Hydrological behavior of distinct types of ferricretes (“cangas”) of the southeast region of the iron quadrangle, Minas Gerais, Brazil

Abstract

Ferricretes (“cangas”) in southeastern Brazil are hardened iron-rich layers that usually cover the itabirites of the Cauê Formation of the Iron Quadrangle – a world-class mineral province well-known for the intense iron-ore mining activities. Despite the significant porosity of ferricretes, there is still some controversy regarding the relationships between such parameter and hydraulic conductivity. Thus, the role of ferricretes in the recharge of underlying aquifers can only be assessed on the basis of hydrological studies. The Cauê Formation is a relevant groundwater resource of the Iron Quadrangle and the removal of ferricretes by mining activities may considerably affect the replenishment of aquifers and consequently water supply. In order to estimate the hydraulic conductivity of the Cauê Formation ferricretes, a topographic ramp located in the southeast portion of the Iron Quadrangle was selected for the physical and hydraulic characterization of two of the most common types of ferricretes – namely structured and detrital ferricretes. Ferricrete samples were selected for mineralogical, microstructural, chemical characterization and quantification of the apparent porosity in the laboratory. Infiltration tests were carried out in the field using sprinkle, double ring and mini-disk infiltrometers. The comparison of these results with other previous studies shows that the porosity and the hydraulic conductivity of ferricretes are variable, depending on their mineralogical, textural and structural characteristics. The apparent porosity and hydraulic conductivity of the ferricretes of this study are equivalent to those of silty sand or silt (10−5 to 10−7 m/s), with water predominantly flowing across pores larger than 1 mm in diameter. The monitoring of the infiltration and percolation of a NaCl tracer solution through the ferricretes via multitemporal electrical resistivity sections obtained using dipole-dipole array configurations indicated the predominance of horizontal flows that are controlled by the transition horizon. Interferences in the flow patterns were caused by a meter-sized cavity present in the transition horizon. Karst-like features are common at greater depths and, as previous studies have already indicated, karstification in this medium is strongly conditioned by geomorphology. Therefore, the recharge of the underlying aquifers tends to be significant but variable in space and time.