Metals in water, sediments, and biota of an offshore oil ... - Springer Link
Environ Monit Assess (2013) 185:4427–4447 DOI 10.1007/s10661-012-2881-9
Metals in water, sediments, and biota of an offshore oil exploration area in the Potiguar Basin, Northeastern Brazil L. D. Lacerda & R. C. Campos & R. E. Santelli
Received: 3 February 2012 / Accepted: 11 September 2012 / Published online: 27 September 2012 # Springer Science+Business Media B.V. 2012
Abstract Metal concentrations were evaluated in water, bottom sediments, and biota in four field campaigns from 2002 to 2004 in the Potiguar Basin, northeastern Brazil, where offshore oil exploration occurs. Analyses were performed by inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectrometry. Total metal concentrations in water (dissolved+particulate) and sediments were in the range expected for coastal and oceanic areas. Abnormally high concentrations in waters were only found for Ba (80 μgl−1) and Mn (12 μgl−1) at the releasing point of one of the outfalls, and for the other metals, concentrations in water were found in stations closer to shore, suggesting continental inputs. In bottom sediments, only
L. D. Lacerda (*) Instituto de Ciências do Mar, Universidade Federal do Ceará, Fortaleza, CE 60165-081, Brazil e-mail: [email protected] R. C. Campos Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, Rio de Janeiro 22450-051, Brazil e-mail: [email protected]
Fe and Mn showed abnormal concentrations closer to the effluent releasing point. Metal spatial distribution in shelf sediments showed the influence of the silt–clay fraction distribution, with deeper stations at the edge of the continental shelf, which are much richer in silt–clay fraction showing higher concentrations than shallower sediments typically dominated by carbonates. Metal concentrations in estuarine (mollusks and crustaceans) and marine (fish) organisms showed highest concentrations in oysters (Crassostrea rhizophorae). Fish tissues metal concentrations were similar between the continental shelf influenced by the oil exploration area and a control site. The results were within the range of concentrations reported for pristine environments without metals contamination. The global results suggest small, if any, alteration in metal concentrations due to the oil exploration activity in the Potiguar Basin. For monitoring purposes, the continental inputs and the distribution of the clay–silt fraction need to be taken into consideration for interpreting environmental monitoring results. Keywords Metals . Water . Sediments . Biota . Offshore oil exploration . Continental platform
Introduction R. E. Santelli Departamento de Geoquímica, Universidade Federal Fluminense, Niterói, Rio de Janeiro 240202-007, Brazil e-mail: [email protected]
Contamination of the marine environment originates from diverse anthropogenic sources. Terrestrial sources contribute with 70 to 80 % of contaminant emissions to
4428
coastal areas, whereas from 20 to 30 % of emissions originate from activities located in situ, mostly associated with maritime transport, exploration of mineral resources, prospecting and exploration of offshore oil, and natural gas and discharges from submarine outfalls (Crossland et al. 2005). Among the contaminants present in these effluents, pathogenic microorganisms, organic matter, nutrients, organic micropollutants, hydrocarbons, and metals are the most significant (Marcovecchio 2000). These substances reach coastal waters through rivers and atmospheric deposition. Nondegradable substances, such as metals, are of particular environmental concern, due their inherent toxicity and possibility of geoaccumulation to relative high concentrations in marine bottom sediments, bioaccumulation and transfer through food chains (Marins et al. 1998; Marcovecchio 2000). Prospecting and exploration of offshore oil and natural gas have recently achieved environmental significance due to increasing known reserves throughout the world and the rising of oil prices, which allowed their economic exploitation under offshore conditions. These activities may be important sources of organic contaminants to the oceans and their visual and shocking impacts when accidents occur resulted in increasing public awareness of their environmental significance. However, offshore oil and gas prospecting and production also emit other contaminants to the marine environment, in particular metals, which are still poorly documented for most exploitation areas worldwide. Metals are ubiquitous components of many exploration platform effluents, perforation fluids, and produced waters. They are also present in effluents from water treatment plants located onshore (Neff 2002; Rezende et al. 2002; Trefry et al. 2003; Pozebon et al. 2005), refineries (Ramos et al. 2012), fly ash from coal burning power stations (Haynes et al. 1997; Haynes and Johnson 2000), and coastal landfills (Hübner et al. 2010). On the other hand, metals are also found in varying concentrations in natural substrates (water, sediments, and biota), depending on geological, oceanographic, and ecological conditions of a given area, making it extremely difficult to differentiate between eventual contributions from anthropogenic sources from the natural background (Freire et al. 2004; Rezende et al. 2004). Produced water is the major effluent of offshore oil and gas production. Globally, it accounts for daily emissions of over 17 million cubic meters. About 40 % only (7 millionm3 day−1) is released to the sea prior to or after
Environ Monit Assess (2013) 185:4427–4447
treatment for reduction of particulate matter and hydrocarbons (OGP 2005). The ratio between oil and produced water in mature fields may reach 1:10 and it increases with aging. Thus, an overall increase in produced water release with time is expected (OGP 2005). Chemical composition of produced water is complex and depends on the oil or gas field and is in equilibrium with the reservoir characteristics and geological formation. Apart from major constituents such as chlorides, sulfate, iron, boron, and barium, some metals of environmental significance, in particular, Cu, Zn, Mn, Ni, Fe, and V, may also present relatively high concentrations (Kennicutt et al. 1996a; Kennicutt et al. 1996b; Utvik 1999; Gabardo et al. 2005). Therefore, due to the naturally very low concentrations of metals found in the pristine marine environment and the increasing release of produced waters from oil and natural gas offshore exploitation, assessment, and monitoring of their concentrations in areas under the influence of offshore operations are mandatory steps in the environmental regulation of this activity. Unfortunately, many inherent aspects of metals distribution in offshore environments make difficult the recognition of impacts from offshore oil and gas exploration on metals concentrations. Metals are poorly soluble in seawater, and the large dilution factors result in extremely low concentrations even close to sources. Such low levels together with the interferences caused by the saline matrix make the analytical detection a challenging task. Therefore, concentrations of metals in the Brazilian continental shelf waters are very poorly known, being restricted to protected areas close to contamination sources located onshore. Sediments, on the other hand, may integrate metals reaching the sedimentary environment of the continental shelf making easier their analysis and this resulted in a few comprehensive studies on their distribution in Brazilian continental shelf sediments (Freire et al. 2004; Rezende et al. 2004; Lacerda and Marins 2006). In the oil exploration area of the Campos Basin in southeastern (SE) Brazil, for example, Rezende et al. (2002) showed no effect on metal concentrations in sediments collected as close as 500 m from producing oil platforms. During drilling, a small increase in the concentrations of Zn, Ni, and Al was reported close to the drilling operation, but it could not be detected after 6 months from closing operations. Apart from the generally low concentrations in effluents from the offshore oil and gas exploration, the natural variability of metals concentrations in continental
Environ Monit Assess (2013) 185:4427–4447
shelf sediments may be very large due to the strong influence of continental inputs of relatively metal-rich materials. For example, at the same Campos Basin exploration area, the input of Hg from rivers may reach the outer continental shelf (Lacerda et al. 1993), making extremely difficult the differentiation between natural from locally generated anthropogenic inputs of Hg. Also, natural inputs from continental sources also present a strong seasonal component (Molisani et al. 1999; Salomão et al. 2001; Carvalho et al. 2002), which may result in varying distribution of continental derived metals in shelf sediments. Distribution of metals in the biota under the influence of oil and natural gas offshore exploitation are poorly known and restricted to offshore areas in the Gulf of Mexico (Trefry et al. 1995), Norway (OGP 2005), and Thailand (Windom and Crammer 1998). No evidence so far, suggested any influence of metal release from the activity on their concentration in the marine biota. Along the Brazilian coast, metals concentrations in marine biota close to exploitation areas are restricted to results obtained in samples of adherent organisms collected on the structure of offshore exploration platforms. These few available results, however, also failed to discriminate between metals originated in effluents from those originated in the metallic structure itself and are of small monitoring significance (Paranhos and Ximenez 2005). The Potiguar Basin is an oil and natural gas production area located in northeastern (NE) Brazil. The production area is 40 % onshore, constituted of about 3,000 small production wells and 60 % offshore, represented by 34 production platforms. Produced water from both areas is treated onshore and the effluents released by two submarine outfalls in the continental shelf. Effluents flux from the outfalls average 160,000 m3 day−1 and the production water treatment plant are in full operation since 2001. Some produced water (80,000 m3 day−1) has been directly released at sea between 1998 and 2001, but re-injection procedures and the completion of the Guamaré Treatment Plant, presently avoid this practice (Gabardo et al. 2005; PETROBRAS [Petróleo Brasileiro S.A.] 2006). This paper presents a synthesis of the results from a 3-year monitoring program on metal distribution in water, bottom sediments, and biota of the Potiguar Basin, in order to test if the presence of operational platform and/or the release of effluents from the production water treatment plant result in alterations of
4429
metal concentrations in these environmental compartments, after nearly 5 years of operation. The monitoring program included the continental shelf proper, where most exploration platforms are located and the shallow coastal areas, which are mostly under the influence of effluents discharged by the submarine outfalls from the produce water treatment plant located onshore.
Study area The coastal zone adjacent to the major oil exploitation basins in NE Brazil is characterized by a relatively narrow coastal plain (50 to 100 km) composed by Tertiary siliclastic gross sediments (Barreiras Formation) limited inland by the granite outcrops of the Pre-Cambrian Brazilian shield. This geology is the major source of continental materials to the ocean. Total continental drainage basin of the region is about 200,000 km2 and fluvial inputs to the ocean reaches about 200 m3 s−1. Climate varies from humid (1,000 to 1,200 mmyear−1) at mountain ranges and coastal valleys, to semi-arid (500 a 700-mmyear−1) in the lowlands. Coastal soils are podsols and oxisols within extensive dune fields and sandy beaches. Mangroves dominate estuaries and coastal lagoons. The continental shelf shows low declivity down to 70 m deep (1:670 to 1:1,000) and width ranging from 40 km at the east extreme to 100 km at the west sector (Martins and Coutinho 1981; Arz et al. 1999). Three parallel sedimentary zones are recognized for this sector of the Brazilian continental shelf. From the beach limit to about 20 m deep, a first zone dominated by quartz sands and siliclastic sediments of continental origin is present. This is followed by a zone dominated by bioclastic sediments, in particular carbonates from calcareous algae, mostly of the Lithothaminium spp. (Rodophyta) and Halimeda spp. (Clorophyta), extending to 70 m of depth. At the edge of the continental shelf, terrigenous siliclastic sediments dominate again (Summerhayes et al. 1975; Nascimento et al. 2010). Continental shelf sedimentation is, therefore, defined by the relative importance of these major sources where the higher the relative importance of calcareous (marine; organogenic) sediments the lower the relative importance of siliclastic (terrigenous) sediments (Milliman and Summerhayes 1975; Knoppers et al. 1999).
4430
Environ Monit Assess (2013) 185:4427–4447
All water and sediment samples were collected in four campaigns at roughly 8-month intervals from the ship N/RB Astro Garoupa from the Brazilian oil company, PETROBRAS S.A. Location of sample stations were the same for water and sediments and DGPS positioning reduced vessel shift between campaigns to less than 100 m. Marine biota samples were collected by hand along estuaries, whereas marine fish were sampled from the ship NOc. Prof. Martins Filho, from the Federal University of Ceará. All biological samples were collected following the biodiversity protocols and authorization from the Brazilian Institute of Renewable Natural Resources from the Ministry of the Environment.
and were analyzed unfiltered, therefore determining total (dissolved + particulate) metal content in the water. The concentrations of Al, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn were performed by inductively coupled plasma mass spectrometry, after 20× sample dilution with ultrapure water using a Model X Series II spectrometer (Thermo Fisher Scientific, Bremen, Germany). Instrumental parameters were optimized for each element, in order to achieve the best instrumental signal to noise ratio. Pneumatic nebulization was used and the individual matrix matched calibration solutions ranged from 0.1 to 50 μgl−1. Mercury was determined by cold vapor atomic absorption spectrometry (CV AAS), using a homemade vapor generator accessory and a model RA 915 + Mercury Analyser (Lumex, St. Petersburg, Russia). In both cases, 50-ml aliquots were used.
Water
Sediments
Surface water samples were collected in 26 stations around the area of influence of the Guamaré submarine outfalls (Fig. 1). Two stations were located within 50 m from the outfalls releasing point; eight and ten stations were distributed within a 500-m and within 2,000-m radius from the releasing points, respectively; whereas six stations were established outside a 4,000-m radius from the outfalls, being considered as control. This sampling grid has been used in other marine exploration sites with the farther stations also considered as controls (Kennicutt et al. 1996a; Gray et al. 1999; EPA 2000). Water column in this area is relatively shallow (5 to 20 m) and showed no stratification, however, bottom water samples were collected in selected seven stations, representing the four radius to check for eventual differences in metal content. Water samples were not collected at the outer shelf area; since earlier studies showed the signature of produced water discharge to disappear only a few meters from the platforms due to enormous dilution factors, up to 2,000 times in the first 10 to 100 m (Gabardo et al. 2005). Samples were collected with Teflon lined Go-Flo bottles and transferred to pre-cleaned polyethylene bottles and kept under low temperature (∼4 °C) until analysis. Acceptable clean protocols (Boutron 1990; Nriagu et al. 1993) were used throughout sampling, transport, and storage to avoid cross-contamination. Samples suffered no pre-treatment previous to analysis
Surface sediment samples were collected in triplicate in the same 26 stations used for water samples plus 43 stations located in the continental shelf area under the influence of oil and gas production platforms. Samples were collected along four parallel lines including coastal samples up to 5 m deep, shallow shelf up to 50 m deep, shelf edge from 50 to 100 m deep and slope from 100 to 450 m deep (Fig. 2). Most samples were collected using a van Veen grab sampler, whereas in those stations closer to the submarine outfall, sediments were collected by divers using small steel cylinders. Based on a tensample comparison, no significant difference was found in metal concentrations in sediments when comparing the two sampling procedures. From the deeper sampling line (>100 m), a box corer was used to retrieve the samples. The samples were transferred to plastic bags, avoiding the contact with any metallic surface. Only the top 2.0 cm layer was used for metal analysis, irrespectively of the sampling device. Samples were kept frozen until analysis. Previous to metals determination, approximately 100 g of oven dried (80 °C) sediments were used for gravimetric determination of the silt–clay fraction (
Metals in water, sediments, and biota of an offshore oil exploration area in the Potiguar Basin, Northeastern Brazil L. D. Lacerda & R. C. Campos & R. E. Santelli
Received: 3 February 2012 / Accepted: 11 September 2012 / Published online: 27 September 2012 # Springer Science+Business Media B.V. 2012
Abstract Metal concentrations were evaluated in water, bottom sediments, and biota in four field campaigns from 2002 to 2004 in the Potiguar Basin, northeastern Brazil, where offshore oil exploration occurs. Analyses were performed by inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectrometry. Total metal concentrations in water (dissolved+particulate) and sediments were in the range expected for coastal and oceanic areas. Abnormally high concentrations in waters were only found for Ba (80 μgl−1) and Mn (12 μgl−1) at the releasing point of one of the outfalls, and for the other metals, concentrations in water were found in stations closer to shore, suggesting continental inputs. In bottom sediments, only
L. D. Lacerda (*) Instituto de Ciências do Mar, Universidade Federal do Ceará, Fortaleza, CE 60165-081, Brazil e-mail: [email protected] R. C. Campos Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, Rio de Janeiro 22450-051, Brazil e-mail: [email protected]
Fe and Mn showed abnormal concentrations closer to the effluent releasing point. Metal spatial distribution in shelf sediments showed the influence of the silt–clay fraction distribution, with deeper stations at the edge of the continental shelf, which are much richer in silt–clay fraction showing higher concentrations than shallower sediments typically dominated by carbonates. Metal concentrations in estuarine (mollusks and crustaceans) and marine (fish) organisms showed highest concentrations in oysters (Crassostrea rhizophorae). Fish tissues metal concentrations were similar between the continental shelf influenced by the oil exploration area and a control site. The results were within the range of concentrations reported for pristine environments without metals contamination. The global results suggest small, if any, alteration in metal concentrations due to the oil exploration activity in the Potiguar Basin. For monitoring purposes, the continental inputs and the distribution of the clay–silt fraction need to be taken into consideration for interpreting environmental monitoring results. Keywords Metals . Water . Sediments . Biota . Offshore oil exploration . Continental platform
Introduction R. E. Santelli Departamento de Geoquímica, Universidade Federal Fluminense, Niterói, Rio de Janeiro 240202-007, Brazil e-mail: [email protected]
Contamination of the marine environment originates from diverse anthropogenic sources. Terrestrial sources contribute with 70 to 80 % of contaminant emissions to
4428
coastal areas, whereas from 20 to 30 % of emissions originate from activities located in situ, mostly associated with maritime transport, exploration of mineral resources, prospecting and exploration of offshore oil, and natural gas and discharges from submarine outfalls (Crossland et al. 2005). Among the contaminants present in these effluents, pathogenic microorganisms, organic matter, nutrients, organic micropollutants, hydrocarbons, and metals are the most significant (Marcovecchio 2000). These substances reach coastal waters through rivers and atmospheric deposition. Nondegradable substances, such as metals, are of particular environmental concern, due their inherent toxicity and possibility of geoaccumulation to relative high concentrations in marine bottom sediments, bioaccumulation and transfer through food chains (Marins et al. 1998; Marcovecchio 2000). Prospecting and exploration of offshore oil and natural gas have recently achieved environmental significance due to increasing known reserves throughout the world and the rising of oil prices, which allowed their economic exploitation under offshore conditions. These activities may be important sources of organic contaminants to the oceans and their visual and shocking impacts when accidents occur resulted in increasing public awareness of their environmental significance. However, offshore oil and gas prospecting and production also emit other contaminants to the marine environment, in particular metals, which are still poorly documented for most exploitation areas worldwide. Metals are ubiquitous components of many exploration platform effluents, perforation fluids, and produced waters. They are also present in effluents from water treatment plants located onshore (Neff 2002; Rezende et al. 2002; Trefry et al. 2003; Pozebon et al. 2005), refineries (Ramos et al. 2012), fly ash from coal burning power stations (Haynes et al. 1997; Haynes and Johnson 2000), and coastal landfills (Hübner et al. 2010). On the other hand, metals are also found in varying concentrations in natural substrates (water, sediments, and biota), depending on geological, oceanographic, and ecological conditions of a given area, making it extremely difficult to differentiate between eventual contributions from anthropogenic sources from the natural background (Freire et al. 2004; Rezende et al. 2004). Produced water is the major effluent of offshore oil and gas production. Globally, it accounts for daily emissions of over 17 million cubic meters. About 40 % only (7 millionm3 day−1) is released to the sea prior to or after
Environ Monit Assess (2013) 185:4427–4447
treatment for reduction of particulate matter and hydrocarbons (OGP 2005). The ratio between oil and produced water in mature fields may reach 1:10 and it increases with aging. Thus, an overall increase in produced water release with time is expected (OGP 2005). Chemical composition of produced water is complex and depends on the oil or gas field and is in equilibrium with the reservoir characteristics and geological formation. Apart from major constituents such as chlorides, sulfate, iron, boron, and barium, some metals of environmental significance, in particular, Cu, Zn, Mn, Ni, Fe, and V, may also present relatively high concentrations (Kennicutt et al. 1996a; Kennicutt et al. 1996b; Utvik 1999; Gabardo et al. 2005). Therefore, due to the naturally very low concentrations of metals found in the pristine marine environment and the increasing release of produced waters from oil and natural gas offshore exploitation, assessment, and monitoring of their concentrations in areas under the influence of offshore operations are mandatory steps in the environmental regulation of this activity. Unfortunately, many inherent aspects of metals distribution in offshore environments make difficult the recognition of impacts from offshore oil and gas exploration on metals concentrations. Metals are poorly soluble in seawater, and the large dilution factors result in extremely low concentrations even close to sources. Such low levels together with the interferences caused by the saline matrix make the analytical detection a challenging task. Therefore, concentrations of metals in the Brazilian continental shelf waters are very poorly known, being restricted to protected areas close to contamination sources located onshore. Sediments, on the other hand, may integrate metals reaching the sedimentary environment of the continental shelf making easier their analysis and this resulted in a few comprehensive studies on their distribution in Brazilian continental shelf sediments (Freire et al. 2004; Rezende et al. 2004; Lacerda and Marins 2006). In the oil exploration area of the Campos Basin in southeastern (SE) Brazil, for example, Rezende et al. (2002) showed no effect on metal concentrations in sediments collected as close as 500 m from producing oil platforms. During drilling, a small increase in the concentrations of Zn, Ni, and Al was reported close to the drilling operation, but it could not be detected after 6 months from closing operations. Apart from the generally low concentrations in effluents from the offshore oil and gas exploration, the natural variability of metals concentrations in continental
Environ Monit Assess (2013) 185:4427–4447
shelf sediments may be very large due to the strong influence of continental inputs of relatively metal-rich materials. For example, at the same Campos Basin exploration area, the input of Hg from rivers may reach the outer continental shelf (Lacerda et al. 1993), making extremely difficult the differentiation between natural from locally generated anthropogenic inputs of Hg. Also, natural inputs from continental sources also present a strong seasonal component (Molisani et al. 1999; Salomão et al. 2001; Carvalho et al. 2002), which may result in varying distribution of continental derived metals in shelf sediments. Distribution of metals in the biota under the influence of oil and natural gas offshore exploitation are poorly known and restricted to offshore areas in the Gulf of Mexico (Trefry et al. 1995), Norway (OGP 2005), and Thailand (Windom and Crammer 1998). No evidence so far, suggested any influence of metal release from the activity on their concentration in the marine biota. Along the Brazilian coast, metals concentrations in marine biota close to exploitation areas are restricted to results obtained in samples of adherent organisms collected on the structure of offshore exploration platforms. These few available results, however, also failed to discriminate between metals originated in effluents from those originated in the metallic structure itself and are of small monitoring significance (Paranhos and Ximenez 2005). The Potiguar Basin is an oil and natural gas production area located in northeastern (NE) Brazil. The production area is 40 % onshore, constituted of about 3,000 small production wells and 60 % offshore, represented by 34 production platforms. Produced water from both areas is treated onshore and the effluents released by two submarine outfalls in the continental shelf. Effluents flux from the outfalls average 160,000 m3 day−1 and the production water treatment plant are in full operation since 2001. Some produced water (80,000 m3 day−1) has been directly released at sea between 1998 and 2001, but re-injection procedures and the completion of the Guamaré Treatment Plant, presently avoid this practice (Gabardo et al. 2005; PETROBRAS [Petróleo Brasileiro S.A.] 2006). This paper presents a synthesis of the results from a 3-year monitoring program on metal distribution in water, bottom sediments, and biota of the Potiguar Basin, in order to test if the presence of operational platform and/or the release of effluents from the production water treatment plant result in alterations of
4429
metal concentrations in these environmental compartments, after nearly 5 years of operation. The monitoring program included the continental shelf proper, where most exploration platforms are located and the shallow coastal areas, which are mostly under the influence of effluents discharged by the submarine outfalls from the produce water treatment plant located onshore.
Study area The coastal zone adjacent to the major oil exploitation basins in NE Brazil is characterized by a relatively narrow coastal plain (50 to 100 km) composed by Tertiary siliclastic gross sediments (Barreiras Formation) limited inland by the granite outcrops of the Pre-Cambrian Brazilian shield. This geology is the major source of continental materials to the ocean. Total continental drainage basin of the region is about 200,000 km2 and fluvial inputs to the ocean reaches about 200 m3 s−1. Climate varies from humid (1,000 to 1,200 mmyear−1) at mountain ranges and coastal valleys, to semi-arid (500 a 700-mmyear−1) in the lowlands. Coastal soils are podsols and oxisols within extensive dune fields and sandy beaches. Mangroves dominate estuaries and coastal lagoons. The continental shelf shows low declivity down to 70 m deep (1:670 to 1:1,000) and width ranging from 40 km at the east extreme to 100 km at the west sector (Martins and Coutinho 1981; Arz et al. 1999). Three parallel sedimentary zones are recognized for this sector of the Brazilian continental shelf. From the beach limit to about 20 m deep, a first zone dominated by quartz sands and siliclastic sediments of continental origin is present. This is followed by a zone dominated by bioclastic sediments, in particular carbonates from calcareous algae, mostly of the Lithothaminium spp. (Rodophyta) and Halimeda spp. (Clorophyta), extending to 70 m of depth. At the edge of the continental shelf, terrigenous siliclastic sediments dominate again (Summerhayes et al. 1975; Nascimento et al. 2010). Continental shelf sedimentation is, therefore, defined by the relative importance of these major sources where the higher the relative importance of calcareous (marine; organogenic) sediments the lower the relative importance of siliclastic (terrigenous) sediments (Milliman and Summerhayes 1975; Knoppers et al. 1999).
4430
Environ Monit Assess (2013) 185:4427–4447
All water and sediment samples were collected in four campaigns at roughly 8-month intervals from the ship N/RB Astro Garoupa from the Brazilian oil company, PETROBRAS S.A. Location of sample stations were the same for water and sediments and DGPS positioning reduced vessel shift between campaigns to less than 100 m. Marine biota samples were collected by hand along estuaries, whereas marine fish were sampled from the ship NOc. Prof. Martins Filho, from the Federal University of Ceará. All biological samples were collected following the biodiversity protocols and authorization from the Brazilian Institute of Renewable Natural Resources from the Ministry of the Environment.
and were analyzed unfiltered, therefore determining total (dissolved + particulate) metal content in the water. The concentrations of Al, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn were performed by inductively coupled plasma mass spectrometry, after 20× sample dilution with ultrapure water using a Model X Series II spectrometer (Thermo Fisher Scientific, Bremen, Germany). Instrumental parameters were optimized for each element, in order to achieve the best instrumental signal to noise ratio. Pneumatic nebulization was used and the individual matrix matched calibration solutions ranged from 0.1 to 50 μgl−1. Mercury was determined by cold vapor atomic absorption spectrometry (CV AAS), using a homemade vapor generator accessory and a model RA 915 + Mercury Analyser (Lumex, St. Petersburg, Russia). In both cases, 50-ml aliquots were used.
Water
Sediments
Surface water samples were collected in 26 stations around the area of influence of the Guamaré submarine outfalls (Fig. 1). Two stations were located within 50 m from the outfalls releasing point; eight and ten stations were distributed within a 500-m and within 2,000-m radius from the releasing points, respectively; whereas six stations were established outside a 4,000-m radius from the outfalls, being considered as control. This sampling grid has been used in other marine exploration sites with the farther stations also considered as controls (Kennicutt et al. 1996a; Gray et al. 1999; EPA 2000). Water column in this area is relatively shallow (5 to 20 m) and showed no stratification, however, bottom water samples were collected in selected seven stations, representing the four radius to check for eventual differences in metal content. Water samples were not collected at the outer shelf area; since earlier studies showed the signature of produced water discharge to disappear only a few meters from the platforms due to enormous dilution factors, up to 2,000 times in the first 10 to 100 m (Gabardo et al. 2005). Samples were collected with Teflon lined Go-Flo bottles and transferred to pre-cleaned polyethylene bottles and kept under low temperature (∼4 °C) until analysis. Acceptable clean protocols (Boutron 1990; Nriagu et al. 1993) were used throughout sampling, transport, and storage to avoid cross-contamination. Samples suffered no pre-treatment previous to analysis
Surface sediment samples were collected in triplicate in the same 26 stations used for water samples plus 43 stations located in the continental shelf area under the influence of oil and gas production platforms. Samples were collected along four parallel lines including coastal samples up to 5 m deep, shallow shelf up to 50 m deep, shelf edge from 50 to 100 m deep and slope from 100 to 450 m deep (Fig. 2). Most samples were collected using a van Veen grab sampler, whereas in those stations closer to the submarine outfall, sediments were collected by divers using small steel cylinders. Based on a tensample comparison, no significant difference was found in metal concentrations in sediments when comparing the two sampling procedures. From the deeper sampling line (>100 m), a box corer was used to retrieve the samples. The samples were transferred to plastic bags, avoiding the contact with any metallic surface. Only the top 2.0 cm layer was used for metal analysis, irrespectively of the sampling device. Samples were kept frozen until analysis. Previous to metals determination, approximately 100 g of oven dried (80 °C) sediments were used for gravimetric determination of the silt–clay fraction (
