228Ra and 226Ra in coastal seawater samples from the Ubatuba region: Brazilian southeastern coastal region

Abstracts

The present work reports 228Ra and 226Ra results obtained during the 4th Submarine Groundwater Discharge (SGD) intercomparison experiment realized in Ubatuba, November 2003, aiming to test the methodologies on a fractured crystalline rocks system. Although with distinct objectives, it was decided to participate in this experiment in order to verify if radium isotopes could be used to determine water mixing rates in this region and to test the laboratory capability to perform, later on, a marine dispersion model validation at Angra dos Reis reactor site, located nearby. The results show that the SGD model constrains seems to be valid, at least for a distance of 20 km offshore, that corresponds to the distance between the reactor site and Ilha Grande, a 193-km² island existing at this site and probably an additional SGD source. Salinity, barium and silica results are also reported and their relation to 228Ra concentrations is discussed.

228Ra; 226Ra; seawater; submarine groundwater discharge; Brazil

ARTICLE

²²⁸Ra and ²²⁶Ra in coastal seawater samples from the Ubatuba region - Brazilian southeastern coastal region

José Marcus GodoyI, II, ^*; Zenildo Lara de Carvalho^I; Flávio da Costa Fernandes^III; Olga M. Danelon^III; Maria Luiza D. P. Godoy^I; Ana Cristina M. Ferreira^I; Luiz Alfredo Roldão^I

^IInstituto de Radioproteção e Dosimetria, Comissão de Energia Nuclear, CP 37750, Barra da Tijuca, 22643-970 Rio de Janeiro-RJ, Brazil

^IIDepartamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, Gávea, 22453-900 Rio de Janeiro-RJ, Brazil

^IIIInstituto de Estudos do Mar Almirante Paulo Moreira, Marinha do Brasil, Rua Kioto 253, 28930-000 Arraial do Cabo-Rio de Janeiro-RJ, Brazil

ABSTRACT

The present work reports ²²⁸Ra and ²²⁶Ra results obtained during the 4^th Submarine Groundwater Discharge (SGD) intercomparison experiment realized in Ubatuba, November 2003, aiming to test the methodologies on a fractured crystalline rocks system. Although with distinct objectives, it was decided to participate in this experiment in order to verify if radium isotopes could be used to determine water mixing rates in this region and to test the laboratory capability to perform, later on, a marine dispersion model validation at Angra dos Reis reactor site, located nearby. The results show that the SGD model constrains seems to be valid, at least for a distance of 20 km offshore, that corresponds to the distance between the reactor site and Ilha Grande, a 193-km² island existing at this site and probably an additional SGD source. Salinity, barium and silica results are also reported and their relation to ²²⁸Ra concentrations is discussed.

Keywords:²²⁸Ra, ²²⁶Ra, seawater, submarine groundwater discharge, Brazil

RESUMO

O presente trabalho apresenta os resultados de ²²⁶Ra e ²²⁸Ra obtidos durante o 4º experimento de intercomparação referente à descarga submarina de água subterrânea (Submarine Groundwater Discharge, SGD) realizado em Ubatuba (SP), em novembro/2003. Este experimento visava testar, em um sistema de rochas cristalinas fraturadas, as diferentes metodologias existentes de determinação da SDG. Embora com objetivo distinto, decidiu-se participar deste experimento de modo a verificar se os isótopos de rádio poderiam ser empregados no estudo de mistura de águas na região e, também, testar a capacidade analítica do laboratório, visando uma futura validação de modelos de dispersão marinha para o local de instalação dos reatores nucleares de Angra dos Reis, distantes apenas setenta quilômetros da região do experimento. Além das concentrações de ²²⁸Ra e ²²⁶Ra, resultados de salinidade, bário e sílica também são apresentados e suas relações com a concentração de ²²⁸Ra são discutidas.

INTRODUCTION

The Angra dos Reis nuclear power plant site is located 130 km south from Rio de Janeiro city. There are two pressured water reactors in operation at the site: the first one started operation in 1985 and has a nominal capacity of 660 MWe and the second one is a 1300 MWe unit and started its operation in 2000. Seawater is used as cooling water for both units; the cooling water is pumped from Itaorna Bay and discharged into Piraquara de Fora Bay, together with the liquid effluents from the two NPP.

The dispersion of discharged radionuclides was modeled by Fernandes et al.¹ in order to allow the assessment of the committed dose incurred by the local population in case of normal operation and accidents. The results obtained suggested that conservative radionuclides released, such as ¹³⁷Cs and ³H, are efficiently exported to the open ocean.

In order to validate this model, it was first thought to use the released tritium as a tracer. However, due to the actually achieved tritium detection limit (2 kBq m^-3), its potential application was restricted to the Piraquara de Fora Bay. For a longer distance an alternative was needed. With the development of the research related to the importance of the submarine groundwater discharge (SGD) as an relevant source of substances to the coastal region and, in particular, with the consequent use of radium isotopes for the determination of coastal mixing rates,² it was decided to investigate its potential application as a way to validate the dispersion model.

An updated review of SGD in seawater was published by Burnett et al.³ In order to compare the different SGD assessment methodologies the Intergovernmental Oceanographic Commission of UNESCO (IOC), the International Hydrological Program (IHP) and the International Atomic Energy Agency (IAEA) are sponsoring intercomparison experiments at different locations around the world. The previous SGD intercomparison experiments were performed in Gulf of Mexico, Florida (2000); Cockburn Sound, western Australia (2000) and eastern Long Island, New York (2002). The experimental setup and results of the Gulf of Mexico SGD intercomparison experiment are found in Biogeochemistry volume 66, number 1-2, November 2003, a special issue dealing with it. The Ubatuba experiment was realized in November 2003 and aimed to test the methodologies on a fractured crystalline rocks system. The first experiments on SGD at the Ubatuba coast were performed by Oliveira et al.⁴ and indicated a significant inflow of several cm per day.

Determining coastal mixing rates using radium isotopes ²

The change in concentration or activity (A) with time (t) as a function of distance offshore (x) for a conservative, non reactive tracer may be expressed as a balance of advection and diffusion:

where K_h is the eddy diffusion coefficient and w is the advective velocity. Lateral effects are ignored in this one-dimensional approximation. If K_h and w are constant, the offshore distribution of a conservative tracer may be used to determine the relative importance of these processes in moving the signal offshore (advection) and diluting if (diffusion). If the offshore distribution is dominated by diffusion with constant K_h, a plot of activity vs. distance will be a straight line connecting the end members. Offshore advection will cause positive curvature and onshore advection will cause negative curvature to this plot. This model assumes there is no additional input of the tracer beyond the nearshore. The existence of a termocline isolate the surface water from inputs from below. Once added to nearshore waters, the two long-lived radium isotopes, ²²⁶Ra (t_1/2 = 1620 years) and ²²⁸Ra (t_1/2 = 5.7 years), are conservative tracers.

For the short-lived radium isotopes, ²²³Ra (t_1/2 = 11.3 days) and ²²⁴Ra (t_1/2 = 3.66 days), a decay term must be included in the equation describing their distributions. Equation (2) describes the distribution of the activity of a radioactive tracer with decay constant l in the same context:

If net advection can be neglected, this reduces to:

In this case, the boundary conditions are:

If K_h is constant and the system is in steady state:

where A_x is the activity at distance x from the coast, A_o is the activity at distance 0 from the coast and l is the decay constant.

A plot of ln²²³Ra or ln²²⁴Ra as a function of distance from the coast may be used to estimate K_h, if the exchange is dominated by eddy diffusion rather than advection and if the system is in steady state:

In this case, the slope is .

Therefore, using long-lived radium isotopes in coastal waters it is possible to verify which mixing mechanics (diffusion and advection) are present. Where the exchange is dominated by diffusion, applying short-lived radium isotopes it is possible to determine the eddy diffusion coefficient.

This methodology was successful applied to Yeoja Bay, morphologically similar to Ribeira Bay, including the presence of several islands inside and nearby the bay outlet, in order to estimate nutrient inputs to.⁵ Although with distinct objectives, it was decided to participate in this SGD experiment in order to verify if these constraints are valid in this region and to test the laboratory capability to perform, later on, the marine dispersion model validation at the Angra dos Reis reactor site, located only 70 km from Ubatuba.

At Angra dos Reis reactor site, two others radium sources have to be taken into account at Angra dos Reis reactor site, Ilha Grande SGD and the current flowing though the Ilha Grande/Angra dos Reis channel. But, despite any morphological difference between Ubatuba and Ribeira bay, the radium contribution from the shoreline should be such that it could be differentiated from the open sea end-member, and this can be evaluated with the Ubatuba SGD experiment due to the geological similarity between both locals (Figure 1), with the presence of granites and migmatites of mountain chains (Serra do Mar), which reaches the shore throughout almost all the study area, and limits the extension of the drainage systems and of the Quaternary coastal plains.

In the future, in order to evaluate the relative influence of these two other sources, 2-3 transects at Ribeira bay surroundings will be performed, one of them, from Mambucaba beach, crossing the bay, in direction to open sea through the channel between Ponta de Juatinga and Ilha Grande. Since multiple tracers as short and long-lived radium isotopes, barium, silica and salinity will be used it should be possible to evaluate their relative contributions.⁵

Experimental

The Ubatuba SGD experiment was performed during the period 16-22 November 2003, at the same region described by Oliveira et al.⁴ Samples of surface ocean water were obtained during two cruises, the first one realized on 18^th November from Flamengo Bay to Vitoria Island (31 km offshore) and, next day, the second one from Vitoria Island to Fortaleza Bay (Figure 1 and 2). The distance between each sampling station was about 5 km and a total of 16 samples were taken. The R/V Albacora (Instituto de Oceanografia, Universidade de São Paulo) was used on both cruises and each sampling station localization was recorded by an on-board GPS, the distances to the coast were calculated based on these records. At each sample station 100 L seawater was taken and pumped through on-line filter cartridge (2 µm pre-filter and 0.45 µm filter) followed by a column of manganese coated acrylic fiber (Mn fiber), to quantitatively remove radium.⁶ The Mn fibers were leached with HCl and hydroxylamine hydrochloride was added in order to produce a clear solution. Radium was co-precipitated with BaSO₄ and ²²⁸Ra and ²²⁶Ra determined by gross beta and alpha counting as described by Godoy et al.⁷ The detection limit achieved was 0.02 and 0.3 Bq m^-3 for ²²⁶Ra and ²²⁸Ra, respectively, and the relative measurement error ranges were between 5-10%. Additionally, a seepage meter (10 L), an artesian well and Picinguaba river (20 L) water sample were collected, in order to obtain the ²²⁸Ra/²²⁶Ra activity ratio signature of the surface and groundwater end members. Seawater samples for ²²⁸Ra and ²²⁶Ra were also collected under the MOMAM program during the year 2003,⁸ in order to determine its variability across the Brazilian Southeastern coast.

At each sampling site, a 100 mL sample was taken and kept frozen for the determination of salinity, silica and barium. Barium and silica were determined by inductively coupled plasma atomic emission spectrometry (Perkin-Elmer ICP-AES Optima 2000 DV), salinity by titulation with silver nitrate.

Results and Discussion

The obtained results are shown in Table 1. The artesian well was located on the granite hills close to the Marine Laboratory, due to its very low radium concentration, the contribution of groundwater coming from these mountains to the radium content of coastal water discharged at this region seems to be not relevant. Similarly, Picinguaba river has crystalline waters with; also, low radium content, and cannot be taken as a relevant radium source for the coastal seawater. On the other hand, the ²²⁸Ra content on the seepage water was 15-20 times higher than that observed seawater concentrations. The ²²⁸Ra/²²⁶Ra activity ratio (AR) in the seepage and the artesian well water are similar to the ²³²Th/²³⁸U AR observed in monazite samples taken at the Rio de Janeiro coast,⁹ therefore, it is valid to suppose that the monazite leaching by groundwater moving to the beach is the reason of the high radium concentration observed in the seepage water sample, and this groundwater can be taken as the coastal end member.

Figure 3 shows the ²²⁸Ra and ²²⁶Ra concentrations as a function of distance offshore. For ²²⁶Ra there is not a clear function of the distance offshore as observed for ²²⁸Ra. For ²²⁸Ra the data within 20 km of the coast fit a line of (-0.0714) Bq m^-3 km^-1 having R² = 0.889, there is a break in the slope at 21 km and another at 26 km but only the first break is observed for the ²²⁸Ra/²²⁶Ra ratio (Figure 4). These two breaks could be due to the existence of offshore groundwater discharge. A break at 21 km is also observed for silica (Figure 5) that reinforces this possibility for this particular sampling point. Nevertheless, the results show that at least for a distance of 20 km offshore, a straight line connecting the end members was observed, showing that the offshore distribution is dominated by diffusion with constant K_h. This distance corresponds to the distance between the reactor site and Ilha Grande, a 193 km² island existing at this site and probably an additional SGD source.

The ²²⁸Ra relationship with other parameters with potential SGD signatures as salinity and barium and silica concentrations are shown in Figure 6a-c. The correlation between ²²⁸Ra and barium (Figure 6a) and silica (Figure 6b) is evident and was already reported by other authors.^10,11 However, the same was not observed for ²²⁸Ra and salinity (Figure 6c) or with ²²⁸Ra/²²⁶Ra AR (Figure 6d). Since the seepage water end member has a ²²⁸Ra/²²⁶Ra AR of 7.5 and a salinity of 26.6, higher the activity ratio higher should be the freshened water contribution, as are related by Moore and Todd.¹² On the other hand, a clear relationship between silica and salinity is observed, the Si-Sal slope of 1.9 µmol L^-1 kg^-1, for the samples in the 34-37 psu range (Figure 7), is similar to that related by Moore et al.¹⁰

It was also decided to test the influence of the different approaches adopted to calculate the distances, distance to the shoreline taking into account the start point at the shoreline or the distance to the nearest land mass. Using this methodology, as applied by Moore,¹³ the initial point, are the sampling points 4 and 14 for the first and second transects, respectively. The distances based on these two methodologies are different as shown in Figure 8, and their influence on the radium, on the ²²⁸Ra/²²⁶Ra and silica relationship with the distance is shown on Figures 9a-f. The point offshore with high ²²⁸Ra/²²⁶Ra activity ratio was omitted in both cases (Figure 9c and 9d). For the sampling points inside the bays there is a large difference for the distance calculate by the different methods. But, in general, taking into account a shift of 8 km for the longest distance achieved, the final results are equivalent as shown in Figures 9c-f.

During the 2003 MOMAM program⁸ seawater samples were taken for ²²⁸Ra and ²²⁶Ra determination and the results obtained are shown in Table 2. It can be seem that the ²²⁸Ra and ²²⁶Ra concentration in Ubatuba are similar to those obtained for the Ilha Grande Bay sample as initially supposed. Despite the large area comprised by the MOMAM program, from Vitoria (ES) to Santos (SP), 1000 km along the Brazilian southeastern coast, a relationship was observed between ²²⁸Ra/²²⁶Ra AR and salinity (Figure 10). Thorium rich heavy minerals are found along all over the Brazilian southeastern coast^14,15 giving rise to the observed ²²⁸Ra/²²⁶Ra AR higher than 1.

Conclusions

The results have shown that the shoreline supply of radium through submarine groundwater discharge in Ubatuba, region nearby the Angra dos Reis nuclear power reactor, is such that it can be used as a tracer for the determination of coastal mixing rates and, consequently, be applied as a tool for the validation of the existing marine dispersion model for this site. However, due to the potential influence of two others radium sources, Ilha Grande SGD and the current flowing through the Ilha Grande/Angra dos Reis channel, additional SGD tracers as short-lived radium isotopes, silica and barium will be needed.

Elevated silica, barium and ²²⁸Ra concentrations and ²²⁸Ra/²²⁶Ra AR were observed in the seepage water end member. Statically valid correlation between ²²⁸Ra and others SGD tracers as barium and silica was observed.

Along the Brazilian southeastern coast, a good relationship between ²²⁸Ra/²²⁶Ra AR and salinity was observed. ²²⁸Ra/²²⁶Ra AR was always higher than 1, probably, as consequence of the thorium rich heavy minerals found all over the Brazilian southeastern coast.

Acknowledgments

To Josilene Oliveira and William Burnett who invited us to participate on the 4^th Submarine Groundwater Discharge intercomparison experiment. To the R/V Albacora crew for help during the sampling cruises. To W.S. Moore for his help and fruitful comments during all this work. To Mauricio Puppin (Departamento de Química, PUC-Rio) for the silica and barium determination by ICP-AES.

Received: May 18, 2005

Published on the web: May 26, 2006

• 1. Fernandes; H. M.; Veiga, L. H. S.; Rochedo, E. R. R.; Vetere, M. I. In Extended Synopses of the International Symposium on Marine Pollution, Monaco, 1998, pp. 569-570.
• 2. Moore, W. S.; Cont. Shelf Res 2000, 20, 1993.
• 3. Burnett, W. C.; Bokuniewicz, H.; Huettel, M.; Moore, W. S.; Taniguchi, M.; Biogeochemistry 2003, 66, 3.
• 4. Oliveira, J.; Burnett, W. C.; Mazzilli, B. P.; Braga, E. S.; Farias, L. A.; Chistoff, J.; Furtado, V. V.; J. Environ. Radioact. 2003, 69, 37.
• 5. Hwang, D-W.; Kim, G.; Lee, Y-W.; Yang, H-J; Mar. Chem 2005, 96, 61.
• 6. Moore, W. S.; Deep Sea Res. 1976, 23, 647.
• 7. Godoy, J. M.; Lauria, D. C.; Godoy, M. L. D. P.; Cunha, R. P.; J. Radioanal. Nucl. Chem. 1994, 182, 165.
• 8. Godoy, J. M.; Carvalho, Z. L.; Fernandes, F. C.; Danelon O. M.; Ferreira A. C. M.; Roldão, L. A.; J. Environ. Radioact. 2003, 70, 193.
• 9. Aronne, C. C.; Godoy, J. M.; Godoy, M. L. D. P. In Proceedings of the IV South American Symposium on Isotope Geology, Salvador, Brazil, 2003, pp. 31-34.
• 10. Moore, W. S.; Sarmiento, J. L.; Key, R. M.; J. Geophys. Res 1986, 91, 2574.
• 11. Moore, W. S.; Edmond, J. M.; J. Geophys. Res. 1984, 89, 2061.
• 12. Moore, W. S.; Todd, J. F.; J. Geophys. Res. 1993, 98, 2233.
• 13. Moore, W. S.; Biogeochemistry 2003, 66, 75.
• 14. Freitas, A. C.; Alencar, A. S.; J. Environ. Radioact 2004, 75, 211.
• 15. Sachett, I. A.; Ph.D. Thesis, Universidade do Estado do Rio de Janeiro, Brazil, 2002, avaliable on line at http://www.ird.gov.br/depra/radonio.htm

Publication Dates

• Publication in this collection
  25 Aug 2006
• Date of issue
  Aug 2006

History

• Received
  18 May 2005