Isotope and hydrochemical systematics of groundwater from a multi-tiered aquifer in the central parts of Indo-Gangetic Plains, India – Implications for groundwater sustainability and security

Highlights: • A discernable contrast in groundwater sources and circulations is found up to a depth of 300 m bgl • Top aquitard groundwater is brackish and recharged by vertical percolation • Aquifer-I holds modern groundwater and dominant recharge is through lateral flows & paleochannels • Aquifer-II groundwater is fresh, less dynamic and mainly recharged from exposed zones in marginal alluvial areas • Aquifer-III holds fresh groundwater and was recharged during an arid climate (3.5–4.7 ka BP) The Indo-Gangetic multi-aquifer system provides water supplies to the most populous regions of the Indian subcontinent, however precise knowledge on the sources and dynamics of groundwater is still missing. Environmental isotopes (²H, ¹⁸O, ¹³C, ³H and ¹⁴C) and hydrochemical modeling tools were used in this study in the multi-tiered aquifers underlying the Middle Gangetic Plains (MGP) to investigate the source of recharge, aquifer dynamics and inter-connectivity among aquifers. Within a depth span of 300 m, three aquifers, with contrasting recharge sources and dynamics, were delineated in this Sone-Ganga-Punpun interfluve region, with limited cross-aquifer hydraulic interconnections. The chemistry evolves from Ca-HCO₃ to Na-Ca-HCO₃ in the shallow semiconfined Aquifer-I with a mean transit time of 20–23 years. The dominant recharge to Aquifer-I is from the river inflows and rainwater percolation through paleochannels. The semi-confined to confined Aquifer-II holds fresh quality groundwater with mixed water facies (Mg/Ca-Na-HCO₃). The modeled age of Aquifer-II groundwater is found to be 205–520 years, which is supported by presence of negligible tritium and minor variations in stable isotopes. Outcrop regions of Aquifer-II sediments in the marginal alluvial areas and deep-seated paleochannels in the southwestern part are the potential zones for Aquifer-II recharge. A deep confined Aquifer-III with fresh quality of groundwater is identified below 220 m. This aquifer is characterized by old age (~3.5 to 4.7 ka BP) and enriched δ¹⁸O (−5.7‰). These results along with the existing paleoclimate records of this region infer that Aquifer-III is recharged during an arid climate. The marginal alluvial plains are the probable recharge zones for Aquifer-III. This study helped in conceptualizing the groundwater flow paths in multi-tiered aquifers of MGP. The knowledge and understanding would extend crucial inputs for the sustainable development of deep aquifers not only in the MGP but also in other regions of Indo-Gangetic Plains.