Is the ‘Khulna Experience' a Useful Indication of the Sustainability of Groundwater Abstraction from Deep Aquifers in Arsenic-Affected Southern Bangladesh?

Deep aquifers (generally considered to be > 150 m) in arsenic-affected southern Bangladesh are recognised as potentially valuable sources of arsenic-safe water, but questions arise concerning the sustainability of long-term groundwater abstraction, in particular the vulnerability of the deep aquifers to vertical leakage from the arsenic-bearing shallow groundwater system. Khulna, a coastal city in southern Bangladesh, has derived its water supply over the past three decades from boreholes to the deeper aquifer, without inducing leakage of arsenic from shallow levels. The success of deep groundwater abstraction at Khulna has encouraged the view that deep aquifer development may be sustainable more widely across southern Bangladesh. We compare the groundwater system of Khulna with that of Kachua, a sub-district of Chandpur in southeast Bangladesh where abstraction from deeper aquifers has become widespread over the past few years. The comparison is based on hydrostratigraphical analysis, augmented by hydrochemical and isotopic profiles. Hydrostratigraphical analysis indicates potentially confining layers bounding a multiple aquifer system in the Khulna area, while at Kachua the sediments are dominantly sandy to depths of at least 300 m. Hydrochemical conditions are reducing in the deep aquifers, with more pronounced reduction at Khulna. Major-ion chemistry in both regions is complicated by salinity invasion; deep groundwater at Khulna is NaHCO₃-NaCl type while at Kachua NaCl type water dominates. Radiocarbon (¹⁴C) activity is lower in the deeper groundwater at Khulna (7-12 pMC) than at Kachua (22-36 pMC). This implies older groundwater in the Khulna deep aquifers despite the long history of groundwater abstraction there. Our observations suggest that the deeper aquifers of Kachua are more vulnerable to vertical leakage than equivalent aquifers at Khulna, and groundwater abstraction from the deeper aquifers more widely across southern Bangladesh may be more vulnerable to vertical leakage, and hence less sustainable, than experience at Khulna initially suggests.