EARTH SURFACE PROCESSES AND LANDFORMS
Earth Surf. Process. Landforms 34, 515–529 (2009)
Copyright © 2009 John Wiley & Sons, Ltd.
Published online 16 January 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/esp.1749
The application of caesium-137 measurements
to investigate floodplain deposition in a large
semi-arid catchment in Queensland, Australia:
a low-fallout environment
Chichester,
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Research Group
Kathryn J. Amos,1*† Jacky C. Croke,1 Heiko Timmers,1 Philip N. Owens2 and Celia Thompson1
1
School of Physical, Environmental and Mathematical Sciences, University of New South Wales @ADFA, Canberra,
ACT 2600, Australia
2
Environmental Science Program, University of Northern British Columbia, 3333 University Way, Prince George,
British Columbia, V2N 4Z9, Canada
Received 17 September 2007; Revised 30 June 2008; Accepted 22 July 2008
* Correspondence to: Kathryn J. Amos, Australian School of Petroleum, The University of Adelaide, SA 5005, Australia. E-mail: [email protected]
†
Current address: Australian School of Petroleum, The University of Adelaide, SA 5005, Australia.
ABSTRACT: Floodplains comprise geomorphologically important sources and sinks for sediments and associated pollutants, yet
the sedimentology of large dryland floodplains is not well understood. Processes occurring on such floodplains are often difficult
to observe, and techniques used to investigate smaller perennial floodplains are often not practical in these environments. This
study assesses the utility of 137Cs inventory and depth-profile techniques for determining relative amounts of floodplain
sedimentation in the Fitzroy River, northeastern Australia; a 143 000 km2 semi-arid river system. Caesium-137 inventories were
calculated for floodplain and reference location bulk soil cores collected from four sites. Depth profiles of 137Cs concentration
from each floodplain site and a reference location were recorded. The areal density of 137Cs at reference locations ranged from
13 to 978 Bq m–2 (0–1367 Bq m–2 at the 95% confidence interval), and the mean value ± 2 (standard error of the mean) was
436 ± 264 Bq m–2, similar to published data from other Southern Hemisphere locations. Floodplain inventories ranged from 68 to
1142 Bq m–2 (0–1692 Bq m–2 at the 95% confidence interval), essentially falling within the range of reference inventory values,
thus preventing calculation of erosion or deposition. Depth-profiles of 137Cs concentration indicate erosion at one site and over
66 cm of deposition at another since 1954. Analysis of 239+240Pu concentrations in a depositional core substantiated the
interpretation made from 137Cs data, and depict a more tightly constrained peak in concentration. Average annual deposition rates
range from 0 to 15 mm. The similarity between floodplain and reference bulk inventories does not necessarily indicate a lack of
erosion or deposition, due to low 137Cs fallout in the region and associated high measurement uncertainties, and a likely influence
of gully and bank eroded sediments with no or limited adsorbed 137Cs. In this low-fallout environment, detailed depth-profile data
are necessary for investigating sedimentation using 137Cs. Copyright © 2009 John Wiley & Sons, Ltd.
KEYWORDS:
caesium-137; low-fallout; Southern Hemisphere; floodplain sedimentation; semi-arid
Introduction
Floodplains are widely recognised as critical components of
the fluvial system and play an integral role in maintaining
catchment and ecosystem health. Floodplains can act as sinks
for substantial volumes of fluvial sediments (e.g. Rumsby, 2000)
and associated nutrients and contaminants (e.g. Walling et al.,
2003), which can be remobilized through processes such as
bank erosion and channel migration. Determining rates of
floodplain sedimentation is thus important for the development of catchment-scale sediment, nutrient and contaminant
budgets. The geomorphology and sedimentology of floodplains
have been the focus of many studies, however, there have
been relatively few studies of dryland river floodplains (Tooth,
2000). The geomorphological processes controlling the form
of floodplains in dryland rivers are difficult to determine: they
often have temporally unpredictable discharge, high-magnitude
short-duration flow events, and access during these events is
often difficult.
While much of the soil lost from Australia’s catchments is
believed to be stored in floodplains (Fryirs and Brierley,
2001), little is known about rates of overbank deposition and
lateral reworking for Australian rivers, nor the significance
and stability of floodplains as long-term sediment sinks. This
is of particular concern in light of the fact that Australian
rivers, and hence the floodplains they produce, are relatively
unusual with respect to key factors such as gradient,
suspended sediment load and channel planform (Tooth and
Nanson, 1999; Nanson and Croke, 1992). The predominance
of multiple channel (anabranching) systems in many of
516
EARTH SURFACE PROCESSES AND LANDFORMS
Australia’s large lowland river systems often makes overseas
literature on floodplain storage along single channels relatively
inapplicable. Large river systems generally include large,
morphologically complex floodplains, with widths of over
several kilometres. The spatial resolution of sample collection
that has been used in many studies of smaller floodplains with
widths of only a few hundred metres is often not logistically
possible for much larger floodplains, which as a consequence
are less well understood.
The use of caesium-137 (137Cs) concentrations for determining
rates and relative amounts of soil erosion and deposition
since the 1950s is well established (e.g. Wallbrink et al., 1998;
Owens and Walling, 2002; Terry et al., 2006). However, it is
not known how useful this technique might be in semi-arid
regions with highly variable discharge and therefore highly
temporally discontinuous floodplain deposition. Coupled
with this, fallout of 137Cs in the Southern Hemisphere
was approximately 25 per cent of that in the Northern
Hemisphere (UNSCEAR, 1982), resulting in total fallout
densities of around one order of magnitude lower (McCallan
et al., 1980), making 137Cs less easily detectable. This study
aims to investigate the applicability of the 137Cs inventory and
depth-profile techniques for determining relative amounts
of floodplain deposition and/or erosion in a moderately
large semi-arid river system in northeastern Australia. A depth
profile of the isotopes 239+240Pu is used at a depositional
location to enable comparison with the 137Cs depth-profile,
and evaluation of its interpretation.
Study Area
The Fitzroy River in northeastern Australia (catchment area
approximately 143 000 km2) discharges into the Coral Sea,
within the Great Barrier Reef Marine Park (Figure 1). The
catchment is of low-relief, with 65 per cent of the catchment
below 300 m asl. Most of the catchment is semi-arid (cf. UN
Aridity Index; UNEP, 1997) with an average annual precipita-
tion of 600–700 mm and mean annual potential evapotranspiration (PET) of 1500–1700 mm (Figure 1 and data from
the Australian Bureau of Meteorology). Some regions to the
south of the catchment and in the eastern coastal ranges have
a higher mean annual rainfall (Figure 1) and are semi-arid to
subhumid. Throughout the catchment, rainfall and streamflow
are summer dominant and highly variable, both on an interand intra-annual timescale. Many channels have an ephemeral
discharge, with high flow events and floods mostly resulting
from intense cyclonic or monsoonal rainfall.
Annual sediment export estimates range from 2 to 10 million
tonnes, based on a sparse range of data and differing methods
(Belperio, 1983; Moss et al., 1992; Neil and Yu, 1996; Prosser
et al., 2001; Franz and Piorewicz, 2003; Furnas, 2003; Joo et al.,
2005; McKergow et al., 2005; Dougall et al., 2006). A recent
study using the SedNet model for the Fitzroy River predicts
that hillslope erosion constitutes 71 per cent of sediment input
to the stream network, with gullying and bank erosion contributing approximately equal amounts of the remaining 29
per cent (Dougall et al., 2006). The model predicts a total annual
input to the river system of 7·3 million tonnes, 62 per cent of
which is delivered to the estuary, and 38 per cent is deposited
within channels and floodplains.
The geology underlying most of the catchment is the PermoTriassic sedimentary and volcanic Bowen basin, which in the
southern parts of the catchment are overlain by younger Triassic
and Jurassic sedimentary rocks, as well as widespread remnants
of overlying Palaeocene sedimentary deposits (Jones, 2006).
Across central areas of the catchment, the older rocks are
overlain by Tertiary and Cretaceous basalts. The most easterly
regions of the catchment are more geologically complex,
comprising a Palaeozoic granitic basement, volcanic magmas
and sedimentary rocks. Erosion from the late Palaeocene to
present resulted in the capturing of inland drainage basins by
small coastal streams to form the present landscape, with the
creation of dissected tablelands, extensive areas of subdued
relief and the formation of floodplains along the major rivers
(Jones, 2006). Quaternary floodplain alluvium occurs along
Figure 1. Map of the Fitzroy River catchment, showing site locations and mean annual rainfall (1961–1990; Bureau of Meteorology). Site 1:
Funnel Creek at Saltbush Park, 650 mm per year. Site 2: Nogoa River at Emerald, 650 mm per year. Site 3: Comet River at Glenwood, 650 mm per
year. Site 4: Fitzroy River at Long Island, 800 mm per year. ‘R’ indicates the location of an additional reference core (RefD) within national park
woodland, 550–650 mm per year. Shading represents the mean annual rainfall. The six main subcatchments are labelled, and the black square
represents the location of Rockhampton City.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 515–529 (2009)
DOI: 10.1002/esp
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CS TO INVESTIGATE SEMI-ARID FLOODPLAIN DEPOSITION IN AUSTRALIA
much of the modern river valleys. Most of the Fitzroy catchment
is within the Brigalow Belt bioregion, once largely covered by
Acacia and eucalypt open woodlands (Furnas, 2003). The Fitzroy
catchment contains three dominant soil types, clay (31%),
duplex (27%) and loam (16%; Furnas, 2003). Land use is
predominantly grazing (82%), with some state forest and
national park (9%) and cropping (7%; Calvert et al., 2000).
Thirty per cent of the catchment was cleared for grazing, and
in another 30 per cent the vegetation was thinned, mostly
during two intense episodes in the 1960s and 1970s (Douglas
et al., 2006). Thirty-six per cent of the catchment remains
uncleared.
Floodplains of the Fitzroy River have a range of morphologies, from confined to unconfined, bedrock gorges to meandering single channel to anabranching. Along the 9600 km of
channels with drainage areas greater than 100 km2, 24 per cent
are anabranching (Amos et al., 2008), and 59 per cent have a
mean floodplain width of over 1 km.
Floodplain Site Descriptions
Four geographically dispersed sites were selected for investigation, each representing a different floodplain morphology
(Figures 1 and 2). These sites were selected in order that they
would represent a range of environmental conditions. Site 1
(148°53′16″ E, 22°8′20″ S), on Funnel Creek, in the Isaacs
River subcatchment, is the most northerly, and is located in a
region for which recent SedNet modelling has predicted the
highest rates of hillslope erosion in the catchment (Dougall et al.,
2006). At this site the river is anabranching, with between
four and six channels (widths = 30–110 m) across a 7 km wide
floodplain, and has a catchment area of 6500 km2 (Figure 2A).
This site is ungauged, but manual records of significant events
(Australian Bureau of Meteorology) indicate that overbank
flows occur with a recurrence interval of 1–1.5 years (Table I).
Inundation of the floodplain occurred in December 1990
following rainfall resulting from ex-tropical cyclone Joy (property
owners, personal communication). From manual records of
gauge height it is estimated that flooding of this magnitude
has occurred seven times since 1954. The property owners
reported that this floodplain site has not been cleared of
brigalow scrub vegetation (personal communication).
Site 2 (148°11′56″ E, 23°30′35″ S) is located on the Nogoa
River, just downstream from the town of Emerald, with a
catchment area of 16 800 km2. This site is 25 km downstream
from the Fairbairn Dam, which was constructed in 1972
with a capacity to impound 1·3 × 109 m3. Since dam construction, it is likely that the incidence of flooding at Site 2
517
has been reduced, influencing overbank deposition. At this
site, the river is anabranching, with two to three channels
(widths = 30–50 m) across a 300 m wide floodplain (Figure 2B).
This modern floodplain is inset within an older floodplain
surface, several kilometres wide, on which several large palaeomeanders are visible. Manual records of overbank events
(Australian Bureau of Meteorology) distinguish significant
events as those which inundate the older floodplain surface,
at a location 5.5 km upstream from the site. These records
indicate that flow onto the older floodplain surface has occurred
once since 1954, in 1956 (Table I). The Fairbairn Dam has
been overtopped on ten occasions since construction, however
the occurrence of inundation of the modern floodplain is not
known.
Site 3 (148°44′7″ E, 24°39′58″ S) is on the Comet River, 30 km
upstream of the town of Rolleston with a catchment area of
5400 km2. At this site the river is anabranching, with five
channels (widths = 30–50 m) across a floodplain 6 km wide.
Inundation history at this site is unknown, although manual
records of significant events recorded 27 km downstream
from the site (Australian Bureau of Meteorology) indicate that
overbank flows occur at that location with a recurrence
interval of 1.5–2·0 years (Table I).
Site 4 (150°24′8″ E, 23°14′46″ S) is located 25 km upstream
from the Rockhampton tidal barrage, and is 80 km from the
river mouth. The catchment area of the river at this location is
139 500 km2. At this site, the river is 250 m wide, and meanders
across a floodplain that is between 3 and 5 km wide, and contains
pronounced scroll-bar topography indicating substantial past
lateral migration of the channel (Figure 2D). Manual records
of significant events (Australian Bureau of Meteorology) are
available for sites 27 km upstream and 26 km downstream from
the site. These records indicate that overbank flow occurs with
a recurrence interval of 1–1·4 and 1·9–2·3 years at these
locations, respectively. Since 1954, the second and third highest
recorded discharges since records began in 1860 occurred in
February 1954 and January 1991, causing widespread flooding
of the lower Fitzroy region.
Methods
Use of caesium-137
Caesium-137 is an anthropogenically derived radioisotope
(half-life = 30·2 years) that was introduced to the atmosphere
through nuclear weapons testing which started in the early
1950s. Appreciable fallout levels of 137Cs were not recorded
in Australia until after 1954 (UNSCEAR, 1982; Longmore et al.,
Table I. Number of significant discharge events likely to have resulted in overbank flow at gauging stations closest to field sites. Data are from
manually reported gauge height records, and therefore might not include a complete record of flood events. High stages reported within days of
another are presented in parentheses, as it is not possible to tell whether these represent a separate discharge peak or are part of the same
hydrograph. Moderate events are those that cause inundation of low lying areas, requiring the removal of stock and/or the evacuation of some
houses. Major events cause inundation of large areas, isolating towns and cities, and cause widespread flooding of farmland. Data provided by
the Australian Bureau of Meteorology
Site
1 – Funnel Creek at Saltbush Park
2 – Nogoa River at Emeralda
3 – Comet River at Glenwood
4 – Fitzroy River at Long Island
a
b
Gauge distance
from site
0 km
5·5 km upstream
27 km downstream
27 km upstream
26 km downstream
Number of moderate
events (1954–2005)
Number of major
events (1954–2005)
6 (+3)
1
10 (+3)b
15 (+10)
10 (+2)
26 (+15)
0
14 (+4) b
22 (6)
12 (+3)
At this site, inundation of a high-level floodplain surface is described and not inundation of the mid-channel islands between anabranches.
Gauge records at this site cover the period 1958–2005.
Copyright © 2009 John Wiley & Sons, Ltd.
Earth Surf. Process. Landforms 34, 515– 529 (2009)
DOI: 10.1002/esp
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EARTH SURFACE PROCESSES AND LANDFORMS
Figure 2. Locations of floodplain and reference cores collected at each of the four field sites. White triangles represent floodplain cores, black
triangles show the location of cores analysed for depth-profiles. (A) Site 1, Funnel Creek at Saltbush Park; (B) Site 2, Nogoa River at Emerald; (C)
Site 3, Comet River at Glenwood; (D) Site 4, Fitzroy River at Long Island. Dashed line of Site 4 transect represents channel cross-section measured
about 500 m upstream from the channel-end of the floodplain transect. This figure is available in colour online at www.interscience.wiley.com/
journal/espl
1983a). After fallout of 137Cs from the atmosphere, primarily
associated with precipitation, 137Cs was rapidly adsorbed
onto fine soil particles, especially silts, clays and fine organic
material. In Australia, peak fallout of 137Cs occurred in 1964
(Zhang and Walling, 2005; Terry et al., 2006) and fallout
effectively ceased by the mid- to late 1970s (Longmore et al.,
1983a). The present distribution of 137Cs in Australian soils,
therefore, is the result of direct fallout that occurred prior to
1980, radioactive decay, and soil erosion and deposition.
Copyright © 2009 John Wiley & Sons, Ltd.
Two approaches are commonly applied in the use of 137Cs
concentrations to determine rates and amounts of floodplain
sedimentation; the depth profile method and the total areal
loading or inventory method. With respect to the former, in a
depositional soil/sediment profile, a peak in 137Cs concentration with depth generally corresponds to the time at which
fallout was at its peak (i.e. 1964). Within the soil profile,
some downward migration of the 137Cs can occur, which
can be measured by determining the depth of peak 137Cs
Earth Surf. Process. Landforms 34, 515–529 (2009)
DOI: 10.1002/esp
USING
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CS TO INVESTIGATE SEMI-ARID FLOODPLAIN DEPOSITION IN AUSTRALIA
concentration at a site that has been unaffected by soil
erosion and deposition since fallout began (e.g. Wallbrink
and Murray, 1993; Owens et al., 1996). For a depositional
profile, if downward migration is taken into account, an
average rate of deposition since peak fallout can be calculated.
However, changes in 137Cs concentration in lake and floodplain
sediments may result from switching between surface and
subsoil erosion upstream, providing some uncertainty when
ascribing the 137Cs concentration peak to the 1964 peak in
fallout.
In Southern Hemisphere locations, low concentrations of
137
Cs and thus relatively large measurement uncertainties can
result in profiles which do not have a clearly identifiable peak
that can be confidently ascribed to the 1964 fallout peak, and
instead the first occurrence of measurable 137Cs may be used
(e.g. Leslie and Hancock, 2008). However, some studies of
Southern Hemisphere sediments have been able to identify a
peak which has been ascribed to the peak in fallout and used
to estimate rates of sediment deposition (Cisternas et al.,
2001; Kuhnen, 2004; Terry et al., 2006). When evaluating
sediment deposition since the onset of 137Cs fallout, it is
generally assumed that the first occurrence of 137Cs in a sediment
profile equates to 1954. However, the precise timing of first
137
Cs detection is uncertain, as the lowest occurrence of 137Cs
is influenced by down-core migration, and its detection is
partly a function of measurement equipment. Leslie and Hancock
(2008) have confirmed that 1954 is a reasonable assumption;
their study indicates that, for the latitude 30–40° S, the first
detectable 137Cs occurrence in depositional environments
equates to 1955 ± 1 year.
The areal loading of 137Cs per unit surface area is described
as the 137Cs inventory, and comparison of 137Cs inventories can
be used to determine whether soil erosion or deposition has
occurred. This method is based upon the following assumptions: that 137Cs fallout input is relatively constant across the
study area; that 137Cs is strongly and rapidly adsorbed onto
soil particles, such that the redistribution of 137Cs occurs as
a result of the movement of soils; and that the deposited
sediments contain adsorbed 137Cs. At a location that has had
no erosion or deposition since the 1950s, where the 137Cs
is the result of direct fallout only, this inventory is described
as a reference inventory. Comparison of 137Cs inventories
from sampling locations with a local reference inventory can
be used to determine whether these sampling locations have
experienced soil erosion or deposition since fallout of 137Cs
began (e.g. Walling and Quine, 1991). A 137Cs inventory
less than the local reference inventory indicates erosion,
and a higher inventory represents deposition. However, the
assumptions required for this to hold true are often compromised. Large particles, or those sourced from bank and
gully erosion can have little or no adsorbed 137Cs, which when
deposited might not significantly alter the inventory at that
location (e.g. Wallbrink and Murray, 1993). In addition to
this, reference inventories measured in close proximity to one
another can vary on a small scale due to differences in soil
properties, vegetation cover, microtopography and bioturbation,
and on a large scale by systematic variation in precipitation
amounts and vegetation cover and soil type, as well as through
sampling variability and measurement precision (Owens and
Walling, 1996). Variation between reference inventories can be
taken into account through the collection of multiple reference
cores and description of a range of reference inventory values
for a region (cf. Owens and Walling, 1996), or through the
combination of multiple cores into a single measured sample
(e.g. Wallbrink et al., 1998).
The analysis of a sample for 137Cs concentration often takes
approximately one day or more, thus the calculation of invenCopyright © 2009 John Wiley & Sons, Ltd.
519
tories from whole-core samples (herein called ‘bulk inventories’)
enables analysis of material from a greater number of
locations than through the measurement of depth profiles,
enabling the investigation of spatial patterns of sedimentation
over a larger scale (Siggers et al., 1999). However, a combination
of both 137Cs inventory and depth profile distribution
information is usually required to provide a more complete
picture of overbank deposition, and both the inventory and
depth profile methods have been applied in this study.
Plutonium-239+240
As with 137Cs, plutonium isotopes are anthropogenically derived
and were introduced into the atmosphere following nuclear
weapons testing, binding effectively to soil particles upon
fallout. Plutonium isotopes have not been widely used as a tracer
for studying sediment redistribution, although plutonium has
some important advantages over 137Cs as a tracer of soil loss
and transport processes (Everett et al., 2008). The sensitivity
of 137Cs as an environmental tracer is significantly decreasing;
due to its half-life of approximately 30 years, levels of 137Cs
have decreased by a factor of three since the last atmospheric
nuclear weapons testing in 1962. Plutonium isotopes have
half-lives of thousands of years, thus their inventory (six times
greater than that of 137Cs) is not much less now than it was at
its peak. A study of the usefulness of plutonium isotopes and
137
Cs as sediment tracers in the Herbert River catchment,
northeastern Australia, found that concentrations of 239+240Pu
in samples from cultivated and uncultivated land-use areas and
from floodplain and channel-bed sites are strongly correlated,
in spite of significant variations in concentrations of the two
isotopes, rainfall, land use, erosion rates, sediment sources
and geology (Everett et al., 2008). This indicates that depthprofiles of 137Cs and 239+240Pu concentrations are likely to vary
in a similar way, recording sediment erosion and deposition
since the onset of fallout.
Different techniques are required to measure concentrations
of 137Cs and plutonium isotopes in soil samples (discussed
in Everett et al., 2008). The very high sensitivity of the AMS
technique used here to measure 239+240Pu concentrations plus
the absence of background (compared with a typical spectral
background of around 50 per cent in the measurement of 137Cs
by gamma spectrometry presented here) produces improved
statistical precision for plutonium measurements compared
with 137Cs. It is therefore likely that in a low-fallout environment,
this increased precision of Pu isotope concentrations might
result in much more useful data than 137Cs for the evaluation
of sediment erosion and deposition. This was found to be the
case by Leslie and Hancock (2008) who present a depth profile
of 137Cs and 239+240Pu concentrations in a lacustrine core from
about 38° S in Australia. In this core, the 239+240Pu profile showed
a distinct peak at 36–38 cm, whereas the 137Cs concentrations from 8 to 40 cm were within measurement uncertainty
(although the confidence interval represented by their uncertainties is not explained).
Sampling methodology
At each site, five sediment cores were collected to a depth of
1·5 m along a floodplain transect, from which the material
from 0 to 50 cm was analysed for 137Cs inventories (Figure 2).
One additional core was also collected from each site, from
which a depth profile of 137Cs concentration was determined
(Figure 2). Core location was primarily determined by
accessibility of truck-mounted coring equipment, although it
Earth Surf. Process. Landforms 34, 515– 529 (2009)
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EARTH SURFACE PROCESSES AND LANDFORMS
Table II. Elevation and location information for inventory and depth-profile cores. Elevation was not recorded at S4D due to equipment failure
Core
Elevation
(m a.s.l.)
S1C1
S1C2
S1C3
S1C4
S1C5
S1D
S2C1
S2C2
S2C3
S2C4
135·83
139·67
135·74
135·00
136·27
135·24
233·53
226·97
231·92
228·80
S2C5
S2D
S3C1
S3C2
S3C3
S3C4
S3C5
S3D
S4C1
S4C2
S4C3
S4C4
S4C5
S4D
236·84
226·60
285·75
285·23
285·12
284·71
284·51
285·54
4·25
7·98
8·06
5·13
6·65
–
Location description
On edge of slight ridge
Close to channel (approximately 20 m)
In depression with gilgai and cracking clays
Close to small channel (approximately 10 m)
On ridge between two depressions
Piles of trash around base of small trees
Approximately 5 m from channel
On ridge top between bench and abandoned channel
On highest point on island
Approximately 5 m from main channel.
Below slope up to floodplain
On ridge
Approximately 7 m from deep narrow channel
A few metres from edge of channel
In area of Brigalow tree regrowth
Close to a region of Brigalow tree regrowth
Edge of Coolabah swamp, sedge ground cover
Coolabah swamp, soft grass ground cover
Approximately 10 m from edge of channel
Swale
Ridge top
Ridge top
Swale
Bench?
Approximately 3 m from edge of water-filled swale
was ensured that the five bulk inventory cores were located
in a range of floodplain environments at each site (Table II).
The depth profile cores were collected from a location on the
floodplain deemed likely to experience enhanced deposition,
or to have experienced recent deposition; close to an abandoned
channel, within a swale, and in an area with substantial trash
deposits (Table II). Core elevations and surveyed transects
were recorded using a Real Time Kinematic GPS and theodolite.
Two 30 cm deep reference cores were collected from
suitable nearby locations at each study site. Because of the
possibility that some of these areas may have been cleared
during the 1960s and 1970s, one additional reference core
was collected from a National Park woodland (Figure 1) from
which a depth profile of 137Cs was recorded. A 137Cs inventory
was calculated for each of the depth profile cores by
summing incremental data. Surface areas for the cores ranged
from 17 cm2 to 28 cm2. All bulk inventory cores plus depth
profile cores S2D and S3D were collected in November
2004, and the other depth profile cores were collected in
November 2005.
Laboratory procedures
Cores were split lengthways, and their stratigraphy was logged.
For the analysis of bulk inventories, 0–30 cm of core material
from the reference cores, and 0–50 cm from the floodplain
cores were analysed. The depth profile cores were analysed
in increments of either 2 or 5 cm. For each core, the material
was combined and then split to provide a representative
sample. The <63 μm fraction was analysed for all floodplain
samples (bulk inventory and depth profile), as most 137Cs is
attached to this fraction. Both the <63 μm and <2 mm
fractions were analysed for the bulked reference inventory
samples (each from a different split of the sample), although
in two cases only the <2 mm fraction was analysed. The reference
Copyright © 2009 John Wiley & Sons, Ltd.
core depth profile increments were analysed using the <2 mm
fraction. The grain size fractions analysed for each bulk
inventory core are listed in Table III.
Samples were oven dried, gently disaggregated, sieved
through a 2 mm mesh and then split into manageable sample
sizes following the methods outlined in Gale and Hoare
(1991, pp. 13–17 and 86–87). The <63 μm fraction of samples
was isolated by wet-sieving, following soaking of the sample
in distilled water and sonication for at least 2 hours, stirring
every 30 minutes. Clay aggregates were observed upon sieving
some samples, and these were re-sonicated until all fine
material could be brushed through the 63 μm sieve. Once the
appropriate fraction had been separated and dried, samples
were ground and then pressed into cylindrical containers
of uniform size, to ensure a constant sample density and
geometry. The mass of the sample was recorded to enable the
influence of sample mass on count rate to be taken into
account when calculating 137Cs concentration.
Samples were analysed for 137Cs concentration using highpurity Germanium gamma-ray detectors at the University of
New South Wales @ADFA (UNSW@ADFA, Canberra) and
the Australian Nuclear Science and Technology Organisation
(ANSTO) between June 2006 and June 2007, and results
relate to this time period. The 137Cs activity of each sample
was determined using the 662 keV peak after subtraction of
the 214Bi peak interference, with a minimum count time of
79 200 s. The detectors were calibrated with IAEA reference
materials to enable calculation of 137Cs concentration (presented
in Bq kg–1). Uncertainties in concentration and inventory values
were calculated using a two standard deviation counting
error (95% confidence interval). Uncertainty associated with
the inventory values also includes a 5 per cent sampling error
which is an arbitrarily derived value used by Owens and Walling
(1996) that approximates the likely sampling error associated
with deformation of a 26 cm2 core tube of 5·4 per cent measured
by Foster et al. (1994). Precise 137Cs concentrations could not
Earth Surf. Process. Landforms 34, 515–529 (2009)
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CS TO INVESTIGATE SEMI-ARID FLOODPLAIN DEPOSITION IN AUSTRALIA
be determined for samples with very low count rates, and in
these instances ‘less than’ values are presented that are accurate
at the 95 per cent confidence interval.
Eight samples from core S4D were analysed for 239+240Pu
concentration, using the 14UD Pelletron Tandem Accelerator
Mass Spectrometry (AMS) system of the Department of
Nuclear Physics at the Australian National University (ANU).
The sample preparation and analysis methodologies are
relatively complex, and so are not described here. The
sample preparation methodology employed is identical to
that described by Everett et al. (2008), which followed the
methodology of Priest et al. (2000). Measurements of 239+240Pu
concentration using AMS were based on the methodology of
Fifield et al. (1996), and are identical to that described by
Everett et al. (2008). In essence, a subsample of 4 g was taken
from the sample analysed for 137Cs concentration. This was
spiked with a certified reference 242Pu standard provided
by the US National Institute of Standards and Technology.
Following sample preparation for AMS analysis, the
239,240,242
Pu isotopes were measured in order to determine
the relative count rates for each isotope. The 239+240Pu
concentration was then determined from the known mass of
the 242Pu spike added to the sample. Uncertainty is presented
at the 95 per cent confidence limit.
Table III.
Floodplain
cores
Reference
cores
a
b
Grain size proportions,
521
For the bulk cores, 137Cs inventories were calculated by
multiplying 137Cs concentration by the dry mass of the grain
size fraction analysed within the increment and dividing this
by the core surface area (presented in units of Bq m–2). For the
depth profile cores, the inventories of each increment were
summed.
Results
Floodplain and reference-site bulk inventories
The grain size composition, 137Cs concentration and 137Cs
inventory values for all bulk-inventory floodplain and reference cores analysed are presented in Table III, as well as the
time each sample was analysed for. None of the cores contained distinct facies boundaries, very few particles greater
than sand size occurred in these cores, and most contained a
high proportion of silt and clay (<63 μm; Table III).
The 137Cs concentrations in all of these samples were low,
with most below 3 Bq kg–1 at the 95 per cent confidence limit
(Table III). These low concentrations result in relatively high
uncertainties associated with both concentration and inventory
results (Table III and Figure 3). In the bulk reference cores for
137
Cs concentrations and inventories for floodplain and reference cores
Core
number
Per cent
<2 mm
Pper cent
<63 μm
Fraction
analysed
Count
Time (s)
Concentration
(Bq kg–1)
Error
(Bq kg–1)
Inventory
(Bq m2)
Error
(Bq m2)
S1C1
S1C2
S1C3
S1C4
S1C5
S2C1
S2C2
S2C3
S2C4
S2C5
S3C1
S3C2
S3C3
S3C4
S3C5
S4C1
S4C2
S4C3
S4C4
S4C5
S1RC1
99·96
99·95
99·93
99·99
99·97
100·00
100·00
100·00
99·97
100·00
99·98
99·81
99·91
99·14
96·38
99·98
100·00
100·00
100·00
100·00
97·62
77·58
90·96
93·70
66·29
93·12
60·46
78·52
46·45
65·49
50·59
86·06
90·72
93·19
97·58
90·01
98·31
90·67
81·99
79·46
63·75
39·71
84·70
S2RC1
99·84
87·94
0·718
0·924
0·309
1·251
0·869
1·514
1·587
1·649
2·226
1·789
<0·843
1·377
1·038
0·974
0·168
0·852
1·361
2·313
1·506
2·085
2·604
1·044
1·400
<0·911
2·300
2·295
0·600
0·934
435
524
190
596
487
610
669
521
1142
619
<373
688
526
470
68·1
370
593
1009
706
727
377
366
466
<356
703
790
341
505
664
556
706
462
476
428
550
476
99·90
288 000
158 400
79 200
70 041
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
79 200
146 880
172 800
64 800
82 800
318 816
82 800
0·527
0·844
1·063
1·104
1·216
1·070
1·051
1·273
0·960
1·023
S1RC2
S2RC2
99·84
–b
S3RC1
98·80
68·36
82 800
86 400
82 800
0·043
2·400
1·433
0·790
1·200
1·018
S3RC2
98·72
–b
S4RC1
99·99
69·14
S4RC2
100·00
95·50
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μm
<63 μma
<2 mm
<63 μma
<2 mm
<63 μma
<2 mm
–
<2 mm
<63 μma
<2 mm
–
<2 mm
<63 μm
<2 mm
<63 μma
<2 mm
82 800
82 800
82 800
144 288
82 800
2·538
1·834
1·609
1·200
1·010
0·882
1·268
1·083
0·800
0·896
1·034
0·977
1·015
1·016
0·945
0·994
0·999
1·002
1·048
1·833
0·662
1·000
Uppermost
inventory limit
(95% c.i.; Bq m2)
219
361
776
1030
854
1152
1193
1072
1145
949
1692
1095
373
1239
1048
984
482
798
1056
1495
1211
1129
661
616
822
356
922
1151
13
663
660
243
365
502
256
1027
1162
978
239
304
554
488
389
177
220
397
457
1367
416
524
951
945
551
522
514
414
429
463
486
505
402
284
250
356
These samples were analysed at ANSTO. All other samples were analysed at UNSW@ADFA.
The <63 μm fraction mass was not recorded for these samples, and analysis of the <63 μm fraction was not conducted.
Copyright © 2009 John Wiley & Sons, Ltd.
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EARTH SURFACE PROCESSES AND LANDFORMS
Figure 3. Caesium-137 inventories for all floodplain and reference cores. Horizontal lines represent the measurement uncertainty associated
with each value at the 95 per cent confidence limit.
which both the <2 mm and <63 μm fractions were analysed,
the 137Cs concentrations and inventory values are within error
of each other. This indicates: that the amount of 137Cs adsorbed
to grains larger than 63 μm is less than that adsorbed to the
smaller grains, as would be expected (cf. He and Walling,
1996); that the varying proportions of sand-sized material
within these soils did not substantially influence the uptake of
137
Cs fallout; and that any variations resulting from these
factors are within the counting uncertainty of 137Cs detection
in these low-concentration samples.
The bulked-core reference inventories occupy the range
13–978 Bq m–2 (Table III). The inventory value for one of the
reference cores was unusually low, and the remaining cores
had values ranging from 239 to 978 Bq m–2. Based on the
measurement uncertainties associated with each sample, the
reference inventory range was 0–1367 Bq m–2 at the 95 per
cent confidence interval (Table III and Figure 3). Following
the method proposed by Owens and Walling (1996), and
supported by Loughran et al. (2002), the range in reference
inventories can also be described as the mean ± 2 SEM (standard
error of the mean). These values, the range in measured
reference inventories and the median reference inventory
values for the <63 μm and <2 mm fractions at the catchment
scale (i.e. using all bulk reference inventories measured) are
presented in Table IV. This method indicates that for the <63 μm
fraction, only floodplain core inventory values greater than
Table IV. Descriptive statistics for reference inventories recorded in this study, and a summary of published
ern hemisphere locations, values decayed to 2007
Study
Location
Latitude
Mean
annual
rainfall
(mm yr–1)
This study, <63 μm
Eastern Australia
22–25° S
550–800
This study, <2 mm
Eastern Australia
22–25° S
550–800
Longmore et al. (1983a)
Elliott et al. (1996)
Everett et al.
(unpublished data)
Owens and Walling (1996)
Schuller et al. (2002)
Schuller et al. (2004)
Eastern Australia
Eastern Australia
Eastern Australia
28° 09′ S
19–27° S
17–18·5° S
Zimbabwe
Chile
Chile
17° 38′ S
38·5–41·4° S
50–54° S
a
Range in
measured
reference
inventories
(Bq m–2)
137
Cs inventories from various south-
x ± 2δ x
(Bq m–2)
Median
inventory
(Bq m–2)
Number
of samples
800
440–1200
750–1600
239–703
(62–1027)a
13–978
(0–1367)a
203·5–547·7
171·9–507·7
160–260
500·3 ± 143·8
(356·5–644·1)a
436·2 ± 263.5
(172·7–699·7)a
447·3 ± 76·3
311·2 ± 54·4
Mean = 220
494·4
464·0
n/a
9
14
3
900–1000
750–4000
248–780
38·1–300·6
349·6–4202·9
168·6–651·4
174·8 ± 56·8
1011·5 ± 342·8
354·2 ± 89·1
195·3
714·7
289·2
24
29
14
510
6
427
8
Values presented in parentheses include measurement uncertainty (at the 95% confidence interval).
Copyright © 2009 John Wiley & Sons, Ltd.
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644 Bq m–2 can be considered as significantly different from
the reference inventory range and thus represent sediment
deposition (cf. Owens and Walling, 1996; Loughran et al., 2002).
The bulked-core floodplain inventories range between
68 and 1142 Bq m–2, but allowing for measurement errors,
the 95 per cent confidence range for these cores is 0–1692
Bq m–2 (Table III and Figure 3). At each of the floodplain sites,
the range of floodplain inventories is within the range of
reference inventories from cores collected at that site. None
of the floodplain bulk inventories measured exceed the
reference inventory mean + 2 SEM value of 644 Bq m–2 at
their lowest range of uncertainty, therefore the inventories of
these floodplain cores cannot be distinguished statistically
from those collected at reference locations when measurement
uncertainties are included.
Reference core depth profile of caesium-137
In the reference core depth profile (RefD), 90 per cent of the
137
Cs is located in the top 8 cm of the soil profile. There is a
peak in concentration at 2– 4 cm, and substantially lower
137
Cs concentrations below 6 cm (Figure 4A). The submerged
peak indicates a down-core migration of 137Cs of approximately
2–4 cm. This reference core depth profile of 137Cs is very
similar to other published depth profiles from cores collected
at undisturbed locations in both the Southern and Northern
Hemispheres, in a range of climatic environments (e.g.
McCallan et al., 1980; Walling and Quine, 1991; Wallbrink
and Murray, 1993; Owens et al., 1996; Owens and Walling,
1996). The down-core migration of the peak of 2–4 cm is
523
similar to that described for a semi-arid environment in
Zimbabwe of 2–3 cm (Owens and Walling, 1996) and can
be used to infer the behaviour of fallout 137Cs within other
soil profiles in the region. The inventory for this core is
283 ± 38 Bq m–2 (Table V).
Floodplain core depth profiles of caesium-137
The depth profiles of 137Cs concentration from each of the four
floodplain sites vary (Figure 4B–E). Cores S1D, S2D and S3D
(Figure 4B–D) have no distinct peak in concentration at the
95 per cent confidence limit. Core S1D contains definite
137
Cs activity (at the 95% confidence limit) in the top 4 cm of
the profile. Below this, 137Cs activity is less than 3 Bq kg–1,
and it appears that there is negligible activity below 12 cm.
Core S2D contains definite 137Cs activity (at the 95% confidence
limit) in the 10–15 and 15–20 cm increments, below which
there is negligible activity. The uncertainty surrounding the
values recorded from core S3D due to their low concentration prevents useful interpretations from being made from
this core. The Site 4 core (S4D) displays a very different 137Cs
depth profile, with approximately constant low 137Cs concentrations from the surface to 48 cm into the profile, higher
concentrations of about 4.5 Bq kg–1 between 50 and 68 cm,
and a lower concentration at 70–72 cm, which is the lowest
increment presently analysed (Figure 4E).
Caesium-137 inventories for the depth-profile cores are
presented in Table V, alongside information about the
approximations used to enable estimation of the inventory
where depth profiles are incomplete (i.e. where precise 137Cs
Figure 4. Depth profiles of 137Cs concentration. Horizontal lines represent measurement precision at the 95 per cent confidence limit. Where a
line occurs with no bar, this represents a ‘less than’ value for those samples which could not be measured more precisely. The fractions analysed
were <2 mm in RefD (A) and <63 μm in the floodplain cores (B–E).
Copyright © 2009 John Wiley & Sons, Ltd.
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EARTH SURFACE PROCESSES AND LANDFORMS
Table V. Caesium-137 inventories calculated for the depth-profile cores. The inventory could not be calculated for S3D as sample masses were
not recorded; the estimate presented is an overestimate of the true inventory. The inventory presented for S4D is not complete, as the base of the
137
Cs profile has not been reached. This is therefore a minimum inventory value
Core
Grain size
fraction
analysed
S1D
<63 μm
72·4 ± 34·2
(38·2 – 106·6)
0–10 cm
Inventory of 6–8 cm used to
approximate that of 8–10 cma.
S2D
<63 μm
758·7 ± 318·0
(440·7 – 1076·7)
0–20 cm
Concentration of 5–10 cm used
to estimate inventory for 0–5 cma.
S3D
<63 μm
<50
0–25 cm
Sample masses were not recorded, and an
increment mass of approximately double that
of other core increments of similar volume
has been used to provide an overestimate of
maximum possible inventorya.
S4D
<63 μm
>1607·6 ± 853·6
(>751 – 2461·2)
0–72 cm
Used the inventory for overlying adjacent
increments to approximate that for
increments not analyseda.
RefD
<2 mm
283·4 ± 38·3
(245·1 – 321·7)
0–12 cm
b
a
b
Inventory (Bq m–2)
Depth of core
used to calculate
inventory
Notes
Samples analysed at ANSTO.
Samples analysed at UNSW@ADFA. Values presented in parentheses include measurement uncertainty (at the 95% confidence interval).
concentration of some increments could not be determined
due to very low count rates, and from core S4D below 20 cm
where only alternate increments were analysed). An inventory
could not be calculated for S3D as sample masses were not
recorded. However, extreme hypothetical sample masses of
approximately double the mass of similar-volume samples
from other cores was used to produce an overestimation of
the inventory. The ranges of these estimated inventories at the
95 per cent confidence limit are within the ranges of those
calculated for the bulk-inventory floodplain cores. The depthprofile inventories estimated for cores S1D and S3D are less
than the mean – 2 SEM, and that estimated for S4D exceeds
the mean + 2 SEM (for the <63 μm fraction m–2; Table IV). The
inventory of RefD is within the mean ± 2 SEM range of the
bulk reference inventories for the <2 mm grain size fraction.
Plutonium-239+240 results
Figure 5 shows the 239+240Pu concentrations analysed for eight
of the S4D increments, plotted with the 137Cs concentration
results to aid comparison. These data depict a similar trend to
the 137Cs data. Lowest concentrations were recorded in the
uppermost samples, a peak of 0·134–0·159 Bq kg–1 occurs in
the 58–60 and 62–64 cm increments, and lower concentrations
occur in the two increments below the peak.
Discussion
Reference inventories
The 137Cs reference inventory values recorded for this study
are similar to the few published reference inventory values
from locations within Queensland; and are comparable with
inventories measured in other Southern Hemisphere locations
(Table IV).
Variations in reference inventories can result from factors
such as the penetration of rain through the tree canopy, soil
bulk density, infiltration capacity and disturbance to the
soil (McCallan et al., 1980; Owens and Walling, 1996). Within
Copyright © 2009 John Wiley & Sons, Ltd.
Australia, Longmore et al. (1983a) recorded a range in reference
inventories of around 350 Bq m–2 from samples collected at
three sites located along an 8·2 km long transect. The range
in reference inventory values presented in this study, from sites
up to 280 km apart, is likely to result from such local variations,
and may also result from variations in the amount of fallout
received as a result of differences in the amount of rainfall
received at these sites. However, evidence from the published
literature summarised below indicates that substantial differences in fallout received at our field sites are unlikely.
Published 137Cs reference inventory data from Queensland
(Longmore et al., 1983a; Elliott et al., 1996), spanning latitudes
from 19º14′ to 28º13′ S and mean annual rainfalls of 440–
1200 mm per year, do not indicate a relationship with mean
annual rainfall (r2 < 0·1). Mean annual rainfall does not vary
substantially across the majority of the Fitzroy catchment
(Figure 1), and Queensland flood history reports describe
numerous heavy rainfall events occurring over all subcatchments of the Fitzroy River between 1950 and 1980
(Australian Bureau of Meteorology). It is also worth noting
that Longmore et al. (1983a) found that their field site in
southern Queensland (mean annual rainfall around 800 mm)
received similar amounts of direct 137Cs fallout to that estimated
for a site approximately 130 km to the northeast, with a mean
annual rainfall of around 1500 mm, and concluded that this
could indicate that the rainfall at their field site was sufficient
to rain-out all available atmospheric 137Cs. It is therefore possible
that, because of the low atmospheric concentrations of 137Cs
over northern Australia, total 137Cs fallout was similar throughout
the Fitzroy River catchment. Variations in reference inventory
between sites within the Fitzroy River catchment are therefore
most likely to result from variations in fallout reaching the
soil that might occur due to rainfall interception by vegetation
and physical soil characteristics, as described by Owens and
Walling (1996).
Floodplain sedimentation
At all four sites investigated, the floodplain bulk inventories are
statistically indistinguishable from the reference inventories,
Earth Surf. Process. Landforms 34, 515–529 (2009)
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Figure 5. Depth profile of 239+240Pu concentrations analysed for the S4D core, plotted with the
Uncertainties are presented at the 95 per cent confidence limit.
which implies that they cannot be considered to be the result
of net soil erosion or deposition using the inventory method
(cf. Owens and Walling 1996; Loughran et al., 2002). However,
because of the relatively large uncertainties associated with
these inventory values, it is likely that deposition or erosion
may have occurred in some cores, but only altered the inventory
value by an amount that is within the range of measurement
uncertainty. This would be particularly significant where
sediments with no, or limited, adsorbed 137Cs have been
deposited. As a result, the most useful information regarding
recent floodplain deposition at our study sites is contained
within the 137Cs depth profiles. The likely contribution of bank
and gully sediments to the fluvial sediment load (estimated to
be 29 per cent by Dougall et al., 2006), with no or limited
adsorbed 137Cs, provides further motivation for looking to
depth profiles for the most useful information.
The floodplain core depth profile from Site 1 (Figure 4B)
indicates that this site has not undergone substantial sediment
deposition over the past 40–50 years. The low concentrations
of 137Cs in this profile and the possible truncation of the profile
when compared with the reference profile (Figure 4A and B)
indicate that around 4 cm of material may have been eroded
from this site. It is known that overbank flow at this site has
occurred, because of the large piles of trash deposits in the
immediate locality, and it is likely that this site has been
inundated several times since the onset of 137Cs fallout, with
known inundation of the entire floodplain in 1991 and an
Copyright © 2009 John Wiley & Sons, Ltd.
137
525
Cs concentration results to aid comparison.
estimated six other events of similar magnitude occurring
since 1954. These results indicate that during these periods
of overbank flow, there was either very little sediment in
suspension, velocities were sufficient that the sediment
load was not deposited, deposited sediments have been
eroded, or that sediments containing 137Cs have been eroded
followed by the deposition of sediment with no or little
adsorbed 137Cs.
The depth profile from Site 2 shows 137Cs activity to at least
20 cm depth (Figure 4C). The occurrence of 137Cs concentration
at depth can indicate sediment deposition, but might also result
from physical disturbance of the profile (e.g. by animals, ploughing or soil desiccation cracking). Due to the uncertainties
associated with the measured 137Cs activity in this profile, it is
not possible to determine which of these is the most likely. If
substantial physical mixing has not taken place and the 15–
20 cm increment represents the base of 137Cs detection (thus
corresponding to 1954), this would provide an approximate
average sedimentation rate of 3.5 mm per year. The history of
floodplain inundation at Site 2 is unknown, and so it is not
possible to determine the number of events responsible for
this deposition. It is important to consider that in 1972 a
reservoir was constructed approximately 25 km upstream of
Site 2. Thus, estimates of overbank sedimentation for Site 2
are likely to have been influenced by sedimentation within
the reservoir and are therefore likely to be underestimates of
floodplain sedimentation under more natural conditions.
Earth Surf. Process. Landforms 34, 515– 529 (2009)
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EARTH SURFACE PROCESSES AND LANDFORMS
The profile of 137Cs concentration with depth at Site 4 shows
a likely peak in 137Cs concentration at between 50 and 68 cm
depth (Figure 4E). The highest concentration was recorded at
66–68 cm depth. However, at the 95 percent confidence limit
this concentration does not exceed that recorded in all other
samples within the profile. Assuming that the true peak in
137
Cs concentration corresponding to the peak in fallout occurs
at between 50–68 cm, this indicates deposition of between
46 and 66 cm since 1964 (50–68 cm minus 2–4 cm of downcore migration as indicated by the reference core RefD) and
an average sedimentation rate of 11·3–16·1 mm per year.
This interpretation is substantiated, and improved on, by the
239+240
Pu analysis conducted on some increments from this
core (Figure 5). The peak in 239+240Pu concentration occupies
a smaller thickness in the depth profile than the 137Cs peak.
A similar observation was made by Leslie and Hancock
(2008) from analysis of 137Cs and 239+240Pu concentrations
with depth in an Australian lacustrine core. The 239+240Pu peak
occurs at 58–64 cm depth. Assuming a similar down-core
migration of 239+240Pu to 137Cs, this results in an estimate of
54–62 cm deposition since 1964, and an average sedimentation
rate of 13·2–15·1 mm per year.
Figure 6 presents a comparison between the Site 4 depth
profile (S4D) with a depth profile of 137Cs concentration
measured from a core collected in Crescent Lagoon, 18 km
from the S4D core (Kuhnen, 2004). Caesium-137 concentrations in the Crescent Lagoon core peak at 52 cm, and decrease
rapidly to approximately zero at 60 cm. The similarities
between these 137Cs profiles indicate that they are the result
of sediment deposition and substantiate the interpretation
that the S4D peak results from peak fallout. However, the
possibility that the S4D and Crescent Lagoon profiles are
the result of factors other than sediment deposition must be
considered. The similarity between the two profiles indicates
that they are unlikely to be affected by physical disturbance
to the soil profile through mechanisms such as bioturbation,
and we are confident that these locations have not been
ploughed. It has been shown that the distribution of 137Cs can
be altered in some depositional environments through processes
such as diffusion of 137Cs from sediments to interstitial water
(e.g. Longmore et al., 1983b; Torgersen and Longmore, 1984)
and replacement of 137Cs by cations such as Na+ and K+ in
saline interstitial water (Foster et al., 2006), these processes
occurring most readily in highly saline or acidic environments. However, redistribution of 137Cs by diffusion usually
results in an elongated ‘tail’ into deeper sediments (e.g.
Krishnaswamy et al., 1971; Ritchie et al., 1973), and this is
not seen in the Crescent Lagoon core. These factors all
indicate that the depth profile of 137Cs concentration at Site 4
depicts the deposition of sediment since the onset of 137Cs
fallout and the 1964 fallout peak. The low 137Cs concentrations recorded above the peaks in 137Cs concentration in the
S4D and Crescent Lagoon cores are likely to result from
mixing between eroded surface material with higher 137Cs
concentration and low/zero concentration material sourced
from gully/bank erosion.
In S4D, the measurement of 137Cs in the lowest-analysed
sample at 70–72 cm, indicates that at least 66 cm of sediment
has been deposited at this site since 1954 (70–72 cm minus
2–4 cm of down-core migration), which provides an average
Figure 6. Depth profile of 137Cs concentration at Site 4 (Long Island) plotted alongside that for a core at Crescent Lagoon, 18 km south of Long
Island (data presented in Kuhnen, 2006; uncertainties provided by Brendan Brooke, Geoscience Australia, personal communication). Horizontal
lines on both data sets represent uncertainties at the 95 per cent confidence interval.
Copyright © 2009 John Wiley & Sons, Ltd.
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CS TO INVESTIGATE SEMI-ARID FLOODPLAIN DEPOSITION IN AUSTRALIA
deposition rate of 12·9 mm per year. At site S4D, inundation
of low-lying areas occurred at least 22–37 times between
1954 and 2005 (Table I), and it is known that sustained and
extensive inundation occurred throughout February 1954
and January 1991. Given the location of this core close to
the typical water level in an abandoned channel that is
connected to the active channel (see Figure 2D), it is expected
that this location acts as a slackwater and that the deposition
recorded in the radioisotope profiles has occurred during
many events. For comparison with Site 4, the average rate of
deposition observed from the core in Crescent Lagoon, assuming
2–4 cm of down-core migration of 137Cs, is 12·3 mm per year
since 1964 and 11·4 mm per year since 1954.
The variability observed between the four floodplain depth
profiles measured ranges from erosion to greater than 66 cm
of deposition in the past 40–50 years. Estimates of average
annual sediment deposition include around 3.5 and 13–15 mm
per year. These floodplain deposition rates are similar to
those typically observed in a range of temperate and tropical
environments of 0–10 mm per year (see data summarised
in Rumsby, 2000 and Terry et al., 2006). Higher floodplain
sedimentation rates of 20–70 mm per year, exceeding those
observed here, are not unusual (Rumsby, 2000; Terry et al.,
2006). Pre- and post-European settlement sedimentation rates
of 3 and 9.5 mm per year, respectively, recorded in an oxbow
lake in semi-arid southern Australia (Leahy et al., 2005) are
also comparable to those observed in this study.
The data presented in this study indicate that where 137Cs
concentrations are low and thus uncertainties are relatively
high, the inventory method used in isolation from depth profile
analysis is not likely to be useful. Comparison of the inventories
calculated from the floodplain depth profiles (Table V) with
the reference inventory mean ± 2 SEM calculated from bulked
core samples of the same grain size fraction (Table IV) substantiates our interpretations of erosion for core S1D and
deposition for core S4D. In these instances, the comparison
of floodplain depth-profile inventories with bulk reference
inventories provides increased confidence in the depthprofile interpretation. The inventory for S2D falls within the
range of the mean ± 2 SEM reference inventory, and the
inventory at S3D is likely to be below the mean – 2 SEM
range. In this low-fallout environment, similarly low inventories
could occur in a depositional setting due to the deposition of
sediment with no adsorbed 137Cs, especially if the 137Cs peak
is either below the depth of the profile analysed or has been
eroded. Therefore, in these instances, comparison of the depthprofile inventories in isolation from the depth-profile data is
not useful. If smaller increments had been analysed for these
cores, it is possible that if any higher-concentration sediments
were contained within the profile, these would have been
less diluted by low/zero concentration sediments and a more
useful profile and inventory may have been recorded. In this lowfallout environment, detailed depth-profile data are required
for an unambiguous interpretation of erosion/deposition history
to be made.
Because 137Cs inventory values in excess of the reference
inventory values could not be calculated for the bulked
floodplain cores, it is not possible in this case to estimate
sedimentation rates for the bulk inventory cores using the
depth profile data, using the approach used by others (e.g.
Owens et al., 1999). Given the large size of the floodplains
investigated in this study, the low 137Cs concentrations with
associated high uncertainties, and the likely deposition of low
or zero 137Cs sediments at downstream floodplain locations,
the appropriateness of such a method would need to be
investigated thoroughly before its application in this environment. A greater spatial resolution of detailed depth-profile
Copyright © 2009 John Wiley & Sons, Ltd.
527
analysis would be required to investigate the spatial characteristics of floodplain deposition in more detail, limiting the
applicability of the use of these radionuclide techniques to
investigate floodplain sediment deposition on both a floodplain
scale and a catchment scale in this region. However, the
combination of inventory and depth-profile data in this study
has proved useful in providing preliminary information on
rates of overbank deposition.
Conclusions
Caesium-137 analysis using bulk-inventory and depth-profile
techniques was conducted on reference and floodplain cores
from sites within the Fitzroy Basin, a moderately large (approximately 143 000 km2) semi-arid catchment in northeastern
Australia. The areal density of 137Cs at reference locations
(latitudes between 22 and 25º S) ranged from 13 to 978 Bq m–2
(0–1367 Bq m–2 at the 95 per cent confidence interval), with
a mean ± 2 SEM value of 436 ± 264 Bq m–2 for the <2 mm
fraction (analysed between June 2006 and June 2007).
These are similar to published data from other Southern
Hemisphere locations (after adjusting for radioactive decay).
This range in reference inventories is likely to result from
local variations in rainfall interception by vegetation and
soil characteristics rather than spatial variations in fallout
amounts, since published 137Cs inventories from locations
within the region do not vary with mean annual rainfall.
Floodplain inventories range from 68 to 1142 Bq m–2 (0–1692
Bq m–2 at two standard deviations uncertainty). At all sites,
137
Cs concentrations are low, and as a result the uncertainties
of concentration and inventory data are relatively large, resulting
in floodplain and reference core inventories that occupy the
same range of values and are not sufficiently distinct to enable
the calculation of erosion or deposition. The similarity between
floodplain and reference inventories does not necessarily
indicate a lack of sedimentation, as the deposition of sediment
with low 137Cs concentrations, due to low fallout and also
probably the influence of gully and bank eroded sediments,
may not increase the inventory such that it is significantly
greater than the reference inventory range. The inventory
method used in isolation from depth-profile analysis is not
useful in this low-fallout environment.
The depth profile of 137Cs concentration at a reference
location contained 90 per cent of the 137Cs in the top 8 cm,
with a similar profile to those described from other reference
locations in both hemispheres and from a range of climatic
environments. Floodplain deposition histories over the past
50 years, interpreted from the measured depth profiles of
137
Cs concentration, range from erosion interpreted for one
core to greater than 66 cm of deposition in a downstream
slackwater-swale location. At this depositional site, average
annual deposition rates (1964–2005) are around 13–15 mm
per year. Analysis of 239+240Pu concentrations from key
increments in this core substantiated the interpretations made
from the 137Cs data, and depicted a peak in concentration that
was more tightly constrained than the 137Cs peak.
Inventory values calculated from the depth-profile data fall
within the range recorded at reference locations from bulked
cores at the 95 per cent confidence limit. However, comparison
of depth-profile inventories with the mean ± 2 SEM reference
inventory range (cf. Owens and Walling, 1996) indicates
an erosive profile for the core interpreted to be erosional, and
deposition for the core in which greater than 66 cm
deposition is interpreted to have taken place. Depth-profile
analysis was inconclusive for two cores, and comparison of
their inventories with the reference inventory mean ± 2 SEM
Earth Surf. Process. Landforms 34, 515– 529 (2009)
DOI: 10.1002/esp
528
EARTH SURFACE PROCESSES AND LANDFORMS
was unable to provide useful information, due to the
uncertainty associated with these low 137Cs concentrations.
Thus, comparison of bulk reference inventory data with
floodplain inventories calculated from depth-profile data
might provide information with which an interpretation based
on the depth profile can be substantiated, but are unlikely to
assist otherwise in the interpretation of these profiles.
In summary, the use of 137Cs inventories from bulked core
samples to determine recent floodplain erosion and deposition
rates is of limited use in environments where the 137Cs fallout
was low, a substantial proportion of sediments may be sourced
from bank and gully erosion, and the rate of floodplain
deposition is not high. Depth profiles of 137Cs concentration
can provide a useful method for investigating recent floodplain
deposition in such an environment. When doing so, it is
important to ensure that the measurement uncertainty is
taken into account when describing variations in concentration
with depth, for example to ensure that the location of the
137
Cs peak concentration can be meaningfully ascribed to the
1964 fallout peak. In a dryland river system with wide and/or
anabranching floodplains and infrequent overbank flow events,
a high spatial resolution of depth profiles would be necessary
to investigate patterns of recent floodplain sedimentation,
however the time-consuming nature of such analysis is likely
to prohibit the detailed investigation of floodplain variability
on a catchment scale.
Acknowledgements—Funding for this research was provided by an
Australian Research Council grant (DP0449886), and an Australian
Institute of Nuclear Science and Engineering Award (AINGRA06003).
Jennifer Harrison and Atun Zawadzki (ANSTO) are gratefully thanked
for their contributions to the design and implementation of this study.
Useful discussions were held with Will Blake, Andrew Hughes, Sarah
Everett, David Purvis-Smith and Rachel Nanson. Jennifer Harrison,
Chris Leslie and Danny Hunt are thanked for generously providing advice
and standard samples during the set-up and testing of UNSW@ADFA
laboratory equipment. Thanks are also due to Bruce Forster, Damian
Kelleher, Andrew Hughes, Simon Mockler, David Purvis-Smith, Chris
Thompson, and Eugene Wallensky for field and laboratory assistance,
and to Julie Kesby for assistance with gathering references and
compilation of the reference list. Sarah Everett conducted laboratory
analysis of 239+240Pu concentrations. We thank the many property
owners and managers who allowed access to their properties,
particularly Mr and Mrs Shannon of Saltbush Park, Mr and Mrs Lloyd
of Beeblee, and the Fitzroy Shire Council (Long Island Environmental
Reserve). This paper was improved as a result of reviews by Robert
Loughran and an anonymous reviewer, and editorial input from
Professor Kirkby.
References
Amos KJ, Croke J, Hughes A, Chapman J, Takken I, Lymburner L.
2008. A catchment-scale assessment of anabranching in the
140 000 km2 Fitzroy River, north-eastern Australia. Earth Surface
Processes and Landforms 33: 1222–1241. DOI: 10.1002/esp.1609
Belperio AP. 1983. Terrigenous sedimentation in the central Great
Barrier-Reef Lagoon: a model from the Burdekin region. BMR
Journal of Australian Geology and Geophysics 8(3): 179–190.
Calvert M, Simpson J, Adsett K. 2000. Land Use Mapping of the
Fitzroy catchment. Fitzroy Catchment Implementation Project Final
Report July 2000. Department of Natural Resources, Coorparoo
DC; 57 pp.
Cisternas M, Araneda A, Martinez P, Perez S. 2001. Effects of
historical land use on sediment yield from a lacustrine watershed
in central Chile. Earth Surface Processes and Landforms 26: 63–76.
Dougall C, Packett R, Carroll C, Sherman BS, Read A, Chen Y, Brodie
J. 2006. Sediment and nutrient modelling in the Fitzroy NRM
region. In The Use of SedNet and ANNEX Models to Guide GBR
Copyright © 2009 John Wiley & Sons, Ltd.
Catchment Sediment and Nutrient Target Setting, Vol. 5, Cogle AL,
Carroll C, Sherman BS (eds). Department of Natural Resource,
Mines and Water: Brisbane; 5.1–5.27.
Douglas GB, Ford PW, Palmer M, Noble RM, Packett R. 2006. Fitzroy
River, Queensland, Australia. II. Identification of sources of estuary
bottom sediments. Environmental Chemistry 3(5): 377–385.
Elliott GL, Loughran RJ, Shields P, Powell B. 1996. A National
Reconnaissance Survey of Soil Erosion: Australia, Queensland. A
Report Prepared for the Australian National Landcare Program, Project
No. 1989-90: No. 8. University of Newcastle: Newcastle; 40 pp.
Everett SE, Tims SG, Hancock GJ, Bartley R, Fifield LK. 2008.
Comparison of Pu and 137Cs as tracers of soil and sediment
transport in a terrestrial environment. Journal of Environmental
Radioactivity 99: 383–393.
Fifield LK, Cresswell RG, di Tada ML et al. 1996. Accelerator mass
spectrometry of plutonium isotopes. Nuclear Instruments and
Methods in Physics Research B 117: 296 –303.
Foster IDL, Dalgleish H, Dearing JA, Jones ED. 1994. Quantifying
soil erosion and sediment transport in drainage basins; some
observations on the use of 137Cs. In Variability in Stream Erosion
and Sediment Transport, Olive LJ, Loughran RJ, Kesby JA (eds).
IAHS Publication 224, IAHS Press: Wallingford; 55–64.
Foster IDL, Mighall TM, Proffitt H, Walling DE, Owens PN. 2006.
Post-depositional 137Cs mobility in the sediments of three shallow
coastal lagoons, SW England. Journal of Paleolimnology 35(4): 881–
895.
Franz J, Piorewicz J. 2003. Evaluation of sediment transport in the
lower part of the Fitzroy River, Queensland. Australian Journal of
Water Resources 6(2): 131–150.
Fryirs K, Brierley GJ. 2001. Variability in sediment delivery and
storage along river courses in Bega catchment, NSW, Australia:
implications for geomorphic river recovery. Geomorphology
38(3–4): 237–265.
Furnas M. 2003. Catchments and Corals: Terrestrial Runoff to the
Great Barrier Reef. Australian Institute of Marine Science:
Townsville, Queensland; 334 pp.
Gale SJ, Hoare PG. 1991. Quaternary Sediments: Petrographic
Methods for the Study of Unlithified Rocks. Belhaven Press,
London; 323 pp.
He Q, Walling DE. 1996. Interpreting particle size effects in the
adsorption of 137Cs and unsupported 210Pb by mineral soils and
sediments. Journal of Environmental Radioactivity 30(2): 117–137.
Jones MR. 2006. Cenozoic landscape evolution in central
Queensland. Australian Journal of Earth Sciences 53(3): 433–444.
Joo M, Yu B, Fentie B, Caroll C. 2005. Estimation of long-term
sediment loads in the Fitzroy catchment, Queensland, Australia.
In MODSIM 2005 International Congress on Modelling and
Simulation, Zerger A, Argent RM (eds). Modelling and Simulation
Society of Australia and New Zealand, December; 1161–1167.
Krishnaswamy S, Lal D, Martin JM, Meybeck M. 1971.
Geochronology of lake sediments. Earth and Planetary Science
Letters 11: 407– 414.
Kuhnen M. 2004. Constraining the source areas and nutrient
transport of sediments entering the Fitzroy Estuary since European
arrival. Unpublished Honours Thesis, The Australian National
University: Canberra; 96 pp.
Leahy PJ, Tibby J, Kershaw AP, Heijnis H, Kershaw JS. 2005. The
impact of European settlement on Bolin Billabong, a Yarra River
floodplain lake, Melbourne, Australia. River Research and
Applications 21(2–3): 131–149.
Leslie C, Hancock GJ. 2008. Estimating the date corresponding to the
horizon of the first detection of 137Cs and 239+240Pu in sediment
cores. Journal of Environmental Radioactivity 99(3): 483– 490.
Longmore ME, Oleary BM, Rose CW, Chandica AL. 1983a. Mapping
soil-erosion and accumulation with the fallout isotope caesium137. Australian Journal of Soil Research 21(4): 373–385.
Longmore ME, Oleary BM, Rose CW. 1983b. Cesium-137 profiles in
the sediments of a partial-meromictic lake on Great Sandy Island
(Fraser Island), Queensland, Australia. Hydrobiologia 103: 21–27.
Loughran RJ, Pennock DJ, Walling DE. 2002. Spatial distribution of
caesium-137. In Handbook for the Assessment of Soil Erosion and
Sedimentation using Environmental Radionuclides, Zapata F (ed.).
Kluwer: Dordrecht; 97–109.
Earth Surf. Process. Landforms 34, 515–529 (2009)
DOI: 10.1002/esp
USING
137
CS TO INVESTIGATE SEMI-ARID FLOODPLAIN DEPOSITION IN AUSTRALIA
McCallan ME, Oleary BM, Rose CW. 1980. Redistribution of cesium137 by erosion and deposition on an Australian soil. Australian
Journal of Soil Research 18(2): 119–128.
McKergow LA, Prosser IP, Hughes AO, Brodie J. 2005. Sources of
sediment to the Great Barrier Reef World Heritage Area. Marine
Pollution Bulletin 51(1–4): 200–211.
Moss AJ, Rayment GE, Reilly N, Best EK. 1992. A Preliminary
Assessment of Sediment and Nutrient Exports from Queensland
Coastal Catchments. Queensland Department of Environment and
Heritage: Brisbane; 27 pp.
Nanson GC, Croke JC. 1992. A genetic classification of floodplains.
Geomorphology 4: 459– 486.
Neil DT, Yu B. 1996. Fluvial sediment yield to the Great Barrier Reef
Lagoon: Spatial patterns and the effect of land use. In Downstream
Effects of Land Use, Hunter HM, Eyles AG, Rayment GE (eds).
Department of Natural Resources: Brisbane; 281–286.
Owens PN, Walling DE. 1996. Spatial variability of caesium-137
inventories at reference sites: An example from two contrasting
sites in England and Zimbabwe. Applied Radiation and Isotopes
47(7): 699–707.
Owens PN, Walling DE. 2002. Changes in sediment sources and
floodplain deposition rates in the catchment of the River Tweed,
Scotland, over the last 100 years: The impact of climate and land
use change. Earth Surface Processes and Landforms 27(4): 403–
423.
Owens PN, Walling DE, He Q. 1996. The behaviour of bomb-derived
caesium-137 fallout in catchment soils. Journal of Environmental
Radioactivity 32: 169–191.
Owens PN, Walling DE, Leeks GJL. 1999. Deposition and storage of
fine-grained sediment within the main channel system of the River
Tweed, Scotland. Earth Surface Processes and Landforms 24(12):
1061–1076.
Priest ND, Poch Merlo M, Leon Vintro L, Fifield LK. 2000.
Accelerator mass spectrometry for plutonium isotopes: methods
and procedures. http://www.mdx.ac.uk/research/centres/ssciences/
risk /docs/nick19.pdf.
Prosser IP, Rutherford ID, Olley JM, Young WJ, Wallbrink PJ, Moran
CJ. 2001. Large-scale patterns of erosion and sediment transport
in river networks, with examples from Australia. Marine and
Freshwater Research 52(1): 81–99.
Ritchie JC, McHenry JR, Gill AC. 1973. Dating recent reservoir
sediments. Limnology and Oceanography 18: 479– 489.
Rumsby B. 2000. Vertical accretion rates in fluvial systems: a
comparison of volumetric and depth-based estimates. Earth
Surface Processes and Landforms 25(6): 617– 631.
Schuller P, Voigt G, Handl J, Ellies A, Oliva L. 2002. Global weapons’
Copyright © 2009 John Wiley & Sons, Ltd.
529
fallout 137Cs in soils and transfer to vegetation in south-central
Chile. Journal of Environmental Radioactivity 62(2): 181–193.
Schuller P, Bunzl K, Voigt G, Ellies A, Castillo A. 2004. Global fallout
Cs-137 accumulation and vertical migration in selected soils from
South Patagonia. Journal of Environmental Radioactivity 71(1): 43–
60.
Siggers GB, Bates PD, Anderson MG, Walling DE, He Q. 1999. A
preliminary investigation of the integration of modelled floodplain
hydraulics with estimates of overbank floodplain sedimentation
derived from Pb-210 and Cs-137 measurements. Earth Surface
Processes and Landforms 24(3): 211–231.
Terry JP, Kostaschuk RA, Garimella S. 2006. Sediment deposition
rate in the Falefa River basin, Upolu Island, Samoa. Journal of
Environmental Radioactivity 86(1): 45–63.
Tooth S. 2000. Process, form and change in dryland rivers: a review
of recent research. Earth-Science Reviews 51(1–4): 67–107.
Tooth S, Nanson GC. 1999. Anabranching rivers on the northern
plains of arid central Australia. Geomorphology 29(3–4): 211–
233.
Torgersen T, Longmore ME. 1984. Cs-137 diffusion in the highly
organic sediment of Hidden Lake, Fraser Island, Queensland.
Australian Journal of Marine and Freshwater Research 35(5): 537–
548.
UNEP (United Nations Environment Programme). 1997. World Atlas
of Desertification (Second Edition), Middleton N, Thomas D (eds).
Edward Arnold: London; 182 pp.
UNSCEAR (United Nations Scientific Committee on the Effects of
Atomic Radiation). 1982. Ionizing Radiation: Sources and Biological
Effects. UNSCEAR 1982 Report to the General Assembly, with
Annexes, United Nations: New York.
Wallbrink PJ, Murray AS. 1993. Use of fallout radionuclides as
indicators of erosion processes. Hydrological Processes 7(3): 297–
304.
Wallbrink PJ, Murray AS, Olley JM, Olive LJ. 1998. Determining sources
and transit times of suspended sediment in the Murrumbidgee
River, New South Wales, Australia, using fallout Cs-137 and Pb-210.
Water Resources Research 34(4): 879–887.
Walling DE, Quine TA. 1991. Use of 137Cs measurements to investigate
soil-erosion on arable fields in the UK – potential applications and
limitations. Journal of Soil Science 42(1): 147–165.
Walling DE, Owens PN, Carter J et al. 2003. Storage of sedimentassociated nutrients and contaminants in river channel and
floodplain systems. Applied Geochemistry 18(2): 195–220.
Zhang XB, Walling DE. 2005. Characterizing land surface erosion
from cesium-137 profiles in lake and reservoir sediments. Journal
of Environmental Quality 34(2): 514–523.
Earth Surf. Process. Landforms 34, 515– 529 (2009)
DOI: 10.1002/esp