Distribution and changes in heavy metal contents of paddy soils in different physiographic units of Bangladesh

The distribution and changes in the background levels of V, Cr, Mo, Ni, Cu, Zn, Pb, U, and As were assessed using 66 archived surface (Ap horizon) soil samples (22 for 1967 and 44 for 1995) analyzed by RIX 2000 x-ray fluorescence spectrometry to determine whether a soil was polluted or not during the period 1967-1995 in Bangladesh. Distribution of the mean V content showed lower values (<115.6 mg kg(-1)) in the soils of the Old Himalayan Piedmont Plain (OHP), Barind Tract (BT), and Madhupur Tract (MT), values ranging between 115.6 and 169.0 mg kg(-1) (mean +/- SD/(2)) in the Tista Floodplain (TF), Brahmaputra Floodplain (BF), Meghna River and Estuarine Floodplain (W), Northern and Eastern Piedmont Plains (NEP), and Chittagong Coastal Plain (CCP), and higher values (>169.0 mg kg(-1)) in the Ganges Floodplain (GF) and Surma-Kushiyara Floodplain (SY.F). The mean Cr contents were lower (<84.7 mg kg(-1)) in OHP, TF, BT, and MT, ranged between 84.7 and 107.1 mg kg(-1) in GF, BF, MF, NEP, and CCP, and a higher content (>107.1 mg kg(-1)) was recorded in SKF. The mean Mo contents ranged between 2.01 and 2.11 mg kg(-1) in all the physiographic units except for a lower content (<2.01 mg kg(-1)) in TF and a higher content (>2.11 mg kg(-1)) in the MT unit. The mean Ni contents were lower (<39.5 mg kg(-1)) in OHP, TF, and BT, ranged between 39.5 and 62.3 mg kg(-1) in GF, BF, MF, MT, NEP, and CCP and a higher content (>62.3 mg kg(-1)) was recorded in SKF. The mean Cu contents were lower (<23.4 mg kg(-1)) in OHP, TF, BT, MT, and NEP, ranged between 23.4 and 39.8 mg kg(-1) in MF, SYCF, and CCP, and a higher content (>39.8 mg kg(-1)) was recorded in GF and BF. The mean Zn contents were lower (<60.3 mg kg(-1)) in OHP, BT, MT, and NEP, ranged between 60.3 and 90.9 mg kg(-1) in TF, MF, and CCP, and a higher content (>90.9 mg kg(-1)) was recorded in GF, BF, and SKF. The mean Pb contents were lower (<21.7 mg kg(-1)) in TF, BT, and NEP, and ranged between 21.7 and 25.3 mg kg(-1) in the other physiographic units. The mean U content was lower (<3.47 mg kg(-1)) in NEP and an almost uniform distribution between 3.47 and 5.29 mg kg(-1) was observed in the other physiographic units except for a higher content (>5.29 mg kg(-1)) in TF. The mean As contents were lower (<6.2 mg kg(-1)) in OHP, TF, and BT, ranged between 6.2 to 8.7 mg kg(-1) in BF, MT, MF, NEP, and CCP, and a higher content (>8.7 mg kg(-1)) was recorded in the GF and SYCF soils. The contents of V, Cr, Mo, Ni, Cu, Zn, Pb, U, and As in the peat soil were 222.2, 111.1, 2.11, 63.4, 57.4, 108.8, 25.4, 5.28, and 12.3 mg kg(-1), respectively. However, the distribution of the V, Cr, Mo, Ni, Cu, Zn, Pb, U, and As contents was in the range of 43.3-275.0, 61.0-176.0, 1.90-2.21, 14.0-134.0, 8.7-65.7, 27.8-138.0, 14.3-31.0, 2.23-5.93, and 3.9-14.0 mg kg(-1), with mean values of 142.3, 95.9, 2.06, 51.0, 31.6, 75.6, 23.5, 4.38, and 7.5 mg kg(-1), respectively, in 1995. The heavy metal contents of the surface soils were, generally, higher on the lower slopes than in the middle or upper parts of the landscapes. A wide variation in the changes of heavy metal contents was observed among the soils of different physiographic units in Bangladesh. Changes in the V, Cr, Mo, Ni, Cu, Zn, Pb, U, and As contents showed higher increases up to 33.1, 25.0, 15.38, 28.5, 35.5, 24.9, 19.0, 22.10, and 34.6% and higher decreases down to 24.3, 13.1, 2.56, 36.4, 51.0, 21.5, 12.3, 18.98, and 40.0% respectively, with mean values remaining almost unchanged (-1.87 to +6.81%) except for a decrease of 17.1% Cu during the period 1967-1995. Results of this study suggested that the soil properties (clay, total Fe and Al contents, ECEC, total C content, pH, total S and P contents) are important factors regulating the natural background levels of heavy metals in the soils of Bangladesh.