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Environmental science

Poisoned waters traced to source

Nature volume 454, pages 415–416 (2008)Cite this article

South Asia's well-water is widely polluted with arsenic, but no one has located the source. A study on the Mekong River finds that contamination begins in pond sediments, and is spread by groundwater flow to wells.

By applying an understanding of these physical processes, Polizzotto et al.1 arrived at two crucial insights. First, they realized that if they selected a location where groundwater flow is primarily perpendicular to the Mekong River, a single transect of wells of different depths aligned with that flow would sample water from the various layers of the subsurface system, fully mapping the local three-dimensional system. Second, they recognized that the chemical composition of groundwater at different depths reflects the different origins of the water — thus avoiding the common mistake of assuming that the composition is purely determined by a gradual process of change as solutes react with aquifer sediments over time. Although it is true that deeper groundwater is usually older, the water found at shallow depths does not flow straight down to deeper levels. By analysing the arsenic content of samples from different depths and locations at their site, Polizzotto et al. conclude that the local arsenic contamination originates from nearby pond sediments. So how can this source be explained?

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Figure 1: Arsenic contamination in south Asian aquifers.

References

  1. Polizzotto, M. L., Kocar, B. D., Benner, S. G., Sampson, M. & Fendorf, S. Nature 454, 505–508 (2008).

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  2. Freeze, R. A. & Cherry, J. A. Groundwater (Prentice Hall, 1979).

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  3. Gelhar, L. W. Stochastic Subsurface Hydrology (Prentice Hall, 1993).

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  4. Harvey, C. F. et al. Chem. Geol. 228, 112–136 (2006).

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  1. Charles F. Harvey is in the Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA. charvey@mit.edu,

    Charles F. Harvey

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  1. Charles F. Harvey
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Harvey, C. Poisoned waters traced to source. Nature 454, 415–416 (2008). https://doi.org/10.1038/454415a

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