In This Article Expand or collapse the "in this article" section Arsenic Contamination in South and Southeast Asia

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Environmental Science Arsenic Contamination in South and Southeast Asia
by
John C. Ayers, Steven Goodbred, Matthew Dietrich
  • LAST REVIEWED: 26 November 2019
  • LAST MODIFIED: 26 November 2019
  • DOI: 10.1093/obo/9780199363445-0087

Introduction

Rural populations in South and Southeast Asia often rely on groundwater for drinking and irrigation. Usually water is obtained by installing a tubewell consisting of PVC pipe with a cast-iron hand pump mounted on top. The well is usually drilled deep enough to reach unconsolidated sand layers that have high permeability and therefore make good aquifers. Many of these tubewells are now known to deliver water with arsenic concentration >10 μg/L, the World Health Organization (WHO) guideline for safe drinking water, and even greater than the Bangladesh government guideline of 50 ug/L. Increased groundwater usage has also exacerbated the threat of previously uncontaminated aquifers being contaminated with arsenic through drawdown, and crops such as rice accumulating high levels of arsenic through irrigation with arsenic contaminated groundwater. The affected areas are generally located in river floodplains and deltas that have relatively low elevation and low-gradient topography. The principal areas fall within five river basins that drain the Himalayas: the relatively well-studied Ganges–Brahmaputra–Meghna River basin that includes portions of Bangladesh, India, Nepal, Bhutan, and China; the comparatively little-studied Indus and Irrawaddy river basins, affecting primarily Pakistan, India, and Myanmar, respectively; and the moderately well-studied Mekong system flowing through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam; and the Red River basin in China and Vietnam. In these areas, at least 100 million people are affected by unsafe levels of arsenic in untreated groundwater, with the greatest number living in Bangladesh and West Bengal. Affected areas continue to be identified. Arsenic poisoning symptoms appeared eight to ten years after residents began drinking contaminated well water. Acronyms used in this review: hydrous ferric oxide or oxyhydroxide (HFO) and dissolved organic carbon (DOC).

Geology and Chemistry of Arsenic

Many areas of Southeast Asia (including Bangladesh) have elevated risk of unsafe levels of arsenic in groundwater. River deltas where sediment burial is rapid are most at risk. The deltas began forming and prograding during the Holocene Climate Optimum about 8,500 to 6,500 years ago, when sea-level rise began to decelerate. The deltas tend to have young (Holocene), organic-rich sediments and are flat, low-lying areas with low hydraulic gradients. These characteristics allow for the development of reducing conditions in old groundwater contained in shallow aquifers, due to the microbial oxidation of young, reactive (labile) dissolved organic carbon. This, in turn, allows for the reductive dissolution of HFOs and release of adsorbed arsenic. Contaminated aquifers tend to occur in grey Holocene fine sands in low gradient areas, often in association with reduced Holocene muds that provide a source of organic matter and impede groundwater flow; aquifers composed of coarser, well-flushed Holocene sands or older (Pleistocene), orange-brown (oxidized) sediments are less likely to have elevated arsenic.

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