Clay Science 10, 375-384 (1998) MINERALOGICAL APPROACH TO ALTERNATION OF DIFFERENT RIVER SEDIMENTS IN MEGHNA FLOODPLAINS SOILS OF BANGLADESH ABU ZOFAR MD. MOSLEHUDDIN,S.M. SAHEED1 and KAZUHIKO EGASHIRA Laboratory of Soils, Faculty of Agriculture,Kyushu University,Fukuoka 812-8581,Japan 1Former Director , Soil ResourceDevelopment Institute, Krishi Khamar Sarak, Dhaka, Bangladesh (Received September 2, 1998. Accepted October 23, 1998) ABSTRACT Mineralogical studies of some Meghna Floodplains soils, in which the sediments as parent materials were possibly transported by the Meghna, Old Brahmaputra, Jamuna (Brahmaputra), and/or Ganges Rivers, were carried out to determine the contribution of different river sediments in Lower Meghna River Floodplain, Young Meghna Estuarine Floodplain, and the part of Old Meghna Estuarine Floodplain which is surrounded by Ganges Floodplains. In general, mica, smectite, vermiculite, chlorite and kaolinite were the major clay minerals in these soils. Soils of Lower Meghna River Floodplain were dominated by mica, vermiculite and chlorite, and were estimated to be developed from the Jamuna (Brahmaputra) sediments while the contribution of the Meghna/Old Brahmaputra sediments increased towards the surface. The Ganges sediments had apparently no contribution. In contrast, soils of Young Meghna Estuarine Floodplain contained good amounts of smectite and chlorite in addition to mica, and were estimated to be developed from the Ganges and Meghna/Old Brahmaputra sediments.Due to the predominance of smectite in the Ap and B horizons or Ap horizon, soils of the part of Old Meghna Estuarine Floodplain which is surrounded by Ganges Floodplains were estimated to consist of the Ganges sedimentsin the upper horizons and of the Meghna/Old Brahmaputrta sediments in the lower horizons. Key words: Mineralogical approach, Interstratification of sediments, Meghna Floodplains soils, Bangladesh INTRODUCTION Eighty percent area of Bangladesh has been developed from sediments carried by different rivers. The Ganges, Jamuna (Brahmaputra) and Meghna are the major rivers of the country. The Meghna River is the third largest river in the country and the area developed on the meander and estuarine floodplains of the Meghna River occupies about 17% of the whole floodplain area. The Meghna floodplain area is divided into four agroecological regions (AEZ): Middle Meghna River Floodplain, Lower Meghna River Floodplain, Young Meghna Estuarine Floodplain and Old Meghna Estuarine Floodplain (FAO-UNDP, 1988). The whole area of Meghna floodplains is not contributed only by the Meghna sediments, but rather there is admixture with the Jamuna (Brahmaputra) and 376 A.Z. M Moslehuddin et al. Ganges sediments. From the viewpoint of geologic history and physiography of rivers in Bangladesh, the Brahmaputra River changed its previous course (presently called as the Old Brahmaputra River) into the present Jamuna River more than 200 years ago. That time the present Meghna River was the estuary of the Brahmaputra River. Now, the Jamuna River joins with the Ganges River at Goalandaghat (about 210km upstream from the coast of the Bay of Bengal) in the Lower Ganges River Floodplain region, keeping the name as the Ganges River. Later, the Meghna River joins them at Chandpur (about 100km upstream from the coast of the Bay of Bengal) in the Meghna Floodplains regions, and the name of the combined three rivers remains as the Meghna River until flowing into the Bay of Bengal. The Old Brahmaputra River joins with the Meghna River at Bhairab (about 80km upstream from the joining point of the Ganges and Meghna Rivers) in the Middle Meghna River Floodplain region. Mineralogy of soils developed on Old Brahmaputra Floodplain and Old Meghna Estuarine Floodplain was almost similar to each other and comprised mainly mica, chlorite, vermiculite with some kaolinite; the soils developed on Active Brahmaputra- Jamuna Floodplain and Young Brahmaputra and Jamuna Floodplain were rich in mica, vermiculite, kaolinite with some vermiculite-chlorite intergrade (Mia, 1990; Alam et al., 1993; Aramaki, 1996; Moslehuddin and Egashira, 1996). Soils developed from the sediments carried by the Ganges River have been reported to be rich in smectite along with mica and sometimes vermiculite (Islam and Lotse, 1986; Egashira and Yasmin, 1990; Alam et al., 1993; Moslehuddin and Egashira, 1996). Among the four AEZs of Meghna Floodplains, mineralogical studies have been concentrated on soils of Old Meghna Estuarine Floodplain (Mia, 1990; Alam et al., 1993; Aramaki, 1996; Moslehuddin and Egashira, 1996). In soils of Middle Meghna River Floodplain the similar mineralogical composition to that of Old Meghna Estuarine Floodplain is expected, but soils of Lower Meghna River Floodplain and Young Meghna Estuarine Floodplain may have some contribution from the Ganges and Jamuna (Brahmaputra) sediments. In addition, the part of soils of Old Meghna Estuarine Floodplain, that is surrounded by Ganges Floodplains, may have the contribution of the Ganges sediments. Therefore, the present study was undertaken to clarify the contribution of different river sediments in Lower Meghna River Floodplain, Young Meghna Estuarine Floodplain, and the part of Old Meghna Estuarine Floodplain which is surrounded by Ganges Floodplains by the mineralogical approach. MATERIALS AND METHODS Soils used Topsoil (Ap horizon), subsoil (B2 horizon) and substratum (C or C1 horizon) samples (in case of Ramgati (1) soil, samples from Ap, C1 and C2 horizons) were collected from five soil profiles: two from Lower Meghna River Floodplain, one from Young Meghna Estuarine Floodplain, and two from Old Meghna Estuarine Floodplain surrounded by Ganges Floodplains. In addition, a sample from exposed C horizon (at 3 m depth) and a Mineralogical Approach to Alternation of Sediments 377 FIG. 1. Soil sampling sites shown on the map of agroecological regions of Bangladesh. Soil samples: a. Faridganj (1); b. Faridganj (2); c. Ramgati (1); d. Ramgati (2); e. Ichakhali khal; f. Wazirpur; and g. Batajor. Agroecological regions: 1. Old Himalayan Piedmont Plain; 2. Active Tista Floodplain: 3. Tista Meander Floodplain; 4. Karatoya-Bangali Floodplain; 5. Lower Atrai Basin; 6. Lower Pur- nabhaba Floodplain; 7. Active Brahmaputra-Jamuna Floodplain; 8. Young Brahmaputra and Jamuna Floodplain; 9. Old Brahmaputra Floodplain; 10. Active Ganges Floodplain; 11. High Ganges River Floodplain; 12. Low Ganges River Floodplain; 13. Ganges Tidal Floodplain; 14. Gopalgonj-Khulna Bils; 15. Arial Bil; 16. Middle Meghna River Floodplain; 17. Lower Meghna River Floodplain; 18. Young Meghna Estuarine Floodplain; 19. Old Meghna Estuarine Floodplain; 20. Eastern Surma-Kusiyara Floodplain; 21. Sylhet Basin; 22. Northern and Eastern Piedmont Plain; 23. Chittagong Coastal Plain; 24. St. Martins Coral Island; 25. Level Barind Tract; 26. High Barind Tract; 27. North-Eastern Barind Tract; 28. Madhupur Tract; 29. Northern and Eastern Hills; and 30. Akhaura Terrace ( FAO-UNDP, 1988). 378 A.Z. M Moslehuddin et al. freshly deposited river mud sample were collected, both from Young Meghna Estuarine Floodplain. The soil samples were brought to Japan by air, and then air-dried, crushed with a wooden pestle, passed through a 2-mm sieve and preserved in plastic bottles for subsequent analyses. Sampling sites are shown on the map of agroecological regions of Bangladesh in Fig.1 with locations of major rivers. Important features of these samples are described in Table 1. Determination of pH, electrical conductivity (EC) and carbonate content The pH was measured by a glass-electrode pH meter in the soil suspension having soil: water ratio of 1: 2.5, after 30-min shaking. The EC was measured by a EC meter in the soil suspension having soil: water ratio of 1: 5, after 30-min shaking. Carbonate content, mostly present as calcium carbonate, was determined by the method developed by Wada (1997). An air-tight plastic bag containing soil sample was first TABLE 1. Description of soils used in the study 1) AEZ: 17. Lower Meghna River Floodplain; 18. Young Meghna Estuarine Floodplain; 19 . Old Meghna Estuarine Floodplain. 2)Land type: MHL , medium highland; MLL, medium lowland. Mineralogical Approach to Alternation of Sediments 379 evacuated, then filled with a definite volume of air followed by the addition of 2molL-1 HCl via an attached rubber septum by using a plastic syringe. After about 20 min, the CO2 concentration of the air inside was determined by a CO2-detector tube. The amount of CO2 released from the soil sample was calculated by adding the amount of CO2 in the air-phase and that dissolved in the aqueous phase, the latter of which was calculated with the aid of the Henry's law and the CO2 concentration of the air in the bag. Particle-size analysis Soil samples were treated with 1M NaCH3COO (pH 5.0) in order to dissolve car- bonate, if present, and washed with water to remove the dissolved carbonate. They were then treated with 7% H2O2 to decompose organic matter, washed with water by cen- trifugation, adjusted to the pH 10 by adding 1M NaOH, and dispersed by ultrasonic vibration (tank-type; 38kHz, 250W). The<2ƒÊm clay fraction was separated by repeated sonification-sedimentation-siphoning. The 2-20ƒÊm fraction was separated by repeated sedimentation and siphoning, and the 20-53, 53-200 and 200-2000ƒÊm fractions were separated by wet-sieving. Weights of each fraction were determined to calculate the particle-size distribution. Mineralogical analysis Specimens for ƒ´-ray diffraction (XRD) were prepared by taking duplicate clay sols containing 50mg of clay (<2ƒÊm). Of the duplicate sets, one was saturated with K and the other with Mg, by washing 3 times with 1M KCl and 0.5M MgCl2, respectively. Excess salt was removed by washing one time with water. One mL of water was added and the sol was suspended thoroughly. An aliquot of 0.4mL of the sol was dropped onto a glass slide (28•~48mm), covering two-thirds of its area, air-dried, and ƒ´-rayed (parallel powder mount). XRD patterns were obtained using a Rigaku diffractometer with Ni-filtered CuKƒ¿ radiation at 40kV and 20mA and at a scanning speed of 2•‹2ƒÆ/min over a range of 3- 30•‹2ƒÆ.