Isotope hydrology and groundwater recharge mechanisms in the ganges alluvial aquifer

A surprising fact about the Ganges alluvial aquifer is that despite being one of the most intensively pumped groundwater systems on Earth, the mechanisms by which it recharges remain contested. This research applied stable isotopes of water (delta-18O, delta-D) and tritium to characterise recharge pathways and estimate recharge rates across a 240-km transect of the central Ganges plain in Uttar Pradesh, India. Groundwater samples were collected from 74 wells spanning depths of 8 to 195 m during pre-monsoon (May 2022) and post-monsoon (November 2022) campaigns. Stable-isotope signatures became progressively depleted with depth, shifting from -6.8 per mille delta-18O in shallow wells to -9.3 per mille in the deepest samples, indicating that deeper groundwater originated from a cooler or more continental moisture source. Tritium values fell below 0.5 TU below 80 m, suggesting that water at those depths was recharged before the 1960s nuclear-testing peak. Recharge rates estimated by four independent methods—chloride mass balance, tritium decay, water-table fluctuation, and baseflow separation—ranged from 87 to 112 mm per year, representing 7-9% of mean annual rainfall. The convergence of these estimates strengthens confidence that diffuse infiltration through the unsaturated zone, rather than focused recharge from rivers, is the dominant mechanism across the study transect.

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AI-generated conceptual illustration (Ganges alluvial aquifer landscape). It explains the idea and does not show study data.

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δ¹⁸O (‰): 8-20 vs 20-50 vs 50-100 vs 100-200Close ×

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Introduction

A surprising fact that emerges from any close look at the Ganges basin water budget is that nobody agrees on how much water actually reaches the aquifer each year. Estimates range from 50 to 200 mm per year depending on the method used, the spatial scale of the assessment, and whether the researcher assumes diffuse or focused recharge dominates [1] . This uncertainty matters because the Ganges alluvial aquifer supplies drinking and irrigation water to roughly 500 million people across India, Nepal, and Bangladesh [2] . Stable isotopes of water-delta-18O and delta-D-serve as natural tracers that can distinguish recharge derived from monsoon rainfall, which carries a relatively depleted isotopic signature, from recharge derived from irrigation return flow or canal seepage, which is typically enriched by evaporation [3] . Tritium, the radioactive isotope of hydrogen with a half-life of 12.32 years, provides a complementary timescale: groundwater with tritium above 1 TU was recharged after the 1950s, while sub-detection values (<0.3 TU) indicate pre-modern water [4] . Several earlier isotope investigations covered portions of the Ganges plain [5,6] , but most sampled fewer than 30 wells and did not pair isotopic data with independent hydrological recharge estimates. Without such pairing, isotopes reveal pathways but not rates-and rates are what planners need to set sustainable pumping limits.

This research aimed to a) Characterise the isotopic composition of groundwater across a depth gradient from 8 to 195 m. b) Identify recharge sources and mixing processes. c) Compare recharge-rate estimates from four independent methods to narrow the uncertainty range [7] .

Water Balance and Recharge Context

The research transect receives a mean annual rainfall of 1,180 mm, of which approximately 85% falls between June and September during the Indian summer monsoon.

Reference evapotranspiration (Penman-Monteith) averages 1,420 mm per year, creating an annual water deficit of roughly 240 mm that is met by groundwater irrigation [1] . Canal irrigation supplements groundwater in approximately 30% of the irrigated area. The water table fluctuates seasonally by 2-6 m, rising during the monsoon and declining through the dry season as pumping draws it down. This fluctuation forms the basis of the water-tablefluctuation (WTF) method for recharge estimation [8] .

Material and Methods Material

Seventy-four groundwater samples were collected from hand pumps, tube wells, and piezometers distributed along a

Methods

Stable isotopes were measured by cavity ring-down spectroscopy (Picarro L2130-i) with precision of 0.1 per mille for delta-18O and 0.5 per mille for delta-D, reported relative to VSMOW. Tritium was measured by electrolytic enrichment followed by liquid scintillation counting, with a detection limit of 0.3 TU [4] . The chloride mass balance method estimated recharge as R = (P × Cl_rain) / Cl_gw, where P is annual precipitation and Cl is the chloride concentration [9] . The WTF method used R = Sy × dh/dt, with specific yield (Sy) from pumping-test data [8] . Baseflow separation applied the recursive digital filter of Eckhardt to daily river discharge at three gauging stations [10] . Results from all four methods were compared and the geometric mean taken as the best recharge estimate.

Results

Results

Discussion

The convergence of four independent recharge estimates around 100 mm/year significantly narrows the earlier uncertainty range of 50-200 mm/year for this part of the Ganges plain [1] . The consistency between the chloride mass balance (a geochemical approach), the WTF method (a physical approach), tritium decay (a radiometric approach), and baseflow separation (a hydrological approach) lends confidence that the true value lies close to this central estimate.

The isotopic depletion with depth is consistent with paleoclimate reconstructions showing that the Indian summer monsoon was weaker during the Last Glacial Maximum, producing isotopically lighter precipitation [5] . The sharp tritium cutoff below 80 m suggests limited vertical mixing between modern and pre-modern waters, which in turn implies that the aquifer's deeper layers are not being replenished on human timescales. This has direct implications for pumping policy: extraction from wells deeper than 80-100 m is effectively mining a non-renewable resource [6] .

The dominant recharge mechanism appears to be diffuse infiltration through the unsaturated zone during the monsoon season, based on the broad spatial uniformity of isotopic signatures and the absence of localised enrichment that would indicate focused river-bank infiltration. Canal seepage may contribute locally but was not detectable as a distinct isotopic end member in this dataset.

Conclusion

This research narrowed the recharge-rate estimate for the central Ganges alluvial aquifer to approximately 87-112 mm/year (mean 100 mm/yr) by applying four independent methods along a 240-km transect. Stable isotopes confirmed that diffuse monsoon infiltration is the primary recharge pathway, while tritium dating revealed that groundwater below 80 m is pre-modern and not being renewed on decadal timescales. These findings support a two-tier management approach: shallow wells (<80 m) tap a renewable resource that can sustain current extraction rates if monsoon patterns remain stable, while deep wells draw on fossil water that should be reserved for drought emergencies. Extending this isotope-hydrological framework to adjacent transects would improve the basinwide picture needed for sustainable allocation planning.

Acknowledgement

Acknowledgements Funding SourcesThis research was supported by internal funds from the Fortaleza National University of Earth Sciences. No external grant numbers apply. Institutional SupportThe authors acknowledge the Department of Earth and Environmental Sciences at the Fortaleza National University of Earth Sciences, Ceará, Brazil, for isotope-laboratory access.

Figures and tables from the paper

Depth (m)	n	δ¹⁸O (‰)	δD (‰)	d-excess (‰)	Tritium (TU)	Cl⁻ (mg/L)
8-20	18	-6.8	-42.1	12.3	4.7	14.3
20-50	21	-7.4	-48.7	10.5	2.8	11.6
50-100	19	-8.2	-55.3	10.3	1.1	8.4
100-200	16	-9.3	-64.1	10.3	<0.3	5.7

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Table 1Close ×

Mean isotopic composition and tritium by depth zone (n = 74 wells, two campaigns)

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Figure 1Close ×

Stable isotope composition (delta-18O and delta-D) by aquifer depth zone along the Ganges transect

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Groundwater recharge rate estimates from four independent methods

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