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Uranium isotopes in the Hooghly estuary, India
ELSEVIER
Marine Chemistry47 (1994) 291-296
Uranium isotopes in the Hooghly estuary, India B.L.K.
Somayajulu
Physical Research Laboratory, Ahmedabad 380 009, India
Received 18 March 1993; revisionaccepted 16 March 1994
Abstract
Uranium isotopes are measured in the estuarine waters, bed sediments and suspended matter of the most polluted estuary of India, the Hooghly, which flows through the metropolis of Calcutta. The uranium concentrations in the freshwater-endmember of the estuary are higher (3.9-3.5 #g/l) than in any other estuary so far studied; the 2~U/238U activity ratios in the river channel (CI ~<7.29 g/l) are low, namely < 1.I0 + 0.02. Uranium behaves non-conservatively in the Hooghly estuary, its concentrations are less than the values expected from the theoretical dilution curve obtained by joining the two endmember-concentrations. Based on one set of measurements, it is estimated that ~ 25% of the uranium entering the estuary in the dissolved form is getting removed from the estuarine waters, most likely onto sediments.
1. Introduction
Uranium and its daughter products have long been used to understand the time constants of many aquatic geochemical processes, notably the marine ones. One of the important inputs for such applications is knowledge of the behaviour of uranium itself which in turn needs a detailed understanding of its input into the oceans via rivers and streams that flow through different geological terrains. It is important to look for polluted estuaries, especially the organically polluted ones, to see the effect of pollutants on U behaviour. In India, mostly unpolluted river-estuarine systems have been studied for uranium and it is found to behave conservatively (Borole et al., 1977, 1982; Satin et al., 1985; Somayajulu et al., 1993) especially in chlorinity regions of ~ 1-19 g C1/1. This is the case for rivers from other parts of the world also (Martin et al., 1978a,b; Maeda and
Windom, 1982; McKee et al., 1987; Toole et al., 1987). In contrast, in the low-chlorinity (< 1 g CI/ 1) and in some mid-chlorinity regions, uranium removal was noted (Martin et al., 1978a; Maeda and Windom, 1982; Satin et al., 1985; Toole et al., 1987). In some of these estuaries, an increase in phosphate was also found that would indicate U removal via the formation of phosphate compounds. Carroll and Moore (1993) observed uranium removal in the Meghna estuary (the Ganges and Brahmaputra rivers combine after flowing into Bangladesh and the river so formed is known as Meghna) upto a chlorinity of 6 g CI/ 1. They attributed the uranium removal to the reducing environment created by the mangrove forests. In the Indian context, the most polluted river is the Hooghly, the smallest branch of the Ganga (Ganges is known as Ganga in India) river system which flows through the metropolis of Calcutta (Fig. 1). In the Amazon shelf, on the other hand,
0304-4203/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-4203(94)00018-9
292
BIL.K. Somayq/ulu/Marine Chemistry 4"," ~ /994 J 291 296
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Fig. 1. Map of the northern Bay of Bengal showing Calcutta and the rivers Ganga and Brahmaputra. The Hooghly estuary alongwith the sampling locations and numbers is shown in the inset. Sampling seaward of Kakdwip was carried out in the smaller channel known as Mooriganga.
diffusion of uranium from the shelf sediments accounts for high U in the overlying waters (McKee et al., 1987). Uranium isotopes were measured in the Hooghly river, the results of which are presented and discussed here.
(1956) and Biswas (1986) and some environmental characteristics have been given by Sengupta et al. (1989). The same characteristics extend to the Bangladesh border on the east. Towards the west, however, there are anorthosites, Raja Mahal traps (basalt) and Upper and Lower Gondwanas, the weathered material from which is brought to the Hooghly river by two of its tributaries on the we~t (Geol. Surv. India, 1974). As expected, the water chemistry of the Hooghly resembles that of the Ganga (Sarin et al., 1989; Sen et al., in prep.). 2.2 Pollution status Of all Indian rivers, the Hooghly is known to be the most polluted with both domestic and industrial wastes (Basu et al., 1970; Gopalakrishnan et al., 1973). The domestic wastes are primarily the Calcutta city sewage from both humans and cattle and the industrial pollution is due to Rayon factories, paper factories, distilleries, paints, rubber and other chemical factories. Inorganic chemicals are added by industries whereas the sewage is mostly biological. The urban pollution load in terms of BOD (biological oxygen demand) amounts to ~ 3 × 105 kg/day (1976--1977 data: WBP&CPB, 1982). The pollution levels get amplified during the summer months (May-June) when freshwater discharge from the Hooghly is minimal. The effect of pollution on plankton growth has been studied by earlier workers (Basu et al., 1970; Gopalakrishnan et al., 1973). Dense mangrove swamps known as Sunderbans, also thrive near the Hooghly river mouth.
2. Material and methods
2.3 Sampling and processing 2.1 Geohydrology Relevant details on the geology of the Ganga drainage basin, of which the Hooghly forms the end part (as far as India is concerned), is given by Sarin et al. (1989), The Hooghly flows mainly over alluvium from the Farakka dam in the north to the Bay of Bengal in the south. As is the case with all other Indian rivers, the Hooghly too has its highest discharge during the southwest monsoon period, i.e. July-September. The geohydrologieal details of the Hooghly estuary can be found in Bose
During 16-23 December 1991, a field trip was undertaken to the Hooghly river-estuarine system wherein waters, sediments and suspended matter were collected for geochemical studies. For U isotopes, 234Th and 21°pb measurements, 20 1 surface water samples were collected using clean plastic bottles from the mid stream onboard motor launches. All samples were collected within three days during the same tidal cycle. Sampling was done starting from upstream of the Diamond Harbour (Fig. I) where the tributary Rupnarayan joins
B.L.K.
Somayajulu/MarineChemistry47 (1994) 291-296
293
Table 1 U isotopes in waters, suspended matter and bed sediments of the Hooghly estuary Dissolved Sample code
Suspended matter/bed sediment Chlotinity (g/l)
234U (dpm/l)
23SU (dpm/1)
234U/238U
238U
234U/238U
(Activity ratio)
(ppm)
(Activity ratio)
1.55 ± 0.04 2.36 ± 0.04 2.80+0.09
1.03 4- 0.03 1.01 + 0.02 1.10 4- 0.03
Suspended matter Sediment Sediment
3.70 ± 0.12 2.64 4- 0.08
1.01 4-0.03 1.06 4- 0.03
Suspended matter Sediment
2.37 + 0.07 2.43 4- 0.07
1.02 4- 0.02 1.03 4- 0.02
Sediment Sediment
(i) 2.54 + 0.06 1.04 + 0.02 (ii) 2.46 5:0.08 1.00 + 0.03
Sediment
Hgy-14
0.013
3.18 4- 0.09
2.944-0.08
1.084-0.01
Hgy-1 Hgy-2 Hgy-3 Hgy-4 Hgy-5
0.014 0.016 0.018 0.25 0.90
2.79 4- 0.08 2.68 4- 0.08 2.88 4- 0.08 2.78 4- 0.08 2.644-0.09
2.66 4- 0.08 2.60 4- 0.07 2.69 4- 0.08 2.63 4- 0.07 2.384-0.08
1.05 4- 0.02 1.03 4- 0.02 1.07 4- 0.02 1.06 -4-0.02 1.11±0.02
Hgy-7 Hgy-8 Hgy-10 Hgy-11 Hgy-12 Hgy-13 E 12a
3.44 4.12 5.97 7.20 8.06 8.21 16.69
2.54 4- 0.07 2.06 4- 0.06 2.354-0.06 2.24 4- 0.07
2.39 4- 0.07 1.93 4- 0.06 2.144-0.06 2.08 4- 0.07
1.06 4- 0.02 1.07 4- 0.02 1.104-0.02 1.08 4- 0.02
1.91 4-0.05 2.48+0.07
1.724-0.04 2.224-0.06
1.124-0.02 1.124-0.02
H 13b E 13b
17.82 17.85
2.57+0.07 2.574-0.07
2.274-0.06 2.20+0.06
1.144-0.02 1.974-0.02
Remarks
Errors quoted are due to propogated one sigma counting statistics only. Sample locations are given in Fig. 1. (i)/(ii) denotes measurements on two separate analyses of the same sample, Hgy-12. aCollected in December 1991 (Satin et al., 1994). bCollected in March 1991 (Satin et al., 1992).
the Hooghly and stopped somewhat south of the Sagar island (on which sample numbers 6-10 are indicated; Fig. 1). An additional freshwater sample (Hgy-14) was collected near the Outram Ghat in the middle of Calcutta (Fig. 1). The Hooghly river waters starting from freshwater side upto a chlorinity of ~ 1 g C1/I (Samples Hgy-4, -5; Table 1) look muddy but relatively clear beyond. The particulate contents are in the range of hundreds of mg/1 in the former case and in tens of mg/1 in the latter. Each evening, the water samples collected during the day were filtered through 3# m Gelman cartridge filters within 6-8 h of sample collection. Known volume of filtered water (~ 201) was acidified to pH 2 with nitric acid followed by the addition of 1 ml of equilibrated 232U228Th spike, 200 lambda of 23°Th spike and stable Pb carrier in the form of lead nitrate and 1 ml of purified Fe cartier. The samples were repeatedly stirred for 6 h using perspex rods so as to completely decompose the bicarbonate. Fe(OH)3
precipitation was done using NH4OH to scavenge the U, Th and Pb isotopes and the precipitate was collected by filtration through Whatman No. 54 filter papers and the precipitates were brought to the Physical Research Laboratory (PRL) for analysis. At PRL, the U, Th and Pb isotopes were radiochemically separated, purified and the U and Th isotopes were electroplated using standard procedures (Krishnaswami and Satin, 1976; Satin et al., 1992) and alpha assayed. Six surficial sediment samples from the estuarine region were also collected using Van Veen type of grab sampler. These, as well as two of the suspended matter samples (which were adequate in amount), were also analyzed for U and Th isotopes using the above mentioned procedures.
3. Results and discussion The results of the uranium concentration and
294
B.L.K. Sornay,ajulu/Marine Chemistry 47 ~1994) 291 296
MARCH1991 DECEMBER 1991
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CHLORINITY (11/,~) Fig. 2. (a) U concentration as a function of chlorinity in the Hooghly estuary. Samples indicated by open circles and stars have been collected from the Bay of Bengal during December 1991 and March 1991 (Satin et al., in prep.), respectively (see also Table 1). The solid line indicates the theoretical dilution curve. Note that all points upto a chlorinity of 8.21 g C1/I fall below the line line indicating probable uranium removal. (b) [234/238] as a function of chlorinity. Despite the large errors, there is.the expected increasing trend beyond a chtorinity of ,,~ 4 g CI/I. Both U and [234/2381 [insets of(a) and (b)] show scatter in the freshwater-endmember region.
234U/238U activity ratio (denoted as [234/238]) in both water samples, suspended matter and in the bed sediments are given in Table 1. At the mouth of the river the ehlorinity encountered was only 8.21 g C1/I whereas normal seawater has 19.4 g C1/1 (Strickland and Parsons, 1972). During 1991 (March and December), the PRL group sampled the Bay of Bengal waters (chlorinity upto 17.85 g C1/1) for U; data from these surface waters are also given in Table 1. 3.1 U in Hooghly waters
The uranium concentrations in the freshwaterend-member of the Hooghly estuary vary from 3.47 to 3.92 #g/l which, as expected, are in good
agreement with the waters of the Ganga at Patna (3.03-- 3.65 #g/t), which is ~ 800 km upstream e.~* the Farakka dam (Sarin et al., 1990) which i,, ~300km north of sample location 14 (Fig. IL Bhat and Krishnaswami (1969), who initiated uranium isotopic measurements of rivers in India, did collect a sample of the Hooghly river water m January 1968 at Nabdeep which is ~ 50 km north of our sample Hgy-14 location (Fig. 1). Whereas their U concentration (1.55 #g/l) is about a factor two lower than ours, their [234/238], 1.07 ± 0.03, is identical with that of Hgy-14 (Table 1). The data are too few to venture a possible explanation. The U/Z cation ratios of the Ganga waters at Patna and the freshwater-endmember values of the Hooghly (Samples Hgy-14, Hgy-l-4; see Table 1) all centre around (4.7 + 0.5) x 10 5. and the U/E cation data of the Hooghly freshwaters correlate well with the U - E cation plot for all analysed Ganga waters upstream of the Patna (Sarin et al., 1990). The Hooghly freshwaters (C1 < 0.025 g/l) have the highest uranium values (3,5--3.9 #g/l; Table 1) so far reported for the freshwater-endmembers of world rivers (Borole et al., 1982; Martin et al., 1978a,b; Maeda and Windom, 1982; Toole et al., 1987; Butts and Moore, 1988; Somayajulu et al_ 1993; Ray et al., 1994). The Meghna river in Bangladesh has U contents of 2.55 iLg/1, which is low compared to that of the Hooghly. While Hooghly is a part of Ganga river system, Meghna is a mixture of both the Ganga and the Brahmaputra and it is known that the Brahmaputra river waters have low uranium contents compared to the Ganga (Sarin et al., 1990). 3.2 U behaviour in the estuary
The variation of 238U concentration and of [234/ 238] as functions of chlorinity in the Hooghly estuary are shown in Fig. 2a and b, respectively. It is seen that the 23SU concentration decreased with chlorinity from the freshwater-endmember side upto a chlorinity of 8.21 g CI/I which is almost near the mouth of the river (Fig, 2a). Though not shown, the same behaviour is exhibited by 234U (Table 1). It is only in the open ocean area of the Bay of Bengal that chlorinities reach upto 17.85 g C1/I closer to the normal seawater value. In the
B.L.K. Somayajulu/Marine Chemistry 47 (1994) 291-296
freshwater region there is scatter in U contents as well as in the [234/238]. The [234/238] increases with chlorinity between -,~ 4 g CI/I to 17.85 g CI/1 (Fig. 2b). Irrespective of whether U is removed or added in small amounts, the [234/238] will show an overall increase with chlorinity as long as the [234/ 238] of the regions closer to the freshwater-endmember have low [234/238]. In most estuaries, especially the least polluted to unpolluted ones, the U isotopes behave conservatively (Martin et al., 1978b; Borole et al., 1982; Maeda and Windom, 1982; McKee et al., 1987; Toole et al., 1987; Somayajulu et al., 1993; Ray et al., 1994). In a few estuaries, like the Forth (Toole et al., 1987), the Ogeechee (Maeda and Windom, 1982) and the Charente (Martin et al., 1978a), both uranium isotopes (234U and 238U) are seen to behave non-conservatively; namely, there is U removal especially in the low-chlorinity regions. Also in the Meghna river estuary, Carroll and Moore (1993) noted uranium removal but the entire mixing zone is beyond the river mouth in the Bay of Bengal. In the Hooghly river, both 23Suand 234Ubehave similarly. All data points fall below the theoretical mixing line obtained by joining the freshwater- and seawater-endmembers of the Hooghly estuary (Fig. 2a). Based on the assumption that the observed lower concentrations are due to removal only, it is estimated that ,,~ 25% of the U introduced into the estuary is removed most likely in regions closer to the river mouth. 3.3 U removal
It is well known that uranium removal from waters takes place in reducing environments, especially near the sediment-water interface (Anderson, 1987). In view of the tremendous sewage load of the Hooghly river, it is a natural consequence that uranium removal takes place since anoxic conditions occur in the sediments. Since no near-bottom waters have been analyzed in the present investigation, the only possible way to show that U removal is taking place is by looking at the U contents and [234/238] of the bed sediments as well as that of the suspended matter of the estuary (Table 1). The [234/238] of the bed
295
sediments and the overlying waters (at the time of collection) are about the same within experimental uncertainties. The [234/238] values of sediments tend to be lower than that of the waters. This is not unexpected as the sediment has its own U content and [234/238], the latter being generally 1.0, the equilibrium value. The U contents of the Hooghly estuarine sediments and suspended matter range from 1.55 to 3.70 ppm which is high compared to the average crustal value, i.e. 1 ppm. The suspended matter and bed sediment in the freshwater-endmember region (Samples Hgy-14 and -1) have a uranium content of 2.24 + 0.6 ppm and a [234/238] of 1.05 4- 0.03. In contrast, the suspended matter and sediments collected from the estuarine region (C1 = 0.9-8.06 g/l) have a mean U content of 2.69 + 0.08 ppm and a [234/238] of 1.03+0.03 (Table 1). Thus, an increase of ,~ 20% in the mean U content of suspended matter and of bed sediments is seen in the estuarine region. In view of the organic pollution it appears that U removal from estuarine waters is taking place onto the sediments, most likely at the sediment-water interface. Therefore, the collection of more suspended matter and sediments along with near-bottom waters for U and organic matter analysis is required.
Acknowledgements I thank Drs. Sheuli Chatterjee, Mala Sen, P.B. Ghosh of the Sea Explorers Institute, Calcutta and Mr. Supriyo Chakraborty of PRL for arrangements and help during the field trip. This research is supported by the Dept. of Space, Govt. of India. The two reviews served to improve the manuscript, I am thankful to the reviewers.
References Anderson, R.N., 1987. Redox behaviour of uranium in an anoxic marine basin. Uranium, 3: 145-164. Basu, A.K., Ghosh, B.B. and Pal, R.N., 1970. Comparison of the polluted Hooghly estuary with the unpolluted Maltah estuary, India. J. Water Pollut. Control, 42:1771-1781. Bhat, S.G. and Krishnaswami, S., 1969, Isotopes of uranium
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B . L K . Somayajulu/Marine Chem&trv 47 (I994) 291 296
and radium in Indian rivers. Proc. Indian Acad. Sci., 70:1 17. Biswas, A.N., 1986. Geohydromorphology of the Hooghly estuary. J. Inst. Eng., 66:61-73. Borole, D.V., Ktishnaswami, S. and Somayajulu, B.L.K., 1977. Investigations of dissolved uranium and silicon and particulate trace elements in estuaries. Estuarine Coastal Mar. Sci., 5:743-754. Borole, D.V., Krishnaswami, S. and Somayajulu, B.L.K., 1982. Uranium isotopes in rivers, estuaries and adjacent coastal sediments of western India: their weathering, transportation and oceanic budget. Geochim. Cosmochim. Acta, 46: 125137. Bose, B.B., 1956. Observation on the hydrology of the Hooghly estuary. Indian J. Fish., 3: 101. Butts, J.L. and Moore, W.S., 1988. Uranium removal in the Ganga-Brahmaputra river mixing zone. EOS Trans. Am. Geophys. Union, 69: 1255. Carroll, J. and Moore, W.S., 1993. Uranium removal within the Ganga-Brahmaputra River mixing zone during low discharge. Geochim. Cosmochim. Acta., 51:4987--4995. Geol. Surv. India, 1974. Geology and Mineral Resources of India: Part I - West Bengal. Geol. Surv. India, Calcutta, Publ. 30, 32 pp. Gopalakrishnan, V., Ray, P. and Ghosh, B.B., 1973. Present status of pollution of the Hooghly estuary with special reference to the adverse effects observed on the fishery resources. In: Proc. Syrup. Environmental Pollution, Nagpur, pp. 1-8. Krishnaswami, S. and Satin, M.M., 1976. The simultaneous determination of Th, Pu, Ra isotopes, 2x°Pb, 55Fe, a2si and 14 , Cm marine suspended phases. Anal. Chim. Acta, 83: 143156. Maeda, M. and Windom, H.L., 1982. Behaviour of uranium in two estuaries of the south eastern United States. Mar. Chem., 11: 427-436. Martin, J.M., Nijampurkar, V.N. and Salvadori, F., 1978a. Uranium and thorium isotope behaviour in estuarine systems. In: E.D. Goldberg (Editor), Biogeochemistry of Estuarine Sediments. UNESCO/SCOR Workshop, Melreux, Belgium. UNESCO UB., pp. 111-127. Martin, J.M., Meybeck, M. and Pusset, M., 1978b. Uranium behaviour in Zaire estuary. Neth. J. Sea Res., 12: 338-344. McKee, B.A., DeMaster, D.J. and Nittrouver, C.A., 1987. Uranium geochemistry of the Amazon shelf: Evidence for uranium release from bottom sediments. Geoehim. Cosmochim. Acta, 51: 2779-2786. Ray, S.B., 1988. Sedimentotogical and Geochemical Studies on the Mahanadi Estuary, East Coast of India. Ph.D. Thesis. Utkal Univ., Bhubaneswar, 204 pp.
Ray, S.B., Mohanti, M. and Somayajutu, B.L.K., 1994. U r a nium isotopes in Mahanadi river estuarine system. India Estuarine Coastal Shelf Sci., in press. Sarin, M.M., Borole, D.V. and Krishnaswami, S., 1979. Geo. chemistry and geochronology of sediments from the Bay of Bengal and equatorial Indian Ocean Proc. Indian Acad. Sci.. 83:131 154. Sarin, M.M., Rao, K.S., Bhattacharya, S.K., Ramesh, R. and Somayajulu, B.L.K., 1985. Geochemical studies of the river estuarine systems of Krishna and Godavari Mahasagar Bull. Natl. Inst. Oceanogr., 18:129 143. Sarin, M.M., Krishnaswami, S., Dilli, K., Somayajulu, B.L.K and Moore, W.S., 1989. Major ion chemistry of the Ganga Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal. Geochim. Cosmochim. Acta, 53:997 10(/9
Sarin, M.M.. Krishnaswami, S.. Somayajulu, B.L.K and Moore, W.S, 1990. Chemistry of uranium thorium and radium isotopes in the Ganga-Brahmaputra river-systems: weathering processes and fluxes to the Bay of Bengal. Geochim. Cosmochim. Acta, 54:831 845 Sarin, M.M., Bhushan, R., Rengarajan, R. and Yadav, D.N., 1992. Simultaneous determination of 2aSU series nuclides in waters of Arabian Sea and Bay of Bengal. Indian J. Mar. Sci., 21: 121-127. Sarin, M.M., Rengarajan, R. and Somayajulu, B.L.K., 1994. Natural radionuclides in the Arabian Sea and Bay of Bengal: Distribution and evaluation of scavenging processes. Proc. Indian Acad. Sci., in press. Sengupta, R., Murty, C.S. and Bhattathiri, P.M.A, 1989. The environmental characteristics of the Ganga estuary. In: A.N. Bose, S.N. Dwivedi, A.K. Danda, D. Mukhopadhyay and K.K. Bandhopadhyay (Editors), Coastal Zone Management of West Bengal Sea. Sea Explorers Inst., Calcutta, pp. E19E48. Somayajulu, B.L.K., Martin, J.M., Eisma, D., Thomas, A.J, Borole, D.V. and Rao, K.S., 1993. Geochemical studies in the Godavari estuary, India. Mar. Chem., 43: 83-93. Strickland, J.D. and Parsons, T.R.. 1972. A Practical Handbook of Seawater Analysis. Bull. 167 (2nd edition). Fish. Res. Board Can., 310 pp. Toole. J., Baxter, M.S. and Thomson, J., 1987. The behaviour of uranium with salinity changes in three UK estuaries. Estuarine Coastal Shelf Sei., 25: 283--297. West Bengal Prevention and Control of Water Pollution Board (WBP&CPB), 1982. Comprehensive survey and studies of Ganga River Basin in West Bengal. WBP&CPB, Calcutta. pp. 34-38.
Marine Chemistry47 (1994) 291-296
Uranium isotopes in the Hooghly estuary, India B.L.K.
Somayajulu
Physical Research Laboratory, Ahmedabad 380 009, India
Received 18 March 1993; revisionaccepted 16 March 1994
Abstract
Uranium isotopes are measured in the estuarine waters, bed sediments and suspended matter of the most polluted estuary of India, the Hooghly, which flows through the metropolis of Calcutta. The uranium concentrations in the freshwater-endmember of the estuary are higher (3.9-3.5 #g/l) than in any other estuary so far studied; the 2~U/238U activity ratios in the river channel (CI ~<7.29 g/l) are low, namely < 1.I0 + 0.02. Uranium behaves non-conservatively in the Hooghly estuary, its concentrations are less than the values expected from the theoretical dilution curve obtained by joining the two endmember-concentrations. Based on one set of measurements, it is estimated that ~ 25% of the uranium entering the estuary in the dissolved form is getting removed from the estuarine waters, most likely onto sediments.
1. Introduction
Uranium and its daughter products have long been used to understand the time constants of many aquatic geochemical processes, notably the marine ones. One of the important inputs for such applications is knowledge of the behaviour of uranium itself which in turn needs a detailed understanding of its input into the oceans via rivers and streams that flow through different geological terrains. It is important to look for polluted estuaries, especially the organically polluted ones, to see the effect of pollutants on U behaviour. In India, mostly unpolluted river-estuarine systems have been studied for uranium and it is found to behave conservatively (Borole et al., 1977, 1982; Satin et al., 1985; Somayajulu et al., 1993) especially in chlorinity regions of ~ 1-19 g C1/1. This is the case for rivers from other parts of the world also (Martin et al., 1978a,b; Maeda and
Windom, 1982; McKee et al., 1987; Toole et al., 1987). In contrast, in the low-chlorinity (< 1 g CI/ 1) and in some mid-chlorinity regions, uranium removal was noted (Martin et al., 1978a; Maeda and Windom, 1982; Satin et al., 1985; Toole et al., 1987). In some of these estuaries, an increase in phosphate was also found that would indicate U removal via the formation of phosphate compounds. Carroll and Moore (1993) observed uranium removal in the Meghna estuary (the Ganges and Brahmaputra rivers combine after flowing into Bangladesh and the river so formed is known as Meghna) upto a chlorinity of 6 g CI/ 1. They attributed the uranium removal to the reducing environment created by the mangrove forests. In the Indian context, the most polluted river is the Hooghly, the smallest branch of the Ganga (Ganges is known as Ganga in India) river system which flows through the metropolis of Calcutta (Fig. 1). In the Amazon shelf, on the other hand,
0304-4203/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-4203(94)00018-9
292
BIL.K. Somayq/ulu/Marine Chemistry 4"," ~ /994 J 291 296
'BB'E
- -
/)I
-
t
~AMOND R 4
-
22* N
~A
ii: .....22..
~ •-
~
20°
/ BAY OF BENGAL I
_1,o.
|
I N
8o= E
9o" E
Fig. 1. Map of the northern Bay of Bengal showing Calcutta and the rivers Ganga and Brahmaputra. The Hooghly estuary alongwith the sampling locations and numbers is shown in the inset. Sampling seaward of Kakdwip was carried out in the smaller channel known as Mooriganga.
diffusion of uranium from the shelf sediments accounts for high U in the overlying waters (McKee et al., 1987). Uranium isotopes were measured in the Hooghly river, the results of which are presented and discussed here.
(1956) and Biswas (1986) and some environmental characteristics have been given by Sengupta et al. (1989). The same characteristics extend to the Bangladesh border on the east. Towards the west, however, there are anorthosites, Raja Mahal traps (basalt) and Upper and Lower Gondwanas, the weathered material from which is brought to the Hooghly river by two of its tributaries on the we~t (Geol. Surv. India, 1974). As expected, the water chemistry of the Hooghly resembles that of the Ganga (Sarin et al., 1989; Sen et al., in prep.). 2.2 Pollution status Of all Indian rivers, the Hooghly is known to be the most polluted with both domestic and industrial wastes (Basu et al., 1970; Gopalakrishnan et al., 1973). The domestic wastes are primarily the Calcutta city sewage from both humans and cattle and the industrial pollution is due to Rayon factories, paper factories, distilleries, paints, rubber and other chemical factories. Inorganic chemicals are added by industries whereas the sewage is mostly biological. The urban pollution load in terms of BOD (biological oxygen demand) amounts to ~ 3 × 105 kg/day (1976--1977 data: WBP&CPB, 1982). The pollution levels get amplified during the summer months (May-June) when freshwater discharge from the Hooghly is minimal. The effect of pollution on plankton growth has been studied by earlier workers (Basu et al., 1970; Gopalakrishnan et al., 1973). Dense mangrove swamps known as Sunderbans, also thrive near the Hooghly river mouth.
2. Material and methods
2.3 Sampling and processing 2.1 Geohydrology Relevant details on the geology of the Ganga drainage basin, of which the Hooghly forms the end part (as far as India is concerned), is given by Sarin et al. (1989), The Hooghly flows mainly over alluvium from the Farakka dam in the north to the Bay of Bengal in the south. As is the case with all other Indian rivers, the Hooghly too has its highest discharge during the southwest monsoon period, i.e. July-September. The geohydrologieal details of the Hooghly estuary can be found in Bose
During 16-23 December 1991, a field trip was undertaken to the Hooghly river-estuarine system wherein waters, sediments and suspended matter were collected for geochemical studies. For U isotopes, 234Th and 21°pb measurements, 20 1 surface water samples were collected using clean plastic bottles from the mid stream onboard motor launches. All samples were collected within three days during the same tidal cycle. Sampling was done starting from upstream of the Diamond Harbour (Fig. I) where the tributary Rupnarayan joins
B.L.K.
Somayajulu/MarineChemistry47 (1994) 291-296
293
Table 1 U isotopes in waters, suspended matter and bed sediments of the Hooghly estuary Dissolved Sample code
Suspended matter/bed sediment Chlotinity (g/l)
234U (dpm/l)
23SU (dpm/1)
234U/238U
238U
234U/238U
(Activity ratio)
(ppm)
(Activity ratio)
1.55 ± 0.04 2.36 ± 0.04 2.80+0.09
1.03 4- 0.03 1.01 + 0.02 1.10 4- 0.03
Suspended matter Sediment Sediment
3.70 ± 0.12 2.64 4- 0.08
1.01 4-0.03 1.06 4- 0.03
Suspended matter Sediment
2.37 + 0.07 2.43 4- 0.07
1.02 4- 0.02 1.03 4- 0.02
Sediment Sediment
(i) 2.54 + 0.06 1.04 + 0.02 (ii) 2.46 5:0.08 1.00 + 0.03
Sediment
Hgy-14
0.013
3.18 4- 0.09
2.944-0.08
1.084-0.01
Hgy-1 Hgy-2 Hgy-3 Hgy-4 Hgy-5
0.014 0.016 0.018 0.25 0.90
2.79 4- 0.08 2.68 4- 0.08 2.88 4- 0.08 2.78 4- 0.08 2.644-0.09
2.66 4- 0.08 2.60 4- 0.07 2.69 4- 0.08 2.63 4- 0.07 2.384-0.08
1.05 4- 0.02 1.03 4- 0.02 1.07 4- 0.02 1.06 -4-0.02 1.11±0.02
Hgy-7 Hgy-8 Hgy-10 Hgy-11 Hgy-12 Hgy-13 E 12a
3.44 4.12 5.97 7.20 8.06 8.21 16.69
2.54 4- 0.07 2.06 4- 0.06 2.354-0.06 2.24 4- 0.07
2.39 4- 0.07 1.93 4- 0.06 2.144-0.06 2.08 4- 0.07
1.06 4- 0.02 1.07 4- 0.02 1.104-0.02 1.08 4- 0.02
1.91 4-0.05 2.48+0.07
1.724-0.04 2.224-0.06
1.124-0.02 1.124-0.02
H 13b E 13b
17.82 17.85
2.57+0.07 2.574-0.07
2.274-0.06 2.20+0.06
1.144-0.02 1.974-0.02
Remarks
Errors quoted are due to propogated one sigma counting statistics only. Sample locations are given in Fig. 1. (i)/(ii) denotes measurements on two separate analyses of the same sample, Hgy-12. aCollected in December 1991 (Satin et al., 1994). bCollected in March 1991 (Satin et al., 1992).
the Hooghly and stopped somewhat south of the Sagar island (on which sample numbers 6-10 are indicated; Fig. 1). An additional freshwater sample (Hgy-14) was collected near the Outram Ghat in the middle of Calcutta (Fig. 1). The Hooghly river waters starting from freshwater side upto a chlorinity of ~ 1 g C1/I (Samples Hgy-4, -5; Table 1) look muddy but relatively clear beyond. The particulate contents are in the range of hundreds of mg/1 in the former case and in tens of mg/1 in the latter. Each evening, the water samples collected during the day were filtered through 3# m Gelman cartridge filters within 6-8 h of sample collection. Known volume of filtered water (~ 201) was acidified to pH 2 with nitric acid followed by the addition of 1 ml of equilibrated 232U228Th spike, 200 lambda of 23°Th spike and stable Pb carrier in the form of lead nitrate and 1 ml of purified Fe cartier. The samples were repeatedly stirred for 6 h using perspex rods so as to completely decompose the bicarbonate. Fe(OH)3
precipitation was done using NH4OH to scavenge the U, Th and Pb isotopes and the precipitate was collected by filtration through Whatman No. 54 filter papers and the precipitates were brought to the Physical Research Laboratory (PRL) for analysis. At PRL, the U, Th and Pb isotopes were radiochemically separated, purified and the U and Th isotopes were electroplated using standard procedures (Krishnaswami and Satin, 1976; Satin et al., 1992) and alpha assayed. Six surficial sediment samples from the estuarine region were also collected using Van Veen type of grab sampler. These, as well as two of the suspended matter samples (which were adequate in amount), were also analyzed for U and Th isotopes using the above mentioned procedures.
3. Results and discussion The results of the uranium concentration and
294
B.L.K. Sornay,ajulu/Marine Chemistry 47 ~1994) 291 296
MARCH1991 DECEMBER 1991
Z
+
20iL--
(o) t
Z
"t U
g
291 ~
I0
:::0.010!+0.018
05
O0
"6 IIC
=
L
!
I
1
I
1
i
I
J
I
t
0.026 I
I
(b)
120 -
, o6--~ io4
e~
0
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i
I 8
I
I 12
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l 16
i 20
CHLORINITY (11/,~) Fig. 2. (a) U concentration as a function of chlorinity in the Hooghly estuary. Samples indicated by open circles and stars have been collected from the Bay of Bengal during December 1991 and March 1991 (Satin et al., in prep.), respectively (see also Table 1). The solid line indicates the theoretical dilution curve. Note that all points upto a chlorinity of 8.21 g C1/I fall below the line line indicating probable uranium removal. (b) [234/238] as a function of chlorinity. Despite the large errors, there is.the expected increasing trend beyond a chtorinity of ,,~ 4 g CI/I. Both U and [234/2381 [insets of(a) and (b)] show scatter in the freshwater-endmember region.
234U/238U activity ratio (denoted as [234/238]) in both water samples, suspended matter and in the bed sediments are given in Table 1. At the mouth of the river the ehlorinity encountered was only 8.21 g C1/I whereas normal seawater has 19.4 g C1/1 (Strickland and Parsons, 1972). During 1991 (March and December), the PRL group sampled the Bay of Bengal waters (chlorinity upto 17.85 g C1/1) for U; data from these surface waters are also given in Table 1. 3.1 U in Hooghly waters
The uranium concentrations in the freshwaterend-member of the Hooghly estuary vary from 3.47 to 3.92 #g/l which, as expected, are in good
agreement with the waters of the Ganga at Patna (3.03-- 3.65 #g/t), which is ~ 800 km upstream e.~* the Farakka dam (Sarin et al., 1990) which i,, ~300km north of sample location 14 (Fig. IL Bhat and Krishnaswami (1969), who initiated uranium isotopic measurements of rivers in India, did collect a sample of the Hooghly river water m January 1968 at Nabdeep which is ~ 50 km north of our sample Hgy-14 location (Fig. 1). Whereas their U concentration (1.55 #g/l) is about a factor two lower than ours, their [234/238], 1.07 ± 0.03, is identical with that of Hgy-14 (Table 1). The data are too few to venture a possible explanation. The U/Z cation ratios of the Ganga waters at Patna and the freshwater-endmember values of the Hooghly (Samples Hgy-14, Hgy-l-4; see Table 1) all centre around (4.7 + 0.5) x 10 5. and the U/E cation data of the Hooghly freshwaters correlate well with the U - E cation plot for all analysed Ganga waters upstream of the Patna (Sarin et al., 1990). The Hooghly freshwaters (C1 < 0.025 g/l) have the highest uranium values (3,5--3.9 #g/l; Table 1) so far reported for the freshwater-endmembers of world rivers (Borole et al., 1982; Martin et al., 1978a,b; Maeda and Windom, 1982; Toole et al., 1987; Butts and Moore, 1988; Somayajulu et al_ 1993; Ray et al., 1994). The Meghna river in Bangladesh has U contents of 2.55 iLg/1, which is low compared to that of the Hooghly. While Hooghly is a part of Ganga river system, Meghna is a mixture of both the Ganga and the Brahmaputra and it is known that the Brahmaputra river waters have low uranium contents compared to the Ganga (Sarin et al., 1990). 3.2 U behaviour in the estuary
The variation of 238U concentration and of [234/ 238] as functions of chlorinity in the Hooghly estuary are shown in Fig. 2a and b, respectively. It is seen that the 23SU concentration decreased with chlorinity from the freshwater-endmember side upto a chlorinity of 8.21 g CI/I which is almost near the mouth of the river (Fig, 2a). Though not shown, the same behaviour is exhibited by 234U (Table 1). It is only in the open ocean area of the Bay of Bengal that chlorinities reach upto 17.85 g C1/I closer to the normal seawater value. In the
B.L.K. Somayajulu/Marine Chemistry 47 (1994) 291-296
freshwater region there is scatter in U contents as well as in the [234/238]. The [234/238] increases with chlorinity between -,~ 4 g CI/I to 17.85 g CI/1 (Fig. 2b). Irrespective of whether U is removed or added in small amounts, the [234/238] will show an overall increase with chlorinity as long as the [234/ 238] of the regions closer to the freshwater-endmember have low [234/238]. In most estuaries, especially the least polluted to unpolluted ones, the U isotopes behave conservatively (Martin et al., 1978b; Borole et al., 1982; Maeda and Windom, 1982; McKee et al., 1987; Toole et al., 1987; Somayajulu et al., 1993; Ray et al., 1994). In a few estuaries, like the Forth (Toole et al., 1987), the Ogeechee (Maeda and Windom, 1982) and the Charente (Martin et al., 1978a), both uranium isotopes (234U and 238U) are seen to behave non-conservatively; namely, there is U removal especially in the low-chlorinity regions. Also in the Meghna river estuary, Carroll and Moore (1993) noted uranium removal but the entire mixing zone is beyond the river mouth in the Bay of Bengal. In the Hooghly river, both 23Suand 234Ubehave similarly. All data points fall below the theoretical mixing line obtained by joining the freshwater- and seawater-endmembers of the Hooghly estuary (Fig. 2a). Based on the assumption that the observed lower concentrations are due to removal only, it is estimated that ,,~ 25% of the U introduced into the estuary is removed most likely in regions closer to the river mouth. 3.3 U removal
It is well known that uranium removal from waters takes place in reducing environments, especially near the sediment-water interface (Anderson, 1987). In view of the tremendous sewage load of the Hooghly river, it is a natural consequence that uranium removal takes place since anoxic conditions occur in the sediments. Since no near-bottom waters have been analyzed in the present investigation, the only possible way to show that U removal is taking place is by looking at the U contents and [234/238] of the bed sediments as well as that of the suspended matter of the estuary (Table 1). The [234/238] of the bed
295
sediments and the overlying waters (at the time of collection) are about the same within experimental uncertainties. The [234/238] values of sediments tend to be lower than that of the waters. This is not unexpected as the sediment has its own U content and [234/238], the latter being generally 1.0, the equilibrium value. The U contents of the Hooghly estuarine sediments and suspended matter range from 1.55 to 3.70 ppm which is high compared to the average crustal value, i.e. 1 ppm. The suspended matter and bed sediment in the freshwater-endmember region (Samples Hgy-14 and -1) have a uranium content of 2.24 + 0.6 ppm and a [234/238] of 1.05 4- 0.03. In contrast, the suspended matter and sediments collected from the estuarine region (C1 = 0.9-8.06 g/l) have a mean U content of 2.69 + 0.08 ppm and a [234/238] of 1.03+0.03 (Table 1). Thus, an increase of ,~ 20% in the mean U content of suspended matter and of bed sediments is seen in the estuarine region. In view of the organic pollution it appears that U removal from estuarine waters is taking place onto the sediments, most likely at the sediment-water interface. Therefore, the collection of more suspended matter and sediments along with near-bottom waters for U and organic matter analysis is required.
Acknowledgements I thank Drs. Sheuli Chatterjee, Mala Sen, P.B. Ghosh of the Sea Explorers Institute, Calcutta and Mr. Supriyo Chakraborty of PRL for arrangements and help during the field trip. This research is supported by the Dept. of Space, Govt. of India. The two reviews served to improve the manuscript, I am thankful to the reviewers.
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