The migration of arsenic (As) enriched groundwater into Pleistocene aquifers as a consequence of extensive groundwater abstraction represents an increasing threat to the precious water resources in Asian delta regions. Pleistocene aquifer sediments are typically rich in FeIII-(hydr)oxides and are capable to adsorb high amounts of As. This results in a pronounced accumulation of As in Pleistocene aquifers, where high As groundwater infiltrates from adjacent Holocene aquifers. However, As retention by Pleistocene aquifers over long-term time scales remains largely unknown. We studied As sorption in situ by placing natural Pleistocene sediments and pure mineral phases directly inside groundwater monitoring wells at a study site near Hanoi (Vietnam). This in situ exposure allows for constant flushing of the samples with unaltered groundwater and the establishment of undisturbed sorption equilibria similar to those in local aquifer sediments, which is not readily attainable in traditional laboratory sorption experiments. The groundwaters in our experimental wells were characterized by different As concentrations (0.01–6.63 μmol/L) and redox states, reaching from suboxic to anoxic conditions (E_h of +159 to −4 mV). Results show that adsorption is the dominant As retention mechanism, independent from the respective groundwater chemistry (i.e. concentrations of dissolved P, HCO₃⁻ and Si). Whilst most of the As sorbed within the first week, sorption further increased slowly but consistently by 6–189%, respectively, within six months. Hence, the As sorption behavior of Pleistocene aquifer sediments should be determined over longer periods to avoid an underestimation of the As sorption capacity. Accompanying desorption experiments revealed that about 51% of the sorbed As was remobilized within six months when exposed to low As groundwater. We therefore conclude that a considerable proportion of the As accumulated in the aquifer sediments is prone to remobilization once the As concentrations in migrating groundwater decline. Remobilization of As should be considered in local water management plans to avoid contamination of precious groundwater resources with this As legacy.

由于广泛抽取地下水，富砷的地下水向更新世含水层的迁移代表了对亚洲三角洲珍贵水资源日益增加的威胁。更新世含水层沉积物通常富含FeIII-（氢）氧化物，并且能够吸附大量的As。这导致在更新世含水层中明显地积累了As，其中高As地下水从相邻的全新世含水层中渗入。然而，由于更新世含水层在长期尺度上的滞留仍然是未知的。我们通过在河内（越南）附近的研究现场将天然更新世的沉积物和纯矿物相直接置于地下水监测井内，从而就地吸附进行了研究。这原位暴露允许用不变的地下水不断冲洗样品，并建立与本地含水层沉积物中相似的不受干扰的吸附平衡，这在传统的实验室吸附实验中是不容易实现的。我们实验井中的地下水具有不同的砷浓度（0.01–6.63μmol/ L）和氧化还原状态，从低氧状态到缺氧状态（E _h为+159到-4 mV）。结果表明，吸附是主要作为保持机构，独立于各地下水化学（即溶解磷浓度，HCO ₃ ^-和Si）。虽然大多数砷在第一周内被吸收，但在六个月内，吸收进一步缓慢但持续地分别增加了6-189％。因此，应该确定较长时期的更新世含水层沉积物的As吸附行为，以避免低估As的吸附能力。伴随的解吸实验表明，当暴露于低砷地下水中时，大约51％的被吸附砷在六个月内被迁移。因此，我们得出的结论是，一旦迁移的地下水中的砷浓度下降，蓄积在含水层沉积物中的相当一部分砷易于迁移。在当地的水管理计划中应考虑As的迁移，以避免这种As遗留物污染宝贵的地下水资源。