Whaley-Martin K J, San Pedro R J, Mailloux B J, Bostick B C, Ahmed K M, Mozumder R, Ellis T, van Geen A, Slater G F
Environmental Resources Management (ERM), Toronto, ON, Canada.
School of Earth, Environment and Society, McMaster University, ON, Canada.
Org Geochem. 2023 Mar;177. doi: 10.1016/j.orggeochem.2023.104556. Epub 2023 Jan 14.
The presence of dissolved arsenic in shallow aquifers of Bangladesh is widely accepted to require microbial dissimilatory iron-reduction in anoxic aquifers utilizing organic carbon as an electron donor. However, the various potential sources of this carbon, and whether organic carbon sources vary with sediment age (i.e. < 12 kyr-old Holocene vs older Pleistocene sediments) are still poorly understood. To shed light on these questions, natural abundance radiocarbon signatures of in situ microbial phospholipids fatty acids (PLFA), concentrations of sterol biomarkers, and aqueous [Cl] and [Br] were compared in two Bangladesh aquifers; a shallow (11-15 m) aquifer low in dissolved arsenic containing oxidized (orange) Pleistocene sands, Dopar Tek (DT), and a shallow (6-21 m) aquifer high in dissolved arsenic containing reduced (grey) Holocene sands, Desert Island (DI). Radiocarbon signatures of PLFA (ΔC = -30 to -63 ‰ and +9 to +25 ‰, respectively) indicate microbial utilization of carbon fixed from the atmosphere within the last several decades, the drawdown of which into the shallow portions of both the Pleistocene Dopar Tek and Holocene Desert Island aquifers was likely enhanced by regional pumping activities. Similar results were previously obtained for two other Holocene aquifers in the same region, but to our knowledge this is the first time modern PLFA has been extracted from Pleistocene sediments. At both sites, high proportions of phytosterols, low sewage contamination indices (SCI < 0.7), and generally low Cl/Br ratios (averaging 434 and 544 at Desert Island and Dopar Tek respectively), are consistent with predominantly plant-derived organic carbon inputs. This contrasts with sewage-derived input inferred from higher sewage contamination index values (>0.7) previously observed at the two other shallow Holocene aquifers in the same region. Overall, our observations show that microbial communities within shallow aquifers, including those of Pleistocene age, utilize very recently fixed organic carbon associated with both plant and/or sewage origin. The microbial utilization of organic carbon fixed within the past several decades, likely derived from plants, in the anaerobic Pleistocene, has not, as of yet, led to iron reduction that would be sufficient to increase arsenic concentrations in groundwater. However, the observed microbial utilization of recently fixed carbon within all Bangladesh aquifers studied to date, indicates that pumping enhanced drawdown represents a potential risk to any systems where it might occur.
孟加拉国浅层含水层中溶解态砷的存在被广泛认为需要在缺氧含水层中利用有机碳作为电子供体进行微生物异化铁还原。然而，这种碳的各种潜在来源以及有机碳源是否随沉积物年龄变化（即小于12 kyr的全新世沉积物与更古老的更新世沉积物）仍知之甚少。为了阐明这些问题，在孟加拉国的两个含水层中比较了原位微生物磷脂脂肪酸（PLFA）的天然丰度放射性碳特征、甾醇生物标志物的浓度以及水相中的[Cl]和[Br]；一个浅层（11 - 15米）含水层，溶解态砷含量低，含有氧化（橙色）的更新世砂层，多巴尔·泰克（DT），以及一个浅层（6 - 21米）含水层，溶解态砷含量高，含有还原（灰色）的全新世砂层，荒岛（DI）。PLFA的放射性碳特征（分别为ΔC = -30至 -63‰和 +9至 +25‰）表明微生物利用了过去几十年内从大气中固定的碳，区域抽水活动可能增强了这种碳进入更新世多巴尔·泰克和全新世荒岛含水层浅层部分的过程。此前在同一地区的另外两个全新世含水层也获得了类似结果，但据我们所知，这是首次从更新世沉积物中提取现代PLFA。在这两个地点，高比例的植物甾醇、低污水污染指数（SCI < 0.7）以及总体较低的Cl/Br比值（荒岛和多巴尔·泰克分别平均为434和544），与主要来自植物的有机碳输入一致。这与之前在同一地区另外两个浅层全新世含水层中观察到的较高污水污染指数值（>0.7）所推断的污水来源输入形成对比。总体而言，我们的观察结果表明，浅层含水层中的微生物群落，包括更新世时期的群落，利用了与植物和/或污水来源相关的最近固定的有机碳。在厌氧的更新世中，过去几十年内固定的、可能源自植物的有机碳的微生物利用，截至目前尚未导致足以增加地下水中砷浓度的铁还原。然而，在迄今为止研究的所有孟加拉国含水层中观察到的微生物对最近固定碳的利用表明，抽水增强的水位下降对任何可能发生这种情况的系统都构成潜在风险。
