Hidrogeoquímica da porção oeste do aquífero Alter do Chão na zona de interface com o aquífero Içá-Solimões, região metropolitana de Manaus e adjacências

Abstract

The Amazon region presents large availability of surface water and groundwater, however surface resources are often inadequate for human consumption. Hence, a growing groundwater exploitation throughout the region occurs. Albeit the expanding demand, scientific knowledge on Amazonian aquifers is still limited. Generating new data and integrated analysis of hydrogeological knowledge on the region is, therefore, becoming increasingly necessary. The aquifer associated with the Alter do Chão Formation - Alter do Chão Aquifer (ACA) - is largely discussed in the Amazon region due to its importance to water supply. Recent studies have indicated its possible continuity throughout the Amazon basin, as part of the great Amazon Aquifer System (AAS). The aim of the work is, therefore, to generate new insights on the ACA functioning with new hydrogeochemical and hydrogeological data in the important interface of ACA with Içá and Solimões Aquifer Subsystems, located west of Manaus. In two field campaigns, measurements of water levels and in situ physical-chemical parameters were carried out, along with groundwater sampling in wells, Water analyses comprised principal cations and anions, dissolved metals, nitrate, phosphate, fluoride and bromide, as well as isotopes (180, 2H, 13C, 14C, 36CI). A survey of the geological characteristics and aquifer boundaries was also made, based on data from existing oil wells, water wells and piezometers. Data were analyzed with usual interpretive tools (diagrams, graphs) and geochemical calculations (Phreeqcl code). Results showed ACA water west of Manaus has apparently two main origins: waters of meteoric origin, poorly mineralized, located in the upper aquifer layers, in an unconfined system, with direct rainfall recharge; and waters of remote origin, more mineralized, with longer residence times in the aquifer, stored in the deeper layers, probably in a confined system. Isotope analyzes suggest an average age of 20.38 thousand years for more mineralized groundwater occurring in deeper wells in Manacapuru and Careiro municipalities. The exercise performed with Phreeqc/ pointed to the geochemical feasibility of this hypothesis. Because of this hydrogeochemical differentiation, correlated with local geology, the waters of Alter do Chão Aquifer System are subdivided in an upper unconfined and less mineralized portion, and another older and mineralized lower portion, both mixed westwards of Purus Arc, with a progressive dilution by rainfall