Arsenic in paddy soils of Bangladesh: levels, distribution and contribution of irrigation and sediments M.R. Islam 1 , M. Jahiruddin 1 , G.K.M.M. Rahman 2 , M.A.M. Miah 3 , A.T.M. Farid 4 , G.M. Panaullah 5 , R.H. Loeppert 6 , J.M. Duxbury 7 and C.A. Meisner 5 1 BAU, Mymensingh; 2 BSMRAU, Gazipur; 3 BRRI, Gazipur, 4 BARI, Gazipur; 5 CIMMYT, Bangladesh; 6 Texas A&M University, USA; 7 Cornell University, USA 1 E-mail: mrislam58@yahoo.com Introduction Arsenic is widely distributed in nature. It is associated with igneous and sedimentary rocks, particularly with sulphide ores. Natural phenomena such as weathering, biological activity, and volcanic activity, together with anthropogenic inputs, are responsible for the emission of arsenic into the atmosphere, where from it is distributed on the earth’s surface by rain and dry fall out. Bangladesh is currently facing the challenge of high arsenic concentration in shallow aquifers ( Nickson et al. , 1998 ). A large number of hand and shallow tube wells (STW) in some of the localities of 59 districts out of 64 districts have been identified to have arsenic (As) concentration above the Bangladesh standard of 0.05 As mg l - 1 (Alam et al ., 2002 ). To meet up the growing demand for food, the farmers had to cultivate high yielding varieties of Boro rice, which requires a large volume of irrigation water. Irrigation with arsenic contaminated groundwater to rice increases its concentration in soil ( Jahiruddin et al., 2000 ; Meharg and Rahman, 2003; Ali et al., 2003) , and eventually arsenic enters into food chains through crop uptake and poses long term risk to human health ((Duxbury et al ., 2003; Islam et al. 2004a ) But detailed study on the levels and distribution of arsenic in irrigated paddy soils, and contribution of different sources of arsenic on build up of soil arsenic levels are still lacking in Bangladesh. This paper presents the levels and distribution and also quantifies the contribution of different sources of As on build up of As in paddy soils of five upazilas of Bangladesh. Materials and Methods Four hundred fifty six geo-referenced samples for each of irrigation water, soil (0-15 cm depth), rice grain and rice straw were collected from 456 STW command areas of five upazila of Bangladesh during the boro season of 2002. The upazilas were B.Baria, Faridpur, Paba, Senbagh and Tala. The soils of B. Baria and Senbagh were developed by the sediments deposited by Old Meghna river while the soils of the rest three upazilas were developed by Ganges river borne sediments. The soils were analyzed for clay, organic matter, pH, available P, free iron oxide along with total arsenic, phosphate extractable and oxalate extractable arsenic. Soil, rice grain and straw samples were digested with concentrated HNO 3 and H 2 O 2 . Total As in the digest and water samples was determined followed by flow injection hydride generation atomic absorption spectrophotometer (HG-AAS) with Unicam model 969 and MHS-10 hydride generator assembly using matrix-matching standard. Results There was a wide variation in total As concentration in the soil samples from different STW command areas of five upazilas (Table 1). The levels of total As in soils over the locations of the upazilas ranged from 1.0 –48.8 μg g -1 with a mean of 12.3 μg g -1 . The soils of the Ganges river floodplain (Paba, Tala and Faridpur) had higher soil As levels compared to those of Meghna river floodplain (B. Baria and Senbagh). The mean total As levels of 4.7, 6.5, 7.2, 19.4 and 19.6 μ g g - 1 were found in Senbagh, B.Baria, Paba, Tala and Faridpur upazila, respectively . Of the 456 samples, 53% of the soils had total As from 0-10.0 μ g g -1 , 26% soils had 10.1 – 20.0 μg g -1 , 17% soils had 30.1-40.0 μg g - 1, and the rest 1% soils had arsenic level >40.0 μ g g -1 (Fig 1).