Groundwater quality parameters in crystalline rocks in the south of Espírito Santo State, southeastern Brazil

Abstract

In hard rock terrains where gneisses and granitoids predominate, groundwater is generally considered to be of good quality due to the low solubility of the silicate minerals that form these rocks. However, in the long term, silicate minerals weathering can interfere in groundwater quality. In the case of fractured aquifers, these changes are less predictable due to the heterogeneous and anisotropic behavior of this aquifer type. The geological framework in the southern of the State of Espírito Santo is mainly formed by hard rocks, where the water quality parameters that serve to know the suitability for different types of use have not been studied. This work aimed to fill this gap by analyzing 62 groundwater samples collected in 38 wells that exploit the Crystalline Aquifer System in the Itapemirim River Catchment in the south of Espírito Santo. The hydrogen potential, electrical conductivity, total dissolved solids, turbidity, and concentrations of Ca, Mg, Na, Fe, Mn, Pb, Zn, Cd, and Co were analyzed. The concentrations of Cd, Co, Pb, and Zn were below the thresholds defined by the Brazilian standards for various uses of water. On the other hand, Fe, Mn, and Na, turbidity, and total dissolved solids were above the permitted limit for various types of water use. The dissolved solids and Na, as well as other groundwater main components, are controlled by geomorphological, lithological, and climatic factors, with increasing values to the central part of the basin. Differently, Fe, Mn, and turbidity show anomalous values randomly distributed in the area, probably due to pedogenetic enrichment of lateritic profiles and local structural control. It is possible that the lack of adequate maintenance of wells also contributes to such alterations.