer System (TAS). The thick clay-rich basal strata of the SAS appear to confine the TAS. The SAS is widely used for both domestic and industrial purposes, providing good quality freshwater. The TAS has varying water quality: it contains freshwater near its recharge areas in the Sub-Andean fault belt zone, brackish to brine water in the Sub-Andean basins, and salty water in the Solimões Basin (Brazil). The interpretation and conclusions provided by an increasing understanding of the area's hydrogeology resulting from this work made it possible to propose an improved and new WAAS regional hydrogeologic conceptual model with data and descriptions not previously available. Some surprising results have been later confirmed as true by looking at unpublished reports, logs and field notes. Therefore, this work resulted in new findings and settled the basis for future works, especially for the poorly understood TAS.

Graphical abstractThe graphical abstract shown above synthesizes the main achievements of this work.•Determined the WAAS is a multi-layer trans-boundary aquifer system, formed by Cretaceous and Tertiary sedimentary sequences;•Delineated the hydrogeologic boundaries of the WAAS;•Described a new hydrostratigraphic WAAS component, the TAS;•Defined the SAS basal confining unit;•Determined the WAAS regional flow direction;•Determined groundwater in the SAS flows regionally from west to east, controlled by the topography and by the hydrologic network.Figure optionsDownload full-size imageDownload as PowerPoint slide