As-free water in As-polluted areas
- http://pubs.acs.org/doi/abs/10.1021/es1032376 [subscription/payment required to access full text]
Palaeosol Control on Groundwater Flow and Pollutant Distribution: The
Example of Arsenic. Environmental Science and Technology. John M. McArthur (1), Bibhash Nath (2), Dhiraj M. Banerjee(3), R. Purohit (3), and N. Grassineau (4). DOI 10.1021/es1032376.
[Paleosol - soil preserved by burial underneath sediments or volcanic deposits - Moderator].
ABSTRACT: The consumption of groundwater polluted by arsenic (As) has a severe and adverse effect on human health, particularly where, as happens in parts of SE Asia, groundwater is supplied largely from fluvial/deltaic aquifers.
The lateral distribution of the As-pollution in such aquifers is heterogeneous. The cause of the heterogeneity is obscure. The location and severity of the As-pollution is therefore difficult to predict, despite the importance of such predictions to the protection of consumer health, aquifer remediation, and aquifer development.
To explain the heterogeneity, we mapped As-pollution in groundwater using 659 wells across 102 square km of West Bengal, and logged 43 boreholes, to reveal that the distribution of As-pollution is governed by subsurface sedimentology.
Across 47 square km of contiguous palaeo-interfluve, we found that the shallow aquifer (<70 mbgl) is unpolluted by As (<10 ug/L) because it is capped by an impermeable palaeosol of red clay (the last glacial maximum palaeosol, or LGMP) at depths between 16 and 24 mbgl.
The LGMP protects the aquifer from vertical recharge that might carry As-rich water or dissolved organic matter that might drive
reduction of sedimentary iron oxides and so release As to groundwater.
Professor of Geochemistry
University College London
London Arsenic Group
In 55 square km of flanking palaeo-channels, the palaeosol is absent, so invasion of the aquifer by As and dissolved organic matter can occur, so palaeo-channel groundwater is mostly polluted by As (>50 ug/L).
The role of palaeosols and, in particular, the LGMP, has been overlooked as a control on groundwater flow and pollutant movement in deltaic and coastal aquifers worldwide. Models of pollutant infiltration in such environments must include the appreciation that, where the LGMP (or other palaeosols) are present, recharge moves downward in palaeo-channel regions that are separated by palaeo-interfluvial regions where vertical recharge to underlying aquifers cannot occur and where horizontal flow occurs above the LGMP and any aquifer it caps.
1. Department of Earth Sciences, University College London, Gower
Street, London WC1E 6BT, U.K.
2. School of Environmental Systems Engineering, The University of
Western Australia, M015, 35 Stirling Highway, Crawley,
WA 6009, Australia
3. Department of Geology, University of Delhi, Delhi 110 007, India
4. Department of Geology, RHUL, Egham, Surrey TW20 0EX, U.K.