A field, laboratory, and modeling study of As in groundwater discharging to Waquoit Bay, MA, shed light on coupled control of chemistry and hydrology on reactive transport of As in a coastal aquifer. Precipitation of Fe(III) oxides, along with oxidation and adsorption of As occur at the redox interfaces above or below the reducing plume migrating toward the bay. Batch adsorption of As(III) onto orange, brown and gray sediments follows Langmuir isotherms, and can be fitted by a surface complexation model (SCM) assuming a diffuse double layer for ferrihydrite. The SCM simulated the observed dissolved As concentration better than a parametric approach based on Kd.
Shallow groundwaters in the Ganges-Brahmaputra-Meghna Delta (GBMD) are frequently elevated in Fe and As, and discharge to rivers during dry season. Sediment As enrichment up to hundreds to thousands mg/kg in the shallow subsurface along the Meghna Riverbank suggests a plausible mechanism of trapping of As by a natural reactive barrier consisting of oxidatively precipitated Fe(III) oxyhydroxides formed at the redox boundary between reducing groundwater and oxic river water during discharge. Depth profiles of sediment Fe(II)/Fe(II+III) ratios and pore water dissolved oxygen and Fe concentrations indicate that there is a redox transition zone from anoxic to suboxic from ∼2 m depth to the surface, which closely associated with sediment As enrichment. Ferrihydrite is the dominant Fe mineral in As enriched Meghna Riverbank sediment by X-ray absorption spectroscopy.
To study the processes relevant to sorption and desorption of groundwater As in GBMD aquifer, brown and gray sandy sediment collected from suboxic and anoxic zones in Meghna Riverbank were subject to batch sorption and desorption experiments. Sorption experimental data were well fitted to Langmuir isotherm, resulting in Kd of 1∼2 L kg-1 for reducing riverbank sediments, while ∼7 L kg-1 for a suboxic sediment at equilibrium with 100 μg L-1 As(III) or As(V). Amendment with 1 mM lactate greatly enhanced As release from a As enriched sediment, mobilizing ∼70% of initial sediment As, while no further mobilization of As occurred for 1 month after rapid desorption of ∼3 mg kg-1 As within 2 days without lactate.
|Commitee:||Eaton, Timothy, Zhang, Pengfei|
|School:||City University of New York|
|Department:||Earth & Environmental Sciences|
|School Location:||United States -- New York|
|Source:||DAI-B 70/11, Dissertation Abstracts International|
|Subjects:||Hydrologic sciences, Environmental science, Geochemistry|
|Keywords:||Arsenic, Bangladesh, Discharge, Groundwater, Oxidation, Sorption|
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