Hun Bok Jung

Redox trapping of arsenic during groundwater discharge in sediments from the Meghna riverbank in Bangladesh

Groundwater arsenic (As) is elevated in the shallow Holocene aquifers of Bangladesh. In the dry season, the shallow groundwater discharges to major rivers. This process may influence the chemistry of the river and the hyporheic zone sediment. To assess the fate of As during discharge, surface (0–5 cm) and subsurface (1–3 m) sediment samples were collected at 9 sites from the bank of the Meghna River along a transect from its northern source (25° N) to the Bay of Bengal (22.5° N). Bulk As concentrations of surface sediment averaged 16 ± 7 mg/kg (n = 9). Subsurface sediment contained higher mean concentrations of As of 4,000 mg/kg (n = 14), ranging from 1 to 23,000 mg/kg As, with >100 mg/kg As measured at 8 sites. X-ray absorption near-edge structure spectroscopy indicated that As was mainly arsenate and arsenite, not As-bearing sulfides. We hypothesize that the elevated sediment As concentrations form as As-rich groundwater discharges to the river, and enters a more oxidizing environment. A significant portion of dissolved As sorbs to iron-bearing minerals, which form a natural reactive barrier. Recycling of this sediment-bound As to the Ganges-Brahmaputra-Meghna Delta aquifer provides a potential source of As to further contaminate groundwater. Furthermore, chemical fluxes from groundwater discharge from the Ganges-Brahmaputra-Meghna Delta may be less than previous estimates because this barrier can immobilize many elements.

Can Arsenic Occurrence Rates in Bedrock Aquifers Be Predicted

A high percentage (31%) of groundwater samples from bedrock aquifers in the greater Augusta area, Maine was found to contain greater than 10 μg L(-1) of arsenic. Elevated arsenic concentrations are associated with bedrock geology, and more frequently observed in samples with high pH, low dissolved oxygen, and low nitrate. These associations were quantitatively compared by statistical analysis. Stepwise logistic regression models using bedrock geology and/or water chemistry parameters are developed and tested with external data sets to explore the feasibility of predicting groundwater arsenic occurrence rates (the percentages of arsenic concentrations higher than 10 μg L(-1)) in bedrock aquifers. Despite the under-prediction of high arsenic occurrence rates, models including groundwater geochemistry parameters predict arsenic occurrence rates better than those with bedrock geology only. Such simple models with very few parameters can be applied to obtain a preliminary arsenic risk assessment in bedrock aquifers at local to intermediate scales at other localities with similar geology.

Redox Behavior of Uranium at the Nanoporous Aluminum Oxide-Water Interface: Implications for Uranium Remediation

Sorption-desorption experiments show that the majority (ca. 80-90%) of U(VI) presorbed to mesoporous and nanoporous alumina could not be released by extended (2 week) extraction with 50 mM NaHCO(3) in contrast with non-nanoporous α alumina. The extent of reduction of U(VI) presorbed to aluminum oxides was semiquantitatively estimated by comparing the percentages of uranium desorbed by anoxic sodium bicarbonate between AH(2)DS-reacted and unreacted control samples. X-ray absorption spectroscopy confirmed that U(VI) presorbed to non-nanoporous alumina was rapidly and completely reduced to nanoparticulate uraninite by AH(2)DS, whereas reduction of U(VI) presorbed to nanoporous alumina was slow and incomplete (<5% reduction after 1 week). The observed nanopore size-dependent redox behavior of U has important implications in developing efficient remediation techniques for the subsurface uranium contamination because the efficiency of in situ bioremediation depends on how effectively and rapidly U(VI) bound to sediment or soil can be converted to an immobile phase.