An elucidation on the distribution of arsenic safe aquifers: Introspection from the Gangetic basin

The obscurity in the distribution of arsenic safe aquifers poses critical challenges in devising mitigation strategies. This study attempts to bridge this knowledge gap by providing insights on the distribution of redox distinct sediments, hydrogeochemical characteristics and the plausible controls that govern As distribution in the safe and unsafe aquifers. In this study, a total of 75 drillings have been conducted in 3 study sites (2 in West Bengal and 1 in Bihar, India) across ~25 km² of area in each of the sites. The results revealed that in the case of North 24 Parganas (NP) in West Bengal, a continuous layer of brown-colored sand was observed at a depth between 40 and 60 m overlain by a grey-colored sand layer, whereas in the case of Nadia (ND) in West Bengal and Bhagalpur (BHG) in Bihar, the grey-colored sand layer was prominent. The brown-colored/brownish-grey-colored sand layer was fragmentary in both ND and BHG, with small lenses found in some parts of the study area. In the case of NP, the brown sand layer was protected by an aquitard layer. Groundwater chemical analysis revealed that the majority of the grey sand aquifers in both NP and ND yielded water with a high As concentration, while >80% of the wells installed in the brown sand layers exhibited a low As concentration (<10 µg/L). However, in the case of BHG, only 56% of the wells installed in brown or brownish-grey sand were As safe. Besides, 32% of the water samples from grey sand aquifers exhibited As safe concentrations in BHG, among which most of them had high Cl/Br, SO₄, and NO₃ concentrations. This states that the ingression of surficial contaminants may have suppressed the release of As concentrations due to the availability of other potential terminal electron acceptors at the BHG study site. Overall, the continuous brown sand layer observed in NP study site can be utilized as a suitable drinking water source however the intermittent layers as observed in ND and BHG study site may not serve as a potent source in terms of safe drinking water supply.