Influence of soil type and properties on distribution and changes in arsenic contents of different paddy soils in Bangladesh

Abstract

Analysis of background As contents in soils is important for determining whether a soil is polluted or not. The total As contents of 66 archived surface soil (Ap horizon) samples (22 for 1967 and 44 for 1995) were determined using the RIX 2000 XRF system to analyze the distribution and changes in the As contents in the different physiographic units during the period 1967–1995 in Bangladesh. Arsenic distribution showed lower mean As contents of <6.22 mg kg⁻¹ in the soils of the Old Himalayan Piedmont Plain (OHP), TIsta Floodplain (TF), and Barind Tract (BT) in the northwestern region. Mean As contents in the soils of the Northern and Eastern Piedmont Plains (NEP) and those of the Brahmaputra Floodplain (BF), Madhupur Tract (MT), Meghna River and Estuarine Floodplain (MF), and Chittagong Coastal Plain (CCP) in the central and the southeastern regions ranged between 6.22 and 8.70 mg kg⁻¹. The soils in the Ganges Floodplain (GF) in the southwestern region and those of the Surma-Kushiyara Floodplain (SKF) in the northeastern region showed the higher mean As contents of <8.70 mg kg⁻¹. The mean As contents of 5.35, 7.85, and 9.52 mg kg⁻¹ were observed in the highland, medium lowland, and the lowland parts of the landscapes, respectively, in this study. An undifferenciated histosol also showed an As content of 12.3 mg kg-I in 1995 in Bangladesh. Results of this study suggested that soil properties (clay, total Fe content, ECEC, total Al and total C contents, pH, total S and total phosphorus contents) are important factors affecting natural background soil-As contents. The baseline soil-As contents should be defined appropriately based on the magnitude of the observed natural variations in the As contents among the different soils. This therefore suggested that site-specific As contents should be used as baseline levels for proper assessment of the potential As contamination in the soils of Bangladesh. Over a 27 year period, the As contents increased in the soils of Atwari (10%) and Jagdal (5%) in OHP; Pirgacha (8%) in TF; Belabo (25%) in BT; Ghior from Faridpur (19%) in GF; Kalma (12%) in MT; Tippera (9%) and Gumti (6%) in MY; Phagu (35%) in SKF and Harta series (12%) for peat soils. Also, the As contents decreased in the Kaunia series (13%) in TF; Dhamrai (20%) in BF; Sidhirganj (3%) in MF; Manu (14%) in NEP and all the soils of GF (4 to 40% decrease with a mean of 15%) except for the Ghior series from Faridpur. The increase or decrease of the As contents in the different soils was possibly due to local differences in the parent materials and biogeochemical activities. However, the mean As content in the surface soils remained almost unchanged (decreased by 4.82%) during the period between 1967 and 1995 in Bangladesh.

Key Words:

• arsenic contents
• Bangladesh
• distribution and changes of As
• paddy soils