Tens of millions of people in south and southeast Asia routinely consume ground water that has unsafe arsenic levels1,2. Arsenic is naturally derived from eroded Himalayan sediments, and is believed to enter solution following reductive release from solid phases under anaerobic conditions. However, the processes governing aqueous concentrations and locations of arsenic release to pore water remain unresolved, limiting our ability to predict arsenic concentrations spatially (between wells) and temporally (future concentrations) and to assess the impact of human activities on the arsenic problem3,4,5,6,7,8,9. This uncertainty is partly attributed to a poor understanding of groundwater flow paths altered by extensive irrigation pumping in the Ganges-Brahmaputra delta10, where most research has focused. Here, using hydrologic and (bio)geochemical measurements, we show that on the minimally disturbed Mekong delta of Cambodia, arsenic is released from near-surface, river-derived sediments and transported, on a centennial timescale, through the underlying aquifer back to the river. Owing to similarities in geologic deposition, aquifer source rock and regional hydrologic gradients11,12,13,14,15, our results represent a model for understanding pre-disturbance conditions for other major deltas in Asia. Furthermore, the observation of strong hydrologic influence on arsenic behaviour indicates that release and transport of arsenic are sensitive to continuing and impending anthropogenic disturbances. In particular, groundwater pumping for irrigation, changes in agricultural practices, sediment excavation, levee construction and upstream dam installations will alter the hydraulic regime and/or arsenic source material and, by extension, influence groundwater arsenic concentrations and the future of this health problem.
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Landsat map ID 039-880, 11 July 2001 (Global Land Cover Facility, University of Maryland); downloaded for Path 126, Row 052 using the Earth Science Data Interface 〈http://glcfapp.umiacs.umd.edu:8080/esdi/index.jsp〉.
This work was supported by Stanford University, US NSF and US EPA STAR. We thank K. Ouch, K. Phan, co-workers at Resource Development International, G. Li, M. Meyer, M. Busbee and A. Aziz for field and laboratory assistance; S. Ganguly for modelling assistance; and A. Boucher for help with spatial analyses.
Author Contributions All authors contributed to the intellectual design, execution, interpretation and analyses presented in this study. M.L.P. assessed groundwater hydrology and geochemistry; B.D.K. analysed near-surface biogeochemistry; S.G.B. established the hydrologic framework (field layout and data collection) and conducted the modelling; M.S. facilitated the field work and provided the scientific history of the area; S.F. provided the project impetus, biogeochemical deduction, and, with S.G.B. and M.L.P., site selection. M.L.P., S.G.B. and S.F. wrote the manuscript with input from B.D.K.
This file contains Supplementary Methods, field area description, groundwater flow calculations, numerical modeling results, dissolved and solid-phase chemical data, and mass balance calculations from our study of the Upper Mekong Delta, Cambodia. The material is organized as 7 sections with an appendix and includes Supplementary Tables 1-4 and Supplementary Figures 1-9. Additional methods are described in Section 1 of the Supplementary Information. Section 2 provides information about the geologic history, stratigraphy (Supplementary Figure 1), aqueous chemistry (Supplementary Table 1), and land use practices within our field area. Groundwater flux calculations are presented in Section 3, including hydraulic heads throughout the field area (Supplementary Figures 2 and 3), hydraulic conductivity results (Supplementary Tables 2), and flow distances (Supplementary Table 2). The results of groundwater flow modeling (Supplementary Table 4 and Supplementary Figures 4 and 5) are shown in Section 4. Section 5 discusses the spatial distribution (Supplementary Figures 6 and 7) and temporal variations (Supplementary Figure 8) of dissolved arsenic concentrations and solid-phase redox profiles of arsenic (Supplementary Figure 9). Section 6 outlines arsenic input and output calculations. Supplementary Information references are given in Section 7, and all groundwater arsenic measurements are presented in the Supplementary Information Appendix. (PDF 3001 kb)
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Polizzotto, M., Kocar, B., Benner, S. et al. Near-surface wetland sediments as a source of arsenic release to ground water in Asia. Nature 454, 505–508 (2008). https://doi.org/10.1038/nature07093
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