Novel finding of occurrence of geogenic nickel in the arsenic-enriched groundwater of the Himalayan Brahmaputra River Basin aquifers

Widespread occurrence of geogenic groundwater contaminants, such arsenic (As), poses a significant health risk to millions of people worldwide through ingestion of contaminated drinking water. However, the (co)occurrence of other, less studied contaminants that may be sourced from similar geological settings, are less documented. Here, for the first time, we document the (co)occurrence of excess concentrations of nickel (Ni) in the intensely As-enriched groundwater from the alluvial aquifers of the Brahmaputra River Basin (BRB). Approximately 30 % of the sampled groundwater (n = 70) exceeds the drinking water guideline value of 20 µg/L for Ni, while around 20 % of samples contain both Ni (>20 µg/L) and As (>10 µg/L). Furthermore, the groundwater also has elevated levels of Fe (up to 19 mg/L) and Mn (up to ∼ 5 mg/L). The groundwater is mildly oxidizing to strongly reducing (Eh 234 mV to −72 mV), with dominant hydrogeochemical facies ranging from Ca-Mg-HCO₃ to Na-HCO₃ type. Linear correlation shows that Ni has a strong positive correlation with Fe (r = 0.9), As (r = 0.84), and Mn (r = 0.68) in the northern bank of the Brahmaputra River, while having a less prominent relationship with Fe (r = 0.78), As (r = -0.35) and Mn (r = 0.46) in the southern bank. We hypothesize that Ni-rich rocks from the Indus-Tsangpo suture zone and the Tidding suture serve as the primary sources of geogenic Ni in the BRB, later weathered, transported and deposited by the Brahmaputra (Siang/Tsangpo) and Lohit rivers, forming extensive alluvial plains. We suggest that the reductive dissolution of Fe-oxyhydroxides is the primary mechanism of Ni and As release in the groundwater on the northern bank of BRB. At the same time, Fe and SO₄²⁻−reduction play an active role in the contaminant mobilization in the southern bank. The emergence of (co)occurring geogenic contaminants, such as Ni, increases the health risk for millions of residents in the study area, who are already exposed to excessive levels of As in groundwater-sourced drinking water at BRB.