Arsenic in groundwater and contamination of the food chain: Bangladesh scenario

Ingestion of arsenic (As) contaminated groundwater is the major cause of As poisoning in Bangladesh. However, poisoning among the population is not consistent with the level of water intake. Moreover, there is also a spatial variation of the manifestation of arsenicosis in the country. This has raised the question about the role of the food habit, nature and amount of food intake in the As dilemma. Even if an As-safe drinking water supply is ensured yet, the same As contaminated groundwater will continue to be the main source of irrigation for about 40% of the net cultivable area as more than 60% of irrigation water comes from groundwater. This leaves a risk of soil accumulation of the toxic element and eventual exposure of the food chain to As contamination through plant uptake and animal consumption. Water (hand tube wells and irrigation pumps), soil and vegetables/crop samples collected from as many as 150 different locations covering 15 districts of the country have revealed that the average background concentration of As in Bangladesh is much below 10 mg/kg soil. However, in some areas where soils receive As-contaminated groundwater irrigation, the concentration has been found to be as high as 80 mg/kg soil. The soil As varies both spatially and vertically. The soil formation and the aquifer characters control the spatial variation, while the vertical distribution is controlled by the clay contents. The maximum As concentration in irrigation water was found to be 0.55 mg/L; irrigating a rice field with this water when the requirement is 1000 mm of water, it has been calculated that the As load will come to 5 kg As/ha/yr. Many crops receiving As contaminated water as irrigation have been found to accumulate As at levels that exceed the maximum allowable daily limit (MADL) of 0.2 mg per kg dry weight (dw). Some vegetables crops like Arum (Colocassia antiquorum), Kalmi (Ipomea aquatica), Amaranthus (Amaranthus spp.) etc. were found to be As accumulators. The transfer factor for As has been found to exceed the value of 0.1 in a number of plants indicating their affinity towards this element. In Arum, the concentration of As have been found to be as high as more than 150mg/kg dw. Rice and wheat receiving As-contaminated irrigation water have been found to sequester the toxic metalloid into roots and stems. However, the quantities of rice consumed per person per day with the content of As in the grain may in many instances, surpass the MADL.