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The fluvial plains of Bihar, in east India, has been recognized as a water surplus state; but, recurrent floods and drought, along with microbial and metalloid pollution and contaminations, pose a great challenge to cope with the increasing demand for clean water. Of these, the most severe health hazard source is groundwater contamination by arsenic (As), presently detected in 20 out of 38 districts of Bihar . Over 42% of the total state population falls below the poverty line, thereby limiting the state's option to agricultural intensification with expanded irrigation facilities. Bihar's agro-based economy has a predominance of bore well irrigation covering 55.3% of its irrigated area. Bihar Ground Water Irrigation Scheme (BIGWIS) aims to provide irrigation to 9.28 lakh hectare of agricultural land of the State by installing 4.64 lakh units of private shallow tube wells dig wells with pump sets over a period of 3 years end by 2011-12. Its ambitious Million Shallow Tubewell program (MSTP) is an important component in the Special Plan for Bihar under the 10th. and 11th. Five Year plans (Dept. of Agriculture, Bihar, 2010). Rice is the most important food crop in Bihar. Due to the procedure of its cultivation, As contaminated irrigation water leads to arsenic accumulation in the soils. So the crop cultivation takes place in As-contaminated soils under anaerobic conditions, at which As is highly available for plant uptake (Meharg, 2004). Hence, dietary intake of As poses a serious health threat to all consumers of the rice grown in the affected areas.
The objectives of this preliminary research in the state of Bihar, therefore, were to examine and reveal the status of As contamination in rice crops, laying emphasis in the locally grown, commonly consumed rice varieties; to study the impact of As accumulation on plant growth; to identify rice varieties with minimum uptake of As by the grain as a part of the As mitigation strategy; and, to generate preliminary data of the spatial extent of As toxicity in the food chain of the state.
Basic information on depth of bore wells, periods of their use, and pH values of the irrigation bore well water samples collected at source were obtained from the field and tested for As content by standard APHA method (APHA, 1989). Upon laboratory confirmation of presence of As in irrigation water, soil and rice plant samples from selected As hotspots, pot experiments were undertaken on six locally produced rice varieties (PNR-381,Turantha, Saroj, Sugandha, Sarna and Katarni). Thereafter, water, soil and plant samples, collected from the field and of the pot experiments, were tested for As content.
The soil samples were dried and tested for arsenic content by standard APHA method. The plant samples were also similarly dried and grounded and tested for As content by the same procedure.
Results and Discussions
The average depth of contaminated bore wells varied from 80 feet in north Bihar Plains up to 200 feet in south Bihar. There was significant spatial variation of As concentration among rice fields Irrigation water samples collected from 7 districts registered As contents of between 10.7 µg/l (Gaya) to 168 µg/l. (Bhojpur and Khagaria) with pH values varying from 6.8 to 7.3 at source. This indicated of the moderately reducing anaerobic environment of the groundwater, as increase in pH value result in oxidation of the arsenic compounds.
The soil samples collected from the plant root depths had 3-4 µg/l of arsenic, while the As uptake by rice plants and seeds were >10 µg/l. In all field samples a general trend of higher concentration of As in rice plants was noted as compared to arsenic in the soil. In Sangeetbaitha, in Kahalgaon Block, Bhagalpur District, the irrigation bore well (100 feet depth) had 102 µg/l , the root soil sample had 4 µg/g and mature paddy plant 13.6 µg/g of As. This suggests bio-accumulation of As by rice plant. Also oxidizing conditions may account for its lesser concentration in the soil.
Table. Arsenic accumulation in selected local rice species under pot experiments.
Provided As As accumulation in
Conc. (µg/L) Local Rice Varieties (µg/g)
Turantha PNR-381 Sarna Katarni
Control 0.010 0.010 0.003 0.002
50 0.013 0.028 0.045 0.023
500 0.136 0.042 0.055 0.052
1000 0.349 0.131 0.039 0.165
1500 0.122 0.150 0.033 0.044
2000 0.077 0.101 0.017 0.030
5000 0.065 0.050 0.012 0.007
In pot experiments, the responses of rates of As uptake, plant growth and appearance for different varieties were varied. Of the six varieties, Saroj and Sugandha were arsenic resistant. Sarna is the lowest hyperaccumulator of As. The other species registered peaks of As uptake with provided As concentration of 1000 - 1500 µg/L. Thereafter, there were abrupt decreases in arsenic accumulation, being negligible in Sarna and Katarni at the provided arsenic solution of 5000 µg/l. The results are indicative of possible inhibited As absorbing mechanism at higher concentrations.
In all pot experiments, lush plant growth was observed in provided concentration up to 1500 µg/l. Thereafter, with increasing concentrations of As solution the plant growths were retarded with gradual chlorosis at leaf margins, which indicated toxic effects of As accumulation. Previous pot studies (Jahiruddin et al., 2004) showed that higher As concentration in irrigation water (0.1 to 2.0 mg/L) resulted in lower yield of a local rice variety (BR-29). Absence of seed formation at higher levels of provided As concentration was also noted. According to Rahman (2007) this is indicative of Straighthead disease, a physiological disorder of rice (Oryza sativa L.) characterized by sterility of the florates/spikelets leading to reduced grain yield; There is a direct correlation of increase of soil arsenic concentration with the severity of Straighthead in rice plants (Rahman, et al, 2007)
This is the first confirmatory study of arsenic infiltration in the food chain of Bihar. The development of cultivars that assimilate less As, or restrict As translocation to fruits/seeds would lead to reduced dietary exposure to As (Meharg and Hartley-Whitaker, 2002), as found in the experiments on Saroj and Sugandha rice varieties.
Since rice is the staple food, any adverse effects on seed content of rice due to arsenic contaminated irrigation water would only enhance the malnutrition problem more so in Bihar where over 42% of the population live below the poverty line.
The physiology of As uptake and retention by different varieties of rice grown in Bihar need to be investigated further. Studies are also required to understand the mechanism behind the enhanced plant growth of these rice varieties against an average arsenic toxicity of 0.05µg/g to 0.50 µg/g.