Tarit Roychowdhury
Geogenic arsenic (As) that pollutes groundwater in South and South-East Asian countries like India and most parts of Bangladesh has become a massive issue of deteriorating human health in recent times. The Ganga-Bramhaputra-Meghna plain which covers West Bengal is at a higher risk. Besides ingestion of As tainted water, the inhabitants of these zones are also exposed to As by consuming crops and vegetables which are generally grown on contaminated fields. Presence of elevated level of As in rice (Oryza sativa L.), being consumed by 50% of the world’s population and human exposure to As through rice consumption is a worldwide health concern. In West Bengal, India, rice is the major staple crop which is cultivated in different agro-ecosystems throughout the year in uplands as well as in rain-fed lowlands. The countries that receive moderate rainfall like India and Bangladesh generally practice two types of paddy cultivation: Boro cultivation (pre-monsoon), which is mainly carried out with natural groundwater and Aman cultivation (monsoon) which sometimes involves groundwater due to insufficient rainwater supplies. The fact is widely evidenced that during pre-monsoon cultivation, As from irrigation water and soil causes phyto-toxicity by translocation through the root of the paddy plants in the rhizosphere, showing accumulation in plant parts like root, shoot, grain etc. On the other hand, during monsoonal cultivation in July-October, accounting for 69 % of the total rice production in West Bengal, As in the top layer of soil is reduced due to seasonal flooding for which the topsoil arsenic concentration decreases with temporal variability. This causes diffusion of As into floodwater, followed by lateral removal with receding water and the As movement increases to the deeper soil layers by infiltration.
Arsenic is translocated from root to aerial parts in descending order and least amount of As is accumulated in rice grain. Monsoonal cultivation provides least As enriched grain, irrespective of the variety of cultivar and area of cultivation, which amounts to one-third of pre-monsoonal grain and so, it is much safer for consumption with respect to As and other micro-nutrients status, like Zn and Se.
Contamination of food and water by Arsenic:
Arsenic-contaminated paddy is further subjected to a greater As accumulation as a result of being processed in contaminated water, as a part of the regular parboiling procedure followed after the crop has been harvested and this showed higher accumulation of As in parboiled rice grain compared to raw or sunned rice grain. Investigation on the role of parboiling procedure of rice being cultivated and processed in As prone areas suggested that parboiling of rice with As-contaminated groundwater contributes majorly in the increment of As concentration in parboiled rice and its by-products. A large part of the aforesaid population uses parboiled rice as their staple food. Arsenic accumulation in paddy due to use of contaminated groundwater during its harvesting and the subsequent impact on As enrichment of the crop are further environmental challenges. What is further worrisome is that transportation of the As-contaminated crops and vegetables grown using As-contaminated groundwater and soil in As-exposed areas to the uncontaminated sites and consequent dietary intakes leads to great threats for the population residing in non-endemic areas, irrespective of drinking As-free water.
Is Arsenic Carcinogenic:
Arsenic has been listed in the category of Group 1 carcinogen by International Agency for Research on Cancer (IARC). Compared to other crops, such as wheat, rice can absorb up to ten times more As. Accumulation of As in rice varies widely from different geographical locations and among different rice cultivars. Inorganic As (mainly As+3 and As+5), the most toxic and carcinogenic contributes approximately 90% of total content of As in rice grain. Arsenic accumulation in rice grain, that is mainly locally grown and the consequently cooked rice poses a major risk to the populations of Bengal delta where rice and its derivatives form a part of staple diet. In terms of chronic As toxicity, the consumption of cooked rice plays a significant health threat. Several factors like raw rice grain As, cooking water As, cooking practice of the inhabitants, rice cultivars, role of micronutrients or other heavy metals and As species distribution might influence As-accumulation in cooked rice. Several other factors like washing of rice grain, rice grain-water ratio might influence cooked rice As-accumulation.
It is a long-standing debate whether the water in which the rice grain is cooked, plays an additive or synergistic or any role at all towards achieving the final As content in cooked rice. While cooking rice in contaminated water, the threshold As concentration of water above which water As accumulates in cooked rice is not well demarcated. The movement of As from rice grain to water and vice versa during the cooking process is a further topics of research interest. For majority of the rice grains (sunned and parboiled) and different cultivars, As moves from rice to water provided low-As (<3 μg/l) or moderate As-contaminated water (36–58 μg/l) is used for cooking with increasing of selenium (Se) concentration and a significant decrease of As has been observed in cooked rice (up to 89% and 61%), respectively. However, this scenario changes with increased ‘water As’, movement of As from water to wet cooked rice has been inferred with increasing water As (84–105 μg/l), which results in a significant increase of As in cooked rice (up to 114%) with decreasing of Se concentration. There exists a threshold value of water As content above which As percolates into the cooked rice.
Arsenic speciation study emphasizes the fact of similar reduction % of As (III), As (V) and total As in wet cooked rice when cooked with low-As containing water. The risk of As toxicity can be reduced by using raw rice, cooking in excess water (1:3) that is not contaminated and discarding the excess water (gruel). SAMOE value in ‘risk thermometer’ supports the higher risk of suffering from wet cooked rice (class 4) with increasing cooking water As concentration (class 3 to class 5).
Sources: Group of recent research publications of ‘School of Environmental Studies’, Jadavpur University.
About Author
Professor of School of Environmental studies, Jadavpur University.
