Clay Science 10, 375-384 (1998)

MINERALOGICAL APPROACH TO ALTERNATION OF DIFFERENT RIVER SEDIMENTS IN MEGHNA FLOODPLAINS SOILS OF

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 studiesof 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 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 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. In addition to the air-dried specimen, the Mg-saturated clay was ƒ´-rayed after solvation with glycerol; the K-saturated clay after heating at 300•Ž and at 550•Ž for 2hr.

RESULTS AND DISCUSSION

Particle-size distribution Particle-size distribution of soils is shown in Table 2. Both Faridganj (1) and Faridganj

(2) soils of Lower Meghna River Floodplain were medium-textured, and the texture was silt loam in all horizons except for B2 horizon of Faridganj (2) soil which was loam. But the distribution of 2-20, 20-53 and 53-200ƒÊm particles was different between horizons, indicating the possible difference in the time of deposition and the source of sediments. Ramgati (1) soil of Young Meghna Estuarine Floodplain had the silt loam texture throughout the profile but had higher clay in Ap and C1 horizons than in C2 horizon. Ramgati (2) (C horizon) sample was of silt loam texture. The river mud sample of Ichakhali khal consisted of clay and 2-20ƒÊm silt particles mainly, having the silty clay loam texture. 380 A.Z.M. Moslehuddin et al.

TABLE 2. Particle-size distribution, pH, EC and carbonate content of soils

1) Soil texture: SiL, silt loam; L, loam; SiCL, silty clay loam; C, clay; SiC , silty clay.

Wazirpur soil of Old Meghna Estuarine Floodplain contained very high amounts of clay in Ap (68.9%) and B2 (64.2%) horizons, the texture being clay. Lower clay content was observed in C1 horizon, indicating the difference in the nature of the original sediments. Clay content of Batajor soil of the same AEZ decreased abruptly from Ap to C1 horizon, giving the texture of silty clay, silty clay loam and silt loam in topsoil, subsoil and substratum, respectively. pH, EC and carbonate content The pH and EC values, and carbonate content of soils are presented in Table 2. The EC values indicate the effect of tidal inundation and the presence of carbonate indicates the contribution from the calcareous Ganges sediments. The absence of carbonate, however, is not necessarily associated with the absence of deposition of the Ganges sediments, because the decalcification process might cause loss of carbonate (Moslehuddin et al., 1998). Faridganj (1), Faridganj (2), Wazirpur and Batajor soils were mostly neutral in reaction. The EC values were low (0.02 to 0.18 dS m-1) and carbonate contents were also low (up to 0.8 g kg-1). Ramgati (1) soil was alkaline in reaction with some carbonates; both pH and carbonate content increased with depth. The lower carbonate content in upper horizons might be due to loss by decalcification and/or the lower proportion of the Ganges sediments. The EC of Ramgati (1) soil was high in Ap horizon with the value of 1.83 dS m-1, but decreased in lower horizons. Both Ramgati (2) (C horizon) and Ichakhali Mineralogical Approach to Alternation of Sediments 381

khal samples were alkaline in reaction, saline and calcareous. The highest salinity (EC 3.74 dS m-1) was observed in the latter while the highest carbonate content (3.5 g kg-1) in the former.

Clay mineralogical composition The relative proportions of minerals identified in the clay (<2 ƒÊm) fraction are shown in Table 3. The calculation was made based on the relative peak intensities of the respective minerals (Moslehuddin and Egashira, 1996). In general, mica was the most dominant mineral for all samples; smectite, chlorite, vermiculite and kaolinite were present in good proportions but not in all samples. Interstratified mica-chlorite or mica-vermiculite was identified in most samples but in minor amounts. As other than layer silicates quartz, feldspars and goethite were also identified. Contribution of different river sediments to the soils or sediments in Lower Meghna River Floodplain, Young Meghna Estuarine Floodplain, and the part of Old Meghna Estuarine Floodplain surrounded by Ganges Floodplains was approached based on the clay mineralogical composition of the present soil samples, by comparing it with the composition reported for the representative soils developed from the Ganges, Jamuna

(Brahmaputrra), Old Brahmaputra and Meghna sediments as shown in Table 4. Since the

TABLE 3. Approximate mineral contents (%) in the clay fraction (<2 ,ƒÊm) of soils

1) Minerals: Mc, mica; St, smectite; Vt, vermiculite; Ch, chlorite; Kt, kaolinite; Mc/Ch, Mc/Vt, interstratified minerals of respective components; Qr, quartz; Fd, feldspars; Gt, goethite. 382 A.Z.M. Moslehuddin et al. sediments of Lower Meghna River Floodplain, Young Meghna Estuarine Floodplain, and the part of Old Meghna Estuarine Floodplain surrounded by Ganges Floodplains which are considered in the present study are one of the youngest sediments in Bangladesh (Hossain, 1995), change of the clay mineralogical composition by weathering after deposition or movement of clay mineral particles from the surface to subsurface layer was estimated to be insignificant, if any.

TABLE 4. Clay mineralogical composition in the clay fraction (<2ƒÊm) of representative soils developed from different river sediments

1)AEZ: 10. Active Ganges Floodplain; 7. Active Brahmaputra-Jamuna Floodplain; 8. Young Brahmaputra and Jamuna Floodplain; 9. Old Brahmaputra Floodplain; 19. Old Meghna Estuarine Floodplain. 2)References: 1. Moslehuddin and Egashira (1996); 2. Alam et al. (1993); 3. Moslehuddin et al. (1998); 4. Aramaki (1996). 3)Minerals: see Table 3 for abbreviations . *Interstratified minerals of mica, chlorite, vermiculite and smectite were calculated together. In each soil series, mineral contents of the upper and lower rows correspond to those of topsoil and subsoil or substratum, respectively. Mineralogical Approach to Alternation of Sediments 383

Lower Meghna River Floodplain Mica, vermiculite and chlorite were the major minerals in both Faridganj (1) and Faridganj (2) soils. Kaolinite was present in good amounts in Ap horizon of both soils. From Table 4, it is supposed that soils developed from the Jamuna (Brahmaputra) sediments contain vermiculite higher than chlorite and that in soils developed from the Meghna/Old Brahmaputra sediments chlorite content exceeds vermiculite content. In Faridganj (1) soil, Ap horizon had more chlorite than vermiculite while B2 and C horizons had more vermiculite than chlorite. Thus it is estimated that the contribution of the Meghna/Old Brahmaputra sediments is higher in Ap horizon, whereas that of Jamuna (Brahmaputra) sediments is higher in lower horizons. In Faridganj (2) soil, all horizons had less chlorite than vermiculite, indicating the contribution of the Jamuna (Brahma- putra) sediments in the whole profile. The relative content of chlorite, however, increased from C horizon towards the surface possibly due to the increased contribution of the Meghna/Old Brahmaputra sediments. There is hardly any contribution from the Ganges sediments in Lower Meghna River Floodplain, due to the very low content of smectite.

Young Meghna Estuarine Floodplain Ramgati (1) soil, and Ramgati (2) (C horizon) and Ichakhali khal samples contained good amounts of smectite and chlorite in addition to mica. Presence of smectite and the calcareous nature of these samples indicate that they have been developed from the Ganges sediments. In addition, presence of high chlorite indicates the contribution of the Meghna/ Old Brahmaputra sediments. Thus these soils have been developed from admixture of the Ganges and Meghna/Old Brahmaputra sediments, and the similarity in the mineralogical composition indicates the similar proportion of admixture in all locations and throughout the profile in a location.

Old Meghna Estuarine Floodplain surrounded by Ganges Floodplains Smectite was present as a predominant mineral in Ap and B2 horizons of Wazirpur soil and Ap horizon of Batajor soil. These soils were taken from the part of Old Meghna Estuarine Floodplain that is surrounded by Low Ganges River and Ganges Tidal Floodplains. So, there is a possibility of deposition of the Ganges sediments over the Meghna/Old Brahmaputra sediments, as indicated from the clay mineralogical composition in which lower horizons contain low smectite but high chlorite and vermiculite similar to the clay mineralogical composition of the soils developed from the Meghna/Old Brah- maputra sediments.

CONCLUSIONS

The predominant clay minerals of Meghna Floodplains soils of Bangladesh were, in general, mica, smectite, vermiculite, chlorite and kaolinite. Contribution of the Meghna/ Old Brahmaputra sediments increased towards Ap horizon from the substratum in the soils of Lower Meghna River Floodplain having hardly any contribution from the Ganges sediments. Soils of Young Meghna Estuarine Floodplain have been developed mainly from the Ganges and Meghna/Old Brahmaputra sediments throughout the profile. Soils of the 384 A.Z.M. Moslehuddin et al. part of Old Meghna Estuarine Floodplain that is surrounded by Ganges Floodplains consist of the Ganges sediments in the upper horizons and of the Meghna/Old Brah- maputra sediments in the lower horizons

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