Microbes enhance mobility of arsenic in pleistocene aquifer sand from Bangladesh.

Dissimilatory metal-reducing bacteria can mobilize As, but few studies have studied such processes in deeper orange-colored Pleistocene sands containing 1-2 mg kg(-1) As that are associated with low-As groundwater in Bangladesh. To address this gap, anaerobic incubations were conducted in replicate over 90 days using natural orange sands initially containing 0.14 mg kg(-1) of 1 M phosphate-extractable As (24 h), >99% as As(V), and 0.8 g kg(-1) of 1.2 M HCl-leachable Fe (1 h at 80 °C), 95% as Fe(III). The sediment was resuspended in artificial groundwater, with or without lactate as a labile carbon source, and inoculated with metal-reducing Shewanella sp. ANA-3. Within 23 days, dissolved As concentrations increased to 17 µg L(-1) with lactate, 97% as As(III), and 2 µg L(-1) without lactate. Phosphate-extractable As concentrations increased 4-fold to 0.6 mg kg(-1) in the same incubations, even without the addition of lactate. Dissolved As levels in controls without Shewanella, both with and without lactate, instead remained <1 µg L(-1). These observations indicate that metal-reducers such as Shewanella can trigger As release to groundwater by converting sedimentary As to a more mobilizable form without the addition of high levels of labile carbon. Such interactions need to be better understood to determine the vulnerability of low-As aquifers from which drinking water is increasingly drawn in Bangladesh.

Promotion of well-switching to mitigate the current arsenic crisis in Bangladesh.

OBJECTIVE: To survey tube wells and households in Araihazar upazila, Bangladesh, to set the stage for a long-term epidemiological study of the consequences of chronic arsenic exposure. METHODS: Water samples and household data were collected over a period of 4 months in 2000 from 4997 contiguous tube wells serving a population of 55000, the position of each well being determined to within +/- 30 m using Global Positioning System receivers. Arsenic concentrations were determined by graphite-furnace atomic-absorption spectrometry. In addition, groundwater samples collected every 2 weeks for an entire year from six tube wells were analysed for arsenic by high-resolution inductively coupled plasma-mass spectrometry. FINDINGS: Half of the wells surveyed in Araihazar had been installed in the previous 5 years; 94% were privately owned. Only about 48% of the surveyed wells supplied water with an arsenic content below 50 micro g/l, the current Bangladesh standard for drinking-water. Similar to other regions of Bangladesh and West Bengal, India, the distribution of arsenic in Araihazar is spatially highly variable (range: 5-860 micro g/l) and therefore difficult to predict. Because of this variability, however, close to 90% of the inhabitants live within 100 m of a safe well. Monitoring of six tube wells currently meeting the 50 micro g/l standard showed no indication of a seasonal cycle in arsenic concentrations coupled to the hydrological cycle. This suggests that well-switching is a viable option in Araihazar, at least for the short term. CONCLUSIONS: Well-switching should be more systematically encouraged in Araihazar and many other parts of Bangladesh and West Bengal, India. Social barriers to well-switching need to be better understood and, if possible, overcome.