As a large proportion (> 80%) of the annual sediment load was found to be transported during the monsoon months characterized by higher discharges, it appears that suspended sediment in the Brahmaputra River is mostly supply-controlled. Since silt was the dominant part of the sediments and the river water was uniform, metal transport in the Brahmaputra River was mainly by adsorption to silt.
INTRODUCTION
Introduction
- Sediment generation processes
- Sediment sources
- Controlling factors for sediment yield
- Classification of sediments
- Sediment properties
- Physical properties
- Sediment composition
- Sediments in fluvial environment
- Entrainment of sediments
- Sediment transport as flocs
- Vertical distribution of sediment in rivers
- Deposition of sediment in rivers
- Meandering-Braiding transition
- Evolution and modification of sediments
- Sediment as a carrier of pollutants
- Classification of pollutants
Suspended sediment is that part of the sediment that is carried in suspension by the turbid components of the liquid or by Brownian motion (ASTM, D 4410-03). Therefore, the transport of sediment-bound pollutants will depend on the size, structure, composition, and density characteristics of the flocs.
Various forms of pollutants
Metals can exist in dissolved forms as free metal ions or complex molecules, or in particulate forms adsorbed on the surface of solid particles or as precipitates (mainly as sulfides). The release of metal ions from sulfide during oxidation can be counteracted by these processes.
Solid-Liquid partitioning of pollutants
If the net charge is not zero, it is balanced by the ions in the diffuse double layer along the surface. Again, the surface is positively charged at low pH and negatively charged at high pH.
Particulate fraction-the major carriers of pollutants
For many biological surfaces (e.g. bacteria and algae), the PZC is in the range of pH 4-5, so that such particles are negatively charged in most natural waters. Many inorganic particles (e.g. sediments) in the aquatic environment are negatively charged due to an adsorbed layer of natural organic matter.
Concentration of metals within sediment flocs
Fe-Mn oxides often manifest as precipitated crystalline forms or coatings and often dominate the geochemical reactivity of the sediment component. Copper analysis revealed a common association with the organic fraction of the sediment (55–90%) (Droppo and Leppard, 2004).
Some sediment related problems 1. Sedimentation in reservoirs
- Erosion of turbine components
- Channel aggradation and degradation
- Occurrence of floods
- Increased cost of water treatment
When the discharge becomes too high, the stream widens its channel by overflowing its banks and flooding the low-lying areas around the stream. Drinking water treatment costs increase as sediment concentration increases due to increased volume of sludge production and erosion of pumping facilities.
Sediment management issues
The Brahmaputra River, its discharge and sediment load 1. The Brahmaputra Basin
- Geology of the Brahmaputra Basin
- Climate in the Brahmaputra Basin
- The Brahmaputra River
- Discharge of the Brahmaputra River
- Sediment load in the Brahmaputra River
- Tributaries and their sediment loads
The bed load of the Brahmaputra River is generally considered to be 15% of the suspended sediment load (Datta and Singh, 2004). Most of the suspended sediment carried by the rivers of mountainous region is deposited in the river bed.
Research Need of studying sediments of the Brahmaputra River
Many hydropower facilities are under planning/investigation in the upstream areas of the study site and some are being installed (Jain et al., 2007). These studies will also contribute to a better implementation of the various projects and schemes planned for the region.
Objectives of the present study
- Analyses of discharge and suspended sediment characteristics
- Particle size studies
- Mineralogical studies
- Analyses of morphology and chemical composition
- Analyses of metals in the sediments
More than 150 hydropower projects are proposed to be installed upstream of the Brahmaputra River to generate 60,000 MW of power. Sediment particle size characteristics shed light on the origin (Weltje and Prins, 2007), transport dynamics (deposition/resuspension) (Singh et al., 2007) and pollutant sorption potential (Fontaine et al., 2000) of sediments in a river. The extent of erosion of turbine components (Bajracharya et al., 2008) and the sorption capacity of pollutants depend mainly on the mineralogy of the sediments (Jha et al., 1990).
The mobility of sediment-bound metals and their bioavailability in the aquatic environment depends on the geochemical distribution of metals in the sediments (Tessier et al., 1979) and the properties of the aquatic environment (Stumm and Morgan, 1996; Warren and Haack, 2001). ; Eby, 2004). Metals that form part of the crystal lattices are unlikely to be released under normal geochemical fluctuations (Tessier et al., 1979).
A note on the data used in the thesis
Sediments retain their mineralogical characteristics from their sources to the delta area (Heroy et al., 2003). Mineralogical data help to identify and differentiate sediment origins and transport pathways (Xu et al., 2009). The morphology and composition of sediment particles gives an idea of their origin and maturity, basin geology, weathering and erosion prevailing in the basin (Sholkovitz et al., 1978; Martinelli et al., 1993; Manjunatha et al., 1996; Singh and France- Lanord, 2002; Pokrovsky et al., 2005).
Scanning electron microscope equipped with energy dispersive X-ray spectroscopy was used to study the morphology and composition of the sediments as an attempt to understand these aspects. Therefore, only the extractable metals (ASTM, 2003) in the sediments were determined using Atomic Adsorption Spectrophotometer to assess its pollution potential.
LITERATURE REVIEW
Introduction
- Sediment quantity 1. Sediment yield
- Sediment load
- Effects of sediment yield
- Grain size studies
- Usefulness of grain size studies
- Some examples of grain size studies
- Mineralogy
- Chemical composition
- Elemental analyses
- Analyses of metals
- Summary of literature review
Singh and France-Lanord (2002) found primary minerals (quartz, muscovite, feldspar, biotite) to be dominant in the Brahmaputra sediments. They reported dominance of kaolinite with minor amounts of illite and chlorite in the clay fraction (≤ 3.9 µm) of the sediment. Whetten and Kelley (1969) observed a downstream decrease in the ratio of quartz to all unstable constituents (feldspar, mafic, and lithic fragments) in the sediments of the Columbia River, whether the unstable constituents were taken individually or as a group.
They found that the slopes of the linear regression lines approximated the average complexation ratios of metals with organic matter in the sediments. She attributed this to a downstream reduction in the size of sediment grains in the river. An excellent correlation has been observed between different metal pairs in the Brahmaputra sediments (e.g. Subramanian et al., 1987).
Metals in the sediments of the Brahmaputra River are mainly contributed by natural sources and their unchanged concentrations even in the lower reaches of Guwahati (the largest urban settlement along the river) indicate dilution due to the huge amount of sediment in the river (Mahanta and Subramanian, 2004). .
MATERIALS AND METHODS
Introduction
- Collection and storage of samples
- Analyses
- Suspended sediment concentration
- Particle Size Analysis
- Morphological and major element analysis
- Mineralogical analysis
- Analyses of metals
- Resource limitations
Changes in the surface topography of a material are described as changes in the gray level of the image. The presence or absence of certain reflections in the diffraction pattern allows determining the type of lattice. It is therefore not necessary to separate the analyte from the rest of the sample (matrix).
The observed concentrations should be in the linear range of the calibration curve for accuracy of the results. Due to strong turbulence in the rivers for most of the year, sampling by boat was not possible.
DISCHARGE AND SUSPENDED SEDIMENT CHARACTERISTICS OF THE BRAHMAPUTRA
RIVER
Introduction
- Discharge
- Suspended sediment concentration
- Summary
A similar pattern was observed in the annual flow and sediment discharge hydrographs of the Brahmaputra River for the years 1971 to 1979 (Goswami, 1985). A similar behavior was observed in case of Brahmaputra River at the study site (Fig. Based on this, the total annual sediment loads of Brahmaputra River approximately fall in the range of 259–483 million tonnes, i.e.
More than 80% of the total annual cargo is transported during the monsoon months (June to September). It was observed in this study that about 65% of the total annual discharge of the Brahmaputra River occurred during the monsoon months (June to September).
PARTICLE SIZE CHARACTERISTICS OF BRAHMAPUTRA SEDIMENTS
Introduction
- Particle size characteristics
The bed sediment particle size curve (the average of the curves for the five-bed sediment samples created using the Mastersizer 2000 software) showed no overlapping zones with the suspended sediment curves (Figure 5.4), except for a few cases (Figure 5.5). The bottom sediments examined in this study are also dominated by silt (Figure 4.24), indicating that the alluvial deposits (consisting mainly of silt and clay) in the Brahmaputra Valley are a major source of these sediments. Although a limited number of bottom sediment samples were considered in the current study, the average value (4.68%, Figure 5.8) was obtained for clay fractions.
Figure 5.11 shows the monthly discharge agents and sediment loads of different size fractions in the period December 2003 to October 2008. The development of shrinkage cracks in these deposited sediments and the texture reveal the dominance of silt in the Brahmaputra River sediments in the study area (Figure 5.13).
3Sand ~2 m
Silt
Shrinkage cracks
Summary
TWLSS particle sizes were occasionally gap-graded, indicating contributions from colluvial soil deposits (e.g., landslide deposits) or from braided bars in the river. Average amounts of coarse, medium and fine fractions in the depth-integrated suspended sediment samples were respectively approx. and 43.2%. Annual loadings (in million tons) of fine, medium, and coarse fractions of the suspended sediments were in the range of 60–71, respectively.
Dominance of fine fractions in the sediments indicates the occurrence of chemical weathering in the basin (Biksham and Subramanian, 1988). Since a large fraction (more than . 80%) of the annual sediment load was found to be transported during the monsoon months, which are characterized by higher discharge (Chapter 4), suspended sediment in the Brahmaputra River appears to be mostly stock-controlled.
MINERALOGY AND CHEMICAL COMPOSITION OF BRAHMAPUTRA SEDIMENTS
Introduction
- Mineralogy
- Morphology and sediment composition
- Summary
About 62% of the samples showed the presence of montmorillonite (Table 6.1), which is generally attributed to volcanic rocks present in the upper drainage basin of the Brahmaputra River (Sarin et al., 1989). The presence of montmorillonite was reported in suspended sediments. of the Brahmaputra River in some other studies also (Mahanta and Subramanian, 2004; Saxena, 2006) The presence of albite in the sediments of the Brahmaputra River has also been reported by some other authors (Kotoky et al., 2006; Saxena, 2006).
Among feldspars, albite was predominant, indicating its abundance in the Brahmaputra basin (Singh et al., 2005). The mineralogy of the sediments thus supports the dominance of both physical and chemical weathering in the Brahmaputra basin.
EXTRACTABLE METALS IN THE SEDIMENTS OF THE BRAHMAPUTRA RIVER
Introduction
- Concentration of metals
The search for a better understanding of the distribution of trace metals in the sediments of large rivers is of great concern in the exogenous cycling of elements through fluvial processes and in assessing the effects of anthropogenic influences. Thus, knowledge of the concentration and distribution of trace metals in sediments is of fundamental importance in a densely populated and highly agricultural region like the Brahmaputra basin (Singh et al., 2003). With an objective of assessing the distribution of metals in Brahmaputra River sediments, ten TWLSS samples and five bed sediments were analyzed for extracted concentrations of aluminium, cadmium, chromium, copper, iron, lead, manganese, nickel and zinc according to standard practices for extraction of trace elements from sediments (ASTM, 2003).
The chemical composition of the sediments was examined using the EDS technique (Chapter 6), which, being selective in nature, does not necessarily reflect the composition of a sample as a whole. Furthermore, the SEM-EDS technique only provides data for the most important elements, with detection limits of approximately 0.1% (Wagner et al., 2008).
