Arsenic in groundwaters of the Lower Mekong

Abstract

Increasing incidence and awareness of arsenic in many alluvial aquifers of South-east Asia has raised concern over possible arsenic in the Lower Mekong Basin. Here, we have undertaken new research and reviewed many previous small-scale studies to provide a comprehensive overview of the status of arsenic in aquifers of Cambodia and the Cuu Long Delta of Vietnam. In general natural arsenic originates from the Upper Mekong basin, rather than from the local geology, and is widespread in soils at typical concentrations of between 8 and 16 ppm; (dry weight). Industrial and agricultural arsenic is localised and relatively unimportant compared to the natural alluvial arsenic. Aquifers most typically contain groundwaters of no more than 10 microg L(-1), although scattered anomalous areas of 10 to 30 microg L(-1 )are also quite common. The most serious, but possibly ephemeral arsenic anomalies, of up to 600 microg L(-1), are associated with iron and organic-rich flood-plain sediments subject to very large flood-related fluctuations in water level, resulting in transient arsenopyrite dissolution under oxidizing conditions. In general, however, high-arsenic groundwaters result from the competing interaction between sorption and dissolution processes, in which arsenic is only released under reducing and slightly alkaline conditions. High arsenic groundwaters are found both in shallow water-tables, and in deeper aquifers of between 100 and 120 m depth. There is no evidence of widespread arsenicosis, but there are serious localised health-hazards, and some risk of low-level arsenic ingestion through indirect pathways, such as through contaminated rice and aquaculture. An almost ubiquitous presence of arsenic in soils, together with the likelihood of greatly increased groundwater extraction in the future, will require continuing caution in water resources development throughout the region.

Arsenic and Other Metal Contamination of Groundwater in the Mekong River Delta, Vietnam

“…In this study, some samples (3/14) were collected from deep wells with a depth of over 300 m, and other samples (11/14) were collected from shallow wells with a depth of about 30 to 60 m. In general, a high concentration of As has been observed in shallow aquifers. [7][8][9] Consistent with this, some samples collected from shallow wells were contaminated with a high concentration of As, whereas samples collected from deep wells were not. In addition, the average number of shallow-well groups had significantly higher concentrations of Mn, Ba, Ca, and Mg than those of the deep-well groups (Table 1).…”

“…In regard to the Mekong River Delta, Stanger et al recently reported on As pollution in groundwater collected from the lower Mekong basin. 7) To our knowledge, however, a study of contamination by multiple elements in the Mekong River Delta has not been reported.…”

Release of arsenic to deep groundwater in the Mekong Delta, Vietnam, linked to pumping-induced land subsidence

“…1). Whereas prior synoptic studies in the Delta have focused on near-river areas, where the highest population densities and arsenic concentrations are found (14)(15)(16)(17)(18), wells in this new survey were sampled in proportion to their abundance in all populated areas, providing unprecedented spatial coverage. Our analysis uses (i) the richness of these observations along with (ii) simulation of the spatiotemporally explicit flow and pumping history of the multiaquifer system and (iii) validation of pumping-induced compaction by radar remote sensing of land subsidence.…”

Leaching of arsenic in response to organic matter contamination in groundwater treatment practice

“…In Cambodia, the natural arsenic originates from the upper Mekong basin, and is widespread in soils. Within the lower Mekong delta, 5.7% of all groundwater samples exceeded 50 μg/L, while 12.9% exceeded 10 μg/L (Stagner et al, 2005). A small-scale health survey conducted in Myanmar in 2002 reported that 66.6% of the water samples from wells have arsenic levels of >50 μg/L (Tun, et al, 2002).…”