Saturday, 15 July 2006

Fate of Arsenic in Some Soils of Bangladesh.

S.M. Imamul Huq1, AFM Manzurul Hoque2, J.C. Joardar3, and J.U. Shoaib2. (1) Dept of Soil, Water and Environment, Univ of Dhaka, Nilkhet, Dhaka, Bangladesh, (2) Soil Resources Development Institute, Krishi Khamar Sarak, Dhaka, 1215, Bangladesh, (3) Bangladeh-Australia Centre for Environmental Research (BACER-DU), University of Dhaka, Dhaka, 1000, Bangladesh

There are concerns about the fate of arsenic in soils irrigated with arsenic contaminated ground water. It is not certain whether the arsenic is accumulating in soils or moving downward through the soil profile to be recharged in the ground water. Study on seasonal mobility of arsenic and its persistence in profiles of selected soils (Aeric Endoaquept) within a catena in the Old Meghna Estuarine Floodplain a geologic formation having arsenic contaminated aquifers - reveals that arsenic accumulates both in irrigated and in non-irrigated lands to different extent during dry season. Land types (based on inundation depth during monsoon) and duration of seasonal flooding were found to have a bearing on the movement and accumulation of arsenic in soil profile. At the same time, internal drainage condition of the soils and intensity of rainfall may also direct the fate of arsenic in soils. Noticeable accumulation of arsenic after irrigation was evident in all soil horizons of highland and medium highland. But in irrigated medium lowland, arsenic accumulated in surface horizons (Ap1g and Ap2g) and deeper C2 horizon (below 0.9 m). Rainfall intensity during dry season, soil physical properties, particularly the compact plough pan (Ap2g horizon) might govern the arsenic movement and accumulation in subsoil horizons. On the other hand, capillary rise of arsenic in all non-irrigated lands was evident in dry season indicating that arsenic could act as a soluble salt. During monsoon, the depth and duration of submergence vary with land types. After monsoon, remarkable variation in leaching and accumulation of arsenic in horizons of different irrigated and non-irrigated land was observed. Soils collected before and after monsoon from different horizons of both irrigated and non-irrigated lands showed varying phenomenon in the retention of arsenic. In imperfectly drained irrigated highland, increased accumulation of arsenic in upper horizons (Ap1g and Ap2g) and concomitant decrease in the arsenic content in lower horizons (Bw1g, Bw2g, B3g, C1 and C2) were apparent after monsoon. But in the same non-irrigated land type, more arsenic accumulation was observed in upper and lower part of the profile (Ap1g, Ap2g, B3g, C1 and C2 horizon) and the content of arsenic decreased in the subsoil horizons (Bw1g and Bw2g) at 0.15-0.65 m. In poorly drained irrigated medium highland submerged with 0.6-0.9 m water column for 2-3 months, arsenic content decreased in horizons (Ap1g, Ap2g and Bw1g) existing in the upper 0.36 m. and As accumulation was clear in Bw2g to C2 horizon (0.36 to 1.20 m). Whereas in non-irrigated medium highland submerged with 0.3-0.6 m water column for 1-2 months, an increasing trend of arsenic accumulation was observed in all horizons after monsoon. On the other hand, in poorly drained irrigated medium lowland, usually inundated for 4-5 months with 1.2-1.5 m water column during monsoon, leaching and accumulation was similar to that in medium highland except for C2 horizon where arsenic content dropped off. Whereas in non-irrigated medium lowland that remains inundated for 5-6 months with 1.5-1.8 m water column during monsoon, arsenic leaching was evident in upper 0.65 m with subsequent accumulation of arsenic in deeper C1 and C2 horizons. Variation in the duration and depth of submergence of soil in the same land type by irrigation or by natural water that control soil properties like clay, iron and organic matter contents might play an important role in determining the fate of arsenic in soils.

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