Radiation Protection Dosimetry (2006), Vol. 121, No. 2, pp. 179–185 doi:10.1093/rpd/ncl011 Advance Access publication 10 March 2006
RADIOLOGICAL CHARACTERISATION OF DISPOSED PHOSPHOGYPSUM IN BRAZIL: EVALUATION OF THE OCCUPATIONAL EXPOSURE AND ENVIRONMENTAL IMPACT A. J. G. Santos1, P. S. C. Silva1, B. P. Mazzilli1, and D. I. T. Fa´varo2 1Instituto de Pesquisas Energe´ticas e Nucleares, Laborato´rio de Radiometria Ambiental, IPEN, Brazil 2Laborato´rio de Ana´lise por Ativaca¸ ˜o, Av. Prof. Lineu Prestes, 2242, Cidade Universita´ria, CEP 05508-000, Sa˜o Paulo, Brazil
Phosphogypsum, a waste by-product derived from the wet process production of phosphoric acid, represents a serious problem facing the phosphate industry in Brazil. This by-product (mainly calcium sulphate dihydrate) precipitates during the reaction of sulphuric acid with phosphate rock and is stored at a rate of about 4 106 kg per day on several piles in Cubata˜o, Brazil. Contents of natural radionuclides from thorium and uranium series were measured in Brazilian phosphogypsum samples from disposal piles, using high-resolution gamma ray spectrometry and instrumental neutron activation analysis (NAA). These phosphogypsum piles present a potential threat to the surrounding environment and to the individual occupationally exposed. The results obtained in this study show that radionuclides, although present in relatively high concentrations in phos- phogypsum, do not imply in significant doses for individuals occupationally exposed. The results obtained for the water activity in the monitor wells showed that the run-off of the piles is influenced by the activity present in the piles, giving indication of a possible groundwater contamination. Sediments from rivers in the area of influence of the pile presented higher concentrations of 238U and 232Th when compared with reference values.
INTRODUCTION Brazilian regulatory agency (Comissa˜o Nacional de Energia Nuclear—CNEN) has recently publi- Phosphogypsum, a waste by-product derived from shed a regulatory guide concerned on mining and the wet process production of phosphoric acid, milling of natural occurrence radioactive material, represents a serious problem facing the Brazilian which may generate enhanced concentrations of phosphate industry. By taking into account that the radionuclides, under the radiological protection ratio phosphogypsum/P O produced is 5:1, this 2 5 point view (Requisitos de Seguranca¸ e Proteca¸ ˜o industry has produced, since 1991, 390 106 Radiolo´gica para Instalaco¸ ˜es Mı´nero-industriais metric tonnes of phosphogypsum in Brazil(1). This CNEN-NN-4.01)(2). Such activities may include, by-product (mainly calcium sulphate dihydrate) for instance, the mining and processing of ores precipitates during the reaction of sulphuric acid as well as storage of raw material, products, with phosphate rock and is stored at a rate of by-products, residues and wastes containing about 4 106 kg per day on several piles in Cubata˜o, radionuclides of the 238U and 232Th series, simult- Brazil. Two main producers, responsible for these aneously or separately, which may incur undue gypsum piles, are named in this paper as A and C. exposures of members of the public and occupation- Both industries use as raw material an igneous ally exposed. phosphate rock from Catala˜o I carbonatite complex. During the reaction of phosphatic rock with The fresh rock is a phoscorite cut by abundant sulphuric acid the radioactive equilibrium between carbonatitic veins. This complex is located in the 238U, 232Th and their decay products is disrupted State of Goia´s, Central Brazil and belongs to a and the radionuclides migrate according to their group of Mesozoic alkaline rocks bordering the solubility: uranium isotopes form highly soluble Parana´ Basin. The main problem associated with compounds with the ion (PO )3- while Ra isotopes, this material concerns the relatively high levels of 4 210Pb and 210Po, concentrate into phospho- natural uranium and thorium decay series and gypsum(3). Thorium behaviour is dependent upon other impurities, which could have an impact on the chemical reaction and is uniformly distributed the environment and prevent its commercial use. in the two reaction products(4). No Brazilian regulation in terms of radiation Phosphogypsum produced during the process is protection principles has been applied to these filtered off and pumped as slurry to nearby ponds, industries so far. Therefore, no data are available where it stays for a period sufficient to allow com- so far concerning the real occupational and plete deposition. The water is not released to the environmental impact of such industries The environment, since the plants operate in closed circuit. The phosphogypsum waste is then moved Corresponding author: [email protected] to nearby storage areas, the so-called gypsum piles.
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Figure 1. Location of industries A and C (comprising pond and piles) and sampling points. Areas 1 and 2 represent phosphogypsum piles from industry A; 4 represents phosphogypsum piles from industry C; 3 represents ponds from industry C; MW ¼ monitor wells; BR ¼ Bugre River and MR ¼ Mogi River.
180 RADIOLOGICAL CHARACTERISATION OF DISPOSED PHOSPHOGYPSUM IN BRAZIL In Figure 1 the location of the two industries energy gamma rays is highly dependent upon sample (comprising pond and piles) and the surrounding composition. The approach used was that sugges- aquatic system are depicted. It can be seen that ted by Cutshall et al.(5) and the absorption correc- there is just one small river, Bugre River (BR), tion was carried out for each sample. Typical lower which is close to gypsum piles from industry C. limits of detection for gamma spectrometry were This river is tributary of Mogi River (MR). 4.5 Bq kg 1 for 226Ra, 3.1 Bq kg 1 for 228Ra and Besides the obvious waste of potentially valuable 25 Bq kg 1 for 210Pb, for a counting time of 50 000 s. by-product, the main problem associated with phos- The 238U and 232Th determination by NAA was phogypsum storage is the potential threat to the carried out by irradiation of 150 mg of each sam- surrounding environment and to the individual ple, for 8 h at a neutron flux of 1012 ncm 2s 1,at occupationally exposed. This paper is mainly con- Instituto de Pesquisas Energe´ticas e Nucleares cerned with the radionuclide characterisation of (IPEN) research reactor IEA-R1. The induced radio- the phosphogypsum piles from industries A and C, activity was measured with a Ge-hyperpure detector, in Cubata˜o region, and with the evaluation of the Intertechnique, with 2.1 keV resolution for the 1332 consequent environmental and occupational impact. keV 60Co photopeak. The concentration of the The aquatic environment near the disposal area was analysed elements was determined by comparing assessed by measuring natural radionuclide activities activities obtained in the samples with certified ref- in monitor wells, river water and sediment samples in erence materials, Buffalo River Sediment (NIST the vicinity of gypsum pile from industry C. As for SRM 2704) and Soil-7 (IAEA). the individual occupationally exposed, the pathways The aquatic environment near the disposal area considered were internal exposure due to inhalation was assessed by measuring natural radionuclides of radon emanated from phosphogypsum piles and activities in monitor wells, river water and sediment external gamma and beta exposures due to immer- samples in the vicinity of the gypsum pile from sion in the radioactive plume and due to direct irra- industry C. One sediment sample was collected in diation from phosphogypsum. Bugre River (BR1) and nine in Mogi River (MR1– MR9). Water samples were collected in four monitor wells around the gypsum piles from industry C MATERIALS AND METHODS (MW1–MW4), in BR and in MR. The location of Contents of natural radionuclides from thorium and the sampling points is depicted in Figure 1. uranium series (226Ra, 210Pb and 228Ra) were meas- The activity concentration in river sediment ured in phosphogypsum samples from piles, samples, in water samples from monitor wells in using high-resolution gamma spectrometry. the vicinity of the gypsum piles and in soil in the 238U and 232Th characterisation were performed by reference area were determined by gamma spectro- instrumental neutron activation analysis (NAA). metry and NAA, in the same way as already Samples of phosphogypsum were randomly col- described for the analysis of phosphogypsum. lected by coring from piles of industries A and C. Activity concentration of 228Ra, 226Ra and 210Pb Activity concentrations of 226Ra, 228Ra and 210Pb in water samples, collected in the rivers and in the were measured in 33 samples of phosphogypsum reference area, was determined by a radiochemical and in soil from the reference area, by gamma spec- procedure, since the radionuclide content was too trometry with a hyper-pure germanium detector, low to be measured by gamma spectrometry. The EGNC 150-190 R, from Eurisys Measures, with res- procedure consisted of an initial precipitation of Ra 60 olution of 1.8 keV for the 1332 keV Co photopeak, and Pb with 3M H2SO4, dissolution of the precipit- for 4096 channels. The detector was calibrated using ate with nitrilo-tri-acetic acid at basic pH, precipita- natural soil, rock and water spiked with radionuc- tion of Ba(Ra)SO4 with ammonium sulphate and 210 lides certified by Amersham. Samples were packed in precipitation of 30% PbCrO4 with sodium chro- 100 cm3 cans and sealed for about 4 weeks prior to mate. The 226Ra and 228Ra concentrations were the measurement in order to ensure that equilibrium determined by gross alpha and beta counting of the 226 210 has been reached between Ra and its decay pro- Ba(Ra)SO4 precipitate and the Pb concentration ducts of short half-life. The 226Ra activities were through its decay product, 210Bi, by measuring the 210 determined by taking the mean activity of three gross beta activity of the PbCrO4 precipitate. The separate photopeaks of its daughter nuclides: 214Pb chemical yields for the radionuclides 226Ra, 228Ra at 295 keV and 352 keV, and 214Bi at 609 keV. The and 210Pb were determined by gravimetric analysis. 228Ra content of the samples was determined by All radionuclides were determined in a low back- measuring the intensities of the 911 and 968 keV ground gas flow proportional detector. gamma ray peaks from 228Ac. The concentration For the evaluation of the occupational exposure, of 210Pb was carried out by measuring the activity the pathways considered were internal exposure due of its low energy peak (47 keV). Self-absorption to inhalation of radon emanated from phos- correction was applied since the attenuation for low phogypsum piles and external gamma and beta 181 A. J. G. SANTOS ET AL. exposures due to immersion in the radioactive plume exposure, that is, taking into account the back- and due to direct irradiation from phosphogypsum. ground levels of the region as well as the anthropo- Internal doses due to inhalation of radon and its genic increment due to the phosphogypsum decay products were estimated by measuring alpha deposition. Since for the radiation protection point particles originating from radon and its decay pro- of view, only the dose increment should be con- ducts in several points on the phosphogypsum piles, sidered, measurements of background radiation using a portable detector RDA-200 from SCIN- levels were performed in a reference area, out of TREX. At least two points were seasonally sampled the influence of the piles. The region chosen is in each pile, one in the maximum height of the pile located 5 km away from the gypsum piles in the and another in the wind direction. The air was south-west direction, the direction opposite to the sampled through a calibrated scintillation cell con- predominant wind direction. In this area, samples nected to one filter holder assembly using a pump. of ground water and soil were collected; external Radon decay products were measured in the filter 1 h gamma dose rates and radon concentration in air after sampling. The scintillation cells were measured were determined. These values were used for the 3 h after the end of sampling in order to determine evaluation of reference doses and the background the 222Rn airborne concentrations. Doses due to of the region. The water was collected at a depth of inhalation were estimated according to the following 3.7 m and presented a pH 5.7. equation: X RESULTS AND DISCUSSION E¼ Ci · Ui · FDi · Fo Characterisation of DISPOSED phosphogypsum 1 where E is the committed effective dose in Sv y , Ci is the radionuclide i concentration in air in Bq.m 3, Results obtained for radionuclides activity concen- 3 1 tration in phosphogypsum from industries A and C Ui is the respiration rate in m y , FDi is the dose conversion factor for inhalation, for radionuclide are presented in Table 1. Mean activity concentra- 1 tions of the same order of magnitude were observed i,inSvBq and Fo is the occupation factor. An 226 210 1 1 occupation factor of 8 h per day was considered in for Ra and Pb (849 Bq kg and 837 Bq kg for industry A and 357 Bq kg 1 and 342 Bq kg 1 for this case, assuming as critical group the individual 232 228 1 occupationally exposed while working on the piles. industry C) and for Th and Ra (222 Bq kg and 229 Bq kg 1 for industry A and 172 Bq kg 1 and Dose conversion factors were taken from 1 UNSCEAR(6). 163 Bq kg for industry C) showing that all these External gamma dose rates were evaluated using radionuclides are concentrating in the phos- thermoluminescence dosemeters. The detectors were phogypsum. These results agree well with previous determination of the same radionuclides in fresh placed in a 4 mm thick plastic holders, fixed at 1 m (3) above ground, in the same points where 222Rn con- phosphogypsum by Mazzilli et al. , showing that centrations were measured. External doses due to the radioactivity present in the disposed phos- direct exposure from the phosphogypsum piles phogypsum is mainly dependent on the amount were also taken into account. In both cases, external supplied by the ore rock, which is different for the doses were evaluated for the individual occupation- two industries. ally exposed, considering an exposure of 8 h per day. Doses due to direct exposure from the piles were Dose assessment (7,8) evaluated according to the following equation : The 222Rn concentrations measured at several points X in the phosphogypsum piles varied from 8 1to E¼ Ci · rS·FDiðÞEi,T · Fo
1 Table 1. Activity concentration in phosphogypsum samples where E is the committed effective dose (Sv y ), Ci from industries A and C and in soil (Bq kg 1). is the radionuclide i concentration in phos- phogypsum (Bq g 1), rS is the density of dry phos- 3 Producer A Producer C Soil phogypsum (g cm ), FDi is the dose conversion factor for a infinite volume source for radionuclide 1 3 Mean SD Range Mean SD Range i (Sv h per Bq cm ) and Fo is the occupation factor (h y 1). For the evaluation of the dose con- 238U42 13 20–69 48 9 37–61 48 4 version factor (FDi), the photon energy of y-emitter 226Ra 849 242 450–1251 357 120 177–594 91 7 radionuclide i in Mev (Ei) and the photon mean- 210Pb 837 243 408–1234 342 122 159–581 ND free-path length in phosphogypsum in cm (T ) were 232Th 222 98 47–346 172 106 63–285 98 8 (9) 228 considered. Ra 229 46 163–334 163 60 90–238 56 6 The final doses obtained for external and internal exposure due to inhalation correspond to the overall ND: Not determined
182 RADIOLOGICAL CHARACTERISATION OF DISPOSED PHOSPHOGYPSUM IN BRAZIL Table 2. Activity concentrations of 228Ra, 226Ra and 210Pb in water samples from the monitor wells and from Bugre and Mogi rivers (Bq L 1).
228 226 210 2 1 Sampling location Ra Ra Pb pH [SO4 ] (mg L ) Depth (m)
MW1 8.0 0.4 1.7 0.1 <1.5 5.6 1330 2.9 MW2 4.5 0.2 1.2 0.1 2.0 0.1 3.7 328 3.6 MW3 4.8 0.2 1.2 0.1 3.1 0.2 5.6 14 4.6 MW4 6.7 0.3 2.9 0.2 <1.5 5.1 50 3.6 Mean SD 6.0 1.4 1.8 0.8 2.6 0.8 Groundwater (8.7 0.4) 10 2 (9.6 1.1) 10 2 (1.7 0.1) 10 2 5.7 3.7 BR1 (3.1 0.3) 10 2 (3.4 0.4) 10 2 (6.6 0.9) 10 3 6.1 MR1 (1.9 0.4) 10 2 (1.6 0.2) 10 2 <2.4 10 3 6.2 MR2 (2.5 0.3) 10 2 (2.3 0.1) 10 2 <2.5 10 3 6.2
48 7Bqm 3 (mean value of 23 14 Bq m 3), respectively, whereas mean activity of 6.0 Bq L 1 giving an internal dose of 6.8 10 3 mSv y 1. was observed for 228Ra. The monitor wells are Radon daughter concentrations varied from 1.2 located close to the gypsum piles(10). These waters 10 2 WLM to 1.6 10 2 WLM, giving a mean dose presented concentrations of radionuclide above the of 0.5 mSv y 1. In the reference area, 222Rn con- level observed in groundwater of the reference area. centrations varied from 7 2to18 4Bqm 3 The results of sulphate concentration and pH also (mean value of 13 4Bqm 3), resulting in an confirm that these waters are strongly influenced by internal dose of 3.9 10 3 mSv y 1. Doses from the run-off of the piles. However, such influence was radon daughters were within the range observed for not observed in the river water samples analysed. the doses in the piles. It should be emphasised that Recent papers(11,12) are concerned with the potential all the doses were estimated considering 8 h of expos- release of radionuclides from phosphogypsum to the ure per day. It can be concluded that the radon aquatic environment. Burnett and Elzerman(11) concentration in air varied widely, depending upon found concentrations ranging from 0.1 to 0.2 Bq 1 L the weather conditions, time of measuring and con- for 226Ra and from 7 to 70 Bq L 1 for 210Pb, in pile ditions of soil. External gamma dose varying from fluids in Florida. Unfortunately, data were not avail- 3.7 10 1 to 6.5 10 1 mSv y 1 were observed in able concerning the radionuclide concentration in the piles and varied from 2.4 10 1 to 3.3 10 1 the groundwater in the vicinity of the gypsum piles mSv y 1 in the reference area. Considering the activ- to conclude about a possible rout of migration of ity concentration of all radionuclides present in radionuclides through water in this environment. It phosphogypsum (Table 1) direct exposure up to is, therefore, advisable to perform a more detailed 4.9 10 1 mSv y 1 was computed for the piles sampling and characterisation of the groundwater from industry C, whereas direct exposure up to system. 2.3 10 1 mSv y 1 was computed for the reference Oliveira et al.(13) found activity concentrations area by taking into account the activity concentra- for 226Ra and 228Ra in drinking water supply of tion of the same radionuclides and 40K in the soil. Sa˜o Paulo State in the range of <0.2 10 3 to The results obtained in this study show that radio- 235 10 3 Bq L 1 and of <3.7 10 3 to 131 nuclides, although present in relatively high concen- 10 3 Bq L 1, respectively. The waters collected in trations in phosphogypsum, do not imply in BR and MR are one order of magnitude higher than significant doses for individuals occupationally these results. The BR sample presented activity exposed. As a general trend the doses are well concentration slightly above the results observed in below the limits recommended for individuals MR, probably reflecting its proximity to phospho- occupationally exposed, reaching values around the gypsum piles. double of the natural levels of the region. Table 3 shows the radionuclide concentration obtained in the sediment samples. In the same table are reference values obtained for a deep core profile Environmental impact collected in the region(14). These values were con- In Table 2 are presented the results obtained for the sidered as the sediment background of the region activity concentration of 228Ra, 226Ra and 210Pb in (BG), since no data are available in the literature so the waters collected in the monitor wells around the far. In Figure 2 are depicted the results obtained for piles of plant C, in waters from the rivers and the sediment concentration normalised by the refer- groundwater from the reference area. The results ence values. As a general trend, it can be seen that from the monitor wells presented 226Ra and 210Pb the concentration of these elements does not present mean concentrations of 1.8 Bq L 1 and 2.6 Bq L 1, significant variations downstream MR, except for 183 A. J. G. SANTOS ET AL. BR1, RM8 and RM9. Sampling point BR1, located with RM8 and RM9 samples. This similarity is prob- in the BR, a tributary of MR, is the closest point to ably due to the presence of phosphogypsum in the the phosphogypsum piles. This point is enriched in sediments of BR, brought by wind or rain erosion. thorium and uranium. Radium isotopes present a Samples RM8 and RM9 are close to a steel industry. quite constant distribution in the sediment samples, These sediments are clearly affected by anthropo- probably due to their great solubility and the same genic activities. The other phosphogypsum samples was also observed for 210Pb. were grouped together with the background values In order to verify the influence of the phos- of the region. phogypsum piles in the environment, cluster analysis was applied to all experimental results, including phosphogypsum samples from industry C, sediment samples and background values, as shown in CONCLUSIONS Figure 3. It can be seen that the BR1 sample, The results obtained in this study show that which is the sampling point more close to the phos- although radionuclide concentration is high in the phogypsum piles, presents higher similarity with phosphogypsum, no contamination was observed in phosphogypsum samples, forming one single group surface waters. The results obtained for the water activity in the monitor wells showed that the run-off Table 3. Activity concentration (Bq kg 1) in river sediment of the piles is influenced by the activity present in the samples. piles, giving indication of a possible groundwater contamination. It is, therefore, advisable to perform 232Th 238U 228Ra 226Ra 210Pb a more detailed monitoring programme of the groundwater system to evaluate the real environ- BR1 198 9 118 11 86 543 256 9 mental impact of the piles. Sediments from rivers in MR1 82 453 686 543 255 9 the area of influence of the piles presented higher MR2 96 569 682 560 368 10 concentrations of U and Th. As for the occupational MR3 92 456 665 431 249 8 exposure, doses due to Rn inhalation and due to MR4 100 564 571 439 252 8 external irradiation on the piles were considered MR5 99 574 575 443 268 9 irrelevant compared with the reference values. How- MR6 89 468 666 441 274 10 ever, it should be emphasised that another important MR7 93 474 670 641 365 13 source of internal occupational exposure is the MR8 95 493 10 65 447 257 9 MR9 95 497 873 444 248 8 inhalation of dust but results were not available in BG 61 54 71 45 57 this study. This source should not be neglected in the evaluation of the total doses for workers, especially
Figure 2. Normalised concentrations in Bugre and Mogi river sediment samples.
184 RADIOLOGICAL CHARACTERISATION OF DISPOSED PHOSPHOGYPSUM IN BRAZIL
Figure 3. Cluster analysis applied for the analysed samples phosphogypsum (C), sediment (BR, MR) and sediment background. when the material is stored in places with poor vent- corrections. Nucl. Instrum. Methods 206, 309–312 ilation during the manufacturing of phosphoric acid. (1983). 6. United Nations Scientific Committee on the Effects of Atomic Radiation. UNSCEAR Sources and Effects of Ionizing Radiation. UNSCEAR 1993 Report to the ACKNOWLEDGEMENTS General Assembly, with Scientific Annexes (NY: UN) (1993). This work was supported by Conselho Nacional de 7. Chen, S. Y. Calculation of effective dose-equivalent Desenvolvimento Cientı´fico e Tecnolo´gico CNPq, responses for external exposure from residual photon grant 300835/95-7 and Fundaca¸ ˜o de Amparo a` emitters in soil. Health Phys. 60(3), 411–426 (1991). Pesquisa do Estado de Sa˜o Paulo FAPESP, research 8. International Commission on Radiological Protection. contract 1997/1597-6 and fellowship contract 99/ Conversion coefficients for use in radiological protection 06952-4. Phosphogypsum samples were provided against external radiation. ICRP Publication 74 by Companhia de Tecnologia de Saneamento (Oxford: Permagon Press) (1997). Ambiental CETESB. 9. Maduar, M. F. and Hiromoto, G. Evaluation of indoor gamma radiation dose in dwellings. Radiat. Prot. Dosim. 3(2), 221–228 (1991). 10. Santos, A. J. G. Avaliaca¸ ˜o do impacto radiolo´gico REFERENCES ambiental do fosfogesso brasileiro e lixiviaca¸ ˜o de 226Ra e 210Pb. Ph.D. dissertation, Instituto de 1. Departamento Nacional de Produca¸ ˜o Mineral. (2001) Pesquisas Energe´ticas e Nucleares, Sa˜o Paulo, p. 165 Balanco¸ Mineral Brasileiro, DNPM. Available on (In Portuguese) (2002). http://www.dnpm.gov.br/mostra_arquivo.asp?IDBanco 11. Burnett, W. C. and Elzerman, A. W. Nuclide migration ArquivoaArquivo¼361 and environmental radiochemistry of Florida phos- 2. Comissa˜o Nacional de Energia Nuclear. Requisitos de phogypsum. J. Environ. Radioactiv. 54, 27–51 (2001). Seguranca¸ e Proteca¸ ˜o Radiolo´gica para Instalaco¸ ˜es 12. Azouazi, M., Ouahidi, Y., Fakhi, S., Andres, Y., Mı´nero-industriais, CNEN. Report No. CNEN-NN- Abbe, J.Ch. and Benmansour, M. Natural radioactivity 4.01 (2004). in phosphates, phosphogypsum and natural waters in 3. Mazzilli, B., Palmiro, V., Saueia, C. and Nisti, B. M. Morocco. J. Environ. Radioactiv. 54, 231–242 (2001). Radiochemical characterization of Brazilian phos- 13. Oliveira, J., Mazzilli, B. P., Sampa, M. H. O. and phogypsum. J. Environ. Radioactiv. 49, 113–122 Bambalas, E. Natural radionuclides in drinking water (2000). supplies of Sa˜o Paulo State, Brazil and consequent 4. Saueia, C. H., Mazzilli, B. P. and Fa´varo, D. I. T. population doses. J. Environ. Radioactiv. 53, 99–109 Natural radioactivity in phosphate rock, phosphogypsum (2001). and phosphate fertilizers in Brazil. J. Radioanal. Nucl. 14. Silva, P. S. C. Caracterizaca¸ ˜o quı´mica e radiolo´gica dos Chem. 262, 445–448 (2005). sedimentos do estua´rio de Santos, Sa˜o Vicente e Baı´ade 5. Cutshall, N. H., Larser, I. L. and Olsen, C.R. Direct Santos. Ph.D. Thesis, Instituto de Pesquisas Energe´tica analysis of 210Pb in sediment samples: self-absorption e Nucleares, Sa˜o Paulo, p. 266 (In Portuguese) (2004).
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