Study on arsenic in Irrigation water and plant-soil environment in Bangladesh

Abstract

Arsenic in groundwater and its filte and transport in the environment have become matters of great concern in Bangladesh, India and several other countries. In Bangladesh tubewell water extracted from an estimated 7.5 to 8.0 million hand tubewells is the primaI)' source of drinkinglcook1ng water for most of its rural population. Besides domestic use, huge quantities of water from shallow aquifer are also used for irrigation during the dl)' season. A total of 865213 shallow tubewells and 23 I 82 deep tubewells were used for irrigation during the 200 I boro season. Widespread use of groundwater for irrigation suggests that ingestion of irrigated crops could be another major exposure route for arsenic. Besides, phytotoxicity due to increased arsenic in soil/water and its long-term impact on agricultural yield is another major concern. According to the present study, an estimated 900 metric tons of arsenic could be cycled each year through irrigation water. The estimates are particularly high for south-western and south-central (excepting the hill districts) regions of Bangladesh, where both irrigation intensity and arsenic concentration in shallow wells are reletively high. In this study, two arsenic affected areas (Srinagar and Sonargaon) have been selected for detailed characterization of irrigation water, soil and crop/vegetables produced in irrigated soils. A few samples were collected from arsenic affected Comilla and Noakhali districts. Water, soil and crop samples were also collected from an unaffected area in the Dinajpur district in northern Bangladesh. Besides, soil and plant samples were also collected from potato fields at Srinagar site and vegetable fields at Sonargaon site. Except for one, all these vegetable fields were irrigated by surface water. Arsenic concentration in the well water used for irrigating the rice field site at Srinagar varied from 220 to 537 ppb; and for the Sonargaon site it varied from about 83 ppb to 354 ppb. At both sites, arsenic concentrations of irrigation well water varied significantly with time. Arsenic concentrations in all groundwater samples from the Dinajpur district were below detection limit (i.e., < Ippb). Concentration of arsenic in the surface water bodies used for irrigation was significantly lower. Arsenic profiles in the irrigated rice fields at the Srinagar, Sonargoan and Comilla sites show higher accwnulation of arsenic primarily in the top layers (typically top 75 to 150 rom) of soil. In general, arsenic concentration in soil decreased with depth. Arsenic accumulation in the irrigation canal samples was relatively higher compared to the field samples. Compared to the rice field, arsenic concentrations in the soil samples collected from the vegetable fields were significantly lower. For example, mean arsenic concentration in the top soil layer (top 75 rom) at the Srinagar site was 7.8 mg/kg, compared to 14.5 mg/kg for rice field samples. For the Sonargoan site, the mean for the top layer (top 75 rom) of vegetable field samples was 3.5 mg/kg, compared to 8.9 mg/kg for the rice field samples. It is apparent that the higher arsenic concentration in the rice field soil samples, both at Srinagar and Sonargoan sites, is due to the presence of high level of arsenic in the irrigation water. Analysis of arsenic content of rice plant samples collected from Srinagar, Sonargaon and Dinajpur sites showed that the roots of rice plants accumulated the maximum level of arsenic, followed by leaf and stem. Rice grain and husk accumulated the least amount of arsenic. Two grain samples (out of nine) from Srinagar and one (out of twelve) from Sonargaon exceeded the Australian food hygiene limit of 1.0 mg/kg. The level of arsenic in root, leaf and stem of the plant samples collected from Dinajpur (an arsenic-free area) were found to be lower compared to those found in the samples collected from the Srinagar and Sonargoan sites. However, statistically significant difference was found only for arsenic contents in rice plant roots. Thus it appears that arsenic present in irrigation water and soil results in higher level of arsenic especially in rice plant roots. There was no significant difference in arsenic levelspresent in the rice grainsand rice husks. Results of analysis of arsenic in different parts of potato and potato plant showed relatively lower level of accumulation. Highest accumulation of arsenic was found in the root of potato plants (up to 2.9 mg/kg). However, these levels were significantly lower than those found in rice roots. Arsenic concentration in the edible parts varied from 0.12 to 0.85 mg/kg [dry (oven dried at 65°C) weight basis], all below the Australian food hygiene standard. Results of arsenic concentration in different parts of tomato, lalshak, datashak, cabbage and cauliflower samples collected from the Sonargoan site also showed relatively low level of accumulation. Among the different parts, arsenic accumulation in roots was found to be the highest. Arsenic concentration in edible parts of lalshak ranged from < 0.39 to 0.96 mg/kg; for datashak it ranged from 0.56 to 1.06 mg/kg, for cabbage 0.38 to 1.6 mg/kg and for cauliflower 0.35 mglkg. Arsenic concentrations in the five tomato samples ranged from 0.18 to 1.33 mg/kg. However, since all the vegetable fields (except one) were irrigated by pond or canal water, effect of arsenic bearing irrigation water on these vegetables could not be assessed from these results alone.