Geological setting of the Southwestern Boa Vista Aquifer, State of Roraima, Brazil, from shallow vertical electric sounding

Abstract

Twenty vertical electrical sounding profiles were executed during the dry season in the Paricana campus of the Federal University of Roraima. Three geoelectrical units were identified: (a) the lower resistivity unit with values ρ_a< 1,000 Ω.m attributed to mudstone layers; (b) the middle resistivity unit with values 1,000 Ω.m < ρ_a < 20,000 Ω.m attributed to sandstone layers; and (c) higher resistivity unit with values ρ_a > 50,000 Ω.m attributed to lateritic ferruginous concretions and/or ferruginous sandstones. From these three units, seven geoeletrictal patterns can be seen. Patterns 1 and 3 (located in the SW and WNW of the study area) configure the best permoporosity zone in the area because the pore-bearing lateritic concretions are neighboured by paleolakes and actual seasonal lakes. In addition, these patterns 1 and 3 display an extremely sand-grained character. Patterns 2, 4 and 6 (NW, mid-eastern, N and NE) present moderate permoporosity because clay-bearing materials are interpreted as reaching to 10 m depth. Patterns 5 and 7 are attributed to a predominance of clay-bearing material in such a way that it acts as a sealant material to the aquifer protecting it from contamination from the surface.