MOBILIZATION OF ARSENIC DURING ONE-YEAR INCUBATIONS OF GREY AQUIFER SANDS FROM ARAIHAZAR, BANGLADESH

Relatively little is known about the rate of As mobilization in Bangladesh aquifers and the potential role of microbes in the process. To provide some new constraints, changes in the composition of groundwater in contact with grey Holocene sediments was monitored for a year. Aquifer sediment and groundwater samples were collected simultaneously in March 2005 in Araihazar, Bangladesh from depths of 15, 38 and 123 ft. Groundwater As concentrations increased from 30 to 120 µg As L^-1 over the same depth range, whereas the P-extractable As levels slightly decreased from 0.6 to 0.3 mg kg^-1 with depth. The sediment slurries were incubated under anaerobic conditions (1) without amendment, (2) amended with a minimal level of acetate (about 2 µmol g^-1 sediment ), (3) amended with antibiotics, (4) autoclaved, and (5) periodic additions of oxygen (10 µmol after each sampling). Sensitive measurements by HR ICP-MS indicate that As was mobilized from the shallow grey aquifer sediments (15 and 38 ft) at a relatively constant rate over the course of a year (18 ± 4 and 22 ± 3.5 ng As g^-1 yr^-1 ). Only at 38 ft did the addition of acetate increase mobilization to 79 ±16 ng g^-1 yr^-1. These rates are equivalent to an increase of 135 to 600 µg As L^-1 yr^-1 in groundwater, assuming a porosity of 0.25. Although the deeper grey aquifer (123 ft) had a higher As groundwater concentration, there was evidence of As adsorption (-98 ± 65 ng As g^-1 yr^-1) at this depth. These results indicate that the depths with the highest groundwater As are not necessarily those where mobilization is occurring. The enhancement of As release at 38 ft by acetate suggests that this zone could play a critical role, although the local dissolved As concentrations are low. In contrast, the dissolved As contained in the groundwater from 123 ft was being adsorbed suggesting either declining As concentrations over time or As transport to this zone after mobilization elsewhere. Our observations also indicate that As mobilization in groundwater is not directly coupled to reductive dissolution of Fe (oxy)hydroxides. As mobilization began before Fe release and continued at a constant rate even as Fe release dramatically increased. The amendments, designed to investigate the role of microbes in mobilization, could not determine whether the observed releases of As were largely due to abiotic mechanisms.