Avaliação do potencial para a produção de água subterrânea nas formações Serra dos Martins e Campos Novos : um estudo geofísico-geológico

Abstract

The present study assessed the feasibility of groundwater production from the Serra dos Martins Formation - FSM (Occurrences 1 and 2) and the Campos Novos/Macau Basalt Formation – FCN/BM (Occurrence 3), using geophysical surveys, petrophysical tests, and physicochemical analyses. The geophysical investigation revealed that Occurrences 1 and 2 exhibited a resistive superficial layer up to 30 m deep (unsaturated zone), a conductive layer between 30 and 80 m (water-saturated zone), and a highly resistive layer from 80 m onward (crystalline basement). The petrophysical tests indicated average porosities of 20.3% (Occurrence 1) and 8.1% (Occurrence 2). The FSM, composed of sandstones underlying a superficial lateritic crust, showed greater potential for groundwater production, with estimated volumes of 65.3 million m³ for Occurrence 1 and 521 million m³ for Occurrence 2. However, these waters require pre-treatment for human consumption due to levels of pH, color, total hardness, sodium, aluminum, total iron, sulfate, ammonia, chloride, and TDS exceeding the limits established by Ordinance 888/2021. As for FCN/BM (Occurrence 3), composed of bentonitic clays (FCN) and basalts (BM), the geophysical survey indicated a superficial conductive layer ranging from 30 to 90 m in depth, overlying a layer of altered rock between 27 and 59 m. The presence of crystalline basement was observed from 59 m onward. The petrophysical tests revealed primary porosities ranging from 4% to 16% in the altered gneisses and from 0.7% to 1.4% in the basalts. The altered gneisses, originating from the Serrinha-Pedro Velho Complex underlying the FCN/BM, were identified as containing the aquifer. For Occurrence 3, an estimated groundwater volume of 59 million m³ was determined. Most of the wells from which water samples were collected were located in the Serrinha-Pedro Velho Complex. The physicochemical results indicated that the water extracted from these wells had concentrations exceeding potable water standards for turbidity, color, total hardness, sodium, iron, chloride, ammonia, and TDS, thus requiring pre-treatment for human consumption. Salinity levels classified these waters as fresh, with pH values close to neutrality.