SEDIMENTOLOGICAL CONTROLS ON ARSENIC MOBILIZATION IN A PERMEABLE NATURAL REACTIVE BARRIER (PNRB)

Elevated concentrations of arsenic (As) in groundwater pose serious health concerns, affecting millions of people globally. Shallow aquifer sediments enriched with Fe-oxy(hydroxides) have been shown to release adsorbed As under reducing conditions. Recent studies have suggested that the mixing of oxic river water and reduced groundwater with ambient dissolved As within shallow riverbank sediments cause Fe-oxide minerals to precipitate and collect As on their surfaces, acting as a permeable natural reactive barrier (PNRB). Studying the occurrence of the PNRB and the chemical interactions between the sediment-water interfaces is crucial to understand the mobilization and fate of As and similar oxyanions under changing conditions in the aquifer or the river.

Sediment cores (n=9) up to 3 m in depth were collected along a transect up to 80 m inland from the edge of the Meghna River, Bangladesh. The elemental abundance of the sediment samples (n=35) from each core was determined by both in-situ handheld X-Ray Fluorescence (XRF) and later with a standalone XRF unit (Rigaku Primus II) to identify zones of enriched As, Fe, and other key elements. Ongoing work is determining the concentrations of Fe, As, and Mn associated with different phases of minerals through bulk and sequential extractions.

Total Fe concentrations of the top 1.2 m of the sediment cores measured in the field varied between ~7,450 and 38,000 mg/kg, with extractable Fe(II) concentrations comprising a fraction of this, varying between 2 and 25 mg/kg. The total Mn concentrations in these samples varied between 0 and 411 mg/kg. Pore-water chemistry is being measured at each coring location between a depth of 0.5 and 2.5 m to determine the concentrations of dissolved As and Fe.

Detailed analysis of the sediment and pore-water chemistry allows us to identify potential sources and sinks of As in the sediment helping to confirm the presence of a PNRB. Preliminary results reveal zones of Fe and Mn enrichment along with higher acid-extractable Fe(II) in the top meter of the sediment columns and near the river edge. This observation suggests that these areas of high Fe concentrations represent the parameters of an oxyanion regulating PNRB in the riverbank sediment.