Natural arsenic contamination of shallow groundwater poses a major human health threat in many parts of the world. Research by us and others indicates that young (Holocene) river floodplain aquifers are most problematic due to their locally high arsenic levels and because they are exploited for drinking by tens to hundreds of millions of people in developing nations. Recent research supports the concept that As is released by reductive dissolution of the As-bearing hydrous ferric oxides (HFO) in the floodplain aquifers. We have recently proposed that the As-HFO hypothesis is actually part of a much larger continental As transport phenomena, which we have called the arsenic geo-, bio-, hydro- (GBH) transport process. The essence of the GBH process is that typical geologic, hydrologic, and microbial processes are linked and lead to arsenic contamination in river floodplain aquifers. The GBH process starts with weathering of mountain ranges by mechanical effects of ice during the Pleistocene glacial events, and then during the Holocene, warmer climates leads to rapid chemical weathering of glacial deposits and bedrock containing minor amounts of As, and its release to the hydrosphere. Arsenic is sorbed from river waters by HFO in stream sediments, and stream sediments ultimately are deposited with organic matter in river floodplains. This in turn causes anaerobic groundwater to develop and anaerobic iron-reducing bacteria (FeRB) to dissolve As-bearing HFO, producing Fe- and As- riched shallow groundwater. Our recent research has generated the first field microbiologic data (from USA) for the As-groundwater cycling processes and has identified genera of FeRB and sulfate-reducing bacteria (SRB). Our research indicates that the conditions that lead to this water chemistry in river floodplains is universal, but only rises to a serious problem in the developing nations where this water is exploited for human consumption. The subject of this proposed research will extend and test our hypotheses and conclusions drawn from our three field areas in USA to a seriously As-affected region of the world. This will be the ultimate test of our hypothesis regarding the universality of biogeochemical controls on this type of natural arsenic contamination. Further, our previous research has also demonstrated an important role that SRB play in nature in removing As from groundwater. We will investigate the geochemistry of As under in situ sulfate-reducing conditions by stimulating indigenous SRB as we have done in our bioremediation research in the USA, which suggests that As may be removed from groundwater by SRB if dissolved iron concentrations remain elevated (e.g., precludes formation of significant As-thio complexes). The proposed field research in Bangladesh will include: 1) drilling and installation of new wells (including some multiport monitoring well) at field sites selected from previous Dhaka University research, and also collection of solid aquifer materials; 2) collection of geochemical data in the field from existing and new wells; 3) collection of bacteria samples by filtering of groundwater for DNA sequencing and also extraction from solid aquifer samples; 4) collection of hypothesized authigenic carbonate and sulfide mineral phases to evaluate their role in controlling As mobility and groundwater geochemistry; and 5) an investigation of the behavior of As under artificially induced sulfate-reducing conditions in the field. In summary, our research team has unique skills and backgrounds that have never been deployed in As-contaminated areas before, and thus this research offers an unparalleled opportunity to document the cause of the arsenic problem, as well as a potential remediation tool. The proposed research includes educational and research activities for graduate students and post-docs. The proposed research has significant technology transfer elements in it, during the progress of the study, we will also by necessity be cooperating with local scientists and government officials, and we plan on presenting technical seminars in Bangladesh. Because As-contamination is a pressing problem in the developing world, lessons we learn in Bangladesh should be applicable to many other countries. This proposed project falls under almost all five "priority research" thematic areas, but particularly more under thematic area IV as identified as "collaborative research areas" at the US-Bangladesh Workshop on the Ganges-Brahmaputra- Meghna Delta, held in Rajendrapur, Bangladesh (January 28-31, 2002). The workshop was sponsored by the US National Science Foundation and the Government of Bangladesh.
