Arsenic (As) in groundwater is a major health concern in Asia and the risks from using shallow tubewells (STWs) for drinking-water are well-known. At present, twelve countries in the region have reported high As levels in part of their groundwater resources. Bangladesh has the highest percentage of contaminated STWs (~20 percent) and an estimated 30 million people are dependent on those wells for domestic purposes. Since an initial investigation on As accumulation in rice undertaken by FAO with support from the United Nations Development Programme (UNDP) in 2001, further scientific studies in the last couple of years have reported potential risks from As in irrigation water because of land degradation affecting agro-ecosystem services. The most well-known concern is As entering the food chain, affecting food safety. This poses a potential dietary risk to human health in addition to the risk from drinking contaminated groundwater. Less well-known but potentially more serious is the risk of As to crop production. Continuous build up of As in the soil from As-contaminated irrigation water may reduce crop yields, thus affecting the nutritional status and incomes of rural farming communities.
As part of the green revolution, millions of STWs have been installed throughout Asia over the last three decades. This has resulted in a sharp increase of groundwater extraction for irrigation. In Bangladesh, of the four million ha under irrigation, 2.4 million ha are irrigated with approximately 900000 STWs. Considering that there are many contaminated drinking-water STWs, a high percentage of contaminated irrigation STWs can also be expected. It has been estimated that water extraction from the shallow aquifer for irrigation adds 1 million kg of As per year to the arable soil in Bangladesh, mainly in the paddy fields.
The form and behaviour of As vary greatly between flooded soils, such as paddy fields, and non-flooded soils. The most important As species are arsenate (AsV) under non-flooded conditions and arsenite (AsIII) under flooded conditions. AsIII has a higher solubility than AsV, resulting in a higher mobility of As in flooded soils. Soil texture is another important factor, with clay soils having a much greater capacity to bind As than sandy soils. Worldwide, natural soil concentrations are around 5 mg/kg, but this can vary depending on the origin of the soil. Data on As in soils in Bangladesh are limited both in number and quality, but there are indications that 5-10 mg/kg is the background level. Various reports indicate that soil concentrations are increasing because of As input via irrigation water, and is a major concern. At this stage, the conditions under which As is building up in the soil and the time frame involved are still uncertain.
One of the major factors determining uptake and toxicity to plants is the form of As. The two most important forms, AsV and AsIII, are taken up by completely different mechanisms. Uptake, accumulation and toxicity vary within and between plant species. In general, more As in the soil leads to higher concentrations in plants, but this depends on many factors. It is not yet possible to predict As uptake and/ or toxicity in plants from soil parameters. Toxicity data are limited in number and in terms of the experimental setups used to obtain them. At this stage, available data cannot be extrapolated to the field situation. Reliable and representative plant uptake and toxicity data are essential to evaluate current and future soil concentrations in Bangladesh and other affected countries.
There are some potential human health risks related to livestock and fresh water fisheries as these can be exposed to As via drinking-water, pond water, and feeds. Although As is less toxic to animals and concentrations of inorganic As in animal products are expected to be relatively low, very limited information is available, and research is thus needed to examine, for example, the transference of As to milk. Because of the lack of information, impacts on livestock and fisheries are not covered in this technical report.
To date, little field research on the relation between As in irrigation water and crop yield has been conducted. It is of concern that a number of studies from Bangladesh and West Bengal (India) have reported increased concentrations in soils and crops because of irrigation with As-contaminated groundwater. Similar situations can be expected elsewhere in the region where As-contaminated irrigation water is used. The increase in soil concentrations may finally result in a reduction of soil quality and crop yields. Assessment of risks to crop production is difficult because of the limited information on current and future As soil concentrations and the lack of reliable plant toxicity data. These gaps need to be addressed urgently.
Recent data on total and inorganic As in rice and vegetables from Bangladesh indicate that rice contributes significantly to the daily intake. A positive correlation between As in groundwater resources, soil and rice has been reported, indicating that food chain contamination takes place because of prolonged irrigation with contaminated water. In order to refine the risk assessment, more scientific data are needed on As in foods, and on food and water consumption patterns. With limited technical capacity and the continuation of current agricultural practices, it can be expected that As in the food chain will further increase. This would, unfortunately, offset the ongoing activities in the drinkingwater sector to reduce As exposure.
The risks of land degradation are likely to increase with the accumulation of As in the soil. Management options should therefore focus on preventing and minimizing As input to soils. While the risks need to be quantified in detail, various management options could be further explored. For example, farmers often use more irrigation water than needed. Optimizing water input would be a sound option to reduce As input while saving water. Furthermore, aerobic growth conditions in paddy fields may reduce bioavailability and uptake of As in rice. Other possible options include breeding crops tolerant to As and/or low accumulation of As in grains, and shifting from rice in the dry season to crops that demand less water, where feasible.
Based on the information available as presented in this literature review, it can be concluded that As in irrigation water can result in land degradation that in turn affects food safety and crop production. A number of reports found evidence that irrigating with As-contaminated water, causes accumulation of As in the soil and that this is reflected in higher levels of As in the edible parts of crops. Major gaps in our knowledge still need to be filled and this currently hampers a comprehensive risk assessment. Action is urgently needed in a concerted manner from multiple stakeholders, particularly from governments and development partners. The urgency of the need to take up this issue is emphasized by the large scale of As-contaminated groundwater resources, by the extensive use of groundwater for irrigation, and by rice being the staple crop in Asia.
It is recommended to initiate an integrated programme to quantify the scale of the problem in combination with the development of a water/soil/crop quality monitoring system for land degradation in agro-ecosystems. This should not only include As, but a range of physical, chemical (nutrients and contaminants) and biological parameters. Further, management options to prevent and mitigate As contamination need to be explored.