Deep fresh groundwater in offshore islands of Bangladesh and its implications for climate-resilient groundwater management in coastal aquifers
摘要
Offshore fresh groundwater can be considered a climate-resilient freshwater resource contributing to reducing the risk of seawater intrusion into terrestrial coastal aquifers. In coastal Bangladesh, potable groundwater exists in aquifers deeper than 150 m below ground level (bgl) overlain by brackish groundwater. Climate-induced sea-level rise and associated landward migration of the seawater–freshwater interface is a major concern in coastal Bangladesh because of its low-lying topography. This study analyzed the coastal and offshore groundwater system by examining groundwater geochemistry of 101 wells from the coastal mainland and multiple islands located tens of kilometers offshore. Results indicate that the majority of deep (≥ 150 m bgl) groundwater samples from the mainland and offshore islands have the same geochemical composition, with the hydrogeochemical facies characterized as Na–HCO3 type water. In contrast, the shallow (< 150 m bgl) aquifer features a diverse array of groundwater facies due to intricate hydrogeochemical processes influenced by contemporary factors. Principal component analysis underscores the homogeneity of deep groundwater samples and the heterogeneity of shallow groundwater samples in terms of hydrogeochemical composition. Collectively, these analyses affirm the uniformity of hydrogeochemical composition in deep groundwater samples across the mainland and islands, characterizing them as freshwater. This suggests that the offshore deep freshwater system is an extension of the mainland aquifer. This finding has implications for climate-resilient coastal aquifer management in Bangladesh. The primary risk to this deep, fresh groundwater is unlikely to be lateral seawater intrusion due to sea-level rise, but rather pumping-induced downward migration from overlying brackish aquifers.