SOURCE APPORTIONMENT OF POLYCYCLIC AROMATIC HYDROCARBONS IN
SEDIMENT CORES FROM THE HUMBER ESTUARY USING MOLECULAR
DISTRIBUTION.
By
100035803
Thesis presented in part-fulfilment of the degree of Master of Science in accordance with the
regulations of the University of East Anglia
School of Environmental Sciences
University of East Anglia
University Plain
Norwich
NR4 7TJ
© 2013 Ryan Brettell
This copy of the dissertation has been supplied on condition that anyone who consults it is
understood to recognise that its copyright rests with the author and that no quotation from
the dissertation, nor any information derived there from, may be published without the
author’s prior written consent. Moreover, it is supplied on the understanding that it
represents an internal University document and that neither the University nor the author are
responsible for the factual or interpretative correctness of the dissertation.
i
Plagiarism and collusion
By submitting this dissertation I confirm that I have read and understood the University's
policy on plagiarism and collusion, and that the attached work is my own.
ii
Abstract
The origins of polycyclic aromatic hydrocarbon (PAH) contamination of sediments collected
from the Humber Estuary (East Riding of Yorkshire – Lincolnshire, UK) and its tributaries the
River Trent and the River Ouse have been investigated. Molecular distributions of PAHs
extracted from sediment cores have been determined by gas chromatography – flame
ionisation detection (GC-FID).
The molecular distribution of PAHs has been used to unambiguously identify the dominance
of pyrolytic PAH sources. Equipment failure resulting in a lack of isotopic data meant that
coal was the only source that could be identified through molecular distribution, and it
remains unclear in what manner it was being used. Vehicular emissions and PAHs
associated smelting an incineration were suggested by isomeric diagnosis ratios.
The change in concentration of PAH compounds over time indicates a small shift in sources
away from wood towards coal, perhaps due to the global increase in use.
The sediment inventory of PAHs is characterised by the dominance of four-ring parent
compounds. Absolute concentrations of PAHs varied greatly between cores and ranged from
626ng g-1 to 79,628ng g-1. High values exist only in older sediment, modern sediments are
lightly polluted.
Relative amounts of PAHs in modern sediment were similar between cores, with only pyrene
featuring at higher concentrations in core A. These changes were tentatively ascribed to an
industrial source. Changes in the concentrations of three PAHs along core B were similarly
ascribed to diesel or incineration emissions.
iii
Acknowledgements
I owe my deepest gratitude to Dr Nikolai Pedentchouk, for all of his guidance and
supervision from this project's conception until its conclusion.
I would also like to thank Graham Chilvers for his assistance through various laboratory
procedures, and Dr Julian Andrews for taking the time to share his expertise in the
geography and the nature of the estuary and its surrounding areas.
iv
Table of Contents
Plagiarism and collusion ........................................................................................................ii
Abstract.................................................................................................................................. iii
Acknowledgements ............................................................................................................... iv
Illustration Index..................................................................................................................... vi
Index of Tables...................................................................................................................... vii
Introduction............................................................................................................................. 1
Polycyclic aromatic hydrocarbons......................................................................................1
The Humber Estuary..........................................................................................................1
Chemical analysis of pollution............................................................................................2
Literature Review.................................................................................................................... 3
Polycyclic aromatic hydrocarbons......................................................................................3
PAHs in ecosystems......................................................................................................3
PAHs in environmental policy and management............................................................4
Molecular distribution......................................................................................................... 5
Stable isotopes of hydrocarbons........................................................................................7
Hydrocarbon formation..................................................................................................7
Using PAH isotopes to determine hydrocarbon sources................................................8
PAHs in the Humber Estuary.......................................................................................10
Aims and objectives.............................................................................................................. 12
Methodology......................................................................................................................... 13
Designing the project.......................................................................................................13
Collection and isolation of PAHs......................................................................................15
Quantitation and identification of PAHs............................................................................18
Gas chromatography - flame ionisation detection........................................................18
Concentration of PAHs in sediment........................................................................18
Gas chromatography - combustion - isotope ratio mass spectrometry........................19
Data analysis................................................................................................................... 20
Compound ratios......................................................................................................... 20
Quantitation of PAH concentrations.............................................................................23
Isotope ratios............................................................................................................... 24
Results................................................................................................................................. 24
GC-FID Chromatograms..................................................................................................24
Compound ratios.............................................................................................................. 27
PAH diagnostic ratios.......................................................................................................30
v
Quantitation of PAH concentrations.................................................................................36
Discussion............................................................................................................................ 38
Description of the sampling sites and samples................................................................38
GC-FID chromatograms...................................................................................................43
PAH composition.............................................................................................................. 44
PAH compound ratios......................................................................................................45
Quantitation of PAH concentrations.................................................................................46
Potential isotope results...................................................................................................47
Conclusion............................................................................................................................ 48
PAH abbreviations................................................................................................................ 50
Glossary of abbreviations.....................................................................................................51
Glossary............................................................................................................................... 52
References........................................................................................................................... 53
Appendices .......................................................................................................................... 63
Appendix 1 ................................................................................................................. 63
Appendix 2 ................................................................................................................. 64
Appendix 3 ................................................................................................................. 65
vi
Illustration Index
Figure 1: Dating point and salt intrusion locations.................................................................11
Figure 2: Salinity and suspended sediment. Data from Freestone et al. (1987)....................12
Figure 3: Typical chromatograms from each sample site......................................................25
Figure 4: Typical PAH composition of sampling sites............................................................26
Figure 5: Plot of phenanthrene/chrysene ratios....................................................................28
Figure 6: Plot of isomeric ratios: flt/pyr & phe/anc.................................................................29
Figure 7: Plot of isomeric ratios: flt/pyr & phe/anc.................................................................30
Figure 8: Plot of isomeric ratios............................................................................................31
Figure 9: Isomeric ratio depth profile 2: flr/(flr+pyr)...............................................................33
Figure 10: Isomeric ratio depth profile 3: bap/(bap+chry).....................................................34
Figure 11: Isomeric ratio depth profile 5: bag/bgp.................................................................34
Figure 12: Isomeric ratio depth profile 7: pyr/bap..................................................................35
Figure 13: Isomeric ratio depth profile 9: baa/chry................................................................35
Figure 14: Location of PAH sources.....................................................................................38
Figure 15: Location of sampling sites...................................................................................39
Figure 16: Float point information taken from Freestone et al. (1987)..................................42
Index of Tables
Table 1: PAH diagnostic ratios for source indication.............................................................23
Table 2: Standard deviations in molecular concentrations....................................................27
Table 3: Historical and modern results for the isomeric diagnosis ratios...............................33
Table 4: Concentration comparison with previous Humber study..........................................36
Table 5: Comparison of PAH concentrations to global sites..................................................37
vii
Introduction
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are a broad range of chemicals that are generally
associated with incomplete combustion and fossil fuel pollution. They have been released
naturally into the environment for millions of years from sources such as forest fires, volcanic
activity, and fossil fuel seeps. Anthropogenic activity has increased the volume of PAHs
being released into the environment, particularly the increase in the use of fossil fuels since
the beginning of the industrial revolution.
A large proportion of the PAHs from anthropogenic sources is understood to enter
freshwater or marine environments (Mazeas et al, 2001), being transported there by
atmospheric fallout, urban run-off, river inflow and direct discharge. Once there, the
hydrophobic molecules tend to bond to organic matter in sediment, resulting in these
freshwater and marine sediments being recognised as major PAH sinks in near-surface
environments.
Some PAH compounds are considered to be largely harmless to organisms at the
concentrations at which they are likely to be found in the environment (Abrajano Jr. et al.,
2007). Others are known to be carcinogenic (Mastrangelo et al., 1996) and mutagenic
(Durant et al., 1999).
Because of the risks these pollutants pose, they are recognised as priority pollutants and
legislation exists that seeks to control the amount being emitted into the environment.
Investigations into the pathways that PAHs take through the environment are important in
discovering which sources affect which sinks, which PAHs threaten food chains and
ecosystems, and which sources produce them.
The Humber Estuary
The Humber is a tidal estuary that drains over a fifth of England's land area to the sea
(Freestone et al., 1987). The River Trent and Ouse which discharge into it both have
catchment areas of over 10,000km2. Stretches of these rivers feature conurbation and
industry which supply the estuary with PAHs from a variety of sources. The estuary has
been used throughout the industrial revolution and any sediment records from that time will
hold record of local PAH sources. Suspended particulate matter from sources further inland
1
of the confluence of the River Trent and Ouse may also be deposited at measurable
quantities in these sediments.
PAHs from urban areas have been found to dominate riverine and estuarial sediment
(Mazeas et al, 2001). Jones (1979) stated that 15.1m3/s of trade effluent were discharged
into the Humber, this represents over 6% of the estuary's flow according to Freestone et al.
(1987). While only a small portion of this effluent may have been comprised of PAHs it
should be detectable amongst the urban run-off component.
In a study of PAH concentrations and composition, Zhou et al. (1998) found indications that
there were multiple sources for PAH pollution in sediment from several sites in the Humber
Estuary and the River Trent and Ouse. The known locations of sites featuring PAH
deposition will be used from this study to guide sediment collection, and attempts will be
made to understand the PAH sources.
Chemical analysis of pollution
Molecular distribution
Molecular distribution has long been used to identify the source of hydrocarbon
environmental pollution. Gas chromatography-flame ionisation detection will be used to
measure the amounts of PAHs in the sediment samples. Various correlations exist between
the amounts of deposited PAH molecules that can reveal information about their sources.
Stable isotopes
Various processes from formation through to use discriminate between PAH compounds
based on the isotopic weight of their component atoms. Different processes isotopically
fractionate the compounds to varying degrees and the level of fractionation can reliably
indicate which processes they have been subjected to.
Many analytical techniques exist for measuring isotopes, the one chosen for this study was
gas chromatography – combustion – isotope ratio mass spectrometry (GC-C-IRMS). It was
chosen because it has the sensitivity necessary to differentiate between otherwise
indistinguishable fuel sources. In tandem with the results from the GC-FID, carbon and
hydrogen isotope values measured by GC-C-IRMS allow for far greater differentiation
between sources than either method alone.
Unfortunately a fault developed with the mass spectrometer and despite several attempts it
could not be repaired in time to provide results for this project.
2
Literature Review
Polycyclic aromatic hydrocarbons
PAHs in ecosystems
Hydrocarbons found in sediments have been shown to originate from several sources,
grouped into the following categories by UNEP/IOC/IAEA (1992), and Clark (1997).
•
Direct release of petroleum products such as oil into the environment.
•
Hydrocarbons released as the result of partial combustion of fuels.
•
Natural sources such as forest fires.
•
Biosynthesis of hydrocarbons by marine or terrestrial organisms.
•
Early diagenetic transformation of natural material into hydrocarbon.
The naturally occurring PAHs are generally present in lower quantities than anthropogenic
PAHs (Witt, 1995) and high environmental concentrations are generally associated with
urban areas (Meador et al., 1995). The sources of the PAHs sampled in this project may
originate from several of these hydrocarbon sources, and the composition of these sources
may vary depending on the location of the individual sampling sites.
Lima et al. (2003) found indications that fluxes to sediments of pyrogenic PAHs do not
appear to be decreasing. Van Metre et al. (2000) also found increases in fluxes of PAHs to
sediments associated with watersheds that have experienced increases in urban sprawl.
These findings agree with those of Meador et al. (1995) that large PAH concentrations are
associated with urban areas and vehicle emissions.
The hydrophobicity of some PAHs causes their accumulation in the lipids of animals that
consume them. They are ubiquitous compounds, once airborne they can be transported far
away from their source, one study of sediment samples in sub-Arctic Canada found in PAHs
from continental North America, Asia, and northern Europe (Sofowote et al. 2011). PAHs
pervade through food chains globally, once there is a flux into a food chain they have a
tendency to bio-accumulate and threaten the safety of food chains due to their ecotoxicity
(Neff, 1979).
Damage to aquatic organisms has been observed. Filter feeding organisms such as mussels
3
have been shown to be damaged by them (Widdows et al., 1995). The incidence of
abnormalities such as liver neoplasms in bottom-dwelling fish has been linked to the PAH
concentrations of the sediment above which they feed (Malins et al., 1988; Vethaak et al.,
1992).
PAH accumulation in sediment can potentially damage human health too. Biomagnification
is a measure of the cumulative effects over successive levels in a food chain (Franke et al.
1994). It is generally thought to be low due to many organisms' ability to metabolise PAHs
(Eisler, 1987) but some biomagnification through food chains, including some leading to
humans is thought to be possible (DeLeon et al., 1988).
PAHs adsorbed to sediment are less bio-available than airborne or water-borne PAH and
therefore less able to cause damage to some organisms (Menzie et al., 1992). Most PAHs
deposited in sediment will have travelled there via pathways that involved them being
airborne and/or water-borne so the risk they posed to non-filter feeding organisms would
have been higher before their deposition. Similarly, PAHs currently on pathways to
deposition pose a higher threat now than they will when deposited. Investigating sediment
records can highlight how the risks posed to environments by dominant fuel types change
over time, and discover ongoing point sources such as unnoticed leaks.
PAH pathways to sediment that take longer than a few days may result in a change in
molecular distribution. McRae et al. (1999) found that alkylated PAHs and their parent
compounds weather at different rates making the ratio between them unreliable to use for
dating. Readman et al. (1987) found that once PAHs enter sediment they become
essentially chemically inert and so can be assumed to accurately represent the molecular
distribution they had at the time of their deposition.
PAHs in environmental policy and management
The accurate study of pollution can be an important tool in environmental management. The
methods used in this project can establish baseline pollution levels in sediment, soil, and
water (Martinez et al., 2004), and be used to inform environmental impact assessments. In
post-decision monitoring these methods can provide information that increases the accuracy
of predictions made in future impact assessments.
Accurate data can also play an important part at a wider scale for developing policies, plans,
and programmes. The United Nations Environment Programme (UNEP) has been
monitoring several known carcinogenic PAHs as persistent organic pollutants. Due to their
4
ubiquity and potential ecotoxicity, PAHs are recognised in Directives such as 76/464/CEE
and the recent water Directive 200/60/CE. These set certain safe limits for compounds
demonstrated to be carcinogenic, mutagenic, steroidogenic or affecting endocrine functions.
More recently the European Environmental Liability Directive 2004/35/EC has added to
these and included the 'polluter pays' principle, meaning that accurate source identification
can potentially affect a genuine change in companies that are at risk of polluting.
Continuing to investigate the PAH distributions in different geographical areas and improving
the methods used to investigate their sources helps to inform decision making bodies. The
information can be used to revise pollution estimates and improve legislation surrounding
polluting industries.
On a local scale it can be useful to allocate contamination to a specific source in order for
the appropriate risk reduction to be realised or to identify the responsible parties.
A recent example of this is Kelly et al. (2009) finding high concentrations of PAHs in snow
near tar-sand processing plants. Their findings drew strong criticism from Dillon (2011) for a
lack of baseline data that could allow comparison of the contamination with historical levels,
and the attribution of the high PAH levels found to the tar-sand processing plant.
Examination of nearby lake sediments by Kurek (2013) subsequently determined
approximately 50 years worth of pollution history. These historical records leant weight to the
findings from Kelly (2009) and gave the government an opportunity to review its procedures
and decisions both surrounding and within the tar-sand industry.
Molecular distribution
Various relationships between PAH compounds have been shown to exist that can indicate
information about the processes that the hydrocarbons have undergone. GC-FID provides
data about the relative abundance of compounds in a sample which will be used to
characterise the likely sources of PAHs to the area.
The aromatic rings that define PAHs can often contain alkylated substitiuents, the
concentrations of these PAHs can be compared against their unalkylated parent
compounds. Alkylated PAHs are more common than the parent compounds in petrogenic
samples, and less common than the parent compounds in pyrolytic compounds (Wilkes,
2012).
The ratio of high molecular weight compounds to low molecular weight compounds can
5
indicate a sample's source, as low molecular weight compounds are more common in
petrogenic PAH samples and high molecular weight compounds are more common in
pyrolytic PAH samples, as higher molecular weight molecules are formed at higher
temperatures (Walker et al., 2005). The typical global PAH profile is characterised by an
abundance of high molecular weight PAHs from high temperature combustion processes
(McCready et al., 2000).
The ratio of phenanthrene/anthracene plotted against that of fluorene/pyrene is often used to
indicate whether PAHs have petrogenic or pyrolytic origins (Vane et al. 2007; Readman et
al. 2002; Fabbri et al. 2003).
Bence et al. (1996) state that due to the higher solubility in water of phenanthrene than
chrysene, weathering can cause the ratio between them to vary. Samples in which the ratio
of phenanthrene to chrysene does not vary have not been subject to weathering. Kim et al.
(2008) compared this ratio to the ratios of other analytes that have not been studied in this
project, and established this ratio's suitability for distinguishing between PAH sources in
different locations.
PAHs with petrogenic sources have relatively low fluoranthene/pyrene ratios and high
phenanthrene/anthracene ratios while pyrolytic sources create PAHs with higher
fluoranthene/pyrene ratios and lower phenanthrene/anthracene ratios (Readman et al.,
2002; Vane et al., 2007).
Ravindra et al., (1996a) state that high levels of pyrene, fluoranthene and fluorene and
moderate levels of benzo[b]fluoranthene and indeno[1,2,3-cd]pyrene are associated with the
combustion of oil.
High levels of pyrene, fluoranthene and phenanthrene are associated with incineration
(Ravindra et al., 1996a).
Diesel emissions typically produce higher numbers of two and three ring PAHs such as
phenanthrene and pyrene (Miguel et al., 1998; Marr et al., 1996)
Dickhut et al. (2000) demonstrate a relationship between isomer ratios that indicates PAH
sources. Their study focussed on vehicular exhaust, coke production, aluminium smelting
and wood burning as these are understood to be the dominant sources in the USA (Bjorseth
& Ramdahl, 1985). They established the isomer ratios for those sources by collecting
samples from various sites. The different sources were analysed by comparing the ratios of
two pairs of isomers, for each sample analysed the values of one pair are plotted against the
6
other. When the relationships between these fuel types are plotted on a graph it can be used
to compare the ratios of PAHs found in other samples and quickly indicate which is likely to
be the dominant source.
Many studies have published the ratios of individual pairs of isomers typical of different PAH
sources. Table 1 shows some of these diagnostic ratios. Because of the effects that the
different pathways taken by PAHs have on them, it is unlikely that the results of these
individual studies will correspond exactly to those found in this project. Taking all of these
diagnostic ratios together however should give a more comprehensive result.
Analysis with GC-FID alone can provide ambiguous results. Several factors can lower or
eliminate the ability of the molecular distribution of PAHs to be used to accurately indicate
their source. The pathways that anthropogenic and natural PAHs take, and a number of
physical and biogeochemical factors that may occur on these paths can alter their molecular
distribution (Niessner et al. 1985; Arey et al. 1986; Atkinson, 1990). These alterations to the
molecular distribution over time mean that GC-FID is suited to the measurement of recent
pollution events such as fuel or oil spills, but its ability to characterise weathered samples is
less effective.
The 16 studied PAHs do not usually give enough detailed PAH distribution data to allow
definitive links between specific PAH sources and the analysed samples, their main value is
in their ability to provide an estimate of the PAH concentrations (Vane et al., 2007).
Measuring the stable isotope ratios of the PAH molecules can provide far greater detail
about the source.
Stable isotopes of hydrocarbons
Hydrocarbon formation
Isotopes are variants of an element with differing numbers of neutrons and corresponding
differences in weight. Various processes discriminate between these isotopes, for example
evaporation, which favours lighter isotopes and results in the relative enrichment of the
heavier isotope in the source of the material evaporating. Stable isotope measurements are
taken by measuring the ratio of the isotope abundance and comparing it to that of a
universal standard such as Vienna Pee Dee Belemnite (VPDB). The difference between the
two is expressed in parts per thousand (‰) and represented in delta notation as shown in
equation 1.
7
δ13Csample = [ (13Csample / 12Csample) / (13Cstandard / 12Cstandard) -1] * 103
(‰)
(1)
Hydrocarbons are derived from organic material produced by photosynthesis. Organic
material is depleted in 13C because photosynthesis fractionates carbon isotopes, with the
lighter 12C isotopes being favoured for incorporation into the organism. This is a form of
kinetic fractionation, molecules with lighter isotopes have bonds that are easier to break so
they proceed more quickly through the process, giving rise to a product that is enriched in
lighter isotopes. Hydrocarbons are derived from organic material produced by
photosynthesis and so share this depletion in 13C.
During the thermal maturation of hydrocarbons they are subject to another kinetic
fractionation process. Under the heat and pressure of burial, organic matter turns into
kerogen and begins to produce hydrocarbon molecules. Because the bonds of the
molecules in the kerogen containing 12C are easier to break, the newly formed hydrocarbon
molecules are enriched in these lighter isotopes.
Using PAH isotopes to determine hydrocarbon sources
The result of fractionating processes is that specific fossil fuels tend to have narrow ranges
of δ13C values which can be used to characterise petroleum. The different pathways used by
manufacturers to synthesize chemicals from fossil fuels can leave also characteristic levels
of fractionation (Dempster, 1997; Smallwood, 2001). These make it possible to identify or
exclude individual production pathways as well as PAH sources.
An early example of 13C isotopes being used in this way is the relationship between the 13C
value of natural gas and maturity of source rocks being demonstrated by Stahl (1979). 13C
values were also used by Stahl (1979) to show that Canadian Jurassic and Tertiary
petroleum reservoirs did not come from an underlying Jurassic source rock. The common
practise of examining the oil type-curves of the reservoirs and underlying source rocks could
not provide differentiation between them as they were nearly identical. But an examination of
the 13C values of the reservoirs and source rock was able to show that neither reservoir was
formed by the underlying source rock.
It was established by O'Malley et al. (1994) that compound specific isotope analysis can be
used to discern between PAHs from two different sources.
The stable carbon isotope values of molecules that have been through processes such as
combustion are expected to reflect the values of the source material (Okuda et al., 2002).
Environmental weathering does not appear to alter isotopic values either, O'Malley (1994)
8
and Mazeas (2002) found that the carbon isotopic weight of a range of PAHs were not
subject to fractionation by weathering reactions such as vaporisation, photolytic
decomposition, or aerobic bacterial degradation. This isotopic stability of these PAHs after
their release into the environment means they can reliably be used to trace their sources.
Faure and Mensing (2005) state that bituminous coal has an average δ13C of -25‰ (PDB).
While to McRae et al., (1996; 1998; 1999) found that for mild conversion processes such as
domestic combustion, parent coal values of between -25‰ and -23.5‰ are typical, and
more efficient conversion processes such as gasification they are less negative than
-23.5‰. Stahl (1979) states the bounds of petroleum as being between -18‰ and -34‰
(PDB).
δ13C values between -25‰ and -26‰ have been shown to indicate that high temperature
coal tar is the major PAH source (McRae et al. 1999). δ13C values between -28‰ and -29‰
indicate that petroleum is the main source.
Methane combustion has been shown to be a significant source of PAHs in coastal
atmospheres (Simcik et al. 1999). McRae et al. (2000) state that methane pyrolysis
produces PAHs with δ13C of between -63‰ and -36‰. Biogenic methane has been shown to
produce PAHs with values of around -70‰ (Mattavelli et al. 1983).
A tendency for the δ13C depletion of PAHs to increase with depth suggests that the PAH may
be derived from reworking of organic matter by microbiota (McRae et al., 2000). This
increasing depletion could lead to sources being correlated to a change over time if
incorrectly diagnosed.
Upper bound Lower bound Dominant source
-12.9‰
-26.7‰
Reference
Gasoline & diesel exhaust
Okuda et al., 2002;
Okuda et al., 2003
-23.5‰
-25‰
Domestic coal combustion
-25‰
-26‰
High temperature coal tar
-28‰
-29‰
Petroleum, diesel
-27‰
-29‰
Automotive crankcase oil
McRae et al., 1999
O'Malley et al.,
1996
9
-36‰
-63‰
Petrogenic natural gas
McRae et al., 2000
-
-70‰
Biogenic natural gas
Mattavelli et al.,
1983
The ranges expected depend to an extent on the constitution of their sources, for example
PAHs from burned wood will reflect the δ13C ranges of the wood being burned (O'Malley et
al., 1994) meaning that it is important to characterise local sources where possible to be
able to correctly interpret results.
Different PAH molecules from a single source can have different δ13C values which allows
sources to be characterised by examining the isotopic value of different molecular weight
PAHs. In a well-mixed sediment the ability to compare individual compounds to the known
values of sources can make this apportionment technique highly accurate.
Sun et al., (2003b) describe the the δ13C and δD values of twelve PAHs from four combusted
fuel products that have δ13C values within 8‰ of each other. This difference is sufficient to
resolve the individual sources, but may not be enough to accurately identify PAHs in a wellmixed combination of sources.
The natural abundance for δ13C is 110‰ (Fry, 2006) while for δD it is around 155‰.
Hydrogen is subject to fractionation in the same ways that carbon is and the δD values of
these fuels have a far wider range than their δ13C values. By combining the PAH δ13C values
against their δD values, a far greater degree of differentiation between sources can be
attained. Sources with δ13C values which are sufficiently similar that they can be confused in
a complex source, such as petrol and jet fuel, are more easily differentiated with this
method.
PAHs in the Humber Estuary
Zhou et al. (1996) found that the ratio of two dissolved PAHs in the Humber remained
constant while their concentrations varied, suggesting that the different compounds had the
same source. In later analysis of material collected at the same time and locations as the
first study, Zhou et al. (1998) found variations in the ratio of the PAH compounds in
suspended particulate matter and surface sediment. They concluded that these variations
indicated different sources but further investigation was outside the scope of their study. No
further studies can be found into the sources of these pollutants in the Humber.
10
Andrews et al. (2008) dated sediment from around point D in figure 1. The findings suggest
that an 800mm sediment core from the marsh front at this site may reach sediment
deposited in the 1940s or earlier.
Freestone et al. (1987) state that saline water intrudes to point between Brough and Hessle
(Fig 1) and affects the rate of sediment deposition. Being denser than fresh water, the saline
water moves along the estuary bed producing a significant current and moving a
considerable quantity of sediment inland. At the limit of this salt intrusion point saline and
fresh water fully mix and sediment deposition is at its highest in the estuary. Figure 2 shows
the correlation of salinity and suspended particulate matter, with peak suspended sediment
occurring at Brough close to the salt intrusion point. O'Conner (1987) estimates that 2.2 ×
106 m3 a− 1 of sediment enter the estuary in this way compared to just 0.3 × 106 m3 a− 1 of
sediment that enters it from the rivers. It is likely that sedimentation rates will be higher and
therefore sediment age younger for sample cores in the estuary compared to cores located
on either river.
Figure 1: Dating point and salt intrusion locations
11
Figure 2: Salinity and suspended sediment. Data from Freestone et al. (1987)
Aims and objectives
The overall aim of this research is to evaluate the sources of PAHs in the Humber and
determine how both the sources and quantities of PAHs have changed over time, and how
these sources and quantities compare to global locations.
The objectives of this research will be to:
•
Determine whether PAHs are mainly petrogenic or pyrolytic.
•
Determine whether the balance of petrogenic and pyrolytic sources has changed
over time.
•
Evaluate whether specific PAH sources can be determined.
•
Evaluate whether the contribution of specific sources changes over time.
•
Evaluate whether PAH profiles are globally typical.
•
Confirm that PAH concentration measurements match previous studies.
•
Compare PAH concentration measurements to global sites.
•
Confirm that isomer ratios indicate Trent and Ouse have different sources as in
previous study.
•
Estimate when the peak ΣPAH values were deposited in the twentieth century.
12
Methodology
Designing the project
Teece and Fogel (2004) summarise the following approach to a general ecological
investigation involving isotopes
1. Decide whether there is a good possibility that stable isotopes will be able to provide
answers to the questions being asked. Will there be a good signal to noise ratio. Are
there methods already established to analyse the stable isotopes in the materials
that are part of this study?
2. Design a survey of the ecosystem to measure baseline isotopic values in the pools
that are of interest. Is the study site accessible year round or only seasonally. Does
the site vary seasonally? Are permits needed? Is specialist sampling equipment
needed?
3. Begin the planning stage for collecting, processing and analysing samples. Assemble
the equipment. Ancillary analyses for sample nutrient concentrations are often
overlooked.
4. Collect two to three times more samples than are needed for analysis as this will
allow greater flexibility at the time of analysis. Run the first set of analyses, they
should show whether sources have distinct isotopic compositions and whether the
stable isotope techniques will address the research objectives.
5. Rethink the experiment and sampling protocols. Often the first field collection is an
adventure that is problematic and incomplete. Analyse the initial data and reformulate
the questions and experimental design to reflect the reality of the ecosystem.
Consider adding additional parameters and eliminating those that will not contribute
to the final phase.
Previous studies have established that stable isotopes can be used to apportion
hydrocarbon sources in sediment and soil (O'Malley, 1994).
The sampling locations are available annually and specialist sampling equipment was
secured.
To provide some flexibility at the time of analysis, a larger volume of samples were collected
and stored than was necessary. The previous study by Zhou et al. (1998) had already
13
showed that these specific sites have distinct PAH molecular compositions and that the
planned techniques could be used to address this project's research objectives, which were
vital time-saving steps given the short project duration.
The experiment and sampling procedures were based around the information gleaned
during the previous study. Satellite images were used to locate the sampling sites at current
mudbanks near to areas that were found to be high in PAHs in the previous study. The first
field collection was effective because of this.
Teece and Fogel (2004) also state that isotope heterogeneity within samples is important,
and that consideration of the spatial scales over which the sediment is heterogeneous is
important. Mixing of sediment is typically reasonable in rivers and very high in estuaries
(Rogers, 2002) and creates high sediment heterogeneity over small areas. Between 5g and
18g per sample was used by Zhou et al. (1998) and is anticipated to provide a
homogeneous sample of the area.
Suggestions by Teece and Fogel (2004) were adopted concerning analytical processes
always involving gloves and minimal handling of samples minimise contamination risk during
their collection and storage. They also state that geochemical material with organic matter
such as light hydrocarbons can be stored at room temperature without risk of degradation.
Meier-Augenstein (1999) states that the increased sensitivity of GC/C-IRMS creates a
corresponding increase in likeliness that components are detected that are introduced during
samples collection and preparation. Accurate analyses depend on careful sample
preparation and high resolution capillary GC.
The following guidelines are made by Meier-Augenstein (2004) for increasing the accuracy
of results during the preparatory stages.
•
Every step of the sample preparation protocol must be scrutinised for potential mass
discriminatory effects to avoid isotopic fractionation.
•
If the potential for fractionation cannot be ruled out an internal standard of similar
chemical nature must be added to the sample prior to the sample preparation step.
•
The isotopic signature of derivitisation agents used must be homogeneous
throughout the duration of the project, achieved by acquiring large stock from same
batch and appropriate storage.
•
When determining trace components after wet decomposition such as this, care must
14
be taken to use clean vessels and dedicated high-purity reagents
It has been shown that isotopic fractionation does not occur when using DCM for ASE, nor
during column chromatography (Muccio and Jackson, 2009).
Gas chromatography does fractionate compounds, as the isomers are separated during gas
chromatography an isotopic fractionation effect is present. As the molecules are partitioned
during their repeated sorbtion and desorbtion between the stationary and the mobile phase,
the isotopically heavier molecules elute more quickly (Liberti et al., 1965; Ricci et al., 1994).
This causes the beginning of a molecule's peak to be enriched in 13C, and the end of its peak
to be depleted (Hayes et al., 1990). The beginning of the peaks of analytes that begin to
elute before the preceding molecule has finished will appear isotopically depleted by the
addition of the end of the preceding molecule's peak. To avoid this fractionation affecting the
results the analytes need to have been separated from compounds with similar elution times
and be sufficiently separated from each other by the gas chromatography.
The DCM used for the ASE solvent extractions, and the hexane and toluene used for column
chromatography, and for the transfer and adjustment of extracts, were from single or
homogenised batches. Vessels due to come into contact with samples or extracts were
either sourced new or carefully washed and heated to remove any potential contaminants.
Collection and isolation of PAHs
Sediment samples were collected on the 29th May 2013, close to the Spring Tide of the
27th. This Spring Tide had a range of 7640mm measured at Hull which is close to the
maximum recorded of 7750mm. The large range provided the opportunity to safely collect
sediment cores from sites around the Humber and its tributary rivers under near-identical
conditions during a single low tide.
Sampling areas were chosen to match those demonstrated to feature high levels of
fluoranthene and pyrene by Zhou et al. (1998). Within these areas, cores were taken at sites
that satellite imagery showed to feature mud rather than sand, ensuring higher levels of
organic material and therefore PAHs.
A mild steel corer with a nominal inner diameter of 70mm was used to collect the sediment
cores. Stainless steel tools were then used to remove 100mm long sections of the cores
which were immediately placed into resealable sample bags. The bags were transported in a
cool box and stored in a cool room at 4° Celsius
15
Freeze drying
The collected samples were first freeze-dried to remove the water which would otherwise
stop the solvents used later from saturating the sample. This method avoids possible
degradative processes that may be caused by drying with heat by keeping the samples cool
and in a vacuum.
The outer layer of the lower sub-samples featured modern material pushed down from the
overlying sediment as the corer was inserted into the sediment, this layer of material was
removed. Zhou et al., (1998) performed analyses on dry sample weights of between 5.8g
and 15g. To ensure the results would be comparable by falling within these bounds, and to
produce the approximately 15ml of dry sediment required for the solvent extraction process,
approximately 19.5g of wet sediment was determined necessary. This amount was removed
from the centre of each sub-sample and transferred to a plastic container. Perforated foil
was secured around the containers. They were frozen to accelerate the freeze-drying
process before being placed into a Heto Drywinner freeze-dryer until dry.
Accelerated solvent extraction
The analytes were then extracted from the sediment samples with the solvent
dichloromethane (DCM) and concentrated to volumes suitable for the next stages in the
process.
Purification processes at this stage can potentially affect the results of subsequent IRMS
analyses. Walker et al. (2005) analysed PAHs in sediment and found different results to
another analysis of the same material, the difference was suspected to be due to the
different extraction and purification procedures. Compared to other extraction methods ASE
is a fast process that uses small amounts of solvent, it achieves this by heating solvent
above its boiling point under elevated pressure that results in it remaining a liquid. It has
been shown in separate studies to perform favourably at extracting a range of lipid
biomarkers from soil compared to a soxhlet extraction (Jansen et al., 2006) and better than a
soxhlet extraction at specifically extracting PAHs from soils and sediments (Popp et al.,
1997; Ran et al., 2007).
Each sample was individually prepared for accelerated solvent extraction (ASE). Each was
first placed into a 52x35mm polypropylene lidded tube with four glass beads. They were
loaded into a Spex SamplePrep mixer/mill 8000M and milled for five minutes until powdered,
homogenising the material and ensuring that the solvent would be able to maximally
penetrate it. Between 12mL and 15mL of the milled sediment was mixed 1:1 with
16
hydromatrix, an inert sorbent that enables more contact between solvent and sediment due
to its high surface area, increasing the amount of PAHs the solvent can dissolve from the
sediment.
The 33mL stainless steel canisters and caps of the Dionex ASE 200 Accelerated Solvent
Extractor were cleaned in an ultrasonic bath in a solution of 1:1 methanol and distilled water
to remove all traces of organic material that could contaminate the samples. Dionex GF/B
filters were placed in the base of the canisters to filter the sediment mixture from the solvent.
The filters were covered in powdered copper that had been activated as described by Shek
et al., (2008). This prevented sulphur present in the samples from adversely reacting with
the stainless steel tubing of the machine and affecting the results. The sediment mixture was
then added to the tube and lightly tamped before the top cap was added.
The ASE uses high performance liquid chromatography (HPLC) DCM to extract the total lipid
extract (TLE), which contains the hydrocarbons, from the sediment. DCM has previously
been shown to extract greater amounts of PAHs than other solvents when used in ASE
(Freddo et al., 2012). The ASE extraction method was adopted from EPA Method 3545,
consisting of a 5 minute 125°C heating time, and a 5 minute extraction time at 10.3MPa.
Nitrogen pressurised to 1MPa was used to purge the solvent fully from the canisters into
60mL clear collection vials.
The TLEs were concentrated to 1mL using a Caliper Life Sciences TurboVap 2 set to 40°C.
They were then transferred to 4mL vials and the DCM was completely evaporated using dry
nitrogen and replaced with 1mL of hexane in order to be able to perform chromatography.
Column chromatography
Column chromatography was used to isolate the fraction of the TLE that contains the PAHs.
As the components of the TLE are washed through the column by a solvent (the eluent) they
are slowed down as they adsorb to the column's solid contents. Different compounds travel
at different speeds, those with lower adsorption strengths such as alkanes and alkenes are
washed through more quickly than those with higher adsorption strengths such as PAHs.
The first eluent, hexane, was chosen from the elutropic series so that it would exclusively
remove the components of the TLE with less adsorption strength than PAHs. The second
eluent, toluene, was chosen as it has a higher eluting power and would remove the PAHs.
The aliphatics must be removed from the aromatics otherwise they would coelute during the
subsequent gas chromatography stages and the PAHs could not be measured separately
(Muccio and Jackson, 2009).
17
A wet slurry comprising a stationary phase of activated silica gel and a mobile phase of
hexane was used to fill 100mm long, 5mm diameter columns which had been plugged with
quartz wool. The columns were washed with 2mL of hexane and a single TLE sample was
added to the top of each column.
The TLE was first eluted with hexane to remove the components of the TLE with less
adsorption strength than PAHs, and the eluate was archived. The columns were then eluted
with toluene to isolate the PAHs from the TLE. The toluene fraction was concentrated to
500μL using pressurised nitrogen to yield PAH concentrations suitable for analysis with GCFID, and transferred to the 4mL vials appropriate for the GC-FID.
Quantitation and identification of PAHs
Gas chromatography - flame ionisation detection
GC-FID was performed on an Agilent Technologies 7820a GC system. The temperature
profile was taken from that recommended by Martinez et al. (2004) as suitable for detecting
PAHs. This was from 60°C (with a holding time of 1 minute), to 175°C at a rate of 6°C/minute
(with a holding time of 4 minutes) to 235°C at a rate of 3°C/minute and finally to 300°C at a
rate of 8°C/min with a final holding time of 5 minutes. Peak detection and integration were
carried out using OpenLab CDS ChemStation Edition software.
A PAH standard containing the 16 PAH compounds originally identified by Chu and Chen
(1985) as being particularly high-risk pollutants, and subsequently declared to be priority
pollutants by United States Environmental Protection Agency was analysed. The PAH peaks
were then located in the samples by comparing their chromatographic retention times with
those of the compounds in the standard.
Concentration of PAHs in sediment
Because the GC-C-IRMS was unavailable for the final eight weeks of this project, the
decision was made to measure the concentration of PAHs in the samples. This would allow
direct comparison between these cores and sites studied globally, adding to the breadth of
information recorded about the sample cores.
PAH concentration was chosen for several reasons. It could be measured with the GC-FID
using a programme that was known to provide results in a short amount of time, this was
important given the late stage in the project that it became apparent that the GC-C-IRMS
18
may not be successfully repaired. The existing samples could be quickly prepared for this
measurement, again useful because of the time constraints. After preparation for GC-FID the
samples were still suitable for use analysis using the GC-C-IRMS if any of the series of
repairs to it actually proved successful.
In order to be able to determine the concentration of the PAHs it is necessary to add a
standard to the samples. The standard provides a known amount of a molecule that is
chemically similar to the analytes. When the standard and analytes are measured with GCFID the amount of analyte can be calculated from the amount of standard.
The standard is normally added to the solution at the beginning of processing so that every
alteration made to the sample has the same effect on the standard as it does on the
analytes. Processes that might alter the analyte concentrations include the evaporation of
solvent using nitrogen, and column chromatography. The kind of changes that could occur
include a general loss of PAHs, a loss of more lighter than heavier molecules, and unequal
loss between parent and alkylated substituents. Because measuring PAH concentrations
was not initially necessary for this project the standard was not added early enough to be
able to measure any losses and therefore the measurements may underestimate the actual
PAH concentrations.
Automatic pipettes were used to dilute a pentadecane standard with toluene until the
concentration of the standard was comparable to the analyte concentration. The full
procedure can be found in appendix 2. The solvent in the samples was completely
evaporated using dry nitrogen and replaced by the standard solution. The samples were
then analysed using the GC-FID programme detailled before.
Gas chromatography - combustion - isotope ratio mass spectrometry
Meier-Augenstein (2004) explains that gas isotope ratio mass spectrometry (IRMS) is one of
the oldest forms of mass spectrometry used in analytical geochemistry. It became a far more
powerful analytical method when coupled to a gas chromatograph (GC) and combustion (C)
interface. IRMS measures isotopic abundance with high precision and accuracy but it lacks
the flexibility of other mass spectrometers such as GC-MS. Its ability to simultaneously
detect specific isotope pairs allows the isotope ratio measurements that are crucial for
differentiating fuel sources. GC/C-IRMS can measure isotopic composition at the low
enrichment and levels of abundance that are expected in these samples. The combination of
GC with IRMS provides the sensitivity that is necessary to identify the isotope ratios in
individual isomers and allow the different PAH sources to be discovered.
19
GC/C-IRMS measures the isotopes of molecules by first combusting them to form CO2. The
CO2 molecules will have a mass of 44 or 45 depending on which form of the carbon isotope
they contain, 13C molecules being the heavier of the two. A reference CO2 is used to
establish a mass to charge ratio and CO2 values of the sample are determined by comparing
the charges they generate with it (O'Malley, 1994). The time that the charges are recorded
represents the time taken to elute through the gas chromatograph, and so identifies the
isomers being measured.
The six PAH isomers naphthalene and acenapthelene, fluoranthene and pyrene, and
benz[a]anthracene and chrysene were to be the target of this GC/C-IRMS investigation. The
first pair of these are often the target of environmental forensic studies (Mikolajczuk et al.,
2009), while all six have been studied by Sun et al., (2003b).
A certified reference material sample from the Biochemical Laboratories at Indiana
University was to be measured along with the PAH samples. Measuring the known values of
the reference material allows the minor inaccuracies of the machine to be accounted for in
the values of the PAH samples, increasing the accuracy of the data. The reference would
have been measured first and then after the tenth and twentieth samples to measure any
drift in accuracy during measurement.
The samples were prepared by concentrating or diluting their volumes so that the voltage of
the six PAHs would be likely to measure between 250mV and 3500mV, as the equipment
can return the most accurate results when the compounds of interest are at the
concentrations that produce these voltages. The GC-FID peak pA values of each sample
were compared to those of samples from a previous study, and the alterations in volume
necessary for GC/C-IRMS analysis were based on what was found necessary for samples
with similar peaks in the previous study. Following the results from the first run, the volumes
of samples containing peaks outside this range would have been adjusted to bring them
within this range, and all samples would have been run in duplicate to confirm the accuracy
of the isotope ratio measurements was within 0.7‰.
Data analysis
Compound ratios
Ratios of parent and alkylated compounds will be compared to determine whether the
samples are likely to have petrogenic or pyrolytic sources (Wilkes, 2012).
20
The ratio of high and low molecular weight compounds will also be compared for indications
of petrogenic or pyrolytic sources (Walker et al., 2005).
A comparison between the PAH profiles of the sites and the typical global PAH profile
characterised by McCready et al., (2000) and demonstrated by Vane et al., (2007) will be
made to determine whether the profiles are globally typical or influenced by local sources.
The ratio of phenanthrene to chrysene will be examined to find whether sample sites appear
to share sources or a similar rate of PAH degradation before deposition (Bence et al. 1996;
Kim et al. 2008).
The fluoranthene/pyrene ratios of the samples will be plotted against their
phenanthrene/anthracene ratios to indicate whether the sources are petrogenic or pyrolytic
(Readman et al., 2002; Vane et al., 2007).
The ratio of benzo[a]anthracene to chrysene will be plotted against that of
benzo[b]fluoranthene/benzo[k]fluoranthene to determine if any of the fuel sources studied by
Dickhut et al. (2000) appear to be present.
The standard deviations of molecular concentrations within the samples of each core,
between the average concentrations of each of core, and between the concentrations of
recently deposited samples in each core will be measured.The deviations within the samples
of each core will show how much the concentrations of various PAH have changed over the
course of the deposition of the sediment in the core. The comparison of the average
concentration values of each core can indicate whether different sources exist for each core.
The comparison of the molecular concentrations of recently deposited samples from each
core can indicate whether different modern sources exist for each core, ignoring the
influence of the potential historical point source in core B.
The compounds in the samples will be examined according to the nine diagnostic ratios in
table 1 to determine if individual PAH sources can be identified.
21
Ratio Diagnosis ratio
number
1
2
3
4
Indeno[1,2,3-cd]pyrene/
(indeno[1,2,3cd]pyrene+benzo[ghi]perylen
e)
Fluorene/(fluorene+pyrene)
Benz[a]pyrene/
(Benz[a]pyrene+chrysene
Benzo[b]fluoranthene/
benzo[k]fluoranthene
Value
Source
Reference
0.18
0.37
Vehicular emissions Grimmer et al. (1983);
Ravindra et al. (2006a,
b) Kavouras et al. (2001)
Diesel
0.56
Coal
0.62
Wood Burning
0.35-0.70
Diesel emissions
>0.5
Diesel
<0.5
Gasoline
0.5
Diesel
0.73
Gasoline
>0.5
Diesel
Rogge et al. (1993a ,b);
Mandalakis et al. (2002);
Fang et al. (2004);
Ravindra et al. (2006a,
b)
Khalili et al. (1995);
Guo et al. (2003)
Pandey et al. (1999);
Park et al. (2002)
5
0.92 0.16
Wood combustion
1.26 0.19
Vehicular emissions
2.69 0.20
Smelting
3.70 0.17
Coal-coke
Benz[a]pyrene/benzo[ghi]pery 0.5-0.6
lene
Dickhut et al. (2000)
Vehicular emissions Pandey et al. (1999);
Park et al. (2002)
>1.25
Brown coal
0.28 0.05
Wood combustion
0.33 0.06
Vehicular emissions
1.03 0.15
Smelting
1.09 0.15
Coal-coke
22
Dickhut et al. (2000)
6
Indeno[1,2,3<0.4
cd)pyrene/benzo[ghi]perylene
7
Pyrene/Benz[a]pyrene
Gasoline
Caricchia et al. (1999)
~1
Diesel
~10
Diesel emissions
~1
Gasoline emissions
0.6
Vehicular emissions Neilson (1998)
8
Fluoranthene/pyrene
9
Benzo[a]anthracene/chrysene 0.53 0.06
Oda et al. (2001)
Vehicular emissions Dickhut et al. (2000)
0.60 0.06
Smelting
0.79 0.13
Wood combustion
1.11 0.06
Coal-coke
Table 1: PAH diagnostic ratios for source indication
Quantitation of PAH concentrations
Equation 2 below shows the formula used to calculate the concentrations of PAHs in the
sediment. A correction is included to account for the amount that was removed during the
first GC-FID analysis. The response factor can account for differences in the way that the
FID measures the analyte and the standard, given the chemical similarities of the analyte
and standard, and the lack of measurement certainty created by adding the standard to the
sample so late, the response factor is assumed to be 1.
b×c×d
)
e
a=(
)×g×h ...........................................................................................(2)
f
(
a = concentration of PAH(s) in dry sediment (ng/g)
b = concentration of C15 in the solvent (ng/uL)
c = peak areas of the PAH(s) from the sample (pA*s)
d = 1, the response factor.
e = peak area of the C15 (pA*s)
f = weight of the dry sediment analysed (g)
23
g = 1.002, a correction for the previous 1uL of solution removed from the 500uL samples
h = volume of the sample analysed (uL)
Isotope ratios
The δ13C and δD values of the studied 16 PAHs in each sample were to be measured and
their values would have been plotted against each other. Comparisons of the values of
individual isomers against the data given by Sun et al., (2003a) to further differentiate
between the sources, identifying possible individual sources.
Isotopic values of PAHs would have been compared between the samples of the cores to
highlight any possible microbiological reworking of molecules in the lower samples that
might otherwise lead to PAHs being attributed to an incorrect source.
Results
GC-FID Chromatograms
Figure 3 shows a typical chromatogram from each core, the section of the x-axis displayed
includes the PAHs phenanthrene through to Benzo[ghi]perylene.
PAH composition
Figure 4 shows typical composition concentrations of PAHs from each sample site. The PAH
abbreviations are listed on page 50. Table 2 lists the standard deviations of molecular
concentrations within the samples of each core, between the average concentrations of
each of core, and between the concentrations of recently deposited samples in each core.
24
Figure 3: Typical chromatograms from each sample site
25
Figure 4: Typical PAH composition of sampling sites
26
Standard deviations
PAH A historical B historical C historical ABC av. historical ABC surface
nap
0.240
0.650
0.520
0.915
0.238
acy
0.177
0.204
0.157
0.258
0.142
ace
0.502
0.503
0.516
0.639
0.387
flu
0.383
0.514
0.273
0.563
0.244
phe
1.398
1.512
0.652
0.524
0.890
anc
0.239
0.546
0.296
0.551
0.341
flt
1.205
3.603
1.208
3.186
1.098
pyr
0.947
3.786
1.292
1.723
2.254
baa
0.511
2.368
0.708
0.383
0.721
chr
0.632
1.658
0.524
1.041
0.533
bbf
1.119
1.379
0.642
0.678
1.259
bkf
0.638
1.812
1.292
0.222
0.087
bap
0.953
1.716
0.845
0.457
0.661
ip
0.567
1.103
1.213
0.822
0.397
da
0.659
3.270
0.951
2.623
1.342
bgp
0.899
0.686
1.303
0.179
0.891
Table 2: Standard deviations in molecular concentrations
Compound ratios
Figure 5 shows the phenanthrene/chrysene ratio plot from Bence et al. (1996). The samples
from core A have a line of best fit y=0.3711x+266.59 with an R2 value of 0.2597. The
samples from core B have a line of best fit y=1.0830x+4.89 with an R2 value of 0.9261.
The samples from core C have a line of best fit y=0.8814x+91.95 with an R2 value of
0.9308.
27
Ratios of phenanthrene vs. chrysene
1550
1350
Chrysene (ng/g)
1150
950
750
550
350
150
200
400
600
800
1000
Phenanthrene (ng/g)
1200
1400
Site A
Linear Regression for Site A
Site B
Linear Regression for Site B
Site C
Linear Regression for Site C
Figure 5: Plot of phenanthrene/chrysene ratios
Figures 6 and 7 show two representations of the ratio of phenanthrene/anthracene plotted
against that of fluorene/pyrene. Figure 4 features the diagnostic boundaries suggested by
Vane et al. (2007). Figure 7 features the boundaries suggested by Readman et al. (2002)
and includes examples of ratios for petrogenic and pyrolytic samples that they recorded.
28
Figure 6: Plot of isomeric ratios: flt/pyr & phe/anc
29
Figure 7: Plot of isomeric ratios: flt/pyr & phe/anc
Figure 8 shows isomeric ratios plotted according to the findings of Dickhut et al. (2000).
PAHs from the four main fuels are plotted with their error bars, the solid lines represent the
range in mixing lines between automotive and coal sources and are based on the upper and
lower ratios for each source. The dotted lines represent the the range in mixing lines
between the other sources, coal-wood, wood-smelter, and smelter to coal, based on their
upper and lower ratios.
PAH diagnostic ratios
Table 3 contains the results from the isomeric diagnostic ratios, the indicated dominant
historical and modern fuel sources are listed together. Figures 9 to 13 show the ratios that
reveal cores with changing fuel sources which correlate with time.
30
Figure 8: Plot of isomeric ratios
31
Core Isomeric ratio
Early source indicated
Modern source indicated
A
ip/(ip+bgp)
Wood
Coal
A
flr/(flr+pyr)
Gasoline
Gasoline
A
bap/(bap+chry)
Diesel
Diesel
A
bbf/bfk
Smelting
Smelting
A
bag/bgp
Brown coal
Coal-coke
A
ip/bgp
Diesel
Diesel
A
pyr/bap
Gasoline
Gasoline
A
flt/pyr
-
-
A
baa/chry
Wood
Coal-coke
B
ip/(ip+bgp)
Coal
Coal
B
flr/(flr+pyr)
Gasoline
Gasoline
B
bap/(bap+chry)
-
Diesel
B
bbf/bfk
Smelting
Coal-coke
B
bag/bgp
Brown coal
Brown coal
B
ip/bgp
Diesel
-
B
pyr/bap
Gasoline
Gasoline
B
flt/pyr
-
-
B
baa/chry
Wood
Coal-coke
C
ip/(ip+bgp)
Coal
Wood
C
flr/(flr+pyr)
Gasoline
Gasoline
C
bap/(bap+chry)
Diesel
Diesel
32
C
bbf/bfk
Smelting
Coal-coke
C
bag/bgp
Brown coal
Brown coal
C
ip/bgp
Diesel
Diesel
C
pyr/bap
Gasoline
Gasoline
C
flt/pyr
-
-
C
baa/chry
Coal-coke
Coal-coke
Table 3: Historical and modern results for the isomeric diagnosis ratios
Isomeric ratio depth profiles
flr/(flr+pyr)
0.14
0.12
flr/(flr+pyr)
0.1
Sample A
Sample B
Linear Regression for Sample
B
Sample C
0.08
0.06
0.04
0.02
0
0
100
200
300
400
500
600
700
sample depth (mm)
Figure 9: Isomeric ratio depth profile 2: flr/(flr+pyr)
33
800
900 1000
Isomeric ratio depth profiles
bap/(bap+chry)
0.6
bap/(bap+chry)
0.5
Sample A
Sample B
Linear Regression for Sample
B
Sample C
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
600
700
800
900
1000
sample depth (mm)
Figure 10: Isomeric ratio depth profile 3: bap/(bap+chry)
Isomeric ratio depth profiles
bap/bgp
8
7
6
Sample A
Linear Regression for Sample
A
Sample B
Sample C
bap/bgp
5
4
3
2
1
0
0
100
200
300
400
500
600
700
sample depth (mm)
Figure 11: Isomeric ratio depth profile 5: bag/bgp
34
800
900
1000
Isomeric ratio depth profiles
pyr/bap
6
5
Sample A
Sample B
Linear Regression for Sample
B
Sample C
Linear Regression for Sample
C
pyr/bap
4
3
2
1
0
0
100
200
300
400
500
600
700
800
900 1000
sample depth (mm)
Figure 12: Isomeric ratio depth profile 7: pyr/bap
Isomeric ratio depth profile
baa/chry
1.4
1.2
sample depth (mm)
1
Sample A
Linear Regression for Sample
A
Sample B
Sample C
Linear Regression for Sample
C
0.8
0.6
0.4
0.2
0
0
100
200
300
400
500
600
700
baa/chry
Figure 13: Isomeric ratio depth profile 9: baa/chry
35
800
900
1000
Quantitation of PAH concentrations
The concentrations of each compound for each sub-sample are listed in appendix 3. Table 4
shows a comparison of the range of PAH concentrations found in this study and those found
in the study by Zhou et al. (1998) at similar locations.
Table 5 shows the PAH concentrations of this study in the context of those found by studies
globally. The values taken from this project are those of the samples with the minimum and
maximum total PAH concentration values. Entries are arranged by peak concentration in
ascending order.
This study
Sample site
Fluoranthene
Study by Zhou et al. (1998)
Pyrene min-max Sample site
min-max (ng g-1) (ng g-1)
Fluoranthene Pyrene (ng g-1)
(ng g-1)
A
182-564
163-509
Flixborough Wharf
193-285
171-262
B
109-16090
308-14607
Salt End jetty North 277-532
235-485
C
430-631
233-645
Whitgift Church
971
1153
Table 4: Concentration comparison with previous Humber study
36
Area
San Quintin Bay, Mexico
Majorca, Mediterranean Sea
Coastline, Black Sea, Ukraine
Crete Sea, Eastern Mediterranean Sea
Sochi, Black Sea, Russia
Danube River
Dee Estuary, United Kingdom
Italy Coast, Adriatic Sea
Bosphorus, Black Sea, Turkey
Lake Burley Griffin, Australia
Danube Coastline, Black Sea, Ukraine
Odessa, Black Sea, Ukraine
Gironde Estuary & Arcachon Bay, France
Abyssal Black Sea
Kyeonggi bay, Korea
Cotonou, Benin, Africa
Baltic Sea
Danube River mouth, Black Sea
Tabasco State Continental Shelf, Mexico
Western Coast, Australia
Victoria Harbour, Hong Kong
River Trent, UK
Lake Sagamore, NY, USA
Mersey Estuary, United Kingdom
River Ouse, UK
North-Western Gulf
Mersey Estuary, United Kingdom
River Mersey, United Kingdom
Rhone River, Mediterranean Sea
River Thames, United Kingdom
France, Mediterranean Sea
Spain, Mediterranean Sea
Tamar Estuary, United Kingdom
Lake Woods, NY, USA
Brisbane River Estuary, Australia
Sarasota Bay, FL, USA
Xiamen Harbour, China
River Tyne, United Kingdom
North-West Coast, Mediterranean Sea
The Humber Estuary, UK
Pialassa, Baiona, Ravenna, Italy
Boston Harbour, USA
Survey
1992
1996
1995
1994
1995
1992
1984
1990
1995
1989
1995
1995
1988–1990
1995
1993
1988–1990
1989
1991
1992
2013
1978
2000-2002
2013
1991–1993
1984
1993–1994
1985–1986
1993–1996
1996
1996
1984
1978
1986
1993
1993–1996
1991
2013
1998-1999
-
Concentration (ng/g) Study
N.D. -<50 (Σ44 PAHs) Galindo et al.,(1998)
0.3–100 (Σ18 PAHs) Baumard et al., (1998)
7.2–126 (Σ17 PAHs) Readman et al., (2002)
14.6–158.5 (Σ28 PAHs) Gogou et al., (2000)
61.2–368 (Σ17 PAHs) Readman et al., (2002)
< 10–3700 (Σ4 PAHs) Equipe Cousteau (1993)
490 (Σ13 PAHs) Readman et al., (1986a)
27–527 (Σ9 PAHs) Guzzella & DePaolis (1994)
13.8–531 (Σ17 PAHs) Readman et al., (2002)
80–538 (Σ8 PAHs) Leeming & Maher (1992)
30.5–608 (Σ17 PAHs) Readman et al., (2002)
66.9–635 (Σ17 PAHs) Readman et al., (2002)
3.5–853 (Σ 14 PAHs) Soclo et al., (2000)
200–1200 (Σ28 PAHs) Wakeham (1996)
9.1–1400 (Σ14 PAHs) Kim et al., (1999)
80–1411 (Σ14 PAHs) Soclo et al, (2000)
9.5–1871 (Σ15 PAHs) Witt (1995)
2400 (Σ28 PAHs) Wakeham (1996)
454–3120 (Σ15 PAHs) Botello et al., (1991)
1.0–3200 (Σ11 PAHs) Burt & Ebell (1995)
350–3450 (Σ9 PAHs) Hong et al., (1995)
1097-3533 (Σ14 PAHs) This study
3660 (Σ19 PAHs) Tan & Heit (1981)
626-3766 (Σ15 PAHs) Vane et al., (2007)
3110-4472 (Σ14 PAHs) This study
< 20–4740 (Σ13 PAHs) Readman et al., (1996)
5310 (Σ13 PAHs) Readman et al., (1986a)
6–6230 (Σ15 PAHs) Woodhead et al., (1999)
1070–6330 (Σ15 PAHs) Bouloubassi & Saliot (1993)
N.D.-6519 (Σ15 PAHs) Woodhead et al., (1999)
36–6900 (Σ18 PAHs) Baumard et al., (1998)
1.2–8400 (Σ18 PAHs) Baumard et al., (1998)
8630 (Σ13 PAHs) Readman et al., (1986a)
12104 (Σ19 PAHs) Tan & Heit (1981)
2840–13470 (Σ17 PAHs) Kayal & Connell (1989)
17–26771 (Σ11 PAHs) Sherblom et al., (1995)
70–33000 (Σ9 PAHs) Hong et al., (1995)
260–43470 (Σ15 PAHs) Woodhead et al., (1999)
86.5–48090 (Σ14 PAHs) Benlahcen et al., (1997)
4171-79648 (Σ14 PAHs) This study
112000 (Σ13 PAHs) Fabbri (2003)
487–718360 (Σ14 PAHs) Shiaris & Sweet (1986)
Table 5: Comparison of PAH concentrations to global sites
37
Discussion
Description of the sampling sites and samples
Figure 14 shows the location and sizes of the point sources around the estuary. Point
sources are marked in red and labelled, conurbation and the main areas of industry are
marked in grey.
Figure 14: Location of PAH sources
Point source I is the Cemex plant which was built in 1938. It traditionally uses coal and petcoke to produce up to 800,000 \\tonnes of cement per year. Since 2002 it has been using
some Secondary Liquid Fuel, and in 2007 it was granted approval to partly replace the fossil
fuel it uses with Climafuel, a fuel made from various waste materials. As of 2011 13,000
tonnes of Climafuel were being used which represented 45% of their fuel mix (Baynes-Clark,
2011). Cement production involves temperatures of approximately 1400°C which are
associated with the formation of high molecular weight PAHs.
Source H is the 8.09km2 steel works in Scunthorpe, steel works have been in the area since
the 1860s. Satellite imagery shows approximately 149,000m2 of open air coal and coke
storage on site which has the potential to provide petrogenic airborne dust and run-off to the
surrounding areas. Smelting iron ore involves temperatures of approximately 1250°C
meaning this smoke stacks of this site have been a local source of high molecular weight
PAHs for over 150 years.
38
In 1974 Britain's biggest peace-time explosion until 2005 occurred at a chemical plant in
Flixborough at source G. Approximately 400 tonnes of cyclohexane which during
uncontrolled combustion can form benzene, PAHs, and soot, exploded at the site. The
resulting fire burned for ten days and destroyed the plant completely. Other chemicals such
as the plant's product, caprolactam, were also burned.
The plumes from several of these point sources may not be associated with any any local
deposition due to release factors such as chimney heights and weather conditions. Due to
the measured sub-samples only representing approximately 10% of the full sediment depth
profile, deposition from discrete events such as fires may be missed entirely
Figure 15 shows the location of the sampling sites, coordinates for each location are
included in appendix 1.
Figure 15: Location of sampling sites
Site A
Local point sources inland of sample site A include a Scottish & Southern Energy PLC power
station at Keadby (source F), approximately 7km inland. This power station used coal from
1952 to 1984 and was reopened as a gas-fired station in 1996. Other potential point sources
include several wharves with open air coal and coke fuel storage facilities (North Lincs
Council, 2012), the closest being Flixborough Wharf (source G) approximately 4km inland.
The sediment was soft and wet and did not appear to contain much sand, it allowed bodies
to sink into it but tended to solidify and resist their removal.
39
The milled sediment of sample A1 did not readily mix with the hydromatrix before ASE.
When emptying the ASE canister it could be seen that small areas (approximately four areas
of 10mm3) of sediment had been isolated from the hydromatrix and may not have been fully
subjected to the solvent. The absence of similar large reductions in PAHs or background
noise in the chromatograms for sample B1 or C1 also suggest this may be the case.
Site B
The sediment was not as wet as site A and seemed to contain slightly more sand. The top
100-200mm were brown, the remaining 600-700mm comprised a grey seemingly anoxic
layer. Stationary solid bodies did not sink in this sediment.
This site is situated to the East of Hull's developed stretch of the Humber Estuary. Many
shipping terminals are trafficked by container ships associated with intentional and
unintentional releases of petrogenic PAHs
Present and historical industrial activity bordering the Estuary is associated with these
shipping terminals and provides various potential sources for hydrocarbon pollution. The
village of Paull has been associated with ship building for over two hundred years. The
sampling site is adjacent to a small shipyard which has been operational for over 25 years.
Less than a mile inland from this site is an open-air coal storage area capable of contributing
coal dust to the area.
The area was bombed in World War 2, a hydrogen-filled barrage balloon also crashed into
Paull in 1943 and started a large fire, associated burned material was often pushed into
rivers before rebuilding.
The B6-B7 samples bear the strongest evidence of a direct and concentrated pollution event
such as oil or coal tar. A cursory comparison of wet and dry sample weights showed these
samples had a particularly low water content indicative of densely packed sediment and/or
high petroleum content. Visually the sample was black, soft, and easily shaped. The sample
beneath B7 also contained very small PAH concentrations, indicating perhaps that PAHs
from the event above it did not permeate down.
British Petroleum plc (BP) have a chemical processing plant at Saltend less than two miles
inland of this site (source K). They consume and produce hydrocarbons and release waste
into the river as part of their operation, although they have committed themselves to
reducing these emissions.
Although these emissions are released from a jetty away from the bank on which this
40
sample was taken, float point data showing tidal flow released by Freestone et al. (1987)
shows a net movement of water North towards the sample site on the ebb tide (fig. 16). This
and the estuarial mixing would mean that BP's emissions should be perceptible at the
sampling point.
In 2004 an over-land oil carrying pipe split and approximately 60,000 litres of crude oil were
spilled into the Humber at South Killingholme (source J), approximately 8km away from
sample site B. The float tracks in figure 16 again illustrate how material from this spill would
have potentially been transported past the site several times. Diffusion of the material and
the receding tide could have resulted in some oil from this spill being deposited at this site.
41
Figure 16: Float point information taken from Freestone et al. (1987)
42
Site C
This sediment was slippery and viscous, solid bodies sank quickly and easily and were
difficult to retrieve. There appeared to be very little sand content.
This site is approximately 16km away from a power station (source E) at Drax (North
Yorkshire), it opened in 1974 and is currently the biggest in the UK. Its turbines were
modernised between 2007 and 2012 and it features flue-gas desulphurisation technology.
The Environment Agency (EA) approved the station's use of pet-coke after a trial between
2005 and 2007 showed no appreciable change in levels of several contaminants (Drax,
2007) including the level PAHs (Drax, 2006). Although the local PAH levels may not have
changed, the PAH signature may be altered by the combustion of up to 300,000 tonnes of
pet-coke a year, and the transport and on-site storage of up to 6000 tonnes of it permitted by
the EA.
Some samples from site B feature unresolved complex mixture (UCM) where the baseline of
the chromatogram forms a broad hump. The isotopic values of the components of the
unresolvable mixture are measured with those of the target analyte which lowers the
accuracy of the GC/C-IRMS. Chemically UCM has been shown to consist mainly of linear
carbon chains that are connected at branch points, creating T-shaped molecules (Gough
and Rowland, 1990). As these molecules resist biodegradation they tend to accumulate in
sediment. UCM is typical of sites contaminated with oil (Okuda et al., 2000) and degraded
petroleum residues (Le Dréau et al., 1997).
There have been several uncontrolled potential point sources of PAHs to the surrounding
environment. The oil leak of 60,000 litres at South Killingholme came four years after an
explosion at the same location. According to the Health and Safety Executive (Allars, 2001)
a total of approximately 180 tonnes of petroleum (90% ethane/propane/butane) leaked and
exploded resulting in fires and damage in a 400m radius and a large cloud of 'back soot'.
This was
GC-FID chromatograms
Figure 3 shows chromatograms typical of each core. The low relative amounts of alkylated
compounds indicate a pyrolytic rather than a petrogenic source. The unresolved shoulders
of most of the peaks for sample core B show that it is noticeably more alkylated than cores A
and C, indicating a higher content of PAHs from petrogenic sources.
Some samples from site B feature unresolved complex mixture (UCM) where the baseline of
43
the chromatogram had formed a short broad hump. The isotopic values of the components
of the unresolvable mixture are measured with those of the target analyte which lowers the
accuracy of the GC/C-IRMS. Chemically UCM has been shown to consist mainly of linear
carbon chains that are connected at branch points, creating T-shaped molecules (Gough
and Rowland, 1990). As these molecules resist biodegradation they tend to accumulate in
sediment. UCM is typical of sites contaminated with oil (Okuda et al., 2000) and degraded
petroleum residues (Le Dréau et al., 1997).
These indications of petrogenic sources could be due to fuel and oil from the ship yard or
simply due to this site being in a far more industrialised area than sites A and C.
The small quantities of analyte in some samples means that the signal to background noise
ratio is around 1:1. All samples from site-A and site-C as well as samples B1-B4 had
quantities of anthracene comparable to the quantities of background noise. The magnitude
of the noisy signal introduces some uncertainty over how accurately these peak area values
are attributed to compounds, as some of the value may represent the unresolved
background noise.
PAH composition
Figure 4 shows typical composition concentrations of PAHs from each sample site. Site B
has strong concentrations of five-ringed compounds compared to the other sites, while the
highest concentrations of each site are four-ringed compounds, this is indicative of dominant
pyrolytic sources. The relative levels of anthracene and the high levels of phenanthrene at
each site indicate a contribution from a petrogenic source as well (Readman et al., 2002).
High levels of benzo[ghi]perylene are typically formed by during the combustion of natural
gas in applications with diffusion-type flames such as industrial generators. Cores A and C
had low levels of this PAH relative to others which rules out this process as a dominant
source. Core B had much higher levels of benzo[ghi]perylene which may suggest gas as a
source.
High levels of pyrene and fluoranthene suggest the possibility of diesel emissions,
incineration, or the combustion of oil. The low concentrations of fluorene indicate oil
combustion may not be a dominant source, whereas the relatively high levels of
phenanthrene suggest that diesel emissions or incineration may be.
Table 2 lists the standard deviations of molecular concentrations between various PAH data.
The concentrations along the historical profiles of cores A and C suggest that the sources
44
have not changed significantly over the course of the sediment being deposited. Core B is
shown to exhibit changes in concentrations of fluoranthene, pyrene, benzo[a]anthracene and
dibenzo[a,h]anthracene over time, although these changes do not correlate with the high
concentration gradient of core B
A comparison in table 2 of the historical averages of the three cores shows that sources of
fluoranthene and dibenzo[a,h]anthracene have differed significantly between the sites. This
change in pyrolytic compounds does not match any of the concentrations typical of
previously studied sources. The isotopic data would have allowed the compounds
associated with these changes to be analysed more accurately and may have identified
them as coming from a specific source or sources.
In table 2 the comparison of the concentrations of samples A2, B2 and C2 which discounts
the historical point source in core B, shows that all sites could share sources of most
compounds except pyrene. Because the large concentration of pyrene at site A which is not
shared by B or C exists without a significant difference in concentration of any other
compounds, it may originate from an industrial source.
The molecular distributions of cores A and C do not match the globally typical profile, there
are fewer five and six ring compounds present in these samples tha typical. This could be
because of local sources increasing the relative amount of four ring compounds or because
the sites are sufficicently distant from urban areas to lack this high molecular weight
signture.
PAH compound ratios
In the phenanthrene/anthracene ratio plot of figure 5, the low R2 value of the line of best fit
for sample site A indicates that the site is subject to PAHs from a wide variety of sources, or
that the PAHs deposited there are subject to weathering before deposition. The high R2
values of samples sites B and C show that the ratios of these sites has not been altered by
weathering and that the differing ratios can be used to tell the sample sites and their sources
apart. The similarity of sites B and C in also indicates that they may share a source or a
similar rate of PAH degradation before deposition.
Figures 6 and 7 show that around fifteen of the samples have ratios that are plotted within
the 'purely pyrolytic' quadrant. All of samples from site B are in this quadrant, while only
around half of the samples from both site A and C are. This suggests that the vast majority of
all PAHs being deposited are pyrolytic, and that sites A and C have a larger petrogenic
45
component than site B.
All of the samples' phenanthrene/anthracene ratios indicated pyrolytic origins, they are
below the ratio of 10 that describes pyrolytic origins and well below the ratio of 15, above
which samples can be described as purely petrogenic (Vane et al., 2007).
Figure 8 shows isomeric ratios plotted according to the findings of Dickhut et al. (2000). The
high Benzo[b]fluoranthene/benzo[k]fluoranthene ratios indicate that the dominant pyrolytic
source is coal or smelt for sites A and C. Plotted against the benzo[anthracene/chrysene
ratios it becomes apparent that most of the samples lie within the range of mixing for coal
and wood.
The site A point that falls outside of the range of mixing indicated by the dotted lines is from
sample 1, the sample that appeared to be only partially extracted. Sample B8 also featured
very low PAH concentrations in comparison to all other sources has a
Benzo[b]fluoranthene/benzo[k]fluoranthene ratio which has either been affected by these
low PAH concentrations or points to a source outside the focus of this study.
The benzo[b]fluoranthene/benzo[k]fluoranthene ratios of each site show no strong
correlation with time. Each site shows a very slight increase in this ratio over time, an
increase in concentration of PAHs attributable to the use of coal. If the other dominant
source is wood as the results from study by Dickhut suggest, it could be explained by a
reduction in the use of wood as a fuel source rather than an increase in coal.
The results of the diagnostic ratios in table 3 when taken as a whole suggest that wood is
not a large contributor of PAHs to the area, with several of the ip/(ip+bgp) and baa/chry
ratios suggesting that a historical dominance of wood has given way to coal. Coal and coke
appear to be the dominant sources. There is a significant contribution from gasoline and
diesel emissions, particularly with the B5-B7 samples, this could indicate historical pollution
from neighbouring boat breaking activities, or any of the listed World War 2 era events.
Quantitation of PAH concentrations
As previously explained, quantitation was not originally part of this study and so the standard
was not added to the samples at a point in the procedure that would produce results with a
high level of certainty. Had it been part of the planned study the samples would have been
run in duplicate to measure the accuracy of the quantitation and the results would have been
analysed as follows.
Table 4 shows the comparison of compound concentrations found in this study with those
46
found in the earlier study by Zhou et al., (1998). The earlier study considered only surface
samples, the values of which approximate the lower values found in this study. The higher
values found at sites A and B were all measured in deeper sediment than was covered in the
earlier study. Whitgift Church had notably higher PAH concentrations than site C, higher than
normal values were noted in the earlier study for this sampling date. Possible explanations
for the unusually high values not being recorded in this study are that the sub-sampling of
the sediment cores doesn't capture them, and that the sites might be sufficiently far apart
that the concentration of a point source at Whitgift Church would have attenuated due to its
hydrophobicity before reaching site C.
Given that the sampling sites were not identical between studies, and that the sediment at
the surface in the earlier study may have been missed by the sub-sampling method of this
study, the concentration values of the earlier study and the surface sediments of this study
are sufficiently similar that the two can be considered comparable.
Table 5 shows the concentration values compared to global sites. Sites A and C, The River
Trent and Ouse, feature low levels of pollution, while site B at the estuary is heavily polluted
(Baumard et al., 1999). Noteworthy is that the modern sediments of all three sites feature
low levels of pollution, and the long-term trends at sites A and B are towards lower levels.
Potential isotope results
Given the extensive mixing of multiple PAH sources in sediment deposited at site B, the
isotopic values of isomers could be expected to represent a mean value of their sources.
Isotope values of the point source indicated by the magnitude of PAH concentration at the
B5-B7 samples would probably dominate. If coal tar was dumped or buried at the site the
isotopic values could be expected to be -25‰.
The levels of benzo[ghi]perylene suggest that natural gas combustion is not a dominant
source of PAHs in cores A and C but may have been in core B. δ13C values associated with
the combustion of natural gas of between -70‰ and -36‰ would not be expected in cores A
and C, and would confirm natural gas as a source if found in core B (Siegmann and Sattler,
2000).
The oldest core sub-samples could show evidence of microbiological reworking in the form
of slight isotopic depletion.
The three to six-ring compounds associated with the industrial and suburban burning of coal
would be expected to show δ13C associated with coal of -25‰ to -23.5‰. If the two to three47
ring lower molecular weight PAHs are petrogenic, as seems possible from the molecular
distribution, they would be expected to have δ13C values of around -28‰ to -29‰ in line with
diesel and petrol fuel.
Conclusion
As expected in such a large area there is no one clear source for PAH pollution. As
documented around other urbanised areas, pyrolytic sources dominate, the sediment
inventory of PAHs is characterised by the dominance of four-ring parent compounds. The
molecular distribution suggests that coal and coke are the dominant PAH source and that
there are contributions from automotive emissions and the combustion of wood. The oil or
degraded petroleum suggested by the UCM at site B may represent a point source adjacent
to the site and may not be representative of the wider local area.
The PAH concentrations measured in this project are similar to those of previous studies in
the area, so given the shortcomings of the quantitation procedure the absolute
concentrations of 626ng g-1 to 79628ng g-1 can be compared to global concentrations with
reasonable confidence. Similarly the rivers Trent and Ouse can be said to be polluted to a
low level while, at depth, the site in the estuary is heavily polluted.
The gradual increase in coal use indicated by molecular diagnosis ratios changing over time
could be explained by local point sources such as the steel and cement works as well as a
gradual global increase in coal use (British Petroleum, 2012).
Natural gas combustion which had been suggested as a significant source in coastal areas
was not apparent at the sites on the rivers, A and C, but may have been one of the pyrolytic
sources at site B.
The very high pyrolytic PAH concentrations at site B in Paull could be explained in a number
of ways, without isotope analysis of the samples it is impossible to be certain. The
approximate date of the high PAH concentrations at site B could be explained by the
associated activity of World War 2. The ambiguity of the molecular distribution also means it
could originate from unrecorded dumping of high temperature coal tar.
Sites A and B have a peak ΣPAH deposition at approximately 600mm below the surface. At
site B this corresponds approximately to the 1940s. The deposition rate at site A is estimated
to be lower than that for site B so its peak ΣPAH concentrations may have been deposited at
an earlier point in time. The concentrations measured at site C show little correlation with
time.
48
Yang et al. (2013) advise that more than one PAH source apportionment technique should
be used to minimise the weaknesses of individual methods. Several analytical methods were
applied to the molecular distribution data but the data were all generated by GC-FID.
Dating the sediment from which the PAH are being measured, for example using
radioisotope dating, would allow more confident assertions about actual causes of PAHs.
Knowing the decade of deposition would aid in discerning whether a fuel type is typical for
the time in which it is deposited, for example PAHs associated with heavy coal soot
deposition being considered typical ten years before the Clean Air Act but less typical ten
years after it.
This project's plan initially involved visiting a forth site close to the estuary's mouth that had
previously been dated and would allow approximate PAH dating by matching depth. Due to
the estuarial mixing of PAHs and differing rates of sediment deposition inland of this point
the dating data could not have been used to reliably date the River Trent and Ouse samples.
Further study would ideally add to this existing dating information by collecting sufficient
samples to date sediment deposited inland of the salt intrusion point allowing cores to be
much more accurately dated.
A larger number of sample sites would allow greater differentiation between PAHs from
diffuse sources and those from point sources, and better highlight the geographical extents
of PAH sinks in these areas.
This investigation initially included road sweep sampling as described by Stark et al., (2003)
at locations next to the sediment sampling sites. Rain on the day of the site visit meant that
this technique could not be used. Analysing PAHs from nearby roads should offer greater
elucidation of the paths taken by PAHs to the sediment and given results directly
comparable with those of Stark, and should be included in future studies.
The diverse sources and complex sediment of this area merit further investigation.
Duplicating the GC-FID runs would allow for more accurate concentration measurements,
and including total organic carbon measurements would also allow for greater
characterisation of the sediment. With results from GC/C-IRMS as well, further study of
shallower sediment cores from a larger number of sites could describe how hydrocarbon
pollution has affected the area in recent history and how attempts to control pollution alter
the composition of PAHs received across this area.
49
PAH abbreviations
nap
– naphthalene
acy
– acenahpthylene
ace
– acenaphthene
flu
– fluorene
phe
– phenanthrene
anc
– anthracene
flt
– fluoranthene
pyr
– pyrene
baa
– benz[a]anthracene
chr
– chrysene
bbf
– benzo[b]fluoranthene
bkf
– benzo[k]fluoranthene
bap
– benzo[a]pyrene
ip
– indeno(1,2,3-cd)pyrene
da
– dibenz(a,h)anthracene
bgp
– benzo[ghi]perylene
50
Glossary of abbreviations
ASE
Accelerated solvent extraction
BP
British Petroleum plc
DCM
Dichloromethane
EA
Environment Agency
FID
Flame ionisation detection
GC
Gas chromatography
HPLC
High performance liquid chromatography
IRMS
Gas isotope ratio mass spectrometry
mL
millilitre(s)
mm
millimetre(s)
mV
millivolt(s)
MPa
megapascal(s)
ng
nanogram(s)
pA
picoamp(s)
PAH
Polycyclic aromatic hydrocarbon
PDB
Pee Dee Belemnite
s
second(s)
TLE
Total lipid extract
UCM
Unresolved complex mixture
VPBD
Vienna Pee Dee Belemnite
51
Glossary
Analyte – a chemical component being analysed.
Biomass – biological material derived from living or recently living organisms.
Carcinogenic - directly involved in causing cancer.
Ebb tide – the period between high and low tide movements.
Elute
Eluant
Heterogeneous – of a non-uniform composition.
Homogeneous – of a uniform composition throughout.
Hydrophobicity - tendency not to combine with water, or incapability of dissolving in water.
Isomer – compounds with identical chemical formulas but different structural formulas.
Mutagenic - directly involved in increasing the frequency of genetic mutation in an organism.
Petrogenic hydrocarbons – hydrocarbons that were formed as buried material became rock.
Petroleum – a mixture of nominally liquid hydrocarbons.
Polycyclic aromatic hydrocarbon (PAH).
Pyrolytic hydrocarbons – hydrocarbons produced by combustion or heat.
Pyrolysis – the decomposition of material at high temperatures in the absence of oxygen.
Sorbent – a material that tends to take up another material through absorption or adsorbtion.
Steroidogenic – involved in the production of steroids in organisms.
Biosynthesis – the formation of a chemical compound by a living organism.
52
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Appendices
Source apportionment of polycyclic aromatic hydrocarbons in sediment cores from the
humber estuary using molecular distribution.
Appendix 1
Sediment sampling site details
Site A. The River Tent
Near Burton Upon Stather
53°38'58.79''N
0°41'57.8''W
Site B. The Humber Estuary
Paull
53°43'23.44''N
0°14'07.01''W
Site C. The River Ouse
Blacktoft
53°42'26.43''N
0°43'51.89''W
63
Source apportionment of polycyclic aromatic hydrocarbons in sediment cores from the
humber estuary using molecular distribution.
Appendix 2
Producing the C15 standard for GC-FID
An existing procedure that gave a standard concentration suitable for addition to exisiting
samples was modified to yield a concentration suitable for replacing the solvent in the
samples. One litre of C15 weighs 769g. PAH concentrations will be presented as ng/g so
this C15 value is shown as 769000ng/µL. The series of dilution ratios are shown below that
were used to lower this value to approximately 16.34ng/µL.
Step
Solution
Ratio
Volumes used
Concentration
No.
C15 Standard
998000ng/µL
1
Solution A
50:1 toluene to C15
1960 µL: 40 µL
Gives 15380ng/µL
2
Solution B
50:1 toluene to Solution
1960 µL : 40 µL
Gives 307.6ng/µL
A
3
Solution C
1:1 toluene to Solution B 500µL : 500µL
Gives 153.8ng/µL
4
Solution D
1:1 toluene to Solution C 500µL : 500µL
Gives 76.9ng/µL
5
Solution E
5:1 toluene to Solution D 5000 µL : 1000µL Gives 12.82ng/µL
The measurements were checked by weighing the solutions after every addition and using
the liquid's densities to calculate volumes.
64
Source apportionment of polycyclic aromatic hydrocarbons in sediment cores from the
humber estuary using molecular distribution.
Appendix 3
Individual PAH concentrations in sediment (ng/g)
65
66