Arsenic in groundwater in Paraíba do Sul river, Brazil: sources, hydrogeochemistry, and correlation with redox parameters

This study assessed the presence of As in wells located near the lower Paraíba do Sul river, southeastern Brazil. Water samples were collected from 19 home wells and wells managed by the utility company in the region in June 2016 and February, May, and August 2017. The metalloid was detected in seven wells (37%). The most abundant ions in the wells were Cl⁻, Na⁺, SO₄²⁻ and Ca²⁺ and the ions with the lowest concentrations were F⁻ and Br⁻. The pH values were approximately 7.0 and redox potential and dissolved O₂ values were typical of reducing environments. Levels of As varied between 0.13 μg L⁻¹ and 38.8 μg L⁻¹, which are above the maximum allowable concentration for human consumption according to Brazilian and international regulations (10 μg L⁻¹). The presence of As in wells correlated positively with levels of sulfate and dissolved organic carbon, and correlated negatively with well depth. The results support that the oxidation of sulfide minerals is the process that favors mobilization of As to the groundwater, due to inputs from erosion and rainwater seepage through the soil. No significant seasonal changes were observed in As levels, with the lowest and highest value observed in August (2.61 µg L⁻¹) and June (10.24 µg L⁻¹), respectively. Principal component analysis revealed positive correlation between As levels and nitrite, and the negative correlation between As levels and redox potential and dissolved oxygen. This suggests that arsenite, which is the most soluble and toxic reduced form As, is the predominant chemical species in wells.