Fault mapping and characterization of a coastal aquifer related to a mangrove ecosystem, using electrical resistivity tomography (ERT), ground penetrating radar (GPR) and hydrochemical data: The case of the Mangue de Pedra Aquifer, Armação dos Búzios, Brazil

Abstract

The Mangue de Pedra is a rare and fragile ecosystem. It stands above a sandstone-conglomerate substratum of the Neogene Barreiras Formation, which also hosts the local coastal aquifer, known as the Mangue de Pedra Aquifer. The Pai Vitório Fault, a regional structure, which represents the south border of the São João da Barra Graben, separates the Paleoproterozoic crystalline basement from these sediments and limits the occurrence of both the mangrove and aquifer. Aside from hydrochemical data, geophysical surveys, including electrical resistivity tomography (ERT) and ground penetrating radar (GPR), allowed to investigate the local substrate and the role of fault structures affecting the aquifer-mangrove ecosystem, as well as to provide a better understanding on groundwater circulation and effects of seawater interface dynamics. The hydrochemical data for 11 water samples were used for interpolation to generate pH, Eh and electrical conductivity (EC) maps, and the results revealed a zone of less saline water flow and a discharge zone at the cliff's base. The survey of five ERT lines, using the dipole-dipole configuration, provided a picture of the saturated zone of the aquifer. The interpreted sedimentary units have been divided in three different layers, based on water content and variation of the water quality: 1) unsaturated zone; 2) good/intermediate-quality freshwater (34.4–137 Ω-m); 3) brackish/poor-quality freshwater (4.46–34.4 Ω-m). This method indicated the presence of at least five normal faults, including major and shallow structures along the mangrove, and the basement 5 m below sea level. The radargrams also confirmed the presence of most of these faults, recognized as complex fault zones at shallow levels. The interpretation on orientation of these structures was then enhanced by geoprocessing techniques, indicating a major NE-SW trend. This set of faults provides some of the requirements for the mangrove existence, including: a) the shape and materials of the beach and surrounding landscape, composed by chalcedony breccias blocks which are resistant to erosion, protecting the local environment from impact of sea waves; b) pathways for groundwater flow; and c) traps or barriers within the aquifer, which channel less saline waters to that specific site. This set of factors creates a unique ecosystem, which normally does not occur in beaches from open sea shorelines.