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Using Sediment Tracers to Map Sediment Transport Pathways
A Primer (1st Edition)
Published by:
Ordering Information 00 44 141 552 3903 or www.partrac.com
Partrac Ltd
48 St Andrews Square
Glasgow
G1 5PP
+44 (0) 141 552 3903
www.partrac.com
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Photographs K Black/P Wilson/I Guymer/SOI St Andrews/Bairds
Design C Doherty
Printing Exactaprint
Author Kevin Black PhD
AUTHOR BIOGRAPHICAL SKETCH
Kevin Black is a geological oceanographer with over 8 years industry and 15 years of research experience. He is one of the UK’s leading experts in coastal
and shelf sea sedimentary processes, with extensive field experience and over 40 peer reviewed publications. Kevin’s field ex perience spans Chief
Scientist roles on ocean research cruises (including leadership and co -ordination of the DTI 2005 Strategic Environmental Assessment #5), to collection of
metocean, sediment transport (scour) and geotechnical data for coastal and offshore projects within the dred ging, marine energy and offshore wind sectors.
He has provided advice and guidance in the sediment management arena to a range of clients in relation to coastal infrastruct ure development projects and
associated EIAs. He led a high level review of the impacts of offshore renewable structures on coastal processes for NERC/Defra, and has been involved
in coastal processes impact assessment and ES delivery for four major OWF developments. Kevin was the chief editor of the Geo logical Society
Publication 139 (Sedimentary Processes in the Intertidal Zone), and Kevin is presently the co-ordinator on behalf of Defra of the Contaminated Dredge
Material Framework Development initiative (2007-2011).
Kevin has led the development of the unique ‘dual signature’ tracer technology within Partrac Ltd over the last 8 years. This development started with a
review paper to explore the current (1997) state of the art technologically, and to overview what types of studies had previously been undertaken. The
introduction of the dual signature attribute has substantially improved the tracer recovery rates, and simplified the analyti cal stage. This has resulted in a
widespread delivery of particle tracking services on a commercial basis. Kevin can be contacted at [email protected].
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PREFACE
The movement of energy and materials within
the geosphere is a fundamental attribute of
the Earth system. Processes of atmospheric
and hydrologic weathering erode the surface
rocks and soils, and overland, fluvial and
marine currents act to transport and
redistribute particles. Anthropogenically
produced particles, such as soot, urban
contaminants, mine tailings and radionuclides
are also subject to the same transport and re -
distribution processes. An understanding of
the mechanisms and processes of re-
distribution is necessary to understanding
both the natural geological consequences of
erosion, and the transport and fate of
anthropogenically produced particles
(contaminants).
‘Particle tracking’, or as it is sometimes
referred to in the geological sciences
‘sediment tracing’ or ‘sediment tracking’,
offers a unique methodology with which to
track the movement through space and time of
environmental particulates. Utilising this
methodology, information can be garnered
into source – sink relationships, the nature
and location of the transport pathway[s] and
the rate of transport. It is a relatively
straightforward, practical methodology which
involves the introduction of particulate
tracers into the environment (water body,
sewer, beach etc.) labelled with one or more
signatures in order that they may be
unequivocally identified following release
(McComb and Black, 2005; Forsyth, 2000)
Sampling at strategic locations and timings is
used to collect tracer which is then returned
to the laboratory for analysis. Measurable and
uniquely identifiable signatures have in the
past included the use of radioactive tracers
(Courtois and Monaco 1969; Heathershaw and
Carr 1978) and Rare Earth elements (Spencer,
in press), fluorescent coated sands (Vila-
Conjeco et al., 2004), fluorescent silts (Sarma
& Iya, 1960; van Leussen, undated; Draaijer
et al. (1984); Louisse et al. (1986)). However,
radioactive tracers are now considered
unacceptable on economic and environmental
grounds. Synthetic, polymer-based have also
been developed (e.g. Black and McComb,
2005). Black et al. (2007) provide a
comprehensive overview of the historic
evolution of these differing approaches to the
present day. Regardless of the tracer
deployed, the approach to any soil or
sediment project is largely the same.
This primer describes the essential elements
of the sediment tracking methodology. It is
intended to give the reader a grounding in
the fundamental concepts behind the
tracking technique, as well providing
information on the steps involved in tracking
studies and some hints on how to conduct
studies on a practical level. This is how we
do it at Partrac; inevitably other people may
do studies differently (any differences are
largely in the type of tracer used and
consequently also the method used to
measure the tracer), but necessarily the steps
involved in undertaking a tracking study are
broadly similar.
Particle tracking: a simple concept in which
uniquely labelled sediment analogues are
introduced into the environment, and strategic
sampling is undertaken to map the transport
pathway and/or ultimate fate. Schematic courtesy
of Baird.
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TECHNOLOGY DESCRIPTION
Dual Signature Tracers
Partrac use proprietary tracers called ‘dual
signature’ tracers’; this means that each
particle (grain) of tracer has two signatures
which are used to identify the particle
unequivocally following introduction into the
environment. The use of two signatures is an
advancement and improvement on previously
used (mono-signature) tracers. The two
signatures are fluorescent colour and
paramagnetic character. Two types of dual
signature tracer are available: coated
particles, and entirely artificial particles.
Coated particles possess a fixed grain density
of ~ 2500 – 2600 kgm-3
whereas that for
artificial particles can be adjusted through the
range 1010 to 3750 kgm-3
. Coated particle
grain sizes range from ~20 m to 5 mm;
artificial particles are commonly used to
mimic low settling velocity particulates, such
as biological larvae, and for engineering scale
model studies. Whilst compositional data for
each tracer type is commercially confidential,
coated particles (used most frequently in
tracking studies) are made from natural
materials plus a geochemically inert
fluorescent pigment.
Four spectrally distinct fluorescent colours
are available with which to label tracer. These
are commercially available fluorescent
pigments, which themselves comprise
polymer nanospheres embedded with a water
insoluble dye. Each pigment is characterized
by specific excitation and emission
wavelengths, which facilitates a targeted
sample analysis procedure, but all are
consistently reactive upon exposure to black
light. Use of multiple colours means that the
technology can be used to label multiple
sources in the same general area, or to
perform consecutive studies in the same area
under differing hydrodynamic conditions (e.g.
high discharge, low discharge).
Every tracer particle is also para-magnetic.
Para-magnetic minerals are not magnetically
attracted to one another (‘magnetic
flocculation’) but the para-magnetism gives
each particle a magnetic attribute which
means that particles will adhere to any
permanent or electro-magnet if they come in
close proximity. This facilities a simple
separation of tracer within environmental
(water, sediment, soil) samples, a process
which can also be exploited in situ (e.g.
through use of submerged magnets in a water
course; e.g. Guymer, et al., 2010). The
integration of tiny magnetic inclusions onto
the kernel particle during tracer manufacture
is a substantial innovation over mono-
signature, fluorescent-only tracers, for which
there was no effective means of tracer
separation within samples prior to analysis.
This profoundly limited tracer enumeration.
The ‘degree of para-magnetism’ of a granular
material i.e. how magnetic grains are in
comparison to quartz-rich beach sand, can be
determined quantitatively through use of a
magnetic susceptibility sensor. This
essentially measures the disruption to an
applied low frequency, low intensity
alternating magnetic field; ferrous materials
naturally possess a greater propensity to
disrupt a magnetic field in comparison to all
naturally occurring non-magnetic minerals
and hence they can be detected using this
technique. Typically manufactured tracer is
~400-500 times ‘more magnetic’ than quartz -
rich beach sand. The para-magnetic attribute
of tracer can also be exploited in situ through
use of a field-portable, hand-held magnetic
‘Dual signature’ tracers present a
substantial innovation over those
previously used. The magnetic attribute
permits a far simpler and higher recovery
rate of tracer in studies.
Photomicrograph of coated sand fluorescent-
magnetic tracer. The image width is ~ 1 mm.
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susceptibility sensor which can be used in a
semi-quantitative fashion to map tracer
concentration on soil or sediment surfaces
(van der Post, 1995; Black et al., 2007).
CONDUCTING A PARTICLE TRACKING
STUDY
The process of setting up and conducting a
particle tracking study involves a specific set
of steps regardless of the application. These
are: 1) conducting a background survey; 2)
designing a tracer[s] and similarity testing; 3)
tracer introduction; 4) sampling; and 5)
enumeration. A brief discussion of these is
presented here.
1) Background Survey:- prior to any study an
initial native particle properties survey is
always required for two purposes (1) to
determine the particle characteristics (size,
density, settling velocity) that will be
matched during manufacture of magnetic
fluorescent particles, and (2) to determine
the abundance and characteristics (e.g.
size) of any naturally occurring magnetic
and fluorescent particles. Information from
(1) is used to design the tracer whereas
information from (2) is essential ancillary
information which is used during tracer
enumeration.
2) Tracer Design and Similarity Testing:- the
size and density (and sometimes settling
velocity) data collected during the
background survey are used to create a
specification for the tracer. A final
specification will include: size range ()
and d50; density (s), color; para-magnetic
attribute; and quantity (kg). The
manufactured tracer is subject to a
similarity analysis (using the same testing
procedures as the background survey).
Similarity testing or, as it is also termed,
‘hydraulic matching’ is the process in
which the physical hydraulic-
sedimentological attributes of the
manufactured racer (size, density, settling
velocity etc.) are compared quantitatively
to those of the native particles. Black et
al., (2007) discuss similitude tolerances
since manufacturing constraints dictate
that tracer can never be the exact copy of
the native sediments. They find, for
example, similarity of direction for
kurtosis and skewness between native
sediments and tracer acceptable, and a
6% tolerance on tracer density.
Stages in any sediment tracking
study.
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(a)
(b)
3) Tracer Introduction:- introduction of the
tracer into the environment varies
according to application. For suspended
sediment transport studies, commonly
tracer is flushed down a tube so that it is
introduced @ 0.5 m beneath the water
surface in the form of a plume. Pre-made
deep frozen (-70C) tracer blocks have
been used to study resuspension of peat
deposits in the Florida Everglades, and
recently dissolving bags have been used to
encapsulate tracer for a study on reef
sediment impacts in Hawaii. Beach
longshore transport, and soil loss to
stream/bank stability studies, both involve
direct placement of tracer onto the land
surface (in non-windy conditions!)
4) Sampling- the dual signature nature of the
tracer provides for a range of sampling
options. Magnets, deployed on line
moorings, on bed-frames or onto fixed
structures, directly within the anticipated
stream flow have been used very
successfully to intercept tracer. ‘Dipped’
magnets can be used to collect
instantaneous samples. Magnets are
covered with a thin acrylic sheath, which
is simply removed and bagged prior to
enumeration and thus sampling is very
quick and simple. In the field pumped
water sampling can also be used, bottom
sediment cores can be collected using
grabs or cores (for deposited tracer), and in
situ or pumped fluorimetry will detect the
fluorescent colour if a cloud of suspended
tracer particles passes (e.g. Guymer et al.,
2010). If bottom sediment cores are
collected, then post-collection
Example of nearly equivalent grain
size spectra for (a) native sand
(top) and (b) sand tracer (bottom)
derived from settling velocity
determinations.
Different means of introducing tracer into
the environment
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separation of the tracer from the native
sediment is required; this is achieved in
the laboratory using a Frantz Isodynamic
Separator.
5) Tracer Enumeration:- the dual signature
permits can be undertaken using a range of
differing approaches to tracer
enumeration. The chief objective of the
enumeration process is to ascertain the
tracer dry mass (grams) rather than
particle number concentration or
presence/absence within each sample,
simply on the basis that geologists and
oceanographers most commonly work on
with mass transport units (White, 1998). A
variety of analytical approaches are
available. These include: dissolution of the
fluorescent coating, centrifugation and
spectroflourometric analysis (e.g. Carey,
1978; Farinato and Krauss; 1981; Forsyth,
2000); flow cytometry (Forsyth, 2000), or
FlowCam™ analysis (see
http://www.fluidimaging.com/); and
filtration/sedimentation followed by digital
image analysis (e.g. Solan et al., 2009). To
an extent, the choice of method is study-
specific and may vary according to the
study objectives. In natural systems, and
especially in industrialized estuarine and
port environments, there is inevitably a
population of non-fluorescent, magnetic
particles within the suspended and bedded
sediment pools. These constitute noise
within samples. Spectrofluorimetry is the
most is most frequently the method of
choice here as this fraction can be
Box 1
Innovative Sampling using in stream magnets
The innovation of magnetic tracer allows for recovery of tracer directly in the field situation ,
which is applicable in particular to suspended sediment studies. Powerful pole magnets can be
deployed in the water (e.g. on moorings, attached to bottom poles/frames or infrastructure, or
dipped and towed) to capture passing tracer. Where magnets are used statically they operate as a
time-integrating tracer sampler. The magnets present a sampling window ~225 cm2. The magnets
are so powerful (11,000 gauss) and the magnetic susceptibility of the tracer so high that tracer is
not washed off were the magnets to be held under a tap. Magnets are deployed with a slim fitting
sheath which is easily removed, and so in situ arrays of magnets can be changed out very quickly.
In environments where there are no other magnetic particles, the samples from the in situ magnets
are dried and weighed.
The use of in situ magnets also means that sampling can be adaptive, rather than reactive, which
improves tracer recovery rates.
A static magnet which has captured tracer; the
variation in tracer mass vertically is due to the
vertical concentration profile close to the bed
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effectively ignored within the analytical
methodology. Furthermore, the
spectrofluorimetric method can measure
two different tracer colours in the same
sample, which increases the flexibility of
the tracer technique, and is extremely
sensitive (mass resolution to ~0.01 g).
Various tracer enumeration methods (l-r) image analysis, fluorimetry, magnetic susceptibility,
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Box 2
Using A Magnetic Susceptibility Sensor to Measure Tracer Transport
Mass-based magnetic susceptibility (denoted , unitless) is a comparative measure of the relativ eease with which a material can acquire a magnetic field when exposed to a
low frequency (100 T) alrnating magnetic field. Ferros or iron-rich particles acquire a magnetic field far more easily that non -ferros particles. The magnetic suscpetibility of
a granular material is measured using a magnetic suscpetibility sensor (e.g. the Bartington Instru ment MS2 device). Magnetic suscpetibility sensors come in a wide range of
configurations; most frequently used in environmental sciences are the desktop vial sensor (a sample is placed in a vial and then inserted into the machine for measurement)
and a loop sensor, for field use. The technique is especially useful in sediment tracking studies as it provides a semi -quantitative method for measuring tracer concentration in
sediments and soils.
Measuring surface magnetic suscetibility using a loop sensor.
Although varies with particle size, typically the dual sensor tracer material has a magnetic susceptibility =15 – 20 *10-5 whereas for non-ferrous, quartztic beach sand is
~ 7 * 10 -6, and this signal can be used to differentiate tracer from background material. Studies such as soil erosion and soil loss st udies, and intertidal and beachface transport
studies, use the magnetic sucspetibiltiy techniuqe to determine tracer loss and tracer accumulation in surface soils. Prior to any study commencement a backgr ound study is
always performed to measure for the native seimdents; the same data are collected immediately following tracer appl ication and then at successive intervals through time.
The graphic on the left shows a spatial map of on an intertidal sandflat surface following application of tracer. The graphc on the right shows the same seabed area follown g
10 tides.
Sand surface at injection Sand surface following 10 tides
Interesting Paper! Although they used a different type of magnetic tracer (thermomagnetically enhanced sand), the application of a magnetic tracer in the environmental
sciences is described.
VAN DER POST, K. D., OLDFIELD, F. & VOULGARIS, G. 1995. Magnetic tracing of beach sand: preliminary results. Coastal Dynamics ’94, Proceedings of International Conference, 323 –334.
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THE IMPORTANCE OF STUDY DESIGN
AND SAMPLING DESIGN
Whilst the underlying concepts behind
sediment tracking studies are quite simple,
development of practical studies which
provide answers to specific objectives is an
involved process. The importance of
developing an appropriate study design, in
which the project sampling resources (which
are never infinite) are optimised, cannot be
overstated. The foundation for developing a
scientifically robust, quality study is to first
set down the project objectives clearly. Too
frequently, a question of the type ‘where does
the sediment [tracer] go’ is framed. Clearly,
since it is not possible to sample everywhere,
this is difficult to answer, particularly where
studies extend over long timescales (e.g.
many months). In all cases a more judicious
approach frames the question as ‘does the
sediment [tracer] go [t]here?’. Since a
geographically precise location is defined
(e.g. does sediment issuing from this outfall
end up on that beach’), then the sampling
resources can be allocated to the specific
receptor thereby optimising the chances of
intercepting tracer.
Tracer studies are unique in that although
sampling within tracking studies is conducted
on an Eulerian (fixed point) basis, tracer
studies provide an assessment of transport
pathways on a Lagrangian basis. Effectively,
the net sediment transport over the monitoring
(sampling) time frame is visualised.
Issues of time- and space-scale are also of
importance. Studies over broad space scales
(> 5 km) and long time scales (> 1 month) are
difficult to perform principally due to
excessive dilution of the tracer. Such studies
run the risk of generating a Type 2 error in
which a false inference about the transport
pathway is made; this states that tracer does
not arrive at a receptor site (i.e. the receptor
is not therefore part of the transport pathway)
whereas the reality may be that it does, but
tracer concentrations are below the level of
detection. This is a serious error which must
be avoided. More tracer can be introduced at
the study commencement, which will likely
raise the probability of detection at receptor
sites, but this may also raise the project costs.
An appropriate practical approach is to collect
measurements at intermediate locations along
the transport pathway, in addition to at the
receptor. Through this an additional line of
evidence is formed, which can be used to
judge the nature of the transport pathway.
This is a recommended approach because for
the most part the dispersion and dilution rates
of tracer within most studies cannot be known
a priori.
For the above reasons it is not a
straightforward matter to assess the quantity
of tracer required for a given study. Several
calculations can be performed which can
inter-relate the general volume of water
within the study zone to an injected tracer
mass i.e. to provide an idea of tracer
concentrations under various minor to major
dispersion extents. This information can then
be reviewed in terms of the sampling design.
In all study cases, as much tracer should be
purchased as the project budget will allow,
and the study designed around this quantity
and the study objective.
In all successful tracer studies a high
proportion of the tracer is lost. However, high
loss rates are not indicative of experimental
failure and should not be regarded as such ;
the appropriate way to view tracer is to
consider it as a ‘disposable capital purchase’
which is being thoughtfully used to provide
knowledge on sediment transport. It is
inevitable that much of the tracer material
will be lost in turbulent, aquatic systems, but
as long as the study objectives are addressed
satisfactorily the study will be a success. For
these reasons rarely is it possible to establish
a sediment budget for a study.
Asking the right question is critical. You
will get an answer, but it might not be what
you wanted!
Loss of most of the tracer isn’t a worry – it’s
part of the tracer methodology. View the tracer
as a ‘disposable capital purchase’.
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Box 3
Needles and Haystacks: Recovering Tracer and the Use of Magnets and Magnetic Separation
The innovation of magnetic tracer introduces has changed the way in which tracer studies are conducted, principally through provision of a means of
recovering tracer from environmental samples. All but two of the historical methods have used mono -signature tracer, dominantly using the fluorescent
tag. Consider a grab or soil sample (say, 0.5 kg) containing 20 grams of tracer. Previously the grab was sub -sampled and then the number of fluorescent
tracer particles counted either manually or using an automated method . Not only is this a time-consuming process but the sub-sampling process means a
much lower sediment volume is sampled and therefore the probability of finding tracer is correspondingly much lower. Often <1 g of sample was
processed. Nor can this method be used for silts.
Nowadays the magnetic element of dual signature tracers means that an entire sample (2 -3 kg) can be flushed through a magnetic separator to recover
tracer. We use a device from the mining industry called a Frantz Isodynamic Separator. Samples are first sieved to be within the tracer size range and then
(freeze) dried. The sample is introduced onto a conveyor belt which passes through a confined, powerful magnetic field and th e FIS quickly and elegantly
produces separate non-magnetic (NMF) and magnetic fractions (MF) from the sample. Use of all sample material obviates any necessity for sub -sampling,
which avoids sub-sampling errors but – more importantly - means the probability of finding tracer is far greater.
Magnetic non-fluorescent sediment (left) and separated fluorescent-magnetic tracer
Signals and Noise: In environments where there are no other magnetic particles, the magnetic fraction from the FIS separation will be the dry mass of
tracer in the sample. No further processing is necessary. However, in many en vironments, and particularly in industrialized settings such as ports and
harbours, there are abundant magnetic (but non-fluorescent) particles. Using the FIS method means that the magnetic fraction (containing the tracer)
contains additional mass and this needs to be accounted for. Several approaches are possible. A simple approach is to undertake a background survey prior
to the tracer study to assess the abundance of natural magnetic particles (per unit area/time) and to subtract this from real sample mass data. Alternatively,
a spectrophotometric method can be used (Carey, 1989) to measure tracer dry mass which ignores the magnetic, non-fluorescent fraction completely. In
this technique the FIS is first used to isolate the magneti c fraction, and then a solvent is added to the sample to extract the dye from the tracer. The solvent
is then diluted and the concentration of dye measured with a highly sensitive multi -wavelength spectrofluorimeter.
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CASE STUDY 1:
TRACKING THE MOVEMENT OF
CONTAMINATED SEDIMENTS IN AN
INDUSTRIALISED ESTUARY (LOWER
DUWAMISH WATERWAY, SEATTLE,
WASHINGTON STATE, USA)
Where: Lower Duwamish Waterway
(Pacific NW, USA)
When: Winter 2009
Tracer Specification: Silt and Sand
Project Objectives:
To identify the short-term sediment
transport pathways and patterns of
sediment accumulation of fluvially-derived
sediment particles
Background
Source control i.e. the reduction of
contamination from upstream or diffuse
sources, is a critical element in any
management plan for contaminated
waterways. If source control measures are not
successfully implemented then a situation
exists in which contamination will continue
through time, and the cleanup of waterway
segments becomes increasingly problematic.
For greater understanding of the issues
surrounding source control, it is essential to
have some appreciation of contaminant
sources and transport pathways of
contaminated particulates. This specific
aspect formed the basis for conducting a
tracking study to understand the transport
pathways of PCB-contaminated sediments
(principally) entering the Lower Duwamish
waterway via the Green River. A sediment
transport model constructed for the waterway
indicates that a significant majority of the
yearly sediment load entering the Lower
Duwamish Waterway (LDW) is derived from
the Green River (QEA, 2007). About one-half
is predicted to be deposited within the LDW,
an area that has now been selected by the
national government for cleanup within the
Superfund initiative (see
http://www.ldwg.org/rifs_docs9.htm#finalfs).
However, the accumulation of sediment and
sediment-associated contaminants differs by
reach, water depth, settling velocity, and
other factors. A tracking study was
commissioned by the State Dept. of Ecology
(Washington, USA) to identify the short -term
sediment transport pathways and patterns of
sediment accumulation of fluvially-derived
sediment particles. Further information can be
found at:
http://www.ecy.wa.gov/biblio/0903048.html.
The Lower Duwamish Waterway Superfund
site is a 5.5 mile stretch of the Duwamish
River that flows into Elliott Bay in Seattle,
Washington (Fig. 5). The waterway is flanked
by numerous industrial corridors, as well as
several residential neighbourhoods. It is 150 –
215 m wide with a mean depth of ~ 6 m. It is
tidally influenced well beyond River Mile
12.4. Fluvial discharges to the waterway
range <10 to 340 m3
s-1
with a median value
of approximately 40 m3
s-1
.
Tracer Specification
The principal focus of the study was on the
fate of silt-sized particles (< 63 m in
diameter), but since sand sized material is
also flushed down the Green River this size
fraction was also included in the study. Table
1 summarises the tracer specification (particle
size, density, settling velocity); these were
derived from both laboratory and field
measurements of the size, density, and
settling velocity of native sediments that were
in the river and within the estuary. 100 kg of
each of the fractions was provided.
Fluorescent
Colour
Size
Range
(m)
Density
(kg m-3)
Settling
Velocity
(m s-1)
Red Sand 60 –
250 2600 0.013 ± 0.01
Yellow Silt 30 –
60 1200
0.00013 –
0.00024
Table 1 Tracer specification
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Tracer was introduced at River Mile 4.4 (see
Fig. 5) synchronous with high water during a
median flow event using the method shown in
Figure 4. A geospatial sampling programme
covering the length (to River Mile 0) and
width of the waterway was devised which
comprised: magnets fixed onto port
infrastructure (see Fig. 3; 18 sites with 6 of
the 18 sites comprising a triple magnet
arrangement); in situ pumped sampling and
dipped magnets via transects (13 on Day 1
only; transects are shown on Fig. 5), and bed
sampling (63 sites) using a van Veen grab.
The waterway was sampled at the end of the
first day, after 1 week, after 1 month, and
after 2 months.
Principal Study Findings
The Day 1 findings indicated that both sands
and silts had been transported downstream,
however the sand sized material was almost
entirely deposited within ~500 m of the tracer
injection point. In situ pumped water samples
showed that by the turn of the tide silt
material had been carried in suspension down
to River Mile 0 i.e. the length of the study
reach (Fig. 6). Data from fixed magnets,
which were only mounted along the riverbank
infrastructure and not mid-channel, showed
that the silt material was mixed laterally the
length of the waterway. These data
unequivocally confirm that (for the given
hydrologic-tidal circumstances) sands are
carried only a limited distance into the
waterway, whereas contaminated natural silts
flushed into the waterway are mixed
longitudinally and laterally.
Aerial photograph of the Lower Duwamish Waterway showing the tracer release site
and location of transect and other sampling areas.
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Thereafter, sands were never found in
suspension indicating no or highly limited
subsequent resuspension under normal
conditions.
After Day 1 the sheaths on all magnets were
replaced. Silt sizes particles continued to be
collected by the magnets and were therefore
by inference found to be in suspension
throughout the subsequent sampling
campaigns (week 1, months 1 and 2), though
a general reduction in concentration occurred
through time, and by Month 2 the silt
particles in suspension were noted to be finer
grained, suggesting some deposition of the
coarser silt fraction. A hypothesis proposed
for the continued presence of tracer in
suspension is that a well-mixed, quasi-
permanently suspended pool of tracer
remained within the waterway, and was
simply advected by tidal action on a daily
basis and retained in the study area by the
tidal excursion distances.
Some deposition evidently occurred (see Fig.
7) as bed samples were found to contain
tracer, and therefore some of the silt found in
suspension over the longer term could be
derived from resuspension of deposited tracer.
The sediment transport model (QEA, 2007)
identifies propeller induced resuspension as a
process likely to occur in the waterway on
account of the shallow water depths. Silt
tracer was also found in bottom samples
upstream of the tracer injection point,
indicating a degree of upstream transport (by
flood tides) and deposition.
Example result: Distribution of (silt) tracer in bottom sediment samples through time.
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CASE STUDY 2:
PREDICTING THE DEPOSITION OF
HIGHWAY-DERIVED SEDIMENTS IN A
RECEIVING RIVER REACH
Where: River Salwarpe (UK)
Tracer Specification:
Dual Signature Silt and Sand (2 colours)
Project Objectives:
To utilise uniquely-labelled tracer particles
to determine the depositional footprint and
downstream fate of highway-derived
sediments in roadside cuvert streams.
Background
Highway runoff is episodic and its
composition varies over short temporal scales .
Most contaminants are washed off highways
at the start of storm events, with many of the
contaminants in highway runoff being
associated with particulate material. The
loads and particle sizes of sediments in
highway runoff are dependent upon storm
characteristics – such as antecedent period,
storm intensity and duration – combined with
the hydraulics of the highway drainage system
(pavement area, carriageway slope and the
presence and maintenance of sediment traps).
The main factors affecting the dispersion,
sedimentation and re-distribution of such
material are the receiving water flow
characteristics – discharge, velocity,
bathymetry, bed material and vegetation – in
association with particle and outfall
characteristics (particle size, material density,
load and outfall design).
Many models of particle settling have been
developed and these include theoretically
based formulae, regression based methods,
and, more recently, mathematical simulation
of the dynamic processes. Disadvantages of
adapting this approach to receiving river
reaches are that it does not allow for any
spatial variations that might arise from
channel geometry, nor does it account for
turbulence, or for any cross-sectional
variations of suspended sediment
concentration throughout the inflow. Further,
the bed microtopography is a major factor
influencing through-reach transport of
sediments, and this is difficult to parameterise
in models.
The objective of the field investigation was to
utilise uniquely-labelled tracer particles to
determine whether sediments would deposit
close to the outfall or be transported away
from the study reach. For any deposited
particles, the spatial distribution on the bed
would be of interest. The approach adopted
was novel, involving the use of two particle
types, both of which were magnetic, uniquely
and brightly coloured and fluorescent. A
single sediment input and monitoring
experiment was undertaken from an
artificially generated discharge event.
Two different coloured size fractions were
specified, nominally <63 µm and 63-150 µm.
The finer ‘green’ tracer particles had a
density of 1936 kg m-3
, whilst the coarser
‘red’ (sand) tracer had a density of 2600 kg
m-3
. Both tracers were para-magnetic in
character.
Particle size distribution for the two tracers.
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Principal Findings
A steady discharge was required from the
outfall pipe to introduce the two sediment
tracers to the river. 15 kg of each sediment
fraction was introduced to the system. The
injection regime involved combining 1 kg of
each of the tracer fractions and premixing
with 6 l of water (plus 5 ml of soap included
to remove surface tension effects). The
mixture was then introduced to the simulated
highway drainage flow in the manhole
chamber over a 2 minute period.
Consequently 15 ‘pulses’ were seen emerging
from the discharge pipe over a half hour
period (10:00-10:30hrs).
In stream Turner Designs SCUFA instruments
detected increases in fluorescence between
the solute traces and a significant signal on
the turbidity channel 80 m downstream of the
outfall. The results show that none of the
coarse ‘red’ particles were detected in the
suspended sediment settling tube sampling
system at the downstream boundary.
A significant quantity of the coarse ‘red’
sediment was deposited in the outfall pipe and
large quantities were deposited on the
receiving channel bed within first 5 m of the
outfall.
Introduction and subsequent in stream tracer
transport. The green coloration is tracer
particles, not dye.
Time series of fluorescence and turbidity from a series
of pulses.
Residual tracer in the culvert pipe at the end of the
study and nearfield deposition of red (sand) tracer
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It is inferred that none of the coarse red tracer
particles exited the reach. Of the fine green
tracer particles, a few were observed to have
been deposited on the channel bed, but not in
large quantities. Most fine ‘green’ particles
were observed to be conveyed through the
reach and the in-stream measurements
detected their transport in suspension. Coring
of the stream bed and data from the magnets
was used to develop estimates of mass budget
for the stream system under the measured
flow conditions, and to delineate the fractions
still within the culvert from those transported
into the stream.
The tracer technique proved to be a simple
and elegant method for demonstrating in a
quantitative manner the principal sediment
transport characteristics of the system. The
combination of in situ fluorimetry, in situ
magnetic capture and direct sampling forms a
powerful set of tools to elucidate the transport
process and fate of the tracer. Post-sampling
particle size analysis (not performed here) is
an additional tool that could be included in
future investigations to assess size
fractionation of the tracer as it is transported
through the system.
The combination of in situ fluorimetry, in situ
magnetic capture and direct sampling forms a
powerful set of tools to elucidate the transport
process and fate of the tracer
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