Accumulation of arsenic in agricultural soil and selected crops

Abstract

Groundwater from arsenic contaminated shallow aquifers is widely used for irrigation in Bangladesh. In 2004, groundwater irrigation through shallow tubewells covered about 60% of the total irrigated area. Considering I m of irrigation for bora rice, the amount of As cycled each year through irrigation water is estimated to be about 1000 metric tons. In this study, twelve bora (dry season paddy) fields in four arsenic affected and two unaffected areas have been monitored during 2003 for assessing As status in irrigation water, soil and paddy plant. Irrigation with arsenic-bearing groundwater is definitely causing an increase of arsenic content of paddy field soils. Arsenic contents of paddy field soils, irrigated with high arsenic-bearing irrigation water, have been found to vary significantly over a paddy field, with depth and with time. In general, arsenic concentrations in the top soil layers (up to ISOmm) increased significantly at the end of the irrigation season. In general, higher increase in soil arsenic content was observed for sampling points located close to the entry points of water into different sub-areas of the paddy fields. For example, for a paddy field in Munshiganj, average arsenic concentration in the top 0-75 mm of soil increased from about 8.4 mg/kg in March 2003 to 14.8 mglkg right at the end of the irrigation season in May 2003. During the same period, it increased from about 4.7 to 8.0 mg/kg for a field in Comilla, 6.7 to 10.3 mg/kg in Brahmanbaria, and 5.7 to 7.2 mg/kg in Faridpur. Arsenic contents in the top soil layers at the end of the irrigation season have been found to be strongly correlated to the arsenic content of irrigation water. In the unaffected areas of Bogra and Naogaon, where arsenic concentration in the irrigation water was below I I-lg/I,arsenic levels in irrigated soils have been found to be comparatively low (varying from about 1.5 to 3.1 mg/kg) and did not vary significantly either with depth or sampling time. Arsenic concentrations in the paddy field soils have been found to be quite dynamic. After the rainy season (which immediately follow the bora season), arsenic concentration in the top soil layer (0-150 mm) of paddy fields in the arsenic affected areas decreased significantly and came down to levels comparable to those found at the beginning of the bora season. Since the majority of the As in the top 0-75 mm segment of soil layer is rassociated with iron oxyhydroxides, this is most likely due to partitioning of As from soil into the aqueous phase during inundation by reductive dissolution of iron oxyhydroxides and desorption. Thus, long-term As accumulation in agricultural soil appears to be counteracted by bio geochemical pathways leading to arsenic removal from soil. In both arsenic affected and unaffected study areas, the roots of paddy plants accumulated the maximum level of arsenic, followed by leaf and stem. Paddy grain and husk accumulated the least amount of arsenic. The arsenic content in top soil was found to be strongly correlated with arsenic concentration in root, moderately correlated with arsenic concentration in leaf and stem, and poorly correlated with husk and grain. Arsenic concentrations in grain and husk of samples from affected and unaffected areas did not differ significantly, although arsenic concentrations in both grain and husk have been found to be statistically higher in samples from the affected areas. Irrigation with arsenic bearing groundwater appears to be causing a minor increase in human exposure to arsenic. Considering average arsenic contents of rice grains from affected and unaffected areas and average rice consumption of 450 gm/person/day, daily As intake in affected areas is estimated to be slightly higher (233 I-lg)compared to that in unaffected areas (176 I-lg).Since As toxicity is strongly dependent on its chemical form and reported values for