Depth-dependent groundwater response to coastal hydrodynamics in the tropical, Ganges river mega-delta front (the Sundarbans): Impact of hydraulic connectivity on drinking water vulnerability

With increasing population and urbanization in the deltas across global mega-river basins, groundwater stress is increasing due to changing ocean-land disposition, hydroclimatic extremes, over-exploitation and natural/anthropogenic contamination. Such change in coastal hydrodynamic process makes an impact in to the resilience of groundwater-sourced drinking water vulnerability. The groundwater responses studied in this work in the ecologically-vulnerable, UNESCO World Heritage site of the Sundarbans delta-front aquifers at the river Ganges-Brahmaputra mega delta, demonstrate a unique example of such intricacies and complexities. High-resolution, temporal, depth-dependent hydraulic connectivity and hydrochemistry was studied for this complex, climate driven, tidal-induced hydrological interactions between surface water (seawater of Bay of Bengal [BoB] and Ganges river water) with shallow and deep groundwater by stable isotopic (delta O-18 and delta D) signatures and piezometry. A wide range of salinity content and delta O-18 composition of groundwater samples for 14-25 (salinity: 3-45 ppT, delta O-18: -3.63 to -1.15 parts per thousand) and 30-50 meter below ground level (mbgl) (salinity: 5-25 ppT, delta O-18: -3.41 to -1.34 parts per thousand) was recorded, while, comparatively, a narrow range of temporal variability was observed for the depths of 115 (salinity: 1-4 ppT, delta O-18: -5.04 to -2.15 parts per thousand) and 333mbgl (salinity: 1 to 3 ppT, delta O-18: -4.43 to -2.81 parts per thousand). Study results revealed that vertical transport of solute has been induced by coastal hydrodynamics. However, human interferences, in terms of groundwater abstraction and land use-land cover change (e.g. brackish aquaculture) do have substantial impact on evolving groundwater response, thereby impacting resilience to drinking water option in this densely populated area. Fluctuations of groundwater level in relation with delta O-18(gw), composition showed shallow depths (14-50 mbgl) are hydraulically connected by local flow and conducive of chemical exchanges. The deeper groundwater (115-330 mbgl depths) flow suggests regional-scale hydraulic connections along flow paths. In-spite of sub-meter scale seasonal groundwater level changes, salinity variability is observed between 1 and 4 ppT at > 100 mbgl depths, suggest mixing between chemically-distinct water reservoirs, conserving hydraulic mass-balance. However, such observation suggests severe implications for groundwater vulnerability, which might aggravate with future intensification of irrigational abstraction, as well as with changing hydroclimatological regime and extreme events. Thus, future planning and management strategies of sustainable supply of safe water in such ecologically-sensitive groundwater systems need to incorporate such dynamic chemical evolution.