Study on the Bioaccumulation of Polonium-210 and Lead-210 in the Abiotic and Biotic Components of Palk Strait, South East Coast of India
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STUDY ON THE BIOACCUMULATION OF POLONIUM-210 AND LEAD-210 IN THE ABIOTIC AND BIOTIC COMPONENTS OF PALK STRAIT, SOUTH EAST COAST OF INDIA. M.M. Shahul Hameed, A. Sadiq Bukhari and P. Shahul Hameed Environmental Research Laboratory, Post Graduate Department of Zoology Jamal Mohamed College, Tiruchirappalli – 620 020, Tamil Nadu, India. E-mail: [email protected] Abstract A systematic study on the bioaccumulation of natural radionuclides such as Po-210 and Pb-210 in abiotic and biotic components of Palk Strait ecosystem was undertaken in order to generate base line data and to understand the radiation profile of Palk Strait coast. In water Po-210 recorded a mean value of 1.54 mBq/l and Pb-210, 3.15 mBq/l and in sediment Po-210 level was 3.8 Bq/kg and Pb-210, 3.2 Bq/kg. Analysis of Po-210 activity in different grain size in Mandapam sediment showed that the smaller particles (50-µm) recorded highest activity of 164 Bq/kg as compared to larger grains. The depth wise analysis of Po-210 activity showed a maximum of 6 Bq/kg in the upper layer (0-5 cm). The mean Po-210 concentration recorded in plankton samples was 47.1 Bq/kg while Pb-210 registered 12.7 Bq/kg. Among the seaweeds analyzed Sargassum wightii showed highest activity of Po-210 accumulation (20 Bq/kg). Among the molluscs, bivalve Meretrix casta registered a maximum activity of Po-210 and hence could be considered as a sentinel organism for biomonitoring of radionuclides. Higher activity of Po-210 among fishes analysed was found in the detrivore bottom feeder Liza vaigiensis. The results of the present study show that levels of these radionuclides are lesser compared to those of the adjacent Kalpakkam and Gulf of Mannar coasts. Introduction The major contribution to the radiation exposure received by mankind comes from natural sources. These include external sources such as cosmic rays and radiation from primordial radionuclides (U238 and Th232) and their decay products in the environment. Information on the levels of naturally occurring radionuclides is important as they also contribute a substantial fraction of the radiation dose to natural ecosystems (Holtzman, 1966). Polonium – 210 and Lead-210, both belong to naturally occurring U238 series having considerable half-life (138 days and 22years respectively) and are present in almost all environmental matrices. The main source of Pb-210 and Po-210 entering the environment is the exhalation of Rn222, a daughter product of Ra226 which continuously escapes from the surface of the earth. Rn 222 decays to Pb-210, Bi 210 and Po-210 which attach themselves to atmospheric particulates and are transported back to earth surface (Holtzman, 1966; Jaworowski and Grzybowska, 1977; Abe and Abe, 1980; UNSCEAR, 1988). The level of Rn 222 and its decay product in atmosphere over land varies considerably, depending on the emanation rate. The nuclide dispersed in the hydrosphere is diluted, transported and concentrated by physical, chemical and biological processes. Aquatic organisms extract and concentrate elements from the environment in which they grow and have no mechanism to discriminate the radioisotopes of an element (Templeton, 1962). Study on the bioaccumultion of natural radionuclides such as Po-210 and Pb-210 in abiotic and biotic components of any ecosystem is essential in order to understand their radiation profile and hence the present study. Study Area Palk Strait is a narrow channel on the south east coast of India connecting Gulf of Mannar and Bay of Bengal (Long.78°50' – 79°55’; Lat. 9°15' – 10° 20'). Seafood organisms of this coast are largely consumed by the local population and also exported to different countries. For the present study eight sampling stations were fixed along the coast of the Palk Strait, viz., Point Calimere (S1), Mallipatnam (S2), Manamelkudi (S3), Kottaipattinam (S4), Thondi (S5), Devipattinam (S6), Attankarai (S7) and Mandapam (S8) (Fig 1). Analyses Abiotic components such as water and sediment and biotic components such as plankton, sea weeds, crustaceans ,mulluscs and fishes were collected from the sampling stations and were analysed for Po-210 and Pb-210 based on the following methods. (i) Po-210 by electrochemical deposition on a silver planchette and subsequent alpha counting (Flynn, 1968; Iyengar, 1983). (ii) Pb-210 by allowing equilibrium growth of Bi-210 (T1/2 = 5 days) and separation on BiP04 and counting in a low beta counter (Kamath et al., 1964). The counting instruments used were an alpha counter with ZnS(Ag) detector with a background of 0.1-0.2 cpm and a counting efficiency of 25% and a low-beta counter with a background of 1.5-2.0 cpm and a counting efficiency of 40%. Result & Discussion The activities of Po-210 and Pb-210 in the coastal water and sediment are depicted in Table 1. Po-210 concentration in coastal water ranged from 0.62 mBq/l (Point calimere) to 2.83 mBq/l (Mandapam) with the mean value of 1.54 mBq/l. This recorded value is lower when compared to the adjacent Gulf of Mannar coast (2.14 Mbq/l to 32.4 mBq/l) (Masilamani 2001) and Kalpakkam coast (6.6 mBq/l) (Iyengar et al., 1980) and these values are comparable to the recorded values of Dutch coast (1.0 mBq/l) by Koster et al., (1990) and south African water (0.30 to 1.5 mBq/l) by Shannan et al., (1970). Concentration of Pb-210 in the coastal water ranged from 2.10 mBq/l to 4.35 m Bq/l with an average value of 3.15 m Bq/l. These values are less than those of Gulf of Mannar coastal water (34.18 mBq/l, Somasumdaram 1998) and North pacific (7.4 mBq/l, Craig et al., 1973) and in broad agreement with those reported by Bangera and Rudran (1999) for Mumbai coastal water (1.95 m Bq/l), by Cherry and Shannan (1974) for Mediterranean water. Po-210 activity in sediment ranged from 1.4 to 8.80 Bq/kg with a mean value of 3.8 Bq/kg which is less than that recorded in Kalpakkam beach sediments (44 Bq/kg; Iyengar et al., 1980), Gulf of Mannar coastal sediment (286.12 Bq/kg Masilamani 2001), but comparable to south west coast of Mangalore coastal sediment (6.20 – 18.89 Bq/kg, Narayanan et al., 1992). The activity of Pb-210 recorded in the present study (0.9-8.5 Bq/kg: Mean 3.2 Bq/kg) is much less when compared to Gulf of Mannar sediment (35.3 Bq/kg) Kalpakkam beach sediment (385 Bq/kg; Iyengar et al., 1980) and Trombay coast (356.8 Bq/kg). In general the Pb-210 activity in water samples was higher than the Po-210 activity. This pattern was identified only in water, whereas sediment and animals recorded higher levels of Po-210 than Pb-210. Iyengar et al., (1980) linked higher Po-210 levels in Kalpakkam area to monazite deposits and Masilamani (2001) reported higher monazite deposits, rendering the Gulf of Mannar a High Background Radiation Area. Hence it is suggested that the differential distribution of Po-210 in the sampling stations of Palk Strait could be related to quantitative differences in the distribution of radioactive mineral deposits especially monazite in the adjoining areas. Maximum levels of Po-210 and Pb-210 were recorded in Manalmelkudi Coast (S3) and minimum levels in Mandapam Coast (S8). Higher Po-210 registered in sediments may be attributed to the accumulation of organic debris and particulates, which in general are enriched sources of Po-210 (Iyengar, 1983). Attempt was made to study Po-210 concentration in relation to different sediment size and depth. Sediment samples of six different sizes ranging from 50 µm to 900 µm were subjected to analysis. It was observed that particles of 900 to 200 µm registered the activity between 5.7 and 24 Bq/kg. However the higher activity of 164 Bq/kg has been registered in the particle size of 50µm (Table 2). This is indicative of chemical affinity of Po-210 activity towards smaller sediments particles. Avadhani et al., (2001) reported that the concentration of radionuclides increases as the grain size decreases and similar finding was made by Masilamani (2001) in the sediment sample of Gulf of Mannar. Siddappa et al., (1994) have also reported grain size variation in monazite sand of Ullal beaches and observed the same trend. The maximum activity of Po-210 was observed in the depth range of 0 to 5 cm and the activity decreased with increasing depth. The highest activity of Po-210 (6 Bq/kg) is confined to the upper layer sediment (0-5cm). The activity reduces in the subsequent depth intervals (5-10 and 10-15 cm) to 5 Bq/kg and 3 Bq/kg respectively (Table 3). The vertical distribution of radionuclides in the soil and sand has been reported by several investigators. Iyer et al., (1974) have studied the variation of monazite content in the beach sands of Kalpakkam in the intervals of 0-20 cm and 20-40 cm. They have observed a decrease in monazite content over 30% from the first interval to the second. Po-210 and Pb-210 levels in relation to depth were also studied in Kaiga environment by Karunakara et al., (2000) and Siddappa (2001). The study revealed that the activities of Po-210 and Pb-210 were higher in surface soil and decreased with increasing soil depth and remains constant after 20 cm depth. Among the primary producers the plankton samples registered higher levels of Po-210 and Pb-210.