|Year : 2010 | Volume
| Issue : 3 | Page : 147-149
Recent sedimentation rate at Trombay Naval Jetty of Mumbai Harbour Bay
Usha Narayanan, Rupali Karpe, Vikram Joshi, PC Verma, AG Hegde
Environmental Studies Section, Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
|Date of Web Publication||22-Oct-2011|
Environmental Studies Section, Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai
Source of Support: None, Conflict of Interest: None
A major tool to study rates of sedimentation is 210 Pb dating of sediment cores. In the present study, two core samples of 36cm long and 4.5 cm diameter were collected from Trombay Naval Jetty which is located at ~3 km away from BARC. 210 Pb was estimated in the core fractions by 210 Po which is in secular equilibrium with 210 Pb. 226 Ra in the core fractions were estimated by High resolution 50% RE HP (Ge) Detector. The unsupported 210 Pb was evaluated by subtracting 226 Ra from total 210 Pb in each fraction. The log of unsupported 210 Pb in each fraction was then plotted against depth and the slope of this line was evaluated. The sedimentation rate was obtained by dividing the 210 Pb decay constant by the slope of the log-linear plot of unsupported 210 Pb versus depth. The mean sedimentation rate thus calculated at Trombay Naval Jetty was 1.32 cmy -1 .
Keywords: Environmental discharge, sedimentation rate, measurement of Pb-210
|How to cite this article:|
Narayanan U, Karpe R, Joshi V, Verma P C, Hegde A G. Recent sedimentation rate at Trombay Naval Jetty of Mumbai Harbour Bay. Radiat Prot Environ 2010;33:147-9
|How to cite this URL:|
Narayanan U, Karpe R, Joshi V, Verma P C, Hegde A G. Recent sedimentation rate at Trombay Naval Jetty of Mumbai Harbour Bay. Radiat Prot Environ [serial online] 2010 [cited 2022 May 20];33:147-9. Available from: https://www.rpe.org.in/text.asp?2010/33/3/147/86293
| 1. Introduction|| |
The study of sediments in coastal areas has great importance to the understanding of the interaction between human activities and marine systems. Furthermore, the establishment of detailed and accurate chronologies for sediments, in other words, the estimation of sedimentation rate, is of central importance to develop a continuum of insight into environmental processes. One of the most promising methods for estimation of sedimentation rate on a time scale of 100 years is by means of 210 Pb, a naturally occurring radioisotope with a half-life of 22.3 years (Goldberg, 1963; Oldfield and Appleby, 1984). The 210 Pb method was initiated by Goldberg (1963), then applied to lake sediments by Krishnaswamy et al. (1971), and subsequently introduced to marine sediments by Koide et al. (1972). It has been very popular in estimating the sedimentation rate of marine sediments (Lesueur et al., 2001; Alperin et al., 2002; Oguri et al., 2003; Owen and Lee, 2004). 210 Pb is a member of the 238 U decay series and is continuously introduced into the marine environment by deposition from the atmosphere. Radioactive disequilibrium between 210 Pb and it's precursor 226 Ra (half-life 1600 years) arises through the mobility of the intermediate gaseous radionuclide 222 Rn. A proportion of the 222 Rn formed by 226 Ra decay in continental soils diffuses into the atmosphere where it decays to 210 Pb (Appleby et al., 1992). This radionuclide is readily attached to airborne particulates and removed from the atmosphere both by wet and dry deposition. Fallout over the sea is subsequently transported through the water column and incorporated in the bottom sediments. Total activity in sediments will include both the atmospherically-derived 210 Pb and the supported 210 Pb derived from the in situ decay of 226 Ra.The supported component is usually assumed to be in equilibrium with the parent 226 Ra.The unsupported atmospheric component used in sedimentation rate is determined by subtracting the 226 Ra activity from the total 210 Pb activity. This is plotted against depth and the slope of this line can be used to calculate the sedimentation rate, knowing the radioactive decay coefficient of 210 Pb. The sedimentation rate can be obtained by dividing the 210 Pb decay constant by the slope of the log-linear plot of unsupported 210 Pb versus depth (Appleby et al., 1992). The present paper describes the evaluation of sedimentation rate for Trombay Naval Jetty located at ~3 km from BARC.
| 2. Methodology|| |
Trombay Naval Jetty is located ~3 km NW of BARC on the shoreline of Mumbai Harbour Bay. Two core samples were collected with a distance of 100m apart using a sediment corer. The depth of the core was up to 36 cm with a diameter of 4.5 cm. The core was cut into nine fractions of 4 cm each and brought to the laboratory. Each fraction was dried at 90 0 C in an oven for 8 hours. The dried samples were weighed and were stored in tight plastic containers more than one month to allow radioactive equilibrium to be reached. Sediment samples were measured by high resolution (2.0 keV at 1.33 MeV), high relative efficiency (50%) and low background HPGe detector to determine 226 Ra.
Assuming 210 Po is in secular equilibrium with 210 Pb, the 210 Po measurements in fractions of the core were carried out on 1.0 g of dried, ground sediment. The dried sample was digested in a microwave digestion system with a 209 Po tracer with HF and HNO 3 . 210 Po and 209 Po were plated onto silver discs from 0.5 N HCl solution (ERL Manual, 1998) at 90°C for three hours after reducing Fe using ascorbic acid. Polonium activities were measured using alpha spectrometry. Chemical yields using a 209 Po tracer ranged from 87 to 92%.
| 3. Results and Discussion|| |
3.1 Distribution of 226 Ra and 210 Pb with depth in core
The activity of 226 Ra varied between 16.2 to 28.5 Bq/kg in core1 and 16.6 to 29.3Bq/kg in core 2. [Figure 1] gives the distribution of 210 Pb (equivalent to 210 Po values) in sediment cores from the same location. The concentration of 210 Pb in size fractions varied between 19.8 to 48.9 Bq/kg in core1 and 21.1 to 47.2Bq/kg in core2. It can be seen in both the [Figure 1] and [Figure 2], high concentrations of 210 Pb are observed in the surface sediment which is due to natural fallout. However, there is a decreasing trend with depth, which is caused by radioactive decay of fallout 210 Pb with time.
|Figure 1: Distribution of 210Pb (equivalent to 210Po values) in sediment cores|
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3.2 Evaluation of sedimentation rate
Total 210 Pb activity in sediments will include both the atmospherically-derived 210 Pb and the supported 210 Pb derived from the in situ decay of 226 Ra. The values of un-supported 210 Pb thus were calculated by subtracting 226 Ra concentration (supported 210 Pb) from total 210 Pb in each fraction of the core.
In [Figure 2], the logarithm of the unsupported (excess) 210 Pb is plotted against depth. The linear trend of this plot shows that excess 210 Pb concentration varies logarithmically with depth. This situation occurs because radioactive decay is an exponential process. The equation of the fitted line is shown in [Figure 2]. The slope of this line was used to calculate the sedimentation rate, knowing the radioactive decay coefficient of Pb- 210. When base 10 logarithms are used and the depths are expressed in cm, the sedimentation rate in cm per year equals 0.03114/slope. In the case shown, it leads to a calculated sedimentation rate of 1.31 cmy -1 and 1.32 cmy - 1 in case of core 1 and core 2 respectively which averages out to 1.32 cmy -1 . Almost similar result (0.81cmy -1) was observed by Jha et al on the studies at Thane creek which is ~5 km away from BARC.
| 4. Conclusion|| |
The use of the natural radionuclide 210 Pb to determine marine and fresh water sedimentation rate is a well-established method. In the present paper, concentration 210 Po which is in equilibrium with 210 Pb was used to calculate the total 210 Pb in sediment core fractions. From the un-supported 210 Pb derived from atmosphere, the sedimentation rate of Trombay Naval Jetty was evaluated. The sedimentation rate thus works out to 1.32 cmy -1 .
| 5. Acknowledgements|| |
The authors express their sincere thanks to Shri H.S. Kushwaha, Director, HS&E Group, BARC and Dr. P.K. Sarkar, Head, HPD, BARC for their keen interest in the work. Authors also like to thank Shri G.L.Teli for the collection of core samples.
| 6. References|| |
- Alperin et al., (2002), Modern organic carbon burial fluxes, recent sedimentation rates, and Particle mixing rates from the upper continental slope near Cape Hatteras, North Carolina, Deep-Sea Research II 49, 4645-4665
- Appleby et al., (1992), Application of lead-210 to sedimentation studies. Uranium Series Disequilibrium: Application to Earth, Marine and Environmental Science. Oxford Science Publications, 731-783.
- Environmental Radiological Laboratory Procedure, Manual, 1998.
- Goldberg, (1963), Geochronology with Pb-210, Proceedings of a Symposium on Radioactive Dating, IAEA, Vienna, Austria, 121-131.
- Koide et al. (1972), Marine geochronology with Pb-210, Earth and Planetary Science Letters, 14, 442-446.
- Krishnaswamy et al. (1971), Geochronology of lake sediments, Earth and Planetary Science Letters, 11, 407-414.
- Lesueur et al., (2001), Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France), Continental Shelf Research 21, 1383-1401.
- Oguri et al., (2003), Sediment accumulation rates and budgets of depositing particles of the East China Sea, Deep-Sea Research, 50, 513-528.
- Oldfield and Appleby (1984), Empirical testing of 210 Pb dating models for lake sediments, Lake Sediments and Environmental History, Leicester University Press, 93-124.
- Owen and Lee (2004), Human impacts on organic matter sedimentation in a proximal shelf setting, Hong Kong, Continental Shelf Research, 24, 583-602.
[Figure 1], [Figure 2]