Coastal groundwater is susceptible to physico-chemical modification from interaction with seawater and other surface waters. Surface water-groundwater (SW-GW) interaction can alter the Sr concentration and radiogenic 87Sr/86Sr signature of both seawater and groundwater from multi-depth aquifers. In this study, we document such an interaction between a tropical ocean (Bay of Bengal [BoB]) and the coastal aquifers of a large mega-deltaic system formed by the Himalayan-sourced Ganges River, at shallow (10–50 m below ground level [bgl]), and deeper (115 and 333 m bgl) depths, using radiogenic strontium isotopes (87Sr/86Sr), stable isotope ratios (δ18O and δD), salinity and dissolved solutes. The mean 87Sr/86Sr for shallow coastal aquifers (10–50 m bgl: 0.71094) suggests that seawater mixes with the terrestrial-sourced shallow groundwater, modifying them to brackish water. This is further supported by the stable isotope signatures (14–25 m bgl: −3.63 to −0.7 ‰ and 30–50 m bgl: −3.5 to −1.2 ‰ δ18O). The radiogenic 87Sr/86Sr (115 m bgl: 0.71681 and 333 m bgl: 0.71995) and depleted δ18O (115 m bgl: −5.04 to −1.61 ‰ and 333 m bgl: −4.43 to −2.38 ‰) suggest relatively less to negligible mixing between seawater and terrestrial-sourced resident groundwater at greater depths. The mixing process is additionally characterized by a significant Sr flux discharged from these coastal aquifers to the BoB, which ranges between 7.7 × 104 and 12 × 105 mol/year for shallow aquifers, and between 1.78 × 104 and 8.26 × 104 mol/year for deep aquifers, respectively. The overall contribution of Sr from old groundwater of deep aquifers is 1.43 % (115 m bgl) and 0.66 % (333 m bgl), whereas shallow aquifers show a higher contribution, ranging from 6.18 to 9.57 % of BoB Sr budget. This study suggests that the discharge of recirculated brackish water to the BoB from the shallow aquifers contributes more than 5 times higher Sr to the oceanic budget than the deep aquifer, contributing as an essential component of the global oceanic budget of Sr.

中文翻译：

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浅层和深层地下水与热带海洋的相互作用：来自放射性 （87Sr/86Sr） 和稳定同位素循环和通量的见解


沿海地下水容易受到与海水和其他地表水相互作用的物理化学改变。地表水-地下水 （SW-GW） 相互作用可以改变来自多深度含水层的海水和地下水的 Sr 浓度和放射性 87Sr/86Sr 特征。在这项研究中，我们记录了热带海洋（孟加拉湾 [BoB]）与由喜马拉雅源的恒河形成的大型巨型三角洲系统的沿海含水层之间的这种相互作用，在浅水区（低于地面 [bgl] 10-50 m）和更深（115 和 333 m bgl）深度，使用放射性锶同位素 （87Sr/86Sr）、稳定同位素比值（δ18O 和 δD）， 盐度和溶解溶质。沿海浅层含水层的平均值 87Sr/86Sr （10–50 m bgl： 0.71094） 表明海水与陆地来源的浅层地下水混合，使它们变成微咸水。稳定同位素特征（14-25 m bgl：-3.63 至 -0.7 ‰ 和 30-50 m bgl：-3.5 至 -1.2 ‰ δ18O）进一步支持了这一点。放射性 87Sr/86Sr（115 m bgl：0.71681 和 333 m bgl：0.71995）和贫瘠的 δ18O（115 m bgl：-5.04 至 -1.61 ‰ 和 333 m bgl：-4.43 至 -2.38 ‰）表明海水和更深的陆地来源居民地下水之间的混合相对较少或可以忽略不计。混合过程的另一个特征是从这些沿海含水层排放到 BoB 的显着 Sr 通量，浅层含水层的通量范围在 7.7 × 104 和 12 × 105 mol/年之间，深层含水层的 Sr 通量在 1.78 × 104 到 8.26 × 104 mol/年之间。深层含水层旧地下水对 Sr 的总体贡献为 1.43 % （115 m bgl） 和 0.66 % （333 m bgl），而浅层含水层的贡献较高，占 BoB Sr 预算的 6.18% 至 9.57 %。 这项研究表明，从浅层含水层向 BoB 排放的再循环苦咸水对海洋收支的贡献是深层含水层的 5 倍以上，是全球 Sr 收支的重要组成部分。