Precipitation of arsenic-bearing solids as a secondary control on arsenic speciation in groundwater: Evidence from field study and geochemical analysis

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 (EH-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 [CaCO3(s)] and rhodochrosite [MnCO3(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.