Widespread pumping has altered groundwater recharge sources in the Bengal Basin: isotopic evidence

Abstract:

Groundwater is a vital resource in the Bengal Basin, providing water to millions of people and sustaining dry-season farming. This groundwater resource is recharged by a combination of direct rainfall, inflow from large rivers (the Ganges and Brahmaputra), and infiltration from managed bodies of standing water (irrigated rice fields, ponds, and ephemeral rivers). In this work, we characterize how the relative quantities of different recharge sources have shifted across the Bengal Basin over millennia, but with a focus on perturbations to recharge in the last fifty years caused by groundwater extraction and construction of standing water bodies. We use dual isotopes of water and take advantage of the fact that standing bodies of water are evaporatively enriched compared to large Himalayan rivers and precipitation. We compiled isotope data from >3000 groundwater, rivers, ponds, rice fields and precipitation samples from the Basin. Precipitation isotopes closely followed the GMWL. Samples from a large river (Ganges) also fell on the meteoric water line. Groundwater δ¹⁸O varied with depth with shallow aquifers (0-75 m deep) having heavier values and exhibiting larger variation (mean ± s.d. = -4.1 ± 1.3‰) than deeper (>75 m deep) samples (-4.8 ± 1‰). The mean d-excess of shallow (5.3‰) and deep (8.1‰) aquifers were lower than the mean precipitation d-excess (9.9‰). We constructed an isotope mixing model to calculate the contribution of unevaporated sources (precipitation and Ganges) and evaporated ‘standing’ sources to groundwater recharge. The moving depth-average suggests that precipitation and river water were the dominant source of recharge for groundwater: Their combined contribution increasing from 65% in the upper 25 meters to 82% below 75 m depth. Recharge from standing water provides 35% of the groundwater in the upper 25 meters, twice as much as below 75 meters where it is 18%. Tritium active water in the Bengal Basin is present up to a depth of 90 m, providing evidence that the large contribution from evaporated standing water in shallow aquifers is modern. Our results suggest that human activities across in the Bengal Basin have shifted the source of groundwater to standing water and away from rainfall and river water.