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 ^226,228Ra 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, Ca²⁺ 3.6-120.8 mg L^-1, Mg²⁺ 2.2-264 mg L^-1, and SO₄^2- 0-195 mg L^-1. F^- varied from 0.07 to 3.18 mg L^-1 (two wells >1.5 mg L^-1), and NO₃^- 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-HCO₃ fields, while Gibbs diagrams indicate evaporation-dominated control; negative chlorine-alkaline indices (CAI <0) point to reverse ion exchange (Na⁺ ↔ Ca²⁺/Mg²⁺) against the micaceous-schist matrix, modulating the chemistry of the cations. Radiologically, ²²⁶Ra ranged from 30 to 930 mBq·L^-1 (≤∼1 Bq· L^-1 screening level), while for the ²²⁸Ra isotope, the concentrations for most samples were below the detection level. Only two samples showed ²²⁸Ra concentrations above Minimum Detectable Activity (55 mBq·L^-1). The dataset shows that climate forcing (evaporation) and not just water-rock interaction controls salinity and the co-mobilization of halides and trace metals in this semi-arid aquifer. Spatial analysis associated critical contamination points with uranium anomalies and fractured aquifers. The findings demonstrate that climatic forces (evaporation) prevail over water-rock interaction in controlling hydrochemistry, a critical paradigm for groundwater management in semi-arid regions. This work establishes a framework for groundwater management in mineralized semi-arid regions on a global scale, emphasizing the synergy between geogenic and climatic controls in water quality degradation.