In the last three decades, the number of STWs has increased dramatically in the Asian region. These STWs are providing a reliable and inexpensive source of irrigation water, which allows farmers to grow additional crops during the dry season, and ensures them of water security during periods of drought. Furthermore, STWs are an inexpensive source of drinking-water mostly free of waterborne diseases. The installation of STWs to provide drinking-water has significantly contributed to the reduction of diarrhoea and has saved millions of lives. And access to groundwater resources has been a major contributor to the green revolution in Asia.
Since the 1980s, evidence has gradually unfolded that As is present in elevated levels in part of tapped groundwater resources, and the World Health Organization (WHO) has set a drinking-water standard of 10 ug/l (or 0.01 mg/l). At present, countries in the region have reported high levels of As in part of their groundwater resources (Afghanistan, Bangladesh, Cambodia, China, India, Lao PDR, Mongolia, Myanmar, Nepal, Pakistan, Thailand, Viet Nam) and more cases are being reported and published (Berg et al., 2001; Chakraborti et al., 2002; Mandal and Suzuki, 2002; Ng etal., 2003; Polya et al., 2005). The high levels of As in groundwaters in the affected countries are predominantly of geogenic origin. Reductive dissolution of iron(hydr)oxides (FeOOH) stimulated by microbial activity and organic materials is regarded as the most important mechanism releasing As into the aquifer (Ahmed et al., 2004; McArthur et al., 2004; Mukherjee and Bhattacharya, 2001; Ravenscroft et al., 2001; Smedley and Kinniburgh, 2002; Smedley et al., 2003; Zheng et al., 2004). Anthropogenic sources of As include various industrial activities, pesticides, herbicides, and fertilizers. Natural contamination is generally regarded as the main mechanism causing the high levels of As in the groundwater in Asia and this will therefore be the focus of this report.
To illustrate the scale of the As problems in the region, a brief description of the As situation in a few countries in the region with regard to As in groundwater resources and people at risk of consuming contaminated drinking-water are presented below.
Bangladesh
In Bangladesh, groundwater from the shallow aquifer is the main source of drinking-water. Part of the shallow aquifer contains As concentrations above the national drinking-water standard of 0.050 mg/l, particularly in the south and southwestern part of the country. The latest data indicate that approximately 20 percent of the STWs exceed the standard and 10000 to 30000 people have been diagnosed with arsenicosis to date (R. Johnston and G. Howard, personal communication, 2005). An estimated 30 million people consume water which exceeds the Bangladesh drinking-water standard for As. The shallow aquifer is also the main source of irrigation water during the dry Boro season. Approximately 95 percent of all groundwater extracted is used for irrigation, mainly for Boro rice production. More detailed information on the situation in Bangladesh can be found in, for example, Ahmad et al., 1997; Alam et al., 2002; Chakraborti etal., 2002; Chowdhury etal., 2000; Mukherjee and Bhattacharya, 2001.
China
During the 1980s, endemic arsenicosis was found successively in many areas in mainland China. At present, the population exposed to As levels in drinking-water exceeding the national standard of 0.050 mg/l is estimated to be over two million and more than 10000 arsenicosis patients were confirmed by 2001. As-contaminated groundwater resources are mainly located in west China and north China. By 2004, high As levels in groundwater had been reported in the following provinces: Xinjiang, Inner Mogolia, Shanxi, Ningxia, Jilin, and Qinghai. In Qinghai, arsenicosis is mainly related to burning As-rich coal indoors. Based on geochemical and hydrological characteristics, more areas with high As concentrations can be expected within these and other provinces in China (Sun, 2004; Xia and Liu, 2004).
India
The presence of naturally elevated levels of As in groundwater was confirmed in seven Indian states, namely West Bengal, Bihar, Uttar Pradesh, Assam, Jharkland, Chattisgarh and Madhya Pradesh. Except for West Bengal, the extent of the problem is not fully known and the number of people at risk is impossible to estimate with any degree of confidence. In West Bengal, investigations suggest that eight districts show As content in well-water to be above 0.050 mg/l with, according to United Nations Children's Fund (UNICEF), over 13.8 million people at risk (R. Nickson, personal communication, 2006). More detailed information can be found in, for example, Ahmad et al., 1997; Alam et al., 2002; Chakraborti et al., 2002; Chowdhury et al., 2000.
Nepal
As contamination in Nepal has been detected in the Terai region of southern Nepal, where nearly half of Nepal's total population is living. In this region bordering India, 90 percent of the people served by approximately 200000 STWs, use groundwater for drinking purposes. Of the 15000 STWs tested, 23 percent exceeds the WHO drinking-water guideline of 0.010 mg/l, whereas 5 percent exceeds the Nepal interim As guideline of 0.050 mg/l. An estimated 0.5 million people are consuming drinking-water with As levels exceeding 0.050 mg/l (Shrestha et al., 2003).
To date, only limited attention has been paid to the risks of using contaminated groundwater for irrigation. Irrigation water with high levels of As may result in land degradation in terms of crop production (loss of yield) and food safety (food chain contamination) (Brammer, 2005; Duxbury and Zavala, 2005). Long-term use of As-contaminated irrigation water could result in As accumulation in the soil. If absorbed by the crops, this may add substantially to the dietary As intake, thus posing additional human health risks. Over time, As accumulation in the soil could reach soil concentrations toxic to crops, thus reducing yields (Figure 1.1).
Note: A: input of As via irrigation water can lead to accumulation of As in the soil over time. B: depending on bioavailability, uptake and transport within the plants, higher soil concentrations may be reflected in higher concentrations in crops. The dotted line indicates that at a certain level the plant growth becomes severely inhibited and As concentrations in the plants are then no longer relevant. C: with an increase in soil concentration, yields are expected to stay more or less constant until a threshold level is reached, after which yield will decline.
Figure 1.1 The possible risks of using As-contaminated irrigation water over time
Reliable and representative data are therefore needed to assess and manage the risks of As-contaminated irrigation water. With millions of irrigation STWs tapping water from the same As-contaminated aquifer as the STWs for drinking-water, the extent of possible risks can be substantial. An overview of the risk analysis paradigm is presented in Box 1.
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BOX 1: RISK ANALYSIS PARADIGM The terms and definitions presented here are taken from the Codex Alimentarius Commission (Codex, 2004). Although the definitions refer to food safety, with minor adaptations they are also applicable to the risks of As to crop production. For a detailed description refer to http://www.codexalimentarius.net/web/ procedural_manual.jsp (Codex, 2004).
Definitions of risk analysis terms related to food safety as used in the Codex Alimentarius (Codex, 2004)
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| Hazard | A biological, chemical or physical agent in, or condition of, food with the potential to cause an adverse health effect. |
| Hazard characterization | The qualitative and/or quantitative evaluation of the nature of the adverse health effects associated with biological, chemical and physical agents that may be present in food. For chemical agents, a dose-response assessment should be performed. For biological or physical agents, a dose-response assessment should be performed if the data are obtainable. |
| Hazard identification | The identification of biological, chemical and physical agents capable of causing adverse health effects and which may be present in a particular food or groups of foods. |
| Dose-response assessment | The determination of the relationship between the magnitude of exposure (dose) to a chemical, biological or physical agent and the severity and/or frequency of associated adverse health effects (response). |
| Exposure assessment | The qualitative and/or quantitative evaluation of the likely intake of biological, chemical and physical agents via food as well as exposures via other sources if relevant. |
| Risk | A function of the probability of an adverse health effect and the severity of that effect, consequential to a hazard(s) in food. |
| Risk analysis |
A process consisting of three components: risk assessment, risk management and risk communication. |
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Risk |
A scientifically based process consisting of the following steps: 1) hazard identification, 2) hazard characterization, 3) exposure assessment, 4) risk characterization. |
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Risk
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The qualitative and/or quantitative estimation, including attendant uncertainties, of the probability of occurrence and severity of known or potential adverse health effects in a given population based on hazard identification, hazard characterization and exposure assessment. |
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Risk
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The interactive exchange of information and opinions throughout the risk analysis process concerning risk, risk-related factors and risk perceptions, community and other interested parties, including the explanation of risk assessment findings and the basis of risk management decisions. |
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Risk |
The process, distinct from risk assessment, of weighing policy alternatives in consultation with all interested parties, considering risk assessment and other factors relevant for health protection of consumers and for the promotion of fair trade practices and, if needed, selecting appropriate prevention and control options. |
