Among the mechanisms controlling elevated arsenic (As) speciation in groundwater, dissolution-precipitation of As-bearing solids, possibly as colloids, has not been systematically evaluated even though reported groundwater saturation states often indicate super- or near-saturation with respect to multiple solids. In this contribution, a detailed geochemical analysis was performed on well-constrained groundwater quality data collected through (a) sustained sampling (n = 84) over 2.5 y at a newly-identified site in the middle Gangetic plain of India; and (b) metadata analysis on studies conducted worldwide (n = 414). Groundwater saturation indices, speciation (E_H-pH), and mineral solubilities (logC-pH) were calculated, consistent with a carefully-selected and updated thermodynamic database. Results suggest that under oxidizing conditions, secondary precipitation of solids similar to scorodite [FeAsO₄·2H₂O_(s)] and pharmacolite [CaHAsO₄·2H₂O_(s)] influences the As(V) concentrations in groundwater. In addition, groundwater at the investigated site was saturated with calcite [CaCO_3(s)] and rhodochrosite [MnCO_3(s)]. Evidence of colloidal forms of As-containing and As-free solids was found from SEM-EDS characterization of solids collected on 0.2 μm filter membranes used to sample groundwater. XPS analysis showed that the relative As(V) and As(III) signatures in these solids were consistent with the prevalence of dissolved As(V) and As(III) in groundwater, independently quantified using IC-ICP-MS. HR-TEM-SAED characterization of these solids indicated the possible presence of poorly crystalline scorodite- and pharmacolite-like phases along with calcite and lepidocrocite [γ–FeOOH_(s)] in a predominantly amorphous matrix. Also, a possible role of Mn in inducing As immobilization in calcite was suggested with the identification of ∼ 1:1 Mn:As atomic ratios in these solids, consistent with significant (p < 0.05) correlation of dissolved total As and total Mn. These findings imply that solubility-driven secondary processes may exert additional controls on the eventual fate and transport of arsenic in mixed-redox state shallow groundwaters, apart from the known primary mobilization mechanisms. To the best of our knowledge, this is the first study that has characterized colloidal arsenic in groundwater and related it to prevailing mechanisms.

在控制地下水中砷 (As) 形态升高的机制中，尚未系统评估可能作为胶体的含砷固体的溶解-沉淀，尽管报告的地下水饱和状态通常表明多种固体的过饱和或接近饱和. 在这篇文章中，对通过 (a) 在印度中部恒河平原新确定的地点持续采样 (n = 84) 超过 2.5 年收集的受良好约束的地下水质量数据进行了详细的地球化学分析；(b) 对全球开展的研究进行元数据分析（n = 414）。地下水饱和指数，物种形成（E _H-pH) 和矿物溶解度 (logC-pH) 的计算与精心挑选和更新的热力学数据库一致。结果表明，在氧化条件下，类似于臭葱石 [FeAsO ₄ ·2H ₂ O _(s) ] 和药石 [ CaHAsO ₄ · 2H ₂ O ₍ s) ] 的固体二次沉淀会影响地下水中的 As(V) 浓度。此外，调查地点的地下水被方解石 [CaCO _3(s) ] 和菱锰矿 [MnCO _3(s)饱和]。从用于采集地下水的 0.2 μm 滤膜上收集的固体的 SEM-EDS 表征中发现了胶体形式的含 As 和无 As 固体的证据。XPS 分析表明，这些固体中的相对 As(V) 和 As(III) 特征与使用 IC-ICP-MS 独立量化的地下水中溶解的 As(V) 和 As(III) 的普遍性一致。这些固体的 HR-TEM-SAED 表征表明可能存在结晶度差的类臭葱石相和药石相以及方解石和纤铁矿 [γ-FeOOH _(s)] 在主要是无定形基质中。此外，通过确定这些固体中约 1:1 的 Mn:As 原子比，表明 Mn 在诱导方解石中固定 As 中的可能作用，这与溶解的总 As 和总 Mn 的显着（p < 0.05）相关性一致。这些发现表明，除了已知的初级迁移机制之外，溶解度驱动的次级过程可能会对混合氧化还原状态浅层地下水中砷的最终归宿和迁移施加额外的控制。据我们所知，这是第一项描述地下水中胶体砷并将其与普遍机制联系起来的研究。