Integrated hydrochemical and radiological assessment of groundwater in the Seridó crystalline basement (Brazil): Evidence of geogenic influences on 226,228Ra and trace elements

To close a critical evidence gap in Brazil's semi-arid Northeast, where groundwater-quality data remain scarce, we established an integrated hydrochemical–radiological baseline for Parelhas (Seridó Belt, NE Brazil), an area with uranium potential in the Seridó region of Brazil that is susceptible to drought. The assessment focuses on physical-chemical and radiological parameters to determine the suitability of water for human consumption. This preliminary research presents analyses of nine wells sampled between March and August 2023. Major ions and trace metals were determined using AAS/ICP and anions by IC/UV–Vis. The pH, electrical conductivity, temperature, and total dissolved solids (TDS) were measured in situ. The activity concentrations of 226,228 Ra isotopes were measured with a Tri-Carb 5110 TR liquid scintillation counter (LSC). The results revealed neutral to strongly alkaline conditions (pH 6.9–10.1) and from fresh to highly saline (TDS 476–3632 mg L −1 ; 40 % brackish), driven by evaporative enrichment (Na-Cl facies; Gibbs ratio >0.7) and weathering of local lithologies (Jucurutu and Seridó Formations). Salinity covaried with geogenic metals and halides: Cl − 180–2021 mg L −1 , Na + 84–690 mg L −1 , K + 3.4–29.6 mg L −1 , Ca 2+ 3.6–120.8 mg L −1 , Mg 2+ 2.2–264 mg L −1 , and SO 4 2− 0–195 mg L −1 . F − varied from 0.07 to 3.18 mg L −1 (two wells >1.5 mg L −1 ), and NO 3 − from 2.3 to 275.5 mg L −1 (several wells >50 mg L −1 ). Peak trace-metal concentrations reached Mn 2.334 mg L −1 , Al 291 μg L −1 , Pb 31 μg L −1 , and Zn 215 μg L −1 — exceeded aesthetic limits, posing non-carcinogenic risks. Piper diagrams place the waters in the Na–Cl to Na–HCO 3 fields, while Gibbs diagrams indicate evaporation-dominated control; negative chlorine-alkaline indices (CAI