The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10-20 m depth range, groundwater ages along the transect determined by the 3H-3He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 µg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push-pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, Kd, of 0.15-1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100-1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources.

中文翻译：

---

孟加拉国浅层全新世含水层中砷的可逆吸附和冲洗

孟加拉盆地浅层地下水中溶解砷 (As) 浓度的空间异质性归因于来自外部来源的 As（和活性碳）的传输或来自灰砂地层内的 As 的释放。在这项详细的水文和地球化学研究中，我们沿着浅层含水层的 300 m 横断面探索了后一种情况，从沙质河道坝内的地下水补给区延伸到其排放到附近的溪流中。在 10-20 m 深度范围内，由 3H-3He 方法确定的断面地下水年龄从补给区的 <10 年增加到河流的最大 40 年。沿着这条横断面，同一灰砂中的地下水 As 浓度从 10 到 100 增加到 500 µg/L。从一系列推拉实验、传统的批量吸附实验和浅层监测井的意外溢流中获得了地下水和沉积物之间可逆吸附砷的证据。假设可逆吸附和从这些观察中推断出的分布系数 Kd 为 0.15-1.5 L/kg，一个简单的冲刷模型表明，该地点以及孟加拉和其他地区的 As 浓度随着深度和地下水年龄的增加而增加。红河流域可归因于含水层在 100-1000 年间的演变，因为含水层砂逐渐冲刷掉其最初的 As 含量。因此，地下水中的砷浓度范围很广，随着深度的增加，受冲刷历史和当地补给率的影响，