Transcript
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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

[email protected]

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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REFERENCES

1) Black, K., S., Athey, S., Wilson, P.A.,

and Evans, D., 2007 The use of particle

tracking in sediment transport studies: a

review. In Balson, P., and Collins, M.B.,

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Geological Society of London, Special

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2) Carey, D.A., 1989 Fluorometric detection of

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dynamics. Limnol. Oceanogr., 34(3), 1989,

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3) Courtois G, and Monaco A (1969) Radioactive

methods for the quantitative determination of

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4) Draaijer, A., Tadema Wielandt, R. and Houpt,

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6) Guymer, I., Stovin, V.R., Gaskell, P., Maltby,

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7) Heathershaw, A. D. and Carr, A. P. 1987.

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14) Spencer, K., The2011 Development of rare

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