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Threat of Arsenic to Agriculture in India, Bangladesh and Nepal

Arsenic-polluted water used for irrigation in certain areas of India, Bangladesh and Nepal is posing a health hazard for people eating food from the crops irrigated. The accumulation of arsenic in the soil is a threat to sustainable agriculture in the areas affected. These problems are not yet widely recognised. Urgent action is required to address them. The most important need is to assess the scale of the problem so that appropriate interventions can be planned.

NOTESEconomic & Political Weekly EPW november 22, 200879Threat of Arsenic to Agriculture in India, Bangladesh and NepalHugh BrammerThe author is grateful to R Brinkman for a critical reading of the text and to P Ravenscroft for preparing the figure.Hugh Brammer (h.brammer@btinternet.com) is an independent researcher, formerly with the Food and Agriculture Organisation.Arsenic-polluted water used for irrigation in certain areas of India, Bangladesh and Nepal is posing a health hazard for people eating food from the crops irrigated. The accumulation of arsenic in the soil is a threat to sustainable agriculture in the areas affected. These problems are not yet widely recognised. Urgent action is required to address them. The most important need is to assess the scale of the problem so that appropriate interventions can be planned. Natural arsenic pollution of drink-ing water supplies is now known to occur in over 70 countries, affecting an estimated 150 million people worldwide [Ravenscroft et al 2008]. Most of the people at risk live on the Indian sub-continent: an estimated 50 million in Bangladesh, 30 million in India and 2.5 million in Nepal. Pollution also occurs in some countries of east and south-east Asia. The problem of arsenic in drinking water is now well recognised and meas-ures are being taken to mitigate it. How-ever, it is now becoming apparent that arsenic-contaminated groundwater used for irrigation poses an equally serious health hazard for people eating food from the crops irrigated; also that arsenic accu-mulating in irrigated soils poses a threat to sustainable agriculture in affected areas [Heikens 2006]. These threats are not yet widely recognised in the affected countries or by international aid agencies and little work has been done to assess the scale of the problem or to identify and test possible mitigation measures. This article first outlines important characteristics of arsenic accumulation in soils and crops, then describes possible mitigation meas-ures that require testing for use in differ-ent environmental conditions.1 Arsenic ContaminationIn this section, we outline the characteris-tics of arsenic contamination in the soil and in crops in the affected areas of the three countries.1.1 Areas AffectedThe figure (p 80) shows the areas of India, Bangladesh and Nepal within which arsenic contamination of groundwater used for drinking has been reported. Where irrigation water and drinking water are drawn from the same polluted aquifers, it may provisionally be assumed that the irrigation water is also polluted. In Bang-ladesh and West Bengal, about 90 per cent of the groundwater that is abstracted is used for irrigation. It comes from aquifers between ca 20 and 120 m in young (Holocene) alluvial sediments. Such flood-plain sediments extend up the Ganges floodplain into Bihar. In northern India and Nepal, the contaminated aquifers are in piedmont fan deposits in the terai zone at the foot of the Himalayas; they also occur in the Kathmandu valley. In central India (Chhattisgarh), the contaminated ground-water is in sediments derived from ancient volcanic rocks [Patel et al 2005].The arsenic pollution of groundwater is natural. It is not due to human activities. The arsenic originates in relatively un-weathered sediments derived from igne-ous and metamorphic rocks. Arsenic is not present in large amounts in these sedi-ments: its importance lies in the toxicity of this element at very low concentrations. Arsenic is originally bound with iron in sediment particles. It is released into groundwater where microbial activity in organic matter (for example, in buried peat layers) reduces iron to the ferrous form. The variable content of organic mat-ter in aquifer sediments, both laterally and vertically, probably accounts for the great variations between tubewells in the arsenic concentrations of the groundwater they deliver. Great variations also occur between areas in the iron-arsenic and phosphorus-arsenic ratios in groundwater. These differences may be significant for arsenic accumulation rates in soils, plant uptake and feasible mitigation measures.1.2 Arsenic in GroundwaterArsenic in irrigation water is much more difficult to deal with than the problem of arsenic in drinking water. Arsenic uptake by soils and crops is highly complex. That is particularly so in soils used for trans-planted rice (paddy), which have been little studied so far. Complexities that need to be recognised in organising sur-vey, research and mitigation programmes are outlined below.Arsenic concentrations vary greatly between tubewells. In Bangladesh in 1998, 25 per cent of domestic wells provided
INDIA Chhattisgarh Mumbai Sri Lanka Kolkata Delhi Nepal Kathmandu Bhutan Dhaka Bangladesh
NOTESEconomic & Political Weekly EPW november 22, 200881was reported for the first time in Bangla-desh in 2006 [Duxbury and Panaullah 2007]. Yan et al (2005) found virtually no yield reduction in one Chinese rice cultivar but up to 80-90 per cent reduc-tion in four of 10US cultivars tested on high-arsenic soils in theUS. In Bangla-desh, Duxbury and Panaullah (2007) re-ported yields of a single rice variety (BR29) decreasing from 8.9 tonnes/ha at 26.3 ppm soil arsenic to 3 tonnes/ha at 57.5 ppm arsenic. Their results suggest that the practical limit for paddy cultivation might lie somewhere between 25 and 50 mg/kg soil arsenic. However, differences in varietal tolerance described above need to be kept in view. So do differences in the amounts of arsenic transferred to rice grain (described below). Safe levels of soil arsenic for dryland crops that are grown with irrigation remain to be determined. 1.4.3 Plant UptakeArsenic taken up from soils by rice accu-mulates in different proportions in differ-ent plant parts in the order roots >stem >leaf >grain [Abedin et al 2002]. In pot trials in Bangladesh, Das et al (2004) found 2.4 mg/kg arsenic in rice roots, 0.73 mg/kg in stems and leaves, and 0.14 mg/kg in grain. However, there are considera-ble differences in uptake between rice varieties and between the kinds of rice grown in different countries. Meharg and Rahman (2003) found grain arsenic contents rang-ing between 0.058 and 1.835 mg/kg in 13 different rice varieties tested in Bangla-desh and 0.063-0.2 mg/kg in Taiwan. Duxbury and Zavala (2005) reported lower mean concentrations, 0.032–0.046 mg/kg arsenic, for aromatic rices from Bangla-desh, Bhutan, India and Pakistan.Duxbury and Panaullah (2007) found that significant amounts of arsenic were taken up by rice grain even at low soil ar-senic concentrations: 0.54 mg/kg in grain at 11.6 ppm soil arsenic versus 0.35 mg/kg at 57.5 ppm soil arsenic (where yields were much lower). These results were obtained in a field trial; even higher grain concen-trations were obtained in pot trials with samples taken from the same soils. These findings suggest that dangerous amounts of arsenic might be taken up by rice at soil arsenic levels below those when rice yields begin to be affected. However, it is dangerous to generalise from a single trial. Therefore, this trial needs to be repeated over a wider range of soils and rice varieties to obtain results that could provide the basis for sound recommend-ations to farmers and also a basis for assessment surveys.The differences between rice types and cultivars need to be taken into account in assessing the dietary intake of arsenic by people living in arsenic-affected areas. Since relatively large amounts of arsenic are taken up by rice stems and leaves, potential health effects on livestock eating contaminated rice straw also need to be examined, and so do the quality of meat and milk products from such livestock. The trials reported by Duxbury and Pan-aullah (ibid) showed that arsenic in rice straw increased with increasing soil arsenic concentrations.1.5 Human Impact1.5.1 Dietary ImplicationsThe arsenic content of rice grain is impor-tant because of the large amounts of rice eaten by people in many parts of India, Bangladesh and Nepal (an assumed450 g/day for a 60 kg adult in Bangladesh). When arsenic in rice grains is 0.2 mg/kg, adults consuming 450 g of rice and 4 litres of water per day at the 10 ppb WHO water standard consume 130 μg of arsenic per day, which is the FAO and WHO provisional tolerable dietary intake standard for a 60 g adult; (persons consuming 4 litres of water at 50 ppb national standards already exceed that level before eating any rice). Uchino et al (2006), who measured arsenic contents of hair and urine samples from members of 37 families in West Bengal villages with respectively <10, 10–50 and >50 ppb arsenic in drinking water, found that total daily arsenic intake of adults from water and food (rice + veg-etables) was approximately 1.5, 2.7 and 6.1 times the FAO-WHO standard in villages in the respective low, moderate and high drinking water classes. Food was the main source of arsenic in families drinking water in the two lower classes.1.5.2 HealthImpactsIn effect, there is no safe level of arsenic intake from food or water. There is a linear dose-response relationship between ar-senic intake and health hazards down to very low levels of intake. Arsenic causes serious skin lesions and various forms of cancer and it can cause deaths from a wide range of other serious diseases [Meharg 2005]. Symptoms may not appear for two to 10 years from the start of chronic exposure and they may also appear long after exposure ceases. Therefore, efforts need to be made to minimise arsenic in-take from all sources as soon as possible.1.5.3 SocialImpactPoor families and women are particularly affected by the health impacts of chronic arsenic consumption [Sultana 2007]. Sick family members may be unable to work or to obtain adequate medical treatment. Women with disfiguring skin lesions may be denied marriage or be divorced. 2 MitigationMeasuresMost studies on the reclamation of arsenic-contaminated soil have been carried out on sites contaminated with mining, indus-trial or urban wastes or on soils contami-nated with arsenical pesticides. Few of these methods appear to be suitable for small-scale rice farmers. Mitigation methods appropriate for paddy soils need to be identified, tested and propagated as soon as possible in areas where arsenic pollution of soils and crops is found to be serious or imminent. Methods that might be practical will vary from place to place according to soil, climate, flooding and socio-economic conditions. Different methods might even be required within the boundaries of individual tubewell command areas. The possible methods that might be appropriate are listed in the table (p 82) and are described below.2.1 SurveysThe most urgent need is to assess the scale of the soil-arsenic contamination problem: which areas are already affected; and which areas might be affected in five to 10 years’ time if present rates of soil-arsenic accumulation continue. An indicative pic-ture could be obtained by superimposing amap showing the areas where drinking- water tubewells are known to be arsenic-contaminated on a map showing the areas where groundwater is used to irrigate rice.
NOTESnovember 22, 2008 EPW Economic & Political Weekly82Field surveys could then be organised to collect specific information on the number and location of irrigation tubewells deliv-ering high arsenic levels in water, the number of years that they have been in use and soil arsenic contents.At tubewell sites where irrigation with high-arsenic water has been practised for many years, topsoil samples should be taken from fields at different distances from the tubewell and from the water in-take in selected fields. That would provide a preliminary indication of the extent of soils within command areas that presently have >25 mg/kg and >50 mg/kg arsenic, or might reach those levels within the next five to 10 years at present rates of arsenic accumulation. (Arsenic levels in uncon-taminated soils are generally <10 ppm, and often <5 ppm.) Such surveys could initially be done on a sample basis to pro-vide a rapid indication of any actual or potential threat to agricultural production and human health, then followed by a full-scale survey in areas found to be most at risk. In this way, the scale of actual or looming threats from soil-arsenic accu-mulation could be obtained to form the basis for planning relevant research and mitigation programmes.2.2 Water TreatmentIt is considered impractical to provide water-treatment methods used for drinking water to treat the enormous quantities of irrigation water used (especially for rice) because of the cost. However, the natural co-precipitation of arsenic with ferric iron that occurs in irrigation distribution chan-nels provides a simple and practical means to reduce the amount of arsenic reaching fields. Methods to enhance this process need to be tested: for example, by increasing turbulent flow and aeration in distribution channels; providing field or overhead settling tanks; and possibly by adding ferric iron material in settling tanks or channels. In all cases, practical methods will need to be tested for periodically removing the arsenic-enriched material and disposing of it safely so that it does not create another health hazard.2.3 Alternative Irrigation SupplyThe best way to limit the threat of soil arsenic contamination is to stop adding arsenic to soils. Therefore, alternative safe sources of irrigation water should be sought and provided as soon as possible wherever that is practical. Possibilities will vary from area to area: sur-face water supplies from rivers or reservoirs; or use of safe deeper aquifers. Such alterna-tive water sources will probably be more costly to provide, oper-ate and maintain than existing shallow tubewells and subsidies may need to be considered. The costs of subsidies should be weighed against the increasing health, social and economic costs of con-tinuing irrigation with contaminated water until soils and crops become highly contaminated, crop yields and production fall to uneconomic levels, and more people need medical treatment.It will not be possible to provide safe irrigation supplies in all areas. Even where such possibilities exist, it may take several years before they can be provided. There-fore, in many areas, the next best alter-native will be to reduce the rate of soil arsenic accumulation. Those methods are described below.2.4 Agronomic Methods2.4.1 Dryland CropsSubstituting dryland crops such as maize or wheat for rice in appropriate climates could reduce the rate of arsenic accumula-tion in soils and food crops. So could the substitution of appropriate vegetables and cash crops. Dryland crops use much less water than paddy rice, so correspondingly less arsenic is added to soils. Arsenic is also more rapidly immobilised by ferric iron in dryland soils, so less is taken up by crops. However, rice is by far the preferred crop option for farmers in most arsenic-affected areas of India, Bangladesh and Nepal, so it would need to be ensured that the economic returns from substituted dryland crops at least match those from paddy cultivation, including the costs to families of buying the rice they may no longer grow. Much of the land currently irrigated is better suited to paddy rice than to dryland crops and drastic alterations to land and soils might be needed in some places to enable dryland crops to be grown reliably, as is described below.2.4.2 RaisedBedsOn floodplain land, dryland crops are likely to be a better option on relatively better-drained ridge soils which generally are lighter-textured and more permeable than are poorly-drained, heavier, basin soils. Rice can be grown as a dryland crop on relatively well drained soils. On relatively lower sites and heavier soils, it would be necessary to form raised beds on which to grow rice as a dryland crop. On heavy basin clays, beds would need to be made high enough to en-sure satisfactory drainage during heavy rainfall and at the beginning of monsoon-season flooding. The practical limits for constructing and maintaining such beds need to be determined in field trials. Studies have recently been initiated in Bangladesh to test rice cultivation on raised beds, with initially beneficial results on rice yields [Duxbury and Panaullah 2007]. However, more years of study are needed to test farm-er acceptability and monitor conditions to see if any problems arise. The method also needs to be tested under a wide range of soil and climatic conditions. Because of the drastic change to soil properties involved in making raised beds on basin clays, initial trials would best be made on research sta-tions rather than on farmers’ fields.2.4.3 System of Rice IntensificationThe system of rice intensification (SRI), which is being promoted in several coun-tries in Asia and Africa appears to be well adapted to growing rice as an irrigated dry-land crop. In this system, single rice seed-lings eight to 15 days old are transplanted Table: Possible Arsenic Mitigation MethodsType Method1 Water treatment a Filtration/chemical treatment b Co-precipitation with iron2 Alternative irrigation supply a Deep aquifer bRiver/lake/pond cReservoir3 Agronomic methods a Substitute dryland crops b Grow rice on raised bed c Change cultivation system d Use arsenic-tolerant varieties eProvidedrainage f Grow rainfed crops4 Soil amendments a Iron bPhosphorus5 Soil rehabilitation a Grow hyperaccumulator plants bRemovetopsoil
NOTESEconomic & Political Weekly EPW november 22, 200883very shallowly at 25 × 25 cm (or wider) spacing. This results in many more tillers (stems) being formed than with conven-tional transplanting practice, which helps to shade out weeds and increase yields [Stoop et al 2002]. This system deserves testing in arsenic-affected areas to find out the range of environmental conditions under which it might be suitable.2.4.4 Arsenic-Tolerant VarietiesResearch is in progress to breed arsenic- tolerant rice varieties. However, it needs to be borne in mind that the use of tolerant varie-ties does not reduce the rate of soil-arsenic accumulation. Therefore, as with the other agronomic practices described above, irri-gation with contaminated water will con-tinue to add arsenic to soils, albeit at a slower rate where dryland crops or dryland rice are grown. The real need is to prevent further additions of arsenic to soils, especially to soils where arsenic levels are already high.2.4.5 DrainageAllowing rice fields to dry out completely for 10-14 days prior to panicle initiation is practised to reduce arsenic uptake in the southernUS but this also reduces potential rice yields [Williams 2003]. This practice may not be acceptable to small farmers, therefore. It would also not be applicable to rice grown on seasonally-flooded soils in rotation with a dry-season crop irrigated with arsenic-contaminated water.2.4.6 Rainfed AgricultureIn areas where an alternative safe irriga-tion supply cannot be provided, farmers should be encouraged to increase crop pro-duction under rainfed conditions. The pos-sibilities will vary with climate, soils and hydrological conditions. Research studies and field trials should aim to maximise soil absorption and retention of rainfall and run-off by appropriate conservation and agronomic measures. Methods such as theSRI described above and minimum-tillage farming [FAO 2005] deserve study under appropriate environmental conditions.2.5 SoilAmendments2.5.1 IronFerric iron in various forms has been used in developed countries toimmobilise arsenic on dryland soils. Materials used include fer-rous sulphate and iron grit. In Asian coun-tries, it might be possible to use crushed brick or burnt soil for the same purpose. However, it needs to be investigated whether such materials would be effective in flood-irrigated soils in which the iron would eventually be reduced; and also whether the adsorbed arsenic would be remobilised to affect a following rice crop on seasonally-flooded soils.2.5.2 PhosphorusThe potential benefits of adding phos-phatic fertilisers to reduce arsenic uptake by crops also deserve testing under a range of soil and agronomic conditions. Conflicting results have been reported from pot trials. In principle, arsenic (present as arsenite) in reduced soils should not compete with phosphorus. Therefore, adding phosphate to paddy soils should not influence arsenic uptake by crops. However, the situation is differ-ent in oxidised soils where phosphorus and arsenic (as arsenate) compete for up-take sites on plant roots and so plant up-take of arsenic might be reduced where large amounts of phosphorus are added. The situation is complicated by the fact that arsenate and phosphorus also com-pete for adsorption by ferric iron, so the content of iron in soils (including that precipitated from high-iron irrigation water) may also influence the effect of added phosphorus on arsenic uptake. Pot trials need to be supplemented by field trials on reduced and oxidised soils as soon as possible.2.6 SoilRehabilitationWhen soil arsenic concentrations reach levels where crop yields are seriously reduced or arsenic uptake by rice exceeds dietary safety standards, methods to remove arsenic from soils will need to be adopted. The only practical methods known at present are described below.2.6.1 Hyperaccumulator PlantsSome plants have the ability to take up very large amounts of arsenic from soils. In pot trials in the US, Ma et al (2001) found that the fronds of brake fern (pteris vitta-ta) growing in soil material containing 6 mg/kg arsenic had accumulated 755 ppm arsenic after two weeks and 438 ppm after six weeks. At 50 mg/kg soil arsenic, they found 5,131 ppm arsenic in fronds at two weeks and 3,215 ppm after six weeks. Information was not provided on the bio-mass produced after these periods. Assuming production of 10 tonnes/ha, fern fronds containing 3,215 ppm arsenic could remove 32 kg/ha arsenic from soil. That is equivalent to several years’ input of arsenic from contaminated irrigation water.Brake fern could probably only be grown under dryland conditions. Trials are needed to see if this fern could be grown as a short-term crop in the dry season before an irrigated rice crop is planted. Several other fern species are also reported to be hyperaccumulators [Wei et al 2005], and so is Indian mustard [Mahimairaja et al 2005]. Water hyacinth (eichhornia crassipes) is also a hyper-accumulator but this plant grows in water, so its use might be to remove arsenic in settling tanks before water is distributed to fields. This possible use needs to be investigated.All these plants need to be tested to assess the range of environmental condi-tions under which they could be grown and their ability to remove arsenic from soils in meaningful quantities. At the same time, practical methods will need to be found for the safe disposal of the large quantities of arsenic-enriched plant material produced and to assess the health risk to people (especially children), livestock and wildlife eating the plants or inhaling dust from burnt material.2.6.2 SoilRemovalAs a last resort, it might be necessary to remove soil material that has become too heavily contaminated with arsenic to pro-duce satisfactory crop yields or rice of satisfactory quality. Fortunately, it is only the topsoil that becomes heavily contami-nated, so only 10-15 cm of soil material would usually need to be removed. Soil removal is not as drastic a remedy as it may seem. In Bangladesh, farmers com-monly sell soil material for brick-making and soil material is commonly removed from fields for making footpaths and em-bankments. After soil removal, farmers add farmyard manure, compost or dry
NOTESnovember 22, 2008 EPW Economic & Political Weekly84water hyacinth plants to restore soil ferti-lity and tilth; they also grow jute and deep-rooting legumes to help restore soil tilth.In fields where soils are already heavily contaminated, topsoil removal might be the quickest and simplest method to re-store crop production to acceptable levels. Trials are needed to find the most appro-priate methods for restoring soil fertility after topsoil removal and for safe use of soil material removed. Where rice is the preferred crop, particular care would need to be taken where topsoil removal might bring a permeable lower layer close to the surface and so lead to excessive irrigation demand and attendant risks of increased arsenic contamination. ConclusionsIrrigation with arsenic-polluted ground-water poses a threat to sustainable agri-culture and health in several parts of India, Bangladesh and Nepal. The scale of that threat is presently unknown. It urgently needs to be assessed, so that appropriate research studies and inter-ventions can be organised. Ongoing soil and crop research studies (mainly in Bangladesh) are providing valuable re-sults but they are far too few to provide the information needed for planning reli-able interventions in the wide range of environments that exist in affected areas. The development of a quick, reasonably reliable, field method for testing soil arsenic would greatly facilitate the assessment and monitoring of soil arsenic levels and methods of laboratory analysis need to be developed that are better adapted to paddy soils. Field trials need to be used much more extensively because it is practically impossible to simulate the physical, chemical and biological envi-ronment of a paddy soil in a pot experi-ment. Trials with possible mitigation measures need to be organised, taking into account the time that will likely be required to provide applicable results. In badly-affected areas, agricultural research, soil survey, soil laboratory, extension and possibly engineering institutions may need strengthening or reorganisation in order to provide appro-priate services to affected farmers. Gov-ernments, international aid donors and NGOs should consider these increased or changing needs in reviewing their devel-opment programmes in affected areas.ReferencesAbedin, Md J, M C Cresser, A A Meharg, J Feldmann and J C Cotter-Howells (2002): ‘Arsenic Accumu-lation and Metabolism in Rice (Oryza sativa L)’, Environmental Science and Technology, Vol 36, No 5, pp 962-68.Das, H K, A K Mitra, P K Sengupta, A Hossain, F Islam, G H Rabbani (2004): ‘Arsenic Concentrations in Rice, Vegetables and Fish in Bangladesh: A Pre-liminary Study’,Environment International, Vol 30, pp 383-87.Dittmar, J, A Voegelin, L C Roberts, S L Hug, G C Saha, M A Ali, A B M Badruzzaman and R Kretzschmar (2007): ‘Spatial Distribution and Temporal Varia-bility of Arsenic in Irrigated Rice Fields in Bangla-desh’, Environmental Science and Technology, Vol 41, No 17, pp 5967-72. Duxbury, J M and Y J Zavala (2005): ‘What Are Safe Levels of Arsenic in Food and Soils? 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