Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial II, 969-972
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Environmental geochemistry of stream sediments from São
Francisco stream (Rio Branco, Acre - Brazil): a preliminary
overview about contamination problems
Geoquímica ambiental de sedimentos da Ribeira de São Francisco
(Rio Branco, Acre, Brasil): uma visão preliminar sobre problemas
de contaminação
E. S. Cadaxo Sobrinho1, J. Ribeiro2,3, C. Neto3*, C. E. Garção de Carvalho1, D. Flores2,3
Artigo Curto
Short Article
© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The scientific research on stream sediments from riverine
systems has long been used for environmental purposes, providing
information on weathering and transport processes, pollutant sources,
emissions pathways, distribution, and fate of contaminants. The main
objectives of the present study are: (i) to identify and quantify the 16
priority polycyclic aromatic hydrocarbons (PAHs) in the stream
sediments from São Francisco stream in the area of Rio Branco city
(State of Acre, Brazil); (ii) to evaluate the environmental risk of
contamination with PAHs; (iii) to identify potential pollutants
sources. In the studied stream sediments, the most abundant PAHs
are fluoranthene and benzo[a]anthracene, followed by minor
concentrations of chrysene, benzo[ghi]perylene and anthracene. The
concentration of PAHs in the sediments does not represent
environmental concerns when considering the Ontario sediment
quality guidelines. However, the predominance of high molecular
weight (HMW) PAHs comparatively with the low molecular weight
(LMW) PAHs indicates that PAHs in sediments may have
combustion sources (combustion of biomass in forest and farmers
fires and/or combustion of fossil fuels). The industrial activities,
mainly those related with wood, should also be considered as a
source of PAHs.
Keywords: Amazonian Basin, São Francisco stream, Environmental
geochemistry, PAHs, Contamination.
Resumo: O estudo de sedimentos dos cursos de água de sistemas
fluviais tem sido muito utilizado para fins ambientais, fornecendo
informações sobre os processos de meteorização do substrato
rochoso e o transporte dos seus produtos, fontes de poluição, vias de
emissão, distribuição e destino final de contaminantes. Os principais
objetivos deste estudo são: (i) identificar e quantificar os 16
hidrocarbonetos aromáticos policíclicos (HPAs) prioritários nos
sedimentos da Ribeira de São Francisco, nas proximidades da cidade
de Rio Branco (Estado do Acre, Brasil), (ii) avaliar o risco de
contaminação ambiental por HPAs, (iii) identificar possíveis fontes
de poluentes. Nos sedimentos de estudados, os HAPs mais
abundantes são fluoranteno e benzo[a]antraceno, seguidos por
menores concentrações de criseno, benzo[ghi]perileno e antraceno.
Considerando as directrizes de Ontario sobre a qualidade de
sedimentos, a concentração de HPAs nos sedimentos não representa
preocupações ambientais. No entanto, a predominância de HPAs de
alta massa molecular comparativamente com HPAs de baixa massa
molecular, indica que os HPAs presentes nos sedimentos podem ter
origem em fontes de combustão (combustão de biomassa em
incêndios florestais ou agrícolas e/ou combustão de combustíveis
fósseis). As atividades industriais, principalmente as relacionadas
com a indústria da madeira, devem também ser consideradas como
uma fonte de HAPs.
Palavras-chave: Bacia do Amazonas, Ribeira de São Francisco,
Geoquímica ambiental, HPAs, Contaminação.
1
Centro de Ciências Biológicas e da Natureza, Universidade Federal do Acre,
Campus Universitário, BR 364, Km 04, Rio Branco, Acre, Brasil.
2
Centro de Geologia da Universidade do Porto. Rua do Campo Alegre 687,
4169-007 Porto. Portugal.
3
Departamento de Geociências, Ambiente e Ordenamento do Território.
Faculdade de Ciências da Universidade do Porto. Rua do Campo Alegre 687,
4169-007 Porto. Portugal.
*
Corresponding author / Autor correspondente: [email protected]
1. Introduction
The study of stream sediments from riverine systems has
long been used as an exploration tool and well as for
environmental purposes, providing information on
pollutants sources, weathering and transport processes
(Ranasinghe et al., 2008). The polycyclic aromatic
hydrocarbons (PAHs) are known as a group of
environmental organic pollutants present in river waters,
sediments and aquifers that are harmful to environment
and human health. The United States Environmental
Protection Agency has defined 16 PAHs as priority
pollutants, due their toxicity, mutagenic and carcinogenic
properties. In recent sediments, PAHs may derive from
natural (produced during thermal maturation of organic
matter - petrogenic source) and anthropogenic sources
(domestic and industrial waste, incineration by-products of
industrial and urban wastes, oil spills, combustion of fossil
fuels and biomass, among others) and therefore, can enter
to the aquatic systems directly by deposition of airborne
particles, leaching, spillage, accidental release and natural
oil seeps, or indirectly through sewers, urban and highway
runoff (Schwarzbauer, 2006).
The Amazonian basin, which is the largest fluvial basin
in the world, was the focus of several studies dealing with
the chemistry of Amazonian waters. However, few studies
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E. S. Cadaxo Sobrinho et al. / Comunicações Geológicas (2014) 101, Especial II, 969-972
reported the geochemistry of sediments from Amazon
River and its tributaries (Roddaz et al., 2006; Vital &
Stattegger, 2000). The São Francisco stream is a tributary
of Acre River, which is a branch of the Purus River that
flows into the Amazon Basin. The drainage system of Acre
River comprises 14 sub-basins with dendritic
characteristics and one of the most important of those subbasins is represented by São Francisco stream that has a
path of 115.6 km and a drainage density of 1.37 km/km2.
The São Francisco stream is the main collector of the
drainage system of Rio Branco geographical area, as can
be observed in figure 1. The path occurs from west to east
flowing into Acre River, immediately downstream of Rio
Branco urban area.
Generally, the outcrops in the São Francisco stream
have three to five meters of thickness, covered by recent
unconsolidated sediments. The majority of sediments of
São Francisco stream are from fluvial origin from
Solimões Formation (Miocene), which is the most
extensive formation in the studied area (Cavalcante, 2005).
This formation includes sediments typically from flood
plains, with cross-stratification, mudstones with laminar
structure, bedding siltstones with lenses, and fine and
coarse sandstones in lenses or interdicted with siltstones
and mudstones (Cavalcante, 2005). The sedimentary
environment is characterized by high energy with
excavation and deposition caused by the drainage
dynamics and seasonality of floods (Arcos et al., 2012;
Lima et al., 2012).
The city of Rio Branco had an accelerated growth in
the last decades, resulting in the inefficient urban
planning and inequalities in the occupation of the urban
spaces. Consequently, the São Francisco stream is
considered degraded due to deforestation of the river
banks for human occupation and also the pollution of
waters attributed to industrial activities and discharge of
residual/urban effluents. Previous works about
physicochemical and microbiological parameters of water
from São Francisco stream evidenced pollution of water
caused by urban/domestic and industrial effluents (Santi
et al., 2012).
Considering the above, the main goal of this study is to
contribute for the geochemical characterization of stream
sediments from São Francisco stream (Acre, Brazil). The
specific objectives are as follows: (i) to identify and
quantify the 16 priority PAHs in the stream sediments
from São Francisco stream in the area of Rio Branco city
(State of Acre, Brazil); (ii) to evaluate the environmental
risk of contamination with PAHs; (iii) to identify potential
pollutants sources.
Fig. 1. (A) Aerial image of Rio Branco city, São Francisco stream and Acre River. (B) Magnification of the studied area and the location of samples
collection. Images from Google Earth (accessed in January 2013).
Fig. 1. (A) Imagem aérea da Cidade de Rio Branco, Ribeira de São Francisco e Rio Acre. (B) Ampliação da área em estudo e identificação dos locais de
amostragem. Imagens do Google Earth (acedido em Janeiro de 2013).
Environmental geochemistry of São Francisco stream
2. Materials and methods
Stream sediment samples were collected along São
Franscisco stream, close to the urban area of Rio Branco,
where samples were taken from both the right and left
banks. Sampling was performed in upstream, midstream
and downstream points of Rio Branco city (Fig. 1). The
upstream point may be affected by industrial activities,
mainly those related with wood; the downstream point
may be influenced by the urban area of Rio Branco. A
total of 6 sediment samples were collected from banks of
São Francisco stream, at about 0.2 m to 0.5 m depth, each
with approximately 1.5 kg. In the laboratory, they were
air dried at room temperature, homogenized and
quartered to obtain representative samples for subsequent
analyses.
The identification and quantification of the 16 priority
PAHs were performed by gas chromatography coupled to
mass spectrometry (GC-MS). For the GC-MS analysis,
PAHs were removed from sediments by soxhlet extraction
using dichloromethane as solvent. The extracts were
separated into saturated, aromatic and polar fractions by
column chromatography. The aromatic hydrocarbon
fractions were analyzed using an Agilent gas
chromatograph 6890N with a 7683B auto sampler and
coupled to a mass spectrometer detector 5975B system.
Samples were analyzed in the selective ion monitoring
mode and quantification was done by preparing five
calibration standard solutions of a standard mixture
containing the priority PAHs. Blanks, duplicate samples
and internal standard addition were employed for
analytical assurance and deuterated pyrene (D10 - pyrene)
was used as internal standard.
3. Results and discussion
The individual and total concentrations of the 16 priority
PAHs in sediments from São Franscisco stream are
presented in table 1. The results demonstrated that 5 of the
16 priority PAHs were detected: anthracene, fluoranthene,
benzo[a]anthracene, chrysene, and benzo[ghi]perylene.
The most abundant PAHs in the samples are fluoranthene,
and benzo[a]anthracene, followed by minor proportions of
chrysene, benzo[ghi]perylene and anthracene. These PAHs
are known to be the result of incomplete combustion of
fossil fuels and also to be associated with particulate
matter, in ambient air, surface, drinking, and waste water,
and in char-broiled foods (Neff, 1979; OMOE, 1990).
Some of them may have natural sources such as volcanos
or thermal maturation of organic matter.
The sum of the priority PAHs ranges between 8.98 ng
g-1 and 17.51 ng g-1 (Table 1) and the highest value is
reported for right bank of downstream point of samples
collection, where the anthropogenic influence caused by
the urbanized area of Rio Branco city is expected to be
more significant. The sum of the 16 priority PAHs does
not exceed the limit value for the “lowest effect level”
(4000 ng g−1) of Ontario sediment quality guidelines,
which means that the PAHs in sediments of the studied
971
section do not affect fish or the sediment-dwelling
organisms (Persaud et al., 1993).
The PAHs are classified as low molecular weight
(LMW) PAHs, containing 2 to 3 aromatic rings, and high
molecular weight (HMW) PAHs, with 4 to 6 aromatic
rings. The relative percentages of LMW PAHs and HMW
PAHs in sediments were calculated and the results are
presented in table 1. The relative proportion between
LMW PAHs and HMW PAHs can be used to assess
combustion and petroleum inputs. Generally, LMW PAHs
derive primarily from petrogenic sources whereas the
HMW PAHs primarily reflects combustion (Yunker et al.,
2001). The data demonstrates the predominance of HMW
PAHs (> 80%) comparatively with the LMW PAHs (<
20%), indicating that PAHs in sediments may have
combustion sources, mainly the combustion of biomass in
forest and farmers fires and/or the combustion of fossil
fuels. The industrial activities, mainly those related with
wood, should also be considered as a source of PAHs.
The transport, deposition and chemical transformation
of the PAHs in sediments depends on their form of
occurrence, either on gas or in particulate phases (Ravindra
et al., 2008). The PAHs bounded to the sediment particles
may be leachate and therefore transported and incorporated
in a variety of others environmental media such as soils,
surface and groundwater’s, in proximate or distal areas and
consequently affecting human health and ecosystems.
Furthermore, HMW PAHs exhibit a high resistance to
biodegradation (Berkowitz et al., 2008).
Table 1. Concentrations of the priority PAHs present in sediments from
São Francisco stream, sum of the priority PAHs and relative percentages
of LMW PAHs and HMW PAHs.
Tabela 1. Concentração dos HPAs prioritários presentes nos sedimentos
da Ribeira de São Francisco, somatório dos HPAs prioritários e
percentagens relativas de HPAs de baixo peso molecular e HPAs de alto
peso molecular.
4. Conclusions
The assessment of the PAHs concentration in stream
sediments from the studied section of São Francisco
stream (a tributary of Amazonian) was the main goal of
this work. The focus of this study on PAHs is justified by
their negative impact in the environment, biodiversity and
effects on human health. In addition, this research
provides, for the first-time, information concerning the
PAHs concentrations of São Francisco stream sediments,
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E. S. Cadaxo Sobrinho et al. / Comunicações Geológicas (2014) 101, Especial II, 969-972
close to the urban area of Rio Branco city, evidencing the
influence of anthropogenic activities.
The concentrations of PAHs in the sediments do not
represent environmental concerns when considering the
Ontario aquatic sediment quality guidelines. However, the
predominance of HMW PAHs comparatively with the
LMW PAHs indicate that PAHs present in sediments
result from combustion, which, in this case, may be the
combustion of biomass in forest and farmers fires or the
combustion of fossil fuels in vehicles; the industrial
activities, principally those related with wood, should also
be considered as a source of PAHs.
The concentrations of PAHs in stream sediments from
São Francisco may adversely impact in soil, water, and
plants and, consequently, affect the human health and
biodiversity, depending mainly on the extent of exposure,
concentrations and characteristics, and whether exposure
occurs via inhalation, ingestion or skin contact.
Acknowledgments
The author J. Ribeiro benefits from a post-doctoral
scholarship financed by FCT - Fundação para a Ciência e
Tecnologia, Portugal, Ref: SFRH/BPD/76230/2011. This
work was partially funded by FEDER funds through the
Operational Program Competitiveness Factors COMPETE and by national funds through FCT - Fundação
para a Ciência e Tecnologia, under the projects Ref:
REEQ/1140/CTE/2005 and PEst-OE/CTE/UI0039/2014.
The authors would also like to thank Professor Waldemir
and William from the Federal University of Acre for the
samples collection and to Federação das Indústrias do Acre
(FIEAC) for the encouragement to develop this research
project.
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