Arsenic (As) mobilization in groundwater is linked to the dissimilatory reductive dissolution of As(V) coated Fe(III)-oxy-hydroxides from the aquifer sediment coupled with the degradation of available dissolved organic carbon (DOC) due to the presence of anaerobic microbes under anoxic conditions. Understanding the seasonal pattern of As mobilization in the shallow groundwater of the Bengal Basin remains a challenging task due to the heterogeneous character of the shallow aquifers and the involvement of multiple factors. To resolve this issue, in the present study, we showcased a comprehensive effort to advance understanding of the seasonal As mobilization process in the shallow groundwater, utilizing multiple geochemical tracers (i.e., the abundance of dissolved total As, Fe, Mn, NO₃^–, SO₄^2-, DOC, Cl^-, and δ¹⁸O, δ²H, δ¹³C-DOC isotopic tracers) between post-monsoon and pre-monsoon periods over multiple years from Nadia district, West Bengal, India. We quantified and explained the seasonal variation of dissolved total As concentrations in the groundwaters with the nature of the aquifer lithology (i.e., grey sand aquifer or ‘GSA’ and brown sand aquifer or ‘BSA’). The present study reported elevated dissolved total As concentrations in the shallow groundwater samples during dry pre-monsoon (‘PRM’) periods compared to post-monsoon (‘POM’) time. However, the magnitude of such seasonal changes in groundwater As concentrations (denoted as Δ_As = As_PRM-As_POM) varied between years depending on the extent of rainfall, surface water infiltration, mixing, and groundwater withdrawal. Our current findings are different from the past studies that reported elevated As concentrations in the shallow groundwater during the monsoon and post-monsoon periods compared to the dry pre-monsoon periods. However, the limitations of the past findings are that most of the previous studies were carried out between seasonal intervals over a single annual cycle without reinvestigating the seasonal trends over different years under variable hydrological conditions. We proposed that the excess groundwater withdrawal during the dry pre-monsoon period drive draw-down and, therefore, trigger infiltration of surface-derived deep pond water into the shallow aquifer. Such surface water infiltration introduces fresh labile organic matter into the shallow aquifer, promoting high As mobilization during the dry pre-monsoon period. A viable alternative approach of ‘squeezing’ of aquifer intercalated clay-peat sedimentary lenses and mixing of organic-rich pore water in the adjoining groundwater can also enhance high As mobilization during the dry time, as examined in this study. This process is triggered by the excessive groundwater withdrawal practices and drawdown encountered during dry time, driving the aquifer intercalated clay-pockets compaction.

地下水中砷 (As) 的迁移与含水层沉积物中 As(V) 包覆的 Fe(III)-羟基氧化物的异化还原溶解以及由于厌氧生物的存在导致可用溶解有机碳 (DOC) 的降解有关缺氧条件下的微生物。由于浅层含水层的异质性和多种因素的参与，了解孟加拉盆地浅层地下水中砷迁移的季节性模式仍然是一项具有挑战性的任务。为了解决这个问题，在本研究中，我们展示了一项全面的努力，以促进对浅层地下水中季节性 As 迁移过程的理解，利用多种地球化学示踪剂（即溶解的总 As、Fe、Mn、NO ₃ ^-， 所以₄ ^2- , DOC, Cl ^- , 和 δ ¹⁸ O, δ ² H, δ ¹³ C-DOC 同位素示踪剂）在印度西孟加拉邦纳迪亚地区多年后季风期和季风前期之间。我们量化并解释了地下水中溶解的总砷浓度随含水层岩性（即灰砂含水层或“GSA”和棕砂含水层或“BSA”）性质的季节性变化。本研究报告称，与季风后 ('POM') 时期相比，在干燥的季风前 ('PRM') 期间浅层地下水样品中溶解的总砷浓度升高。然而，地下水中 As 浓度的这种季节性变化的幅度（表示为 Δ _As  = As _PRM-作为_POM) 根据降雨量、地表水渗透、混合和地下水抽取的程度在不同年份之间变化。我们目前的研究结果与过去的研究不同，过去的研究报告称，与干燥的季风前相比，季风和季风后期间浅层地下水中的砷浓度升高。然而，过去研究结果的局限性在于，之前的大多数研究都是在单个年度周期内的季节间隔之间进行的，而没有重新研究不同水文条件下不同年份的季节趋势。我们提出，在干燥的季风前时期，过量的地下水抽取会导致水位下降，因此会触发地表衍生的深水池水渗入浅层含水层。这种地表水入渗将新鲜的不稳定有机物引入浅层含水层，在干燥的季风前促进了高砷的迁移。正如本研究中所研究的那样，一种可行的替代方法是“挤压”含水层夹层粘土泥炭沉积透镜体以及在相邻地下水中混合富含有机物的孔隙水，这也可以提高干旱期间的高砷迁移率。这一过程是由过度的地下水抽取实践和干旱期间遇到的下降触发的，从而推动了含水层夹层粘土袋的压实。正如本研究中所研究的那样，一种可行的替代方法是“挤压”含水层夹层粘土泥炭沉积透镜体以及在相邻地下水中混合富含有机物的孔隙水，这也可以提高干旱期间的高砷迁移率。这一过程是由过度的地下水抽取实践和干旱期间遇到的下降触发的，从而推动了含水层夹层粘土袋的压实。正如本研究中所研究的那样，一种可行的替代方法是“挤压”含水层夹层粘土泥炭沉积透镜体以及在相邻地下水中混合富含有机物的孔隙水，这也可以提高干旱期间的高砷迁移率。这一过程是由过度的地下水抽取实践和干旱期间遇到的下降触发的，从而推动了含水层夹层粘土袋的压实。