Magnitude of arsenic pollution in the Mekong and Red River Deltas — Cambodia and Vietnam

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Abstract

Large alluvial deltas of the Mekong River in southern Vietnam and Cambodia and the Red River in northern Vietnam have groundwaters that are exploited for drinking water by private tube-wells, which are of increasing demand since the mid-1990s. This paper presents an overview of groundwater arsenic pollution in the Mekong delta: arsenic concentrations ranged from 1–1610 μg/L in Cambodia (average 217 μg/L) and 1–845 μg/L in southern Vietnam (average 39 μg/L), respectively. It also evaluates the situation in Red River delta where groundwater arsenic concentrations vary from 1–3050 μg/L (average 159 μg/L). In addition to rural areas, the drinking water supply of the city of Hanoi has elevated arsenic concentrations. The sediments of 12–40 m deep cores from the Red River delta contain arsenic levels of 2–33 μg/g (average 7 μg/g, dry weight) and show a remarkable correlation with sediment-bound iron. In all three areas, the groundwater arsenic pollution seem to be of natural origin and caused by reductive dissolution of arsenic-bearing iron phases buried in aquifers. The population at risk of chronic arsenic poisoning is estimated to be 10 million in the Red River delta and 0.5–1 million in the Mekong delta. A subset of hair samples collected in Vietnam and Cambodia from residents drinking groundwater with arsenic levels > 50 μg/L have a significantly higher arsenic content than control groups (< 50 μg/L). Few cases of arsenic related health problems are recognized in the study areas compared to Bangladesh and West Bengal. This difference probably relates to arsenic contaminated tube-well water only being used substantially over the past 7 to 10 years in Vietnam and Cambodia. Because symptoms of chronic arsenic poisoning usually take more than 10 years to develop, the number of future arsenic related ailments in Cambodia and Vietnam is likely to increase. Early mitigation measures should be a high priority.

Introduction

In some countries, arsenic is the most important chemical pollutant in groundwater and drinking water. The Bengal delta region is particularly affected as an estimated 35 million people have been drinking arsenic-rich water for the past 20–30 years (Smedley and Kinniburgh, 2002). Examination for arsenical dermatologic symptoms in 29 thousand people showed that 15% had skin lesions (Chowdhury et al., 2000). Regions with arsenic-rich drinking water can be found around the globe (Smedley and Kinniburgh, 2002). Natural contamination of groundwater by arsenic is also an emerging issue in some countries of Southeast Asia, including Vietnam, Thailand, Cambodia, and Myanmar (Berg et al., 2001, Buschmann et al., submitted for publication, Polya et al., 2005). Vulnerable areas for arsenic contamination are typically young Quaternary deltaic and alluvial sediments comprising highly reducing aquifers.

Chronic levels of 50 μg arsenic/L can cause health problems after 10–15 years of exposure (Smith et al., 2000). The development of symptoms of chronic arsenic poisoning (arsenicosis) is strongly dependent on exposure time and the resulting accumulation in the body. The various stages of arsenicosis are characterized by skin pigmentation, keratosis, skin cancer, effects on the cardiovascular and nervous system, and increased risk of lung, kidney and bladder cancer. The European Union allows a maximum arsenic concentration of 10 μg/L in drinking water, and the World Health Organisation (WHO) recommends the same value. In contrast, developing countries are struggling to establish and implement measures to reach standards of 50 μg/L in arsenic-affected areas.

Drinking water supplies in Cambodia and Vietnam are dependent on groundwater resources (Berg et al., 2001, Berg et al., 2006, Feldman and Rosenboom, 2001, Fredericks, 2004). The Mekong and the Red River deltas are the most productive agricultural regions of South East Asia (see Fig. 1). Both deltas have young sedimentary deposits of Holocene and Pleistocene age. The groundwaters are usually strongly reducing with high concentrations of iron, manganese, and (in some areas) ammonium. The Mekong and the Red River deltas are currently exploited for drinking water supply using installations of various sizes. In the last 7–10 years a rapidly growing rural population has stopped using surface water or water from shallow dug wells because they are prone to contamination by harmful bacteria. Instead, it has become popular to pump groundwater using individual private tube-wells, which is relatively free of pathogens.

The Vietnamese capital Hanoi is situated in the upper part of the 11,000 km2 Red River delta, which is inhabited by 11 million people and is one of the most populous areas in the world. The exploitation of groundwater in the city of Hanoi began more than 90 years ago and has since been expanded several times (Berg et al., 2001). Today, ten major well-fields are operated by water treatment facilities, which collectively process 650,000 m3/day. Due to naturally anoxic conditions in the aquifers, the groundwaters contain large amounts of iron and manganese that are removed in the Hanoi drinking water plants by aeration and sand filtration (Duong et al., 2003). The urban water treatment plants exclusively exploit the lower aquifers in 30–70 m depth, whereas private tube-wells predominantly pump groundwater from the upper aquifers at 12–45 m (Hydrogeological Division II, 2000).

Based on geological analogies to the Ganges delta, elevated arsenic concentrations in the aquifers of the Red River basin were expected (Berg et al., 2001). A first screening by us in 1998 confirmed this assumption and we studied the extent of arsenic contamination in a comprehensive survey from 1999 to 2000. The upper and lower Quaternary aquifers were investigated by analysing groundwaters from small-scale tube-wells and pumped by the Hanoi drinking water plants.

Groundwater arsenic contamination was identified in the Cambodian Mekong delta area in 2000 (Feldman and Rosenboom, 2001), and has since been investigated and addressed through close collaboration of local authorities and NGOs. The first international paper on arsenic groundwater contamination in Cambodia was published by Polya et al. (2005).

In this paper, the arsenic levels in groundwater of the Mekong delta are presented including data for the Vietnamese delta part, which is reported for the first time. In addition to an overview of the magnitude of arsenic poisoning in this region, the limited information available in the international literature on the geology and genesis of the Mekong and Red River delta is summarised.

Section snippets

Sample collection

Based on a projected density of one sample per 10 km2, private tube-wells were randomly sampled over areas of 2000 km2 in Cambodia, 2000 km2 in Southern Vietnam, and 700 km2 in the Red River delta. Groundwater was collected at the tube by hand or electrical pumping. Samples were taken after 10 min pumping, when the oxygen concentration in the water reached a stable value, which was measured online by using a dissolved oxygen electrode (PX 3000, Mettler-Toledo). Redox potential, pH, oxygen

Mekong delta: Cambodia and Southern Vietnam

The Mekong delta is located in southern Vietnam and neighbouring Cambodia between 8°30′ to 11°30′ N and 104°40′ to 106°50′ E and is confined by the South China Sea in the southeast, the Gulf of Thailand in the west, the Vamcodong River in the northeast and a well-defined Late Pleistocene terrace to the north (Nguyen et al., 2000). The Mekong River is 4300 km long and has a catchment area of 520,000 km2. It originates in the Tibetan Plateau, and flows through China, Myanmar, Laos, Thailand,

Conclusions and outlook

Based on the data presented here, arsenic groundwater pollution in Cambodia and Vietnam is evident and its impact to humans clearly reflected in the high arsenic levels measured in hair of people consuming such groundwater. We currently estimate that 10 million people in the Red River delta and 0.5–1 million people in the Mekong delta are at risk of chronic arsenic poisoning. Considering the magnitude of arsenic pollution, early mitigation measures are urgently needed to protect the people from

Acknowledgements

We thank for the financial support by the Swiss Agency for Development and Cooperation. We acknowledge Nguyen Van Dan, Tong Ngoc Thanh, Nguyen van Hung, Nguyen Kim Quyen and Nguyen Trac Viet for fruitful discussions and for providing hydrological information. We are very grateful to Mengieng Ung, Kagna Ouch, Sovathana Vong and Rachna Oum for assisting in the sampling campaigns in Cambodia; to Vu Van Trinh, Phan Van Han for assisting in the sampling campaigns in the Vietnamese Mekong delta; to

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