Contamination of soil due to irrigation with arsenic laden water and its impact on phosphorus leading to crop production in Bangladesh

Md. Arif Hasan, Research Associate, Department of Soil, Water & Environment, University of Dhaka, Dhaka, Bangladesh

Bangladesh occupies a territory in the north-western part of the Indian subcontinent above the Bay of Bengal. It has an area of 147,570 km2 and a population of 125 million with 75 to 80% living in the rural areas. Currently 97% of the population of Bangladesh use tube well water for drinking, cooking and irrigation purposes, and surface water is also used for some domestic and agricultural purposes. According to the Department of Public Health and Engineering (DPHE) sources, there are 855,996 shallow tube wells in rural areas. During the last seven years, clinical symptoms relating to arsenic toxicity have been detected in millions of rural Bangladeshis. Many deaths have also been reported. This discovery leads to numerous monitoring studies in Bangladesh. Of the 8065 tube wells sampled from 60 districts, tube wells in 40 districts revealed arsenic concentrations exceeding 0.05 ug/ml (the ma ximum permissible limit of arsenic for drinking water, WHO) with the rest showing arsenic concentrations exceeding 0.01 ug/ml, the WHO recommended value of safe water.

Arsenic is widely distributed in nature and classified as metalloid, which can exist both in solid and liquid states. Although arsenic occurs in rocks and minerals like arsenopyrites, realgar, orpiment and arsenolyte, it is also obtained from smelting of copper, lead, zinc, gold and silver. Anthropogenically, it also enters into the environment from burning of fossil fuels, agricultural and sivicultural products, pesticides, wood preservatives, etc. Arsenate, arsenite methyl arsonic acid and dimethyl arsonic acid are usually found in water bodies. When arsenic comes from these above sources to water and soil, then human, animal and plants are severely affected. It causes deaths to several kinds of lives. The three major biochemical actions of arsenic are coagulation of protein, complexion with coenzymes and uncoupling of phosphorylation. The ability of As(III) to inhibit ATP production results in the c essation of organ functions rapidly especially in the event of acute arsenic poisoning. In contrast, As(IV) induces its toxicity in uncoupling mitochondrial oxidative phosphorylation by acting as inorganic phosphate.

Geologically, arsenopyrites are thought to be the sources of arsenic in the sediments., varying in depth from 40-250 ft. below the surface soil. This depth is subjected to oxidation and reduction of arsenopyrites due to withdrawal of water for domestic and irrigation purposes in agricultural soils. Such oxidation-reduction and some other physico-chemical changes are supposed to be responsible for soluble arsenic whose concentration varies up to 30 mg/l (permissible limit 0.01 mg/l). The irrigation water, which is uplifted from ground level by shallow and deep tube wells in rural areas of Bangladesh, add arsenic to the surface soil. The plants then uptake arsenic from this surface soil and become contaminated. The allowable concentration in fruits, crops and vegetables is 2.6 mg As/kg fresh weight. Permissible limit of arsenic in agricultural soils is 20 mg/kg soil, while 5 ppm arsenic in soil is found toxic to sensitive crops. The range for cereals is highly variable as the critical values in rice plants, both top and roots and barley seedlings were found 100, 1000, 20 ppm respectively. These values are much higher if crops are grown on contaminated soils that are irrigated with arsenic laden water. Many of the reported incidences relating to arsenic uptake by plants have been conducted with soils that have high capacity to retain arsenic. In such soils, the phyto-availability of arsenic is low unless competing anions such as phosphate are introduced through fertilizer applications into the soil system.

Plant availability of arsenic may vary depending on the soil pH, nature of minerals constituting the soil and other competing ions present in soil solution. For instance, in soils with high pH, the plant availability of arsenic may be high due to the presence of soluble arsenic in soil solution, whereas soils rich in oxidic material exhibit low arsenic bioavailability. The plant availability of arsenic may be influenced by the presence of competing ions such as phosphate, sulphate and dissolved organic carbon. Of these anions, numerous investigators have reported the effects of phosphorus on arsenic uptake. Both synergistic and antagonistic effects of phosphorus on uptake of arsenic by plants have been reported. Phosphate has been shown to either increase or decrease the uptake of arsenic by plants from soil solution. The increased availability of arsenic is particularly noticeable in low arsenic sorbi ng soils, such as sandy soils where the added phosphorus may displace some of the bound arsenic into the soil solution. In soils with adequate phosphorus supply, preferred uptake of nutrient phosphorus may reduce the potential for plants to hyper accumulate of arsenic. There appears to be a higher affinity for phosphorus than arsenic with a discriminate ratio of 4:1, while studying the potential for removal of arsenic from solution by water hyacinth (Eichhornia crassipes Solms). It has been found that high concentrations of phosphorus inhibit arsenic by the plants. Other soil amendments including lime, sulfur and nitrogen have also been observed to alleviate or depress the availability of arsenic to plants.

Reviewing the literature on arsenic, some scientists concluded that soil type is the only significant variable when considering plant phyto-toxicity for inorganic arsenic. The nature of arsenic species in soil solution may also determine the phyto-toxicity. Although arsenic is primarily present as As(V) or As(III) in soil-water environments, monomethyl arsonic acid (MMAA) and dimethyl arsonic acid (DMAA) may also be present. Both As(III) and monomethyl arsonic acid are phyto-toxic to rice plants grown in nutrient solutions and the degree of arsenic uptake by rice followed by trend as below:

As(III) > MMAA > As(V) > DMAA

The symptoms of arsenic phyto-toxicity vary with plant species. Tomato plants grown in soils with high arsenic background concentrations (100-130 mg As/kg) show leaf dieback from the tip and poor quality fruit set. Fruit plants grown on replanted orchard sites commonly exhibit retarded early growth, to which arsenic toxicity may contribute, similarly, rice grown on former cotton producing soils that had a history of repeated MMAA applications show indications of susceptibility to straight head disease (abnormally developed or sterile flowers resulting in low grain yields) under flooded conditions.

Socio-economic significance of the project

The rural economy in Bangladesh is primarily based on agriculture, which employs about 70 to 80% of the labour force. Annual income of farmers in this country ranges from 5000 to 8000 taka and amongst the lowest in the in the world. Primarily because of the low income, the rural population relies largely on local crop production for survival. Many people in Bangladesh rely on vegetables for consumption four to five days per week with the rest of meals being either fish or chicken. Local vegetables and rice production has increased substantially during the rest 30 years following the installation of tube wells. Crops including paddy rice and a range of vegetable crops are generally used for local consumption, selling to urban communities and for export. For the production of these crops, these are irrigated with groundwater from tube wells in aquifers that are rapidly recharged during the wet summer mon ths. Arsenic poisoning of local residents has major impacts on the local economy including:

  • Costs incurred for remedial purposes (taka 40 per hospital visit plus pharmaceutical costs) following arsenocosis;
  • Loss in income due to declining ability of farmers to work in the field following arsenocosis;
  • Loss of crop productivity because of arsenic phyto-toxicity. Public Health Engineering and Water Supplies report 10% loss in productivity due to arsenic phyto-toxicity based on Japanese studies;
  • Economic loss through the impact of arsenic on food quality and subsequent impact on marketability of the crops with local losses ranging from 1000 taka to 9000 taka per week per vegetable growing family;
  • Impact on soil quality where arsenic is present in elevated levels; and
  • Partial economic loss through the decline in consumer confidence. This is now becoming a major issue as evident from labels on bottled water and signs in the vegetable markets warning people of As-free products. Crops originating in those regions where severe cases of arsenic toxicity have been recorded may not be marketable. This impacts on the economic sustainability of the farmers.

    Thus the consequent adverse impact of arsenic ingestion to human and animal health has led to significant economic loss through human loss, health and medical costs and decline in crop productivity. The impact on human health and associated medical costs will be an ongoing problem given that in Bangladesh, arsenic contaminated crops are sold in the urban and city environment for local consumption.

    The continued presence of arsenic in soil will have long-term effect on crop productivity and quality. This will in turn impact on the local and international economy although this is difficult to quantify at this stage.

    Although arsenic associated diseases are not contagious, people suffering from such diseases are facing serious social problems. They can't mix with people who hate them and have matrimonial problems. They are not allowed to sit in a tea stall and waiting for losing jobs. People suffering from arsenic induced diseases are working in their offices in most cases concealing their diseases.

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