C. T. P. Barbosa, 1,2J. A. Souza Neto*, 3C. A. Silva Filho

Transcription

1 ISSN ARSENIC AND ANTIMONY DISTRIBUTION IN THE STREAM SEDIMENTS OF THE CANTO DO AMARO AND ALTO DA PEDRA OIL FIELDS, NORTHEASTERN BRAZIL C. T. P. Barbosa, 1,2J. A. Souza Neto*, 3C. A. Silva Filho 1 1 Programa de Pós-Graduação em Geociências, Universidade Federal de Pernambuco Departamento de Geologia, Universidade Federal de Pernambuco 3 Centro Regional de Ciências Nucleares, Comissão Nacional de Energia Nuclear 2 * To whom all correspondence should be addressed. Address: Av. Acadêmico Hélio Ramos, s/n, Cidade Universitária, Recife - PE - Brazil - CEP Telephone / fax numbers: / adauto@ufpe.br Abstract. With the purpose of evaluating inorganic contamination in the two main oil fields located in the on-shore part of the Potiguar basin, sediments from streams crosscutting the Canto do Amaro and Alto da Pedra fields were sampled in rainy (30 samples) and dry seasons (55 samples). Fine fractions (< 63 µm) of the sediments were digested in acqua regia and analyzed for a multi-element package using an ICP-AES. Among the elements investigated, arsenic and antimony showed anomalous concentrations (6 to 69 ppm, and 5 to 28 ppm, respectively) when compared to background levels established for the area. A highly positive correlation was observed between these two elements, suggesting a common source for them. Oil and/or combustion of fossil fuels in the area could be the source of As and Sb detected. Alternatively, these elements could be related to a particular chemical composition of the rock substrate in the area. Keywords: arsenic and antimony; stream sediments; oil exploitation; Canto do Amaro and Alto da Pedra oil fields; Northeastern Brazil 1. INTRODUCTION During the last century, industrialization promoted an increase in uses of petrochemical substances. As a consequence, the number of contaminated areas by oil and its derivatives has increased. Several environmental reservoirs (e.g. soil and water) are liable to be contaminated by these substances, which are considered to produce mutagenic and carcinogenic effects in human (Manahan, 1994). As the result of burning of fossil fuels, several microscopic pollutants are released and incorporated into the atmosphere. The composition of the particulate material in the atmosphere is significantly diversified. Most common constituents are salts, oxides, nitrogen and sulfur compounds, some radionuclides, and minor amounts of Cu, Pb, Ti, Zn, Sb, Be, Bi, Cd, Co, Cr, Cs, Li, Mn, Ni, Rb, Se, Sr and V (Manahan, 1994). These pollutants can pass from the atmosphere to the surface water bodies and soil, through rain water or as precipitated solid particles (wet and dry deposition of pollutants). Pollutants deriving from oil and different products used in its production, such as barium from drilling fluids (Carls et al., 1995), can be detected in the sediments from streams and rivers crosscutting oil fields. Hydrocarbon compounds and metals, such as Ba, Cr, Pb, Zn, Ni, V, Cu, Cd, and Mn, are the most common substances among these pollutants (Carls et al., 1995; Metwally et al., 1997; Macías-Zamora et al., 1999). Hydrocarbons can be oxidized or decomposed by bacterial activity, but metals are not decomposed in the environment, and consequently are incorporated into the 45

2 sediments. Metals are hazardous to humans when present in elevated amounts, and could present biomagnification through the trophic levels of the food chain (Manahan, 1994). This work has the main purpose of reporting the occurrence of anomalous concentrations of As and Sb detected in the sediments from the streams crosscutting the Canto do Amaro and Alto da Pedra Brazilian oil fields. 2. MATERIALS AND METHODS This work was conducted within an area of 415 km² in the Canto do Amaro and Alto da Pedra oil fields, located in Mossoró County, State of Rio Grande do Norte, in Northeastern Brazil. The Canto do Amaro oil field occupies an area of about 85 km², and is crosscutted by rivers and seasonal streams. The Alto da Pedra oil field has an area of 25 km², and is crosscutted by seasonal streams, and by the Mossoró and do Carmo rivers (Figure 1). These oil fields are located in the Potiguar sedimentary basin, and the two main geological units exposed on the region contain limestones (Jandaíra Formation of Mesozoic age), and conglomerate and sandstones (Barreiras Formation of Tertiary age). In order to evaluate the inorganic chemical signature of possible contaminants that could be linked to the oil activities, sediment samples were collected in stations located in the main streams crosscutting the investigated oil fields. Sampling was conducted in two different seasons (rainy and dry) in order to show the distribution of metals in the stream sediments during the time of maximum dilution (lowest contents) and maximum concentration (highest contents) of metal amounts, respectively. Thirty samples were collected at the end of the rainy season (May 2003) and 55 samples at the end of the dry season (December 2003). Ten replicate samples were collected: 4 samples during the rainy season, and 6 samples in the dry season. A sampling station was chosen to represent the background levels (natural concentrations) of the investigated area. This station was strategically located about 13 km to the Northwest from the studied area, in a portion showing the dominant 46 geological substrate (limestone) with respect to the focused oil fields, and where there are no streams under the influence of these fields. Sediment samples were collected using plastic materials in order to avoid any metallic contamination. In each sampling station, about 2 kg of sediment were collected, from a layer with a depth of 0 to 3 cm deep in the middle of the stream channel, in order to sample only the more recently deposited material and to avoid contamination by material from the channel margins. Each sample was composed by several aliquots randomly collected in an area of about 12 m2 around the sampling station. In the laboratory, the samples were wet sieved using ultra pure water and a set of plastic sieves so as to obtain the fine fraction of sediment (< 63 µm). This particle size is generally accepted as the fraction that accumulates higher concentrations of contaminants, and is equivalent in size to the suspension material that remains longer in the fluvial system (Mantei and Sappington, 1994; Mudroch and Azcue, 1995; Herr and Gray, 1997; Datta and Subramanian, 1998; Soares et al., 1999; Luiz-Silva et al., 2002). After sieving, the samples were dried in an oven at temperature ranging between 40-50ºC to avoid losses of organic matter and adsorbed water, as well as the volatilization of compounds with some metals adsorbed (Kralik, 1999). Subsequently, the samples were digested in acqua regia and analyzed for a multi-element package (Ag, As, Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sc, Sb, Sn, Sr, Ti, V, W, Y, Zn, and Zr) using an ICP-AES, at the Lakefield-Geosol laboratories (Belo Horizonte, Brazil). Replicate samples produced in laboratory and those collected in some sampling stations show good reproducibility of results, which attest the validity of the analytical procedures and the natural homogeneity of the material sampled. 3. RESULTS For each sampling station, the results obtained for all elements were compared to the background levels established to the area. Among all investigated elements, arsenic and

3 Figure 1. Location map of the Canto do Amaro and Alto da Pedra oil fields showing the sampling stations. Sampling stations presenting As and Sb anomalies (rainy and dry seasons), and oil spilling observed in the stream sediments are also shown. 47

4 Sb showed anomalous values with regards to the background. Arsenic and Sb concentrations varied between 6 and 69 ppm, and 5 and 28 ppm, respectively. In some sampling stations, arsenic and Sb concentrations also exceeded values of the shale composition ( ppm of As, and ppm of Sb; according to Govett, 1983). Comparison with shale composition could be considered valid, since both granulometric and mineralogical characteristics of this rock are similar to those of the sediments (< 63 µm) used in geochemical analyses. Analyses of seven samples collected in the dry season (stations 49, 52, 56, 59, 67, 68, and 70) showed that As and Sb concentrations exceeded the background values (Figure 2). On the other hand, two samples collected during the rainy season (stations 7 and 23) presented concentrations of these elements above the background of the area (Figure 3). Only As and Sb concentrations above 5 ppm (detection limit of the used method) are shown in the Figures 2 and 3. Correlation matrix gathering the data revealed a highly positive correlation between As and Sb, with correlation coefficients of 0.85 and 0.78 (Figure 4), to the results of dry and rainy seasons, respectively. This behavior could also be observed in the Figures 2 and 3, where the distribution curves for As and Sb are perfectly parallel. 4. DISCUSSIONS AND CONCLUSIONS This work revealed anomalous concentrations of As and Sb in the stream sediments of main drainages crossing the Canto do Amaro and Alto da Pedra oil fields, in Northeastern Brazil. A high correlation was observed between these elements that could be interpreted as an indicative of a common source for them. The similar behavior of As and Sb can be explained by their geochemical similarity, which are chalcophile elements (prefer to bond with sulfur). Common occurrence of sulfur in oil composition suggests that As and Sb could be originated from oil produced in the investigated fields. Additionally, arsenic and Sb, as well as Hg and Zn, are relatively volatile elements occurring in relatively high concentrations in sub-micrometric particles coming from burning of fossil fuels. These particles are considered to be formed by volatilization-condensation processes taking place during combustion (Manahan, 1994). 80 As Sb 70 Concentration (ppm) B B Sampling stations Figure 2. Arsenic and antimony concentrations in stream sediments (dry season) of the Canto do Amaro and Alto da Pedra oil fields. Background (B) values are also shown. Background value for As is < 5 ppm, but was plotted as 2.5 ppm for clarity. 48

5 Figure 3. Arsenic and antimony concentrations in stream sediments (rainy season) of the Canto do Amaro and Alto da Pedra oil fields. Background (B) values are also shown. In this scenario, the combustion of oil or its derivatives taking place during extraction and production of these substances in the investigated area could be the source of As and Sb found in the stream sediments. Thus, the results presented in this work suggest the possibility of using As and Sb as inorganic indicators of the activities of oil production in the Canto do Amaro and Alto da Pedra fields. Data of oil composition from investigated fields should be considered when evaluating this hypothesis. Alternatively, arsenic and Sb concentrations reported in this work could reflect an anomalous chemical composition of the geological substrate of the area. This hypothesis could be supported by the relatively elevated concentrations of As and Sb (31 and 14 ppm, respectively) also found for the background station (rainy season). Further Figure 4. Arsenic versus antimony concentrations in stream sediments (dry and rainy seasons) of the Canto do Amaro and Alto da Pedra oil fields. Background values are shown (green triangle). 49

6 investigations concerning mineralogical composition of samples should be carried out in order to help elucidating this issue. Additionally, a more complete geochemical survey including other environmental reservoirs, such as soil and water, should be carried out to provide a better understanding of the As and Sb sources in the area. It must be noted that the chemical digestion (hot acqua regia) used in the analyses can not be considered as the most appropriate technique to relatively volatile elements, such as arsenic and Sb. Part of the concentration of these elements in samples could be vaporized during the chemical digestion used. Hydrate generation is best indicated to analyze these elements. The current approach to this issue is to use this hydrate technique in further analyses, whereby As and Sb concentrations can even be detected at higher levels than those reported, since the total concentration of these elements in the samples can be measured. ACKNOWLEDGEMENTS This work was financially supported by the Project Monitoring and diagnostic of risk areas of the onshore portion of the Potiguar sedimentary basin, Northeastern Brazil of the Brazilian Agency for Financing Studies and Projects (FINEP) and the Sectorial Funds for Oil and Natural Gas (CTPETRO; Accord n 1277/01). The authors would like to thank the staff of Petrobras Oil Company for help in accessing the sampling sites. REFERENCES Carls, E. G.; Dennis, B. F.; Scott A. C. Soil contamination by oil and gas and production operations in Padre Island National Seashore, Texas, USA. Journal of Environmental Management, v. 45, p , Datta, D. K.; Subramanian, V. Distribution and fractionation of heavy metals in surface sediments of Ganges-Brahmaputra-Meghna river system in Bengal basin. Environmental Geology, v. 36, p , Govett, G. J. S. Rock Geochemistry in Mineral Exploration. In: Govett, G. J. S. (ed). Handbook of Exploration Geochemistry. v. 3. Amsterdam: Elsevier Scientific Publishing Company, p. Herr, C.; Gray, N. F. Sampling riverine sediments impacted by acid mine drainage: problems and solutions. Environmental Geology, v. 29, p , Kralik, M. A rapid procedure for environmental sampling and evaluation of polluted sediments. Applied Geochemistry, v. 14, p , Luiz-Silva, W.; Matos, R. H. R.; Kristosch, G. C. Geoquímica e índice de geoacumulação de mercúrio em sedimentos de superfície do estuário de Santos-Cubatão (SP). Química Nova, v. 25, p , (in Portuguese). Macías-Zamora, J. V.; Villaescusa-Celaya, J. A.; Muñoz-Barbosa, A.; Gold-Bouchot, G. Trace metals in sediment cores from the Campeche shelf, Gulf of Mexico. Environmental Pollution, v. 104, p , Manahan, S. Environmental Chemistry. 6th ed. New York: Lewis Publishers, p. Mantei, E. J.; Sappington, E. J. Heavy metal concentrations in sediments of streams affected by a sanitary landfill: A comparison of metal enrichment in two size sediment fractions. Environment Geology, v. 24, p , Metwally, M. E. S.; Al-Muzaini, S.; Jacob, P. G.; Bahloul, M.; Urushigawa, Y.; Sato, S.; Matsmura, A. Petroleum hydrocarbons and related heavy metals in the near-shore marine sediments of Kuwait. Environment International, v. 23, p , Mudroch, A.; Azcue, J. M. Manual of Aquatic Sediment Sampling. Boca Raton: Lewis Publishers, p. Soares, H. M. V. M.; Boaventura, R. A. R.; Machado, A. A. S. C.; Esteves da Silva, J. C. G. Sediments as monitors of heavy metal contamination in the Ave river basin (Portugal): multivariate analysis of data. Environmental Pollution, v. 105, p , 1999.