Security of Deep Groundwater in the Coastal Bengal Basin Revealed by Tracers. Issue 16 (19th August 2018)

• Record Type: Journal Article
• Authors: Lapworth, D. J.; Zahid, A.; Taylor, R. G.; Burgess, W. G.; Shamsudduha, M.; Ahmed, K. M.; Mukherjee, A.; Gooddy, D. C.; Chatterjee, D.; MacDonald, A. M.
• Is Part Of: Geophysical research letters. Volume 45:Issue 16(2018)
• Page Start: 8241
• Page End: 8252
• Publication Date: 2018-08-19
• Subjects: Indo‐Gengetic -- groundwater -- security -- tracers -- residence times -- deep groundwater; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL078640
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Uncertainty persists regarding the vulnerability of deep groundwater across Asia's megadeltas. In the coastal Bengal Basin aquifer system, shallow groundwater (<100 m) commonly features high salinity or arsenic concentrations, and deep, better‐quality, groundwater supplies drinking water to >80 million people. Here we report new radiocarbon evidence from a network of nine dedicated, multilevel monitoring wells, which indicates residence times of between 10 3 and 10 4 years for groundwater at depths >150 m. Modern groundwater detected in some deep abstraction wells using anthropogenic tracers (SF6, CFCs) is attributed to short circuiting of shallow groundwater within wells. Age‐depth profiles and hydrochemical data in monitoring wells confirm the regional resilience of deep groundwater to ingress of shallow contaminated groundwater. Our results are consistent with high regional anisotropy in the aquifer and support continued use of deep groundwater though the potential for leakage of shallow contaminated groundwater in deep abstraction wells requires careful monitoring. Plain Language Summary: In the Bengal Basin of Bangladesh and India where shallow groundwater (<100 m below the land surface) commonly features high salinity or elevated arsenic concentrations, deep groundwater provides an invaluable source of drinking water to over 80 million people. Pressures on this resource are growing with rapidly increasing urban populations. Understanding how deeper groundwater (>100 m) is replenished and how vulnerable it is to the ingress of more saline and arsenic‐rich groundwater from shallow and intermediate depths is critical to safeguarding public drinking water supplies. To date understanding of the vulnerability of deep potable groundwater here is largely based on modeling studies, with very few direct observations from samples taken from deep groundwaters. We assess the age and chemistry of deep groundwater in this region. Deep groundwater ages are between 1, 000 and 10, 000 years at monitoring sites and reflect long flow paths away from locations of modern replenishment. At a small proportion of wells, with a long history of pumping, there is evidence of a small amount of modern groundwater inflow to wells due to poor well seals but no evidence yet of salinity or arsenic ingress into the wider groundwater. Ongoing monitoring at deep sites needs to be prioritized in order to provide early warning of potential deterioration in the quality of deep groundwater. Key Points: Resilience of deep Bengal Aquifer System to ingress of contaminated shallow groundwater is revealed by multitracer study Detection of modern recharge at pumped sites indicates short circuiting of vertical leakage at selected public supply wells