Alter do Chão aquifer system west of Manaus city, Amazonas State, Brazil: hydrochemical processes, origin of salinity and relations with adjacent aquifers

Abstract

The aquifer associated with the Alter do Chão Formation (ACF) — Alter do Chão Aquifer subsystem (ACA) — is the best known in the Amazon region. Recent studies have indicated its possible connection with the Cretaceous Tikuna Subsystem, which is continuous throughout the Amazon basins, both parts of the great Amazon Aquifer System (AAS). The aim of the article was to generate new insights on the ACA functioning, in the west of Manaus. The methodology included measurements of water level and in situ physical-chemical parameters in wells, along with groundwater sampling for chemical and isotopic analysis. Geological characteristics and aquifer boundaries were estimated with data from existing oil and water wells. Results show that ACA waters in western Manaus have two main origins: 1) meteoric origin, with poorly mineralized waters, situated in the upper aquifer layers in an unconfined system with local recharge; 2) remote origin, more mineralized, with longer residence time in the aquifer, stored in the deeper layers, probably in a confined system. Its origin is presumably in the recharging areas on the AAS western edge. Calibrated isotope analyzes suggest an average age of 21,380 years for more mineralized groundwater occurring in deeper wells from Manacapuru and Careiro municipalities. This water is probably the result of a mixture of older waters with more recent recharge waters. Due to the hydrogeochemical water variation, correlated with local geology, Alter do Chão Aquifer System was divided in an upper unconfined and less mineralized portion, and another older and mineralized lower portion, both mixed eastwards, with a progressive dilution by rainfall.