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Deposition of artificial radionuclides in sediments of Loch Etive, Scotland
Hamza Al-Qasmia, Gareth T.W. Lawa, L. Keith Fifieldb, John A. Howec, Tim Brandc,
Gregory L. Cowied, Kathleen A. Lawa, Francis R. Livensa
a Centre for Radiochemistry Research, School of Chemistry, The University of Manchester,
Oxford Road, Manchester, M13 9PL, UK
b Department of Nuclear Physics, Research School of Physics and Engineering, The
Australian National University, Canberra, ACT 0200, Australia
c Scottish Association for Marine Science, Oban, Argyll, PA37 1QA, UK
d School of Geosciences, The University of Edinburgh, King's Buildings, Edinburgh, EH9
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AbstractThe nuclear fuel reprocessing plants on the Sellafield site (UK) have released low-level
effluents into the Irish Sea under authorisation since 1952. This has led to the labelling of
nearby offshore sediments with a range of artificial radionuclides. In turn, these sediments act
as a long-term secondary source of both soluble and particle-associated radionuclides to
coastal areas. These radionuclides are of interest both in assessing possible environmental
impacts and as tracers for marine processes. Here we present results from a study of the
geochemistry of natural (234, 238U) and artificial (137Cs, 241Am, 238Pu, 239+240Pu, and 236U)
radionuclides and their accumulation in sediments from Loch Etive, Scotland. The data are
interpreted in the context of the historical radioactive discharges to the Irish Sea and
biogeochemical processes in marine sediments. Loch Etive is divided into two basins; a
lower, seaward basin where the sedimentation rate (~ 0.6 cm/yr) is about twice that of the
more isolated upper basin (~ 0.3 cm/yr). These accumulation rates are consistent with the
broad distribution of 137Cs in the sediment profiles which can be related to the maximum
Sellafield discharges of 137Cs in the mid-1970s and suggest that 137Cs was mainly transported
in solution to Loch Etive during that period. Enrichments of Mn, Fe and Mo in sediment and
porewater from both Loch Etive basins result from contemporary biogeochemical redox
processes. Enrichments of 238U and 234U in the lower basin may be a result of the cycling of
natural U. By contrast, the Sellafield-derived artificial isotope 236U does not seem to be
affected by the redox-driven reactions in the lower basin. The 238Pu/239,240Pu ratios suggest
contributions from both historical Sellafield discharges and global fallout Pu. The uniform
sediment distributions of Pu and Am, which do not reflect Sellafield historical discharges,
suggest the existence of a homogenous secondary source. This could be the offshore ‘mud
patch’ in the vicinity of Sellafield from which the supply of radionuclides reflects time-
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integrated Sellafield discharges. This source could also account for the continuing supply of
Cs to Loch Etive, even after substantial reductions in discharge from the Sellafield site.
Keywords: Americium; uranium; plutonium; cesium; Sellafield
1 IntroductionThe behaviour of artificial radionuclides in marine sediments and their interaction with
biogeochemical processes is of fundamental importance in developing an understanding of
their long-term fate in the environment. Such understanding can be used to underpin both
radionuclide tracing of environmental processes and assessment of possible environmental
impacts of radionuclide releases to coastal waters, such as occurred in 2011 as a result of the
accident at the Fukushima Daiichi Nuclear Power Plant.
The absence of oxygen in sediments drives successive reduction of other terminal electron
acceptors (e.g. nitrate, Fe and Mn oxyhydroxides, and sulfate) by anaerobic microorganisms.
These processes could impact the fate of natural and artificial radionuclides in sediments
either by direct cycling of redox sensitive radionuclides by anaerobic microorganisms or by
interaction of radionuclides with biogeochemical redox cycles of other elements (Brookshaw
et al., 2012; Campbell et al., 2015; Kimber et al., 2012; Lovley et al., 1991; Malcolm et al.,
1990). Because of the biogeochemical gradients present, sea-lochs provide an ideal natural
laboratory to explore these interactions (Sholkovitz, 1983; Williams et al., 1988).
Loch Etive is a sea-loch on the west coast of Scotland with well documented geochemistry
and hydrography (Edwards and Edelsten, 1977; Overnell, 2002; Ridgway and Price, 1987)
which provides perfect conditions (high sedimentation rates, anaerobic conditions, restricted
exchange, and reduced erosion) to investigate the geochemical behaviour of natural and
artificial radionuclides. Sellafield, a nuclear site located on the NW coast of England, is the
dominant source of artificial radionuclides in the NE Irish Sea. Authorised radioactive
discharges of low level liquid effluents from Sellafield to the Irish Sea started in 1951 with a 3
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maximum in the mid 1970’s (Figure 1) (Gray et al., 1995). Other sources, such as weapons
testing which had a peak input in 1963 (MacKenzie, 2000), and accidental releases,
particularly the Chernobyl accident in 1986 (Camplin et al., 1986; Mitchell et al., 1986) have
made smaller contributions to the radioactive inventory of the Irish Sea. Further, accidental
release of 238Pu from the burn-up of a US satellite in the atmosphere in 1964 increased the
activity ratio of 238Pu/239+240Pu from global fallout in the northern hemisphere from 0.024 to
0.036 (Hardy et al., 1973).
Figure 1. Total annual quantities of (a) 137Cs, (b) 241Am, and (c) 239,240Pu released from
Sellafield between 1952 - 1992 (data from Gray et al., 1995).
Due to the seawater circulation in the NE Irish Sea, the discharged radionuclides disperse
northwards (Jefferies et al., 1973; Mitchell et al., 1999). Soluble radionuclides such as Cs
mostly remain in seawater and are transported northwards along the Scottish coast out of the
Irish Sea (Dahlgaard, 1995; Jefferies et al., 1973; Kershaw and Baxter, 1995), whereas Pu
and Am (particle-reactive species) mainly associate with suspended particles and are then
focussed into areas of muddy sediment (the ‘mud patch’) close to Sellafield. This mud patch
then acts as a long-term secondary source of soluble and particle-associated artificial
radionuclides, the composition of which are time-integrated due to sediment mixing
processes (Brown et al., 1999; Kershaw et al., 1984; MacKenzie et al., 1998). The ingrowth
of 241Am from decay of 241Pu (t1/2 = 14.3 years) has also dominated the direct discharges of
241Am from Sellafield since the late 1970s (Hunt et al., 2013).
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Uranium occurs naturally almost entirely as a mixture of three isotopes: 238U, 234U and 235U
but natural 236U also exists in the environment in ultra-trace quantities with a 236U/238U atom
ratio of ~ 10-14 (Zhao et al., 1997). Uranium is generally soluble in oxic seawater and present
as dissolved U(VI) complexed with carbonate (Cochran et al., 1986), which may also inhibit
reduction to U(IV). Diffusion of seawater to the underlying sediments may result in the
reduction of U(VI) to insoluble U(IV) at about the depth of Fe(III) reduction, which can lead
to accumulation of U at depth in the sediment (Barnes and Cochran, 1990; Cochran et al.,
1986). The Sellafield discharges to sea contain irradiated U which would be masked by the
natural U in seawater (Zhao et al., 1997) and historical, non-nuclear discharges to the Irish
Sea from the Marchon phosphate plant on the Cumbrian coast (Kershaw et al., 1990).
However, irradiated U is unique in its enhanced level of 236U (t1/2 = 2.3 x 107 years) (Zhao et
al., 1997) which provides a clear signature for irradiated U and hence, in this setting,
Sellafield-derived U, and can be used as a tracer (Al-Qasmi et al., 2016; Marsden et al.,
2006). However, 236U has not been widely used as a tracer of environmental processes due to
the difficulties of detecting its low mass or activity concentrations with traditional methods.
Accelerator mass spectrometry can overcome these difficulties (Eigl et al., 2013; Zhao et al.,
1997).
The inventory of naturally occurring 210Pb (t1/2 = 22.3 years) in marine sediment has two
components: supported and unsupported. The supported Pb is generated in situ by decay of
226Ra contained in the sediment minerals themselves. The term ‘supported’ reflects the fact
that this component is in radioactive equilibrium with its parent isotopes and its activity
concentration will therefore not change over the timescales of interest. The unsupported Pb
originates from decay of 222Rn and this accumulates in surface sediments. Since it is not in
radioactive equilibrium with its parent isotopes, the rate at which unsupported 210Pb decays
allows estimation of the rate at which the sediment accumulates (Swan et al., 1982). The 210Pb
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dating method has been widely used to date cores from sea-lochs from the west coast of
Scotland (Krom et al., 2009; Teasdale et al., 2011).
The main aim of this paper is to investigate relationships between artificial radionuclides
released from Sellafield over a well-defined time period and the biogeochemical cycles which
are well documented in Loch Etive, the study site. This will provide deeper understanding of
the behaviour of natural and artificial radionuclides in the marine environment, underpinning
both assessments of possible environmental impacts and radionuclide tracing of
environmental processes.
2 Methods
2.1 Study Site and Sampling
Loch Etive is a sea-loch on the west coast of Scotland which can essentially be divided into
two distinct parts, a lower and upper basin (Figure 2). The seaward, lower basin has
a maximum depth of ~ 70 m and experiences tidally-induced mixing, leading to
an oxygenated water column (Overnell et al., 2002). The upper basin is separated from the
lower by a submerged sill of 13 m depth and has a maximum depth of 145 m (Howe et al.,
2002). In contrast to the lower basin, which is tidally mixed, only the surface water of the
upper basin is tidally exchanged. As a result, the bottom water of the upper basin is only
exchanged irregularly depending on freshwater input, with a mean repetition time of
16 months (Edwards and Edelsten, 1977) and as a result it is often hypoxic. These
environmental conditions are also reflected in the underlying sediments, which generally
show oxic conditions in the top 5 cm of surface sediments in the lower basin, and suboxic
conditions in top 1 cm in the upper basin (Ridgway and Price, 1987). The upper basin surface
sediment is high in Mn beneath the generally hypoxic waters, contrasting with lower surficial
Mn in the lower basin beneath water which is usually well mixed (Overnell et al., 2002).
Malcolm (1985) studied the geochemistry of Mo in both basins of the loch and established
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an association of Mo geochemistry with the cycling of organic matter, Mn and sulfur.
Previous studies of radioactivity in Loch Etive have used 210Pb and 137Cs as dating tools and
showed sedimentation rates between 0.3 and 0.9 cm/yr (Krom et al., 2009; Ridgway and
Price, 1987; Shimmield, 1993). A study of 25 surficial sediment samples from the length of
the loch showed that Am and Cs had recorded activity concentrations between 1.9 and 8.0
Bq/kg of 241Am and between 88 and 353 Bq/kg of 137Cs (Williams et al., 1988).
Figure 2. Right: Locations of Sellafield and the study site, Loch Etive. The Irish Sea “Mud
Patch” is located just offshore from Sellafield. Left: Map of Loch Etive and its bathymetric
depth profile (Howe et al., 2002) showing the sampling stations in the lower and upper
basins.
For this study, two sediment cores were collected from the near deepest point in the lower
basin (~ 67 m) (Figure 2) in November 2013 using a megacorer. The first core (30 cm) was
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sliced into 25 sections in a nitrogen-filled atmosbag (20 sections from the top 20 cm and 5
sections from the next 10 cm). Porewaters (> 3 ml per sample) were then extracted via
centrifugation (~ 1600 g), filtered through 0.22 µm Acrodisc syringe filters, and preserved
via addition of 100 µl HNO3 (ARISTAR® Grade). The sediment samples were freeze-dried
and then disaggregated with a mortar and pestle to obtain homogeneous fine samples which
were used to conduct all further analysis except gamma spectroscopy. The second core (28
cm) was sliced into 24 sections (20 sections from the top 20 cm and 4 sections from the next
8 cm) which were oven-dried at 40 °C, then disaggregated by mortar and pestle and used for
gamma spectroscopy. Another sediment core was collected from the deepest point (142 m) in
the upper basin (Figure 2) in June 2014 using a Sholkovitz corer. The core and sediment /
porewater samples were processed similarly to the first core from the lower basin.
2.2 Radionuclide Measurements
Samples (2 g dry) were spiked with a known activity of internal standards and digested using
aqua regia. Uranium and Pu isotopes were chemically separated using UTEVA and AG1-X8
resins. The U separation procedure used was based on extraction chromatography (Eichrom
Technologies, 2005). The Pu separation procedure used was based on ion exchange
chromatography (Keith-Roach, 1998). Certified 232U and 242Pu standards (AEA Technology,
Harwell and NPL Teddington, respectively) were used as internal standards in α-
spectrometry. Uranium and Pu were prepared as sources for alpha spectroscopy by
electroplating onto stainless steel planchettes. The alpha sources were counted using passivated
implanted planar silicon (PIPS) detectors (Canberra, Belgium, model A450), with counting times
of up to two weeks, as required, to obtain suitable counting statistics. Uranium (234 and 238)
and Pu (238, 239+240) activities were obtained from alpha spectra. The 235U/238U atom ratio
was measured by ICP-MS, and the 236U/234U atom ratio by accelerator mass spectrometry
(AMS; see 2.2.1 below for full details). Gamma spectrometry was performed using an Ortec
LO-AX 51370/20-S hyper-pure germanium (HPGe) detector housed in a 5 cm thick lead shield
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(15 -1200 keV). The activities of 210Pb, 241Am, and 137Cs were evaluated from the gamma ray
peaks at 46.5, 59.5 and 661.6 keV respectively by employing γ-spectrometry standards. Here,
matrix-matched standards were prepared by spiking one of the Loch Etive samples with
known activities of certified 210Pb, 241Am, and 137Cs standards (AEA Technology, Harwell).
The standards were then counted in the same geometry as the samples. The 210Pb data was
used to date the cores using the following equation and assuming that the initial activity of
210Pb and the rate of sediment accumulation are constants (Faure, 1977):
t=2.303λ log( Pb210 0
Pb210 )A
where: 210PbA = activity of 210Pb per unit weight of sample at depth h, 210PbA0 = activity of
210Pb at the surface (h = 0), λ = decay constant of 210Pb (3.11 X 10-2 y-1) and t = age of the
sample.
2.2.1 236U/234U ratio Measurement
The AMS samples were prepared using the procedure described by Marsden et al. (2001)
with slight modification (Al-Qasmi et al., 2016). The U was separated as described above
with no tracer added to the sample. The purified U was eluted from the separation column by
15 ml 1 M HCl which was reduced in volume to ~ 3 ml and then 1 ml of iron (III) nitrate
nonahydrate in 0.1 M HCl (5 mg Fe per ml) was added. This mixture was evaporated to
dryness on a hotplate and 15 ml 1 M HNO3 was added to remove any residual chlorides. This
was then taken to dryness and baked on a hotplate for 1 h at 250 °C to convert the iron nitrate
into iron oxide. The oxide residue was placed in a furnace at 450 °C for 8 h to ensure
complete conversion to oxide. The oxide was mixed with 1 mg of aluminium powder and the
mixture packed into an aluminium sample holder. Samples were then loaded into a 32-sample
wheel for insertion into the ion source of the AMS system. The measurements were
performed using the 14UD pelletron accelerator at the Australian National University. The
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detection system is based upon combined high-resolution velocity measurement by time-of-
flight and energy measurements (Fifield, 2008; Fifield et al., 2013). The counting times per
cycle were 1 minute at 234U followed by 5 min at 236U. This cycle was performed twice for
each sample, ending with a 234U count.
2.3 Stable Metal Measurements
Homogenised, freeze-dried sediment samples were totally digested in acid matrices (cHCl,
cHNO3 and cHF in a ratio 6:3:2) assisted by a microwave digestion system (CEM Mars)
based on that of Law et al. (2009). After digestion, the supernatant was evaporated to
incipient dryness on a hotplate and the residue was dissolved in 30 ml 2 % HNO3. Inductively
coupled plasma mass spectrometry (ICP-MS, Agilent 7500cx) was used to analyse U and Mo
while inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin-Elmer
Optima 5300) was used to analyse Fe (238.2 nm) and Mn (257.6 nm) in the porewaters and
the digestion supernatants.
3 Results and Discussion
3.1 Stable Metal Profiles
The metal concentrations in porewater and solid phases are plotted against the core depth in
the upper and lower basin cores (Figure 3). Iron and Mn data are similar to those previously
reported for Loch Etive (Malcolm, 1985; Overnell, 2002). Solid phase Fe data show
approximately uniform profiles in both the lower basin (4.2-5.3 wt%) and the upper basin
(3.9-4.6 wt%) whereas porewater Fe shows concentration changes with depth at both sites.
In the lower basin, solid phase Mn decreased from ~ 0.33 wt% at the surface to ~ 0.06 wt% at
5 cm depth, which was then maintained to the bottom of the core (30 cm depth). There was
also an increased Mn concentration in the porewater profile between 2 and 9 cm. In the upper
basin, the solid phase Mn concentration decreased with depth from ~ 0.53 wt% at the surface
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to ~ 0.23 wt% at 12 cm and then stayed approximately constant to the bottom of the core
(30 cm depth). There was also a gradual decrease of porewater Mn concentrations with depth,
from 9-10 mg/l at the surface to 5-6 mg/l at 30 cm. The profile distributions of Fe and Mn in
porewater in both sites indicate active microbially-mediated metal reduction (Overnell et al.,
1996). Reductive dissolution of Fe and Mn oxyhydroxides occurs a few centimetres below
the sediment/water interface when the sediment becomes suboxic, releasing some Fe and Mn
into the porewater. This proposition is supported by the surficial enrichment of the Mn solid
phase at both sites, which is consistent with dissolution and precipitation, driven by the redox
cycling of Mn. During coring, sediments collected from the lower basin also changed colour
from brown/grey to black between 10-15 cm and the core smelt of H2S. In contrast, the core
from the upper basin remained brown/grey beyond 30 cm and had no noticeable smell of
H2S.
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Molybdenum concentrations showed minor enrichments in the surface sediments in both the
lower and upper basins (top 5 cm and top 11 cm, respectively), coincident with surface
increases in solid phase Mn. Due to the affinity of MoO42- for Mn oxyhydroxides, Mo is
scavenged onto Mn oxyhydroxides in the water-column. As the Mn oxyhydroxides are buried
in the sediment and used for microbial respiration, the Mo is lost to the porewater (Crusius et
al., 1996). In the lower basin, solid phase Mo concentrations then show increases below
11 cm, peaking at ~ 13 cm. These features have been observed in other marine sediments and
ascribed to sulfate reduction leading to reduction of Mo(VI)(aq) to particle reactive Mo(IV),
followed by Mo incorporation into sulfide minerals such as pyrite (Chaillou et al., 2002;
Morse and Luther, 1999; Zheng et al., 2000). In the upper basin solid phase Mo showed no
increase with depth which indicates that this sediment does not undergo sulfate reduction at
the depths sampled in this study (Crusius et al., 1996; Zheng et al., 2000). Similar behaviour
has been observed in Loch Etive’s lower basin before where solid phase Mo shows two zones
of elevated concentrations in the sediment, coincident with a Mn redox cycling zone near the
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sediment surface and sulfate reduction at greater depth (Malcolm, 1985).
Figure 3. Porewater and solid phase profiles of selected redox-sensitive metals in the lower
(top) and the upper basin (bottom) sediment cores.
3.2 Radionuclide Profiles
3.2.1 Lower Basin
Activity concentrations (Bq/kg) of natural U isotopes (238 and 234), together with 236U,
239+240Pu, 241Am, 137Cs, 210Pb and atom ratios of 236U/238U are plotted against core depth for the
lower basin core (Figure 4). Activity ratios of 238Pu/239+240Pu are between 0.10 ± 0.01 and
0.17 ± 0.02 which indicate a binary mixture of Sellafield-derived Pu with an average ratio of
0.18 (MacKenzie et al., 1998) and fallout (weapons testing and SNAP-9A accident) with
a ratio of 0.036 in the northern hemisphere (Hardy et al., 1973). The activity profile of 210Pb
was used to date the sediment core and gives an estimated accumulation rate of ~ 0.6 cm/yr.
A plot of the date against the depth is also shown in Figure 4. 13
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Figure 4. Radionuclide activity concentrations with depth in the lower basin core, including
atom ratios for 236U/238U. Error bars are ± 1σ, based on counting statistics. Reference lines for
the years ~ 1977 (maximum Sellafield discharge) and ~ 1986 (introduction of operations on
site to reduce discharges) are provided for reference. For more detail on the discharges see
Figure 1.
The 210Pb data provide estimated ages (uncertainty ± 1-3 years) for the core and help with
interpretation of other radionuclide profiles. The activity profile of 137Cs shows a broad peak
dated to the mid-1970s which could be related to the maximum Sellafield discharges of Cs
from ~ 1970 to the mid-1980s (Gray et al., 1995). This would suggest a direct solution input
of Sellafield-derived Cs to Loch Etive until the mid-1980s, consistent with a significant
proportion of the Cs discharges remaining in the solution phase, rather than associating with
particulate matter (Baxter et al., 1979). Although Sellafield discharges of Cs decreased
significantly after the commissioning of the SIXEP effluent treatment plant in 1985 (Kershaw
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et al., 1992), and this would diminish the supply of Cs in solution, the sediment profile shows
that a Sellafield input of Cs to Loch Etive continues to the present day. This probably reflects
a continuing supply of 137Cs, either re-dissolved from the Irish Sea mud-patch or similar
sediment sources, or that associated with supply of fine grained, resuspended particulates
(Hunt and Kershaw, 1990; MacKenzie et al., 1998).
The activity profile of 239,240Pu is uniform (~ 25 Bq/kg) to about 17 cm and then decreases
gradually to reach the limit of detection at 30 cm depth (which was dated to be ~ the mid-
1960s). The activity concentrations of 241Am remain approximately constant (~ 11 Bq/kg) to
about 24 cm and then drop to ~ 5 Bq/kg. It is clear that the Pu and Am profiles do not show
peaks corresponding to the maximum historical discharges from Sellafield in the mid-1970s
as it would be expected for particle-reactive radionuclide profiles if the sediment simply
preserves the historical discharges of Sellafield (e.g. Al-Qasmi et al., 2016; Lindahl et al.,
2011; Marsden et al., 2006).
A similar Pu profile has been observed in the shelf sediments off the west coast of Scotland
in which 239,240Pu activities range between 20 and 25 Bq/kg in the top 10 cm and then
gradually decrease to less than 10 Bq/kg at 20 cm depth (MacKenzie et al., 2006). Since Pu
and Am are strongly particle-associated, this was interpreted as demonstrating a continual
supply of Pu and Am from a homogeneous secondary source, specifically the offshore
sediment described in MacKenzie et al. (1998), which has accumulated Pu and Am from the
Sellafield discharges over time and thus supplies material reflecting time-integrated Sellafield
discharges to Loch Etive. This indirect transport pathway for Pu and Am to Loch Etive means
that the sediment functions as a reservoir or buffer. This interpretation is certainly consistent
with the 210Pb data, which show an exponential decrease and hence do not suggest extensive
post-depositional mixing of the Loch Etive sediments. Ridgway and Price (1987) similarly
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concluded that 137Cs and 210Pb chronologies do not suggest extensive bio-mixing in Loch
Etive.
Some differences in the distribution of Pu and Am, similar to those seen here, have been seen
before in the Irish Sea basin, and were attributed to a combination of the greater affinity of
Am than Pu for particulate matter, and the in situ generation of 241Am from 241Pu decay
(Marsden et al., 2006). The differences in Pu and Am in Loch Etive sediments probably
reflect similar effects. Alternatively, remobilisation of Pu may be occurring in the deeper
sediment sections. Under oxic to suboxic conditions particle-reactive Pu(IV) is thought to
dominate speciation. However, under more reducing conditions in in the presence of organic
ligands, soluble Pu(III)-species can form (e.g. Boukhalfa et al., 2007; Plymale et al., 2012).
As such, sulfate reduction at depths > 10-15cm in the lower basin may serve to slowly
remobilise Pu relative to Am, as is unresponsive to changes in redox conditions.
The 238U/235U atom ratios (136-146; data not plotted) in the lower basin core are typical of the
natural baseline. Uranium-238 activities are approximately uniform in the top 10 cm
(~ 30 Bq/kg) followed by a rise to 44 Bq/kg and then fluctuate in the range between 39 and
50 Bq/kg. Uranium-234 shows a similar trend to 238U, with a generally higher activity than
238U which might suggest that it is predominantly marine-derived (Cochran et al., 1986). The
increase of 238U and 234U activities below 10 cm coincides with the depletion of porewater
Fe(II), and suggests a potential response to the redox chemistry in this core as the redox
cycling of U in marine sediments is closely coupled with the redox cycling of Fe and the
onset of sulfate reduction (Zheng et al., 2002). Here, either biotic reduction of soluble U(VI)
to insoluble U(IV) by Fe(III)-reducing bacteria, or abiotic reduction of U(VI) with reduced
by-products of microbially metabolism can lead to U(IV) enrichment in then sediment
(Campbell et al., 2015; Lovley et al., 1991). Attributing any of the 238U and 234U enrichment
in this core to Sellafield discharges difficult given that U released from the nuclear site
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rapidly equilibrates with seawater natural U and U released from the Marchon phosphate
plant (Kershaw et al., 1990). However, atom ratios of 236U/238U measured in Loch Etive
sediments are around 10-8. This is well above natural baseline levels (10-14) (Zhao et al., 1997)
suggesting that Sellafield-derived U is present in Loch Etive sediments.
As anthropogenic 236U cannot be incorporated in rock-forming minerals, it is expected that it
would respond to redox changes in marine systems (Al-Qasmi et al., 2016). However, the
236U profile in the lower basin (Figure 4) shows no systematic variations with depth that can
be attributed to redox processes in this core (i.e. 236U is not enriched in or below the area of
microbially mediated Fe(III) reduction). In fact, the 236U profile in the lower basin is
approximately uniform to 26 cm depth and then significantly decreases at 30 cm, and as such
is qualitatively similar to Pu and Am. Reflecting this, it appears that Sellafield derived 236U is
transported to Loch Etive in a form that appears unresponsive to redox change (e.g.
particulate bound U rather than dissolved or readily exchangeable U).
3.2.2 Upper Basin
Activity concentrations (Bq/kg) of 239+240Pu, 241Am, 137Cs, 238U, 234U, 210Pb and activity ratios of
238Pu/239+240Pu are plotted against core depth for the upper basin core (Figure 5). Both 238U and
234U profiles are relatively uniform and do not show any clear evidence of redox-driven
redistribution in this core. This may reflect that this core is not sufficiently reducing for
aqueous U(VI)-carbonate reduction. Indeed, there was no evidence for sulfate reduction in
this core. Activity ratios of 238Pu/239+240Pu are between 0.055 ± 0.005 and 0.147 ± 0.010 which
indicate a binary mixture of Sellafield-derived Pu with an average ratio of 0.18 (MacKenzie
et al., 1998) and fallout Pu with a ratio of 0.036 in the northern hemisphere (Hardy et al.,
1973). The activity ratios of 238Pu/239+240Pu start to decrease gradually at 12 cm (~ 1972),
which is consistent with a higher contribution from fallout Pu, the dominant source of Pu in
the mid-1960s, at greater depth.
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Dating using the 210Pb data shows an estimated accumulation rate in the upper basin of
~ 0.3 cm/yr, slower than in the lower basin (~ 0.6 cm/yr) since, as described earlier, the upper
basin is isolated from the open sea source by a sill of 13 m depth (Figure 1). The activity
profile of 137Cs shows a broad peak dated to the mid-1970s which would be consistent with
the Sellafield maximum discharges. The broad peak of 137Cs is followed by a distribution of
Cs to the core bottom which could be attributed to a redissolution and redistribution of 137Cs
within the core (Hunt and Kershaw, 1990; MacKenzie et al., 1998). In contrast, Pu and Am
show uniform profiles to about 11-12 cm then a gradual decrease to less than 5 Bq/kg for Pu
and to below detection limits for Am. The deep distribution of Pu to the bottom of the core
could reflect a partial redissolution of Pu as has been observed in the Irish Sea (Cook et al.,
1997; Hunt and Kershaw, 1990; MacKenzie et al., 1998; McCartney et al., 1994) or input of
fallout plutonium to the Etive sediments before Sellafield-derived Am was transported into
the Sea Loch. Interestingly, and in contrast to the lower basin, Pu was also not lost from the
solids relative to Am in the deep basin core. This may reflect that the deep basin sediments do
not transition to sulfate reduction by 30 cm. Otherwise, the Pu and Am trends are similar in
both cores, whereas Cs behaves in a different way in both. That would suggest a predominant
solution input of Cs to Etive and a particulate input of Pu and Am from a homogeneous
secondary source such as the Irish Sea Mud Patch.
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Figure 5. Radionuclide profiles for the upper basin core, including activity ratios for 238Pu/239,240Pu. Error bars are ± 1σ, based on counting statistics.
4 ConclusionsDespite the significant decrease in Sellafield discharges, Sellafield-derived radionuclides
continue to arrive to Loch Etive suggesting that the offshore sediments in the vicinity of
Sellafield continue to play an important role as a source of radionuclides over time.
Plutonium and Am distribution profiles in Loch Etive do not preserve the Sellafield historical
discharges, while 137Cs profile showed a broad peak corresponding to the Sellafield maximum
discharges. Although U is expected to behave as 137Cs, it actually behaves similarly as Pu and
Am with a pseudo-uniform profile. The Loch Etive settings develop anoxic sediments which
results in the redox cycling of several metals (Mn, Fe, Mo and natural U). However, redox-
active artificial radionuclides did not seem to be affected by the redox chemistry in this
system.
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Acknowledgements
This work has been supported by the Natural Environmental Research Council grant
‘LO-RISE’ – Long-lived Radionuclides in the Surface Environment (NE/L000547/1; part of
the RATE programme (Radioactivity and the Environment) co-funded by the Environment
Agency and Radioactive Waste Management Ltd.). Al-Qasmi would like to acknowledge the
financial support from Al-Baath University, Syria and the British Council. Law and Livens
would also like to acknowledge support from NERC grant NE/M014088/1.
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