Distribution of Rare Earth Elements and Heavy Metals in the Surficial Sediments of the Himalayan River System
Total Page：16
File Type：pdf, Size：1020Kb
Geochemical Journal, Vol. 34, pp. 295 to 319, 2000 Distribution of rare earth elements and heavy metals in the surficial sediments of the Himalayan river system R. RAMESH,1 AL. RAMANATHAN,2 S. RAMESH,1 R. PURVAJA1 and V. SUBRAMANIAN3 1Institute for Ocean Management, Anna University, Chennai 600 025, India 2Department of Geology, Annamalai University, Annamalai Nagar 608 002, India 3School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India (Received May 25, 1999; Accepted March 23, 2000) Rare earth elements (REEs) are considered as useful tracers of various surface geological processes in the river system. REEs and heavy metals (V, Pb, Cr, Co, Ag, Zn, Cd, Ni) were analyzed from the sus- pended sediments of Ganges and surficial sediments of Yamuna, Brahmaputra, Jamuna, Padma and Meghna, using VG Thermal Ionization Mass Spectrometer. Physical weathering process seems to be a major con- trolling factor for the distribution of REE and trace metals in the sediments of the Himalayan rivers. Weathering of the surface crustal area in the drainage basin shows significant variations due to changes in lithology and influence of tributaries. The distribution of REE shows an almost uniform pattern due to factors such as river transportation processes and high level of terrigenous mixing in the bed sediments. The results show that finer grain size and high content of clay mineral (illite) in these sediments are possible traps for the accumulation of metals. Anthropogenic activities seem to have very little influence in controlling the elemental distribution in the Himalayan rivers. predominantly in the solid phase during transport INTRODUCTION in the river systems. Rare Earth Elements (REEs) and heavy met- The Ganges-Brahmaputra (Himalayan river als possess some unique but, identical physical and system) one of the world’s largest river ranks first chemical properties, which make them useful in terms of sediment transport and fourth in terms probes of low temperature geochemical reactions. of water discharge (Fig. 1). Despite its global sig- REE in terrigenous grains are largely unreactive nificance, the distribution of REE in surficial (Sholkovitz et al., 1994) and contribute an inher- sediments of these Himalayan rivers is still poorly ited signature. The role of sediments as carriers known (Martin and Meybeck, 1979). However, and potential sources of REEs and metals is well studies have been carried out by various research- established and a number of studies on river ba- ers on the heavy metal distribution and sin and estuaries have yielded fundamental fea- geochemical characteristics of the Himalayan riv- tures of the aquatic geochemistry of the REEs ers (Galy and France-Lanord, 1999; Subramanian, (Martin et al., 1976; Goldstein and Jacobsen, 1993; Subramanian and Jha, 1988; Subramanian 1987; Sholkovitz and Elderfield, 1988; Elderfield et al., 1985, 1987a, 1987b, 1988; Krishnaswami et al., 1990; Sholkovitz, 1992, 1993, 1995; et al., 1992; Modak et al., 1992; Sarin et al., 1989, Sholkovitz et al., 1992; Ramesh et al., 1999). 1990). These studies indicate that chemical-weathering All the above research work on the Himalayan reactions on the continent leads to extensive rivers, have been carried out within the territorial fractionation between the dissolved REEs and that boundary of the Indian subcontinent. Nearly 80% of the river suspended particles and continental of the lower reaches of Himalayan rivers draining rocks. Fractionation of REEs and metals occur in the Bengal Basin falls under the political bound- 295 296 R. Ramesh et al. Fig. 1. Location map of the Ganges, Brahmaputra and the Bengal Basin. ary of Bangladesh, but only scant information is continent (Rao, 1975), covering virtually the en- available on the distribution and fractionation of tire Northern India (Fig. 2(a)). The Ganges origi- heavy metals in this region (Datta and nates from the Gangothri and drains in the Subramanian, 1997, 1998). Furthermore, to the Himachal-Kumaon region of Himalayas, where authors’ knowledge, there has been no study on the dominant lithology is fine-grained sericite the REE composition of river sediments in India, chlorite schist. The outer Kumaun Himalayas are in particular the Himalayan rivers. Thus, this pa- composed of Miocene to Pleistocene age per attempts to provide a comprehensive database sediments. In the central and lower Himalayas, for one of the highest sediment dispersal systems there are three main zones: a) the outer Krol belt of the world and also to understand the behavior of upper carboniferous age consisting of dolomitic of REEs during weathering and transport in a sec- limestone, calcareous shale and sandstones; b) the ondary sedimentary environment. inner sedimentary belt consisting of thick lime- stone, over-lined by a sequence of shales and quartzites and c) the zone of metamorphic and STUDY AREA igneous crystalline rocks separating the two units Ganges above. These rocks are dominated by biotite- The drainage basin of the Ganges River, occu- chlorite schist, augen gneisses and granites pies an area of about 106 km2 of the Indian sub- (Valdiya, 1980; Wadia, 1981). The tributaries join- REEs and heavy metals in the Himalayan river system 297 ing from the northern side drains mostly in outer which is draining into the recent alluvium of Himalayas of Siwalik system. The main channel Assam apart from the Siwalik sediments, which of the Ganges predominantly drains alluvial is dominated by the gritty slate, feldspathic quartz- sediments of recent origin (Valdiya, 1979). ite, cherty quartzite and phyllite and Darjeeling The tributaries of Yamuna (Fig. 2(b)) drain into gneiss in the south (Acharya, 1979). two different lithological domains. The tributar- ies joining from the southern side of the river Bengal Basin drains into the Bundlekhand plateau, which is a The Ganges and Brahmaputra river merge in stable block made up of Bundlekhand gneiss and Bangladesh and subsequently break up into a schist of the Precambrian age. The headwaters of number of distributaries called the Bengal Basin the tributaries Chambal, Betwa and Ken draining before they drain into the Bay of Bengal (Fig. into the Deccan basalts, contribute to most of the 2(d)). Most of the drainage area consists of the mafic-rich elements into the river system (Valdiya, deltaic recent alluvium and estuarine sediments. 1979). They are estimated to cover an area of approxi- mately 200,000 km2 within the political bounda- Brahmaputra ries of India and Bangladesh. The Bengal basin is The Brahmaputra River originates from the one of the largest deltas of the world (Coleman, Chamyungdung glacier at an elevation of 5200 m 1981), which is mainly drained by the rivers in the Tibetan Himalayas (Fig. 2(c)). The lithol- Jamuna, Padma and Meghna. The river system ogy of the southern part of Tibetan plateau mostly supplies sediments to the Bay of Bengal and then comprises the Tethyan sedimentary series, com- to the biggest submarine-fan (Bengal Fan) in the posed primarily of the palaeozoic-mesozoic car- world (Kuehl et al., 1989). This region has a low bonates and clastic sediments. Himalayan crystal- level of industrialization, but extensive areas are line rocks are the main formations in the higher under irrigation. This region also has a high popu- ranges and consist of ortho- and para-gneisses, lation density that is one of the highest in the migmatites and marbles (Burg et al., 1984). world, ranging from 400 to 1200 people per km2 Gansser (1983) described leucogranite sheets, tens (Milliman et al., 1989). of hundreds of meters thick within the high The Bengal Basin is underlain by Pre- Himalayan crystallines in the southern wall of the Cambrian basement rocks, bordered on the west Khula Khangri massif and reported that this rock by the Indian Shield, north by the Shillong Shield, type occurs in the total Khula Khangri range of east by the Naga-Lusai orogenic belt and is open Nepal. The headwaters of the river drain mostly to the Bay of Bengal in the south (Sengupta, 1966). into the Nepal-Sikkim Himalayas and Bhutan- Nearly 85% of the basin is covered by alluvium Arunachal Himalayas. The Nepal-Sikkim region of Pleistocene and recent age (Alam et al., 1990). is dominated by the quartzopelitic series, quartz- The bedrock that is exposed in the Bengal Basin ite, gray schist, biotite gneiss and calcareous blu- can be broadly grouped into the Neogene (mainly ish black shale (Remy, 1979). Apart from clastic sediments with intercalated sandstone and metamorphics, high Himalayan crystallines are shale) and Palaeogene (alternative sandstone, also exposed in central Bhutan (Edwards et al., siltstone, shale and limestone). 1999). The Bhutan-Arunachal Himalayas is domi- nated by the high-grade metamorphic rocks Materials and methods (migmatites), ortho gneiss, mica schist, marble, The Ganges-Brahmaputra river system, the cal-silicates, apart from the Tethyan sediments like Bengal Basin along with the sampling stations is limestones, phyllites and schists (Janpangi, 1979). shown in Figs. 1 and 2(a) to 2(d) respectively. For The samples for the present study has been col- this study, freshly deposited bed sediments were lected from the main channel of the Brahmaputra collected from 43 locations and suspended 298 R. Ramesh et al. (a) (b) Fig. 2. Location map of the rivers a) Ganges, b) Yamuna, c) Brahmaputra and d) Jamuna-Meghna-Padma show- ing locations of sediment sampling. REEs and heavy metals in the Himalayan river system 299 (c) (d) Fig. 2. (continued). 300 R. Ramesh et al. sediments from 12 locations in the Himalayan results were within the 95% confidence limits of river system. Allegre et al. (1996) demonstrated the recommended values given for these two cer- that the composition of river suspended sediments tified materials. Overall analytical precision was would be highly useful in tracing the chemical ±3% for the trace metals and REE analyzed. The composition of the continental crust.