Morris_Lake_sediments_Canada.pdf

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Journal of Applied Geophy

Lake-based magnetic mapping of contaminated sediment

distribution, Hamilton Harbour, Lake Ontario, Canada

M.R. Pozza, J.I. Boyce*, W.A. Morris

School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada

Received 16 October 2003; accepted 20 August 2004

Abstract

The remediation of toxic sediment in harbours and urban waterways requires detailed mapping of contaminated sediment

distribution and thickness. Conventional methods rely on interpolation of pollutant concentrations from widely spaced core

samples but can lead to significant errors in estimating sediment distribution. An improved approach, as demonstrated by recent

work in Hamilton Harbour in Lake Ontario, is to estimate pollutant levels from proxy measurements of sediment magnetic

properties. Measurements from 40 core samples collected within the harbour show that the magnetic susceptibility of a

contaminated upper layer of sediment is one to two orders of magnitude greater than in the underlying uncontaminated dpre-colonialT sediments. The susceptibility contrast results from elevated levels of urban-source magnetic oxides and is sufficient to

generate a total field anomaly (ca. 5–40 nT) that can be measured with a towed magnetometer. Systematic lake-based magnetic

surveying (N500 line km) of the harbour using an Overhauser marine magnetometer identifies well-defined positive magnetic

anomalies that coincide with mapped accumulations of contaminated sediments on the harbour bottom. Forward modelling of

the anomalies shows that the magnetic response is consistent with a contaminated upper layer thickness of up to 5 m. Apparent

susceptibility maps calculated from magnetic survey data show a close spatial correspondence with core-derived magnetic

susceptibilities and provide a rapid means for classifying contaminated sediments. Detection of shallow magnetic anomalies is

dependent upon a closely spaced survey grid (b75 m line spacing) and careful post-cruise processing to remove diurnal,

regional and water-depth related variations in the magnetic field intensity.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Environmental magnetism; Contaminated sediment; Magnetic susceptibility

1. Introduction

The remediation of contaminated sediments in

urban waterways and coastlines is now recognized

0926-9851/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jappgeo.2004.08.005

* Corresponding author. Fax: +1 905 546 0463.

E-mail address: [email protected] (J.I. Boyce).

as a major environmental priority (US-EPA, 1998). A

major requirement before remediation work can begin

is that areas of contaminated sediment be adequately

characterized in terms of their areal distribution,

thickness and pollutant levels (NRC, 1997). Conven-

tionally, this is carried out on small-scale projects by

chemical and physical property analysis on a limited

sics 57 (2004) 23–41

M.R. Pozza et al. / Journal of Applied Geophysics 57 (2004) 23–4124

number of core samples. These are typically acquired

on a grid pattern and interpolated to produce maps of

sediment thickness and pollutant concentrations. In

the case of large contaminated sites, interpolation

between widely spaced core locations can be unreli-

able and often leads to errors in estimating contam-

inant distribution and sediment volumes (Versteeg et

al., 1997; Boyce et al., 2001). A simple solution

would be to increase the core-sampling density, but in

practice this is often too expensive and impractical for

mapping large contaminated sites.

An alternative approach is to use geophysical

measurements of sediment physical properties to assist

in estimating sediment pollutant levels. Several recent

studies have investigated the use of acoustical,

electrical and magnetic property measurements for

classifying and mapping contaminated sediments in

lakes and rivers. Guigne et al. (1991) demonstrated the

advantages of parametric acoustic arrays for mapping

thin contaminated sediment layers and Caufield and

Filkins (1999) developed a system for classifying

pollutant levels by comparing acoustic adsorption

(bottom loss) in clean versus contaminated sediments.

Other work has focussed on acoustic classification of

bottom sediment texture and other properties (i.e.

density, mineralogy) that have a direct influence on

contaminant sorption in sediments (Chivers et al.,

1990; Leblanc et al., 1992; Rukavina, 1997, 2001). A

major drawback of acoustic methods is that they are

often unsuccessful where there is significant bottom

vegetation, or where the presence of gasified sedi-

ments limits acoustic penetration. These problems led

the US-EPA to suspend further testing of acoustic

methods for contaminant studies following an exten-

sive 4-year trial (US-EPA, 1998). Marine resistivity

surveys employing multi-element arrays show promise

as a method for classifying bottom sediment type

(Manheim et al., 2002), but their ability to detect the

presence of contaminant phases in marine sediments

has not been demonstrated.

Magnetic property measurements provide a further

alternative to conventional analytical methods and

have gained increasing use in soil contamination

mapping and atmospheric pollution studies. Initial

work by Locke and Bertine (1986) and Beckwith et al.

(1986) identified elevated levels of magnetic oxides in

soils and linked them to magnetic particles in airborne

pollution. Several subsequent studies confirmed these

findings and showed a direct correlation between the

magnetic susceptibility (ease of magnetization) of

contaminated soils and the presence of hydrocarbons,

heavy metals and other combustion-related pollutants

(Flanders, 1991; Morris et al., 1995; Kapicka et al.,

1999; Petrovsky and Ellwood, 1999). A positive

correlation between contaminant concentrations and

magnetic oxides has also been identified in lake and

marine sediments. Morris et al. (1994) and Versteeg et

al. (1995, 1997) analysed magnetic properties in core

samples from Hamilton Harbour in western Lake

Ontario (Fig. 1) and were able to map the thickness of

a highly magnetized layer of contaminated sediment

across the harbour. Their results show that concen-

trations of hydrocarbons and certain heavy metals (Pb,

Zn, Fe) are closely tied to magnetic oxide content.

Mayer et al. (1996) found a similar relationship in their

analysis of suspended sediments in the harbour, and

showed that susceptibility and contaminant levels were

unaltered by post-depositional processes. Chan et al.

(1997) conducted a study of contaminated sediments

in Hong Kong harbour and found that metal concen-

trations in sediments were directly correlated with

sediment magnetic susceptibility. They advocated the

use of magnetic properties as a rapid and inexpensive

method for mapping contaminated sediments.

While magnetic susceptibility provides a rapid and

reliable method for assessing contaminant levels in

sediments, it suffers from some drawbacks. Firstly, it

requires the collection of physical core samples and,

secondly, it provides point measurements that must be

interpolated to produce maps of contaminated sedi-

ment distribution. An improved approach, as de-

scribed in this paper, is to combine core-based

magnetic property analysis with remote measurements

of sediment magnetic response using a magnetometer

towed above the lake bottom. The primary advantage

of a magnetic survey is that large areas can be mapped

rapidly and with a high density of magnetic measure-

ments. Here we report on the application of high-

resolution magnetic surveying to mapping contami-

nated sediment distribution in heavily industrialized

Hamilton Harbour, in western Lake Ontario (Fig. 1).

The results of systematic magnetic mapping of the

harbour are evaluated by comparison with the

previous contaminant mapping of Versteeg et al.

(1995). This shows that total field magnetic anomalies

are spatially correlated with known accumulations of

Fig. 1. Hamilton Harbour study area showing generalized land use and locations of major sediment and contaminant input sources.

M.R. Pozza et al. / Journal of Applied Geophysics 57 (2004) 23–41 25

contaminated sediment and that these can be mapped

at a much higher resolution than previously achieved

with core data alone. While it is unlikely that

magnetic surveys will replace conventional core data,

they provide an efficient and complimentary method

for reconnaissance-scale mapping of large areas of

sediment contamination. The accumulation of toxic

sediments is a common problem in many urbanized

waterways and the methods and results reported here

have broader applications to remediation of other

contaminated sites.

2. Study area

Hamilton Harbour is a 22-km2 embayment

located at the western end of Lake Ontario (Fig.

1). The basin is separated from Lake Ontario in the

east by a barrier beach and has a maximum water

depth of about 24 m. The harbour drains a large

urbanized watershed area that includes the cities of

Hamilton and Burlington, with a population of

about 800,000 residents. The bottom sediments in

the harbour are heavily contaminated by the direct

discharge of untreated urban and industrial effluents

during the last century. The contaminants of most

concern include polycyclic aromatic hydrocarbons

(PAH), polychlorinated biphenyls (PCB), heavy

metals, phenols and a range of other toxins

(Poulton, 1987; Mayer and Nagy, 1992). The

primary pollution sources are discharges from four

sewage treatment plants, steel-making operations

and other heavy industries situated on the south

shore of the harbour (Fig. 1). The steel mills are a

primary contributor of PAH and heavy metals to

the harbour through atmospheric loadings and the

discharge of more than 2�106 m3 day�1of process

water used in contact cooling (Morris et al., 1994).

The harbour also receives large volumes of

untreated runoff from the urbanized areas of the

watershed during storm overflow events (Poulton et

al., 1996) (Fig. 1). The basin water quality is

moderated to some extent by direct exchange with

Lake Ontario, via a single connecting channel at

the eastern end of the harbour (the Burlington

Canal; Fig. 1).


Due to the presence of toxic levels of contaminants

in the harbour sediments and other impacts (eutro-

phication, toxic organics in fish, poor aesthetics), the

harbour has been designated as one of the most

contaminated water bodies in the Great Lakes (IJC,

1985). A remedial action plan is being implemented to

clean-up and restore the most heavily impacted areas

of the basin (RAP, 1991); central to the remediation

efforts is the need for detailed mapping of the

distribution and volume of contaminated sediments.

3. Previous magnetic property work

A key factor in the selection of Hamilton Harbour

as a test site for this study was the availability of a

large sediment core and magnetic property database

(Fig. 2). More than 40 core samples have been

collected and analyzed during several previous studies

of contaminant distribution in the harbour (Morris et

al., 1994; Versteeg et al., 1995, 1997). Fig. 2A shows

a typical magnetic susceptibility (j) profile from a

1.3-m long core collected in Hamilton Harbour. The

approximate sediment ages are also shown based on210Pb dates (Turner, 1994). The onset of industrializa-

tion in the harbour in the 1890s is recorded by a rapid

increase in j at a depth of approximately 60 cm. This

horizon marks the base of the dpost-colonialT sediment

layer and provides a useful marker horizon for

estimating the thickness of contaminated fill within

the harbour. The profile reaches a peak j value in the

late 1970s of about 2�10�4 cgs (Fig. 2A). For

comparison, this is roughly equivalent to the suscept-

ibility of basalt and represents more than an order of

magnitude increase above the background suscepti-

bility of the natural harbour sediment. Morris et al.

(1994) attributed this increase to the presence of

magnetite spherules produced by oxidation of pyrite

during steel plant coking operations. Other magnetic

phases were also found in the bottom sediment (e.g.

greigite, hematite) but their contribution to the

induced and remanent magnetic components was

minor.

Comparisons of the contaminant levels and mag-

netic properties of Hamilton Harbour cores showed

that levels of PAH and certain heavy metals (Pb, Zn,

Fe) are strongly positively correlated with sediment

magnetic susceptibility (Morris et al., 1994). The

levels of these contaminants are closely tied to

magnetic oxide content (i.e. flyash) because they are

products of the same combustion processes (e.g.

Petrovsky and Ellwood, 1999). Versteeg et al. (1997)

investigated the use of magnetic susceptibility as a

proxy for estimating PAH levels in lake sediment and

were able to map the distribution of the contaminated

sediment layer across the harbour by interpolating core

j data for various depth horizons (Fig. 2B). Their

maps identify a distinct magnetic susceptibility anom-

aly along the southeast shore of the harbour that is

related to discharges from nearby steel mills and urban

effluents from the city of Hamilton. The maximum janomaly on the south shore identifies the thickest

accumulation of contaminated sediments in the basin

(N6 m, Randle Reef) and is associated with toxic levels

of PAH and heavy metals (Murphy et al., 1990) (Fig.

2B). Other zones of contamination are indicated by

high magnetic susceptibilities adjacent to the steel

works dockyards and at the mouth of Windermere

Basin, which is the receiving water for Hamilton’s

main sewage treatment plant (Fig. 2B). A primary

objective of the present study was to evaluate whether

a total field magnetic survey would be capable of

detecting magnetic susceptibility contrasts within the

harbour bottom sediments.

4. Rationale and methods

The motivation for applying a total magnetic field

survey in this study is the large contrast in magnetic

susceptibility between contaminated and clean sedi-

ments in the harbour (N10–102 cgs; Fig. 2). A

magnetometer towed above the harbour bottom will

respond primarily to near-field variations in the

bottom sediment magnetic susceptibility, provided

that the underlying deeper sediments and bedrock

have low levels of magnetization (Boyce et al., 2001).

The total field or magnetic flux density (B-field) is

directly proportional to magnetic susceptibility j as

(Telford and Sheriff, 1991):

B ¼ H þ 4pM ¼ 1þ 4pjð ÞH ð1:1Þ

where H=field strength (Oersteds), M=intensity of

magnetization (Oersteds) and j=magnetic suscepti-

bility (emu cgs).

Fig. 2. (A) Magnetic susceptibility (j) versus depth in bottom sediment core sample. Approximate sediment ages based on 210Pb dating (from

Turner, 1994). (B) Bottom sediment magnetic susceptibility (j) interpolated from 37 core samples at 10-cm depth intervals (after Versteeg et al.,

1997).



Thus, while magnetic flux density B is the

parameter measured, information is also obtained

indirectly about variations in bottom sediment sus-

ceptibility. The effect is exploited widely in land-

based magnetometry for mapping near-surface mag-

netic anomalies in archaeological and environmental

investigations (Breiner, 1973).

The remanent component of magnetization in soils

and surficial sediments is often also significant

(Morris et al., 1994) but cannot be obtained from

total field measurements. However, if the remanence

is small or negligible in comparison to induced

magnetization, the apparent magnetic susceptibility

ja can be estimated using a geometrical model that

approximates the magnetic source bodies as a series of

square-ended prisms (Bhattacharyya, 1964; Talwani,

1965). ja is obtained using the following compound

filter:

ja r; hð Þ ¼ 1

2pFH rð ÞC hð ÞS r; hð Þ ð1:2Þ

where H(r)=e�hr is a downward continuation operator

(Telford and Sheriff, 1991), C(h)=[sin Ia+icosId

cos(D�h)]2 represents reduction to the pole, S(r,h)=sin arcoshð Þsin arsinhð Þ

arcosharsinh

��is a geometric factor for a square-

ended prism (Spector and Grant, 1970), r=wave-

number in radians/ground units, h=wavenumber

angle, I=geomagnetic inclination, Ia=pole reduction

amplitude inclination, D=geomagnetic declination,

F=total geomagnetic field strength in nT, a=half the

grid-cell size and h=depth in ground units, relative to

the observation level at which to calculate the

susceptibility.

The filter operator downward continues to a

specified source depth, corrects for the geometric

effect of a vertical square-ended prism, and divides

by the total magnetic field F, to yield apparent

susceptibility (cgs units). The model requires that the

International Geomagnetic Reference Field (IGRF)

has been removed and that the magnetization is

parallel to the Earth’s field direction (induced

magnetization).

The apparent susceptibility parameter is

employed in this study for comparison of survey

results with core-based j measurements (Fig. 2A)

and as a means of classifying sediment contaminant

impact levels (see below). Because the sediment

remanence cannot be determined directly from field

data, the apparent susceptibility values may be

overestimated and must be regarded as qualitative

indicator of actual bottom sediment magnetic

susceptibility.

4.1. Magnetic survey parameters

A total of 500 line km of magnetic and

bathymetric data were collected within the harbour

over a 5-day period. The magnetic surveys were

acquired using an Overhauser marine magnetometer

towed behind a 6-m survey boat. The Overhauser

magnetometer (Marine Magnetics SeaSPY) has the

advantages of high sensitivity (0.01 nT/MHz) and is

an omnidirectional sensor, free of heading errors and

dead zones that complicate the use of optically

pumped magnetometers. The magnetometer was

towed at a distance of 30 m behind the survey boat

at a depth of b1 m. Survey positioning was provided

by an onboard differential-GPS and navigational

chart plotting system with a horizontal positioning

error of less than 3 m. Digital bathymetry data were

acquired simultaneously with magnetics using a 200-

kHz echo sounder system. The bathymetry data were

critical for later correction of depth-related variations

in the magnetic field strength and also aided in

interpretation of the magnetic data.

The survey was acquired along southeast–north-

west oriented lines with a nominal spacing of 65 m

and orthogonal tie lines at 200 m (Fig. 3). Two

higher-resolution surveys (50 m line spacing) were

also conducted over Randle Reef and a zone of

previous dredging on the eastern margin of the

harbour (Fig. 3). All data were acquired with 4 Hz

sampling, which provided an in-line sample spacing

of less than 1 m at typical survey speeds of 10–15

km/h. Prior to the start of each survey, a base station

magnetometer was deployed in a magnetically quiet

area adjacent to the harbour to record diurnal field

variations.

4.2. Signal processing

Several processing steps were applied to the

magnetic total field data to obtain a residual

magnetic map that emphasizes the contributions from

shallow magnetic sources. The general processing

Fig. 3. Magnetic survey track lines (ca. 500 line km). Windowed areas show locations of detailed magnetic surveys (nominal 50 m line spacings)

conducted on Randle Reef and zone of dredging in eastern harbour.


flow (Table 1) included editing of line data, base

station corrections to remove diurnal field variations

and a lag correction to account for the magnetometer

lay back behind the boat. The remaining residual

errors were removed using tie-line levelling. The

levelled line data were then gridded using a minimum

curvature algorithm (Briggs, 1974) with a grid cell

size of 10 or 25 m. The gridded data were micro-

leveled to remove any remaining uncompensated

levelling errors (Minty, 1991). Correction for varia-

tions in magnetic field strength related to changes in

water depth were performed using the chessboard

technique of Cordell (1985). The routine employs a

Fourier domain upward and/or downward continu-

ation of the magnetic grid to a series of parallel

surfaces on the bathymetry grid. The magnetic field

between the new surfaces is then interpolated to

produce a new grid at a constant observation height

above the sediment/water interface. For this study, an

observation height of 15 m was chosen as a suitable

level to maintain sufficient high frequency content,

while minimising the use of the downward continu-

ation operator on the grid to avoid potential aliasing

and noise enhancement (Reid, 1980).

The near-surface magnetic signal was then

enhanced by performing a regional-residual separation

(Cowan and Cowan, 1993; Hearst and Morris, 2001).

This involves subtraction of an upward continued grid

(50 m) from the total field to obtain a residual

magnetic field map that is enhanced in short spatial

wavelengths. As a final step to aid in interpretation,

the data were reduced to the magnetic pole. The fully

corrected magnetic residual maps are shown in (Figs.

4B, 5B and 7B). Areas of high magnetic intensity (hot

colours) identify more magnetite-rich sediments,

while areas of lower magnetic intensity (cool colours)

indicate less magnetized bottom sediments.

The processing of bathymetric data involved tie-

line leveling and spline smoothing of profile data.

As a final step, the profile data were gridded using

Table 1

Processing steps employed in processing of magnetic survey data

Processing step Effect

1. Base-station correction Correction for diurnal magnetic

variation

2. Lag correction Corrects for magnetometer layback

from DGPS antenna

3. Manual editing Removal of large amplitude spikes and

areas of extreme

magnetic gradient

4. Tie-line leveling Removal of systematic cross-line error

5. Gridding Interpolation using minimum

curvature algorithm

6. Micro-leveling Removal of remaining

uncompensated cross-line error

7. Drape-correction Correction for water depth variations

of the magnetic field

8. Reduction to pole Correction for inclination and

declination of the geomagnetic field

9. Regional-residual

separation

Removal of magnetic field

associated with deep magnetic sources


minimum curvature with a 10 m or 25 m grid cell

size.

5. Results

5.1. Bathymetry

The gridded echosounder data provide a high-

resolution image of water depth variations across the

harbour (Fig. 4A). The water depth reaches a

maximum of over 24 m within a deep central basin

and is less than 15 m across the rest of the harbour. A

second area of deep water in the southeast corner of

the harbour is an area of bottom dredging (Fig. 4A).

More than 10 m of sediment was excavated from this

area as part of land reclamation activities conducted

between 1968 and 1978. The bathymetry map also

reveals a distinct meandering feature on the harbour

bottom (Fig. 4A) that is interpreted as a submerged

and partially infilled river valley (Boyce et al., 2001).

The submerged channel records the existence of a

Fig. 4. (A) Colour-shaded bathymetry map of harbour (illuminated from n

dredging. Meandering feature is a drowned river valley that existed during

Residual magnetic field map of harbour. Numbers identify magnetic anom

works outfall, (4) north shore anomaly, (5) central basin anomaly, (6) nor

flood plain that formed during an earlier phase of

lower lake levels in Lake Ontario (ca. �30 m) at about

6000 YBP (Anderson and Lewis, 1987). The adjacent

mound-like Randle Reef marks the location of a thick

accumulation (N5 m) of heavily contaminated sedi-

ment and has been a major focus of recent remediation

efforts within the harbour (Poulton, 1987; Murphy et

al., 1990; Morris et al., 1994).

5.2. Magnetics

The results of the magnetic survey work are shown

in the residual magnetic map in Fig. 4B. The residual

magnetic map was created by subtraction of a 50-m

upward continuation of the total field data and thus

represents the shallow magnetic response of the

sediments infilling the harbour basin. Although the

regional field has been removed, the magnetic

variation across the harbour is still considerable

(N160 nT), as a result of the high contrast in the

bottom sediment susceptibility. The Randle Reef and

industrialized southern shore of the harbour stand out

as zones of highest magnetic intensity, with a residual

field anomaly N100 nT above the central basin area.

Positive magnetic anomalies are also associated with

the dredge basin in the eastern harbour and the north

shore of the harbour (Fig. 4A).

The magnetic anomalies can be interpreted by

comparison with the core-derived magnetic suscept-

ibility maps of Versteeg et al. (1997) (Fig. 2B). It

should be noted that the core-derived susceptibility

maps are smoother surfaces because they are based on

only 40 points and are gridded at a much larger cell

size (100 m) than the residual magnetic map.

Comparison of the two datasets reveals that areas of

high bottom sediment susceptibility are related to

areas of high magnetic intensity on the residual field

map (Figs. 2B and 4A). The broad band of high

magnetic susceptibility that extends from Randle Reef

to the dredge basin (30–40 and 40–50 cm depth

interval, Fig. 2B) is spatially coincident with a

corresponding magnetic residual field anomaly. The

residual magnetic field map, because of the higher

orthwest). Rectangular excavation in southeast is an area of bottom

a period of lower lake level (ca. 6000 YBP)(Boyce et al., 2001). (B)

alies discussed in text: (1) Randle Reef, (2) dredged basin, (3) steel

theast magnetic low, (7) basin axis anomaly.


Fig. 5. (A) Bathymetry map of Randle Reef survey area. Line A–B indicates location of forward modelled profiles in Fig. 6. (B) Magnetic

residual map of same area (50 m line spacings). Note high magnetic intensity of sediments in submerged meander channels, indicating down

slope movement of sediment from Randle Reef. Point magnetic targets are from ferrous refuse on the harbour bottom.



sampling density, shows clearly that this broad zone is

in fact three separate anomalies corresponding with

the Randle Reef, the steel works outfall and the

dredge basin (anomalies 1, 2 and 3; Fig. 4B). The

positive anomaly at the steel works outfall has a

plume-like appearance and likely reflects the transport

and dispersal of Fe-oxide rich sediments from the

discharge point on the shoreline. The Randle Reef and

dredged basin were surveyed at high resolution and

are discussed in further detail below.

The residual map also identifies several smaller

magnetic anomalies on the north shore of the harbour

that include a known area of bottom sediment

contamination (anomaly 4, Fig. 4B). Poulton (1987)

mapped the concentration of toxic organics in the

harbour sediments and found anomalously high con-

centrations along the north shore. An area of increased

magnetic intensity in the deep central basin of the

harbour identifies a further possible area of contami-

nated sediment accumulation (anomaly 5, Fig. 4B).

Areas of relatively low magnetic intensity, such as the

northeast and western parts of the harbour (anomaly 6,

Fig. 4B) indicate areas of low magnetic susceptibility

sediments. Versteeg et al. (1997) noted these lows in

their susceptibility mapping and attributed them to

areas of clean sandy bottom sediments.

Other distinctive features in the residual field map

include a broad (ca. 200 m) linear magnetic anomaly

trending along the axis of the harbour (anomaly 7,

Fig. 4B). The anomaly parallels the direction of the

tie-lines but is not attributable to levelling error since

it is clearly visible in the north–south line data. The

origin of the anomaly is enigmatic, but it likely

records thickening of Quaternary sediments within a

deep bedrock valley that underlies the western end of

the harbour (buried Dundas Valley). A recent syn-

thesis of seismic and available borehole data show

that the bedrock valley is more than 150 m deep in the

Hamilton area, and is structurally controlled by a

system of west–east trending faults in the underlying

Paleozoic-age bedrock (Edgecombe et al., 1998;

Boyce et al., 2002). The linearity of the magnetic

anomaly has been interpreted as evidence for a fault-

bounded basin below the harbour (Boyce et al., 2002).

5.2.1. Randle Reef high-resolution survey

Fig. 5 shows the results of a detailed survey of the

Randle Reef acquired with a 50-m nominal line

spacing. The close line spacing greatly enhances the

bathymetric and magnetic boundaries defining the

reef and the adjacent meander channel. The reef is

defined by a mound-like rise in the bottom top-

ography and a corresponding area of high magnetic

intensity (N40 nT) and rugged magnetic relief on the

residual magnetic map (Fig. 5B). The presence of

several ferrous objects on the bottom (metal scrap) is

indicated by small dipole anomalies. The magnetic

intensity decreases rapidly to the north of the reef and

is consistent with the pattern of declining magnetic

susceptibility that is evident in the core-based map-

ping (Fig. 2B). This pattern can be interpreted as a

basin-ward thinning of an uppermost layer of con-

taminated sediment away from the primary sediment

accumulation area at Randle Reef (Versteeg et al.,

1995, 1997).

The thinning of the contaminated layer is also

evident in the pattern of magnetic intensity within the

adjacent meander channel (Fig. 5B). The segment of

the channel closest to the reef shows high magnetic

intensities, while sections of the meander further out

from the reef in deeper water show progressively

lower values. It is also noted that the meander channel

is defined by a positive magnetic anomaly when

compared to the central basin floor. This is intuitively

the reverse of what is expected for a topographic

depression (channel) on the harbour bottom, since the

fall-off in amplitude with distance from the source

should result in a relative magnetic low. The positive

anomaly indicates that the meander channel is

partially infilled with a layer of relatively high

susceptibility sediment. The likely source of the

sediment is the downslope movement of sediment

from the adjacent Randle Reef. Brassard and Morris

(1997) and showed that sediment resuspension by

waves was an important process in moving sediment

into the deeper parts of the basin.

The magnetic response of the meander channel

(Fig. 5B) provides further evidence that the total field

survey is responding to a shallow layer of high

magnetic susceptibility sediment. In order to verify

this result, and to constrain the probable thickness of

the contaminated layer, 2-D forward models were

constructed for a west–east profile across the western

edge of Randle Reef (Fig. 6). The models were

constructed using a proprietary modelling package

(GM-SYSk) which implements the Talwani polygon



method to calculate the magnetic response of source

bodies of various geometries (Talwani, 1965; Won

and Bevis, 1987). The models were constructed by

extraction of the bathymetric and residual field values

along a single transect across Randle Reef and the

average j values assigned based on the core data of

Versteeg et al. (1997) (Fig. 2B). The magnetic residual

field data used in the model were not drape corrected

so that the effects of water depth variation across the

reef could be evaluated.

Fig. 6 shows the model results for two different

scenarios: (A) a single layer of homogeneous sediment

with j=5�10�5 cgs and (B) a two-layer case with an

upper layer of high susceptibility sediment (j=5�10�4

cgs) overlying a low susceptibility (pre-colonial) sedi-

ment layer (j=1�10�5 cgs). The single layer case

demonstrates the magnetic response for a constant j, inwhich the resultant magnetic anomaly is due solely to

changes in bottom relief and water depth (Fig. 6A). The

calculated response shows clearly that the magnetic

field strength mimics the bottom relief, with bathy-

metric lows (i.e. the meander channels) corresponding

with relative magnetic lows. Comparison of the

calculated with the observed residual field, however,

shows that two are anti-correlated and that the

observed magnetic profile cannot be explained by

water depth variations as in this simple model (Fig.

6A). The maximum variation in the calculated

magnetic intensity due to water depth changes is about

28 nT (Fig. 6A) and serves to illustrate the importance

of draping corrections in high-resolution marine

magnetic surveys.

In the two-layer case (Fig. 6B), the magnetic

susceptibility of the upper layer was set as constant,

equal to the average j value obtained from core data

(j=5 x=10�4 cgs). The thickness of the upper layer

was then varied along the profile until the calculated

residual field closely matched the observed signal with

a small residual error (b4 nT). The result shows that

the observed response is consistent with the presence

of an upper layer of relatively high susceptibility

sediment that thickens within topographic lows and

Fig. 6. 2-D forward modelled magnetic profiles across western edge of Ran

5B). The observed residual magnetic profiles were not drape corrected to a

case of sediments with uniform j=5�10�5 (no upper contaminated layer)

urban-sourced sediment (j=5�10�4 cgs) overlying lower susceptibility pr

100 m in both cases (not shown).

thins across the basin highs. The estimated maximum

layer thickness within the meander channels is about 5

m, which is comparable to the estimated maximum

depth of contaminated fill within the Randle Reef area

(N6 m) (Versteeg et al., 1997). The depth of contami-

nated sediment here reflects the extended history and

large volume of discharge of industrial and urban

effluents into this part of the harbour.

The estimated thickness of the sediment layer

obtained through 2-D modelling (Fig. 6B) can only be

considered as an estimate, as the model assumes

layers with a constant magnetic susceptibility. In

reality, the magnetic susceptibility varies laterally

and also from the bottom to the top of the

contaminated layer as shown in the core data in Fig.

2A. Several model runs were attempted with higher

and lower bulk magnetic susceptibility values for the

upper and lower layers. These runs resulted in a

thicker or thinner contaminated layer but in all cases,

the model showed thickening of the infill within the

bathymetric lows as in Fig. 6B. The constant

susceptibility model shown in Fig. 6B best replicates

the available core data and is considered to be

reasonable estimate of the upper layer thickness along

the profile. Most importantly, the model results

indicate that the magnetic variations across the reef

cannot be simply the result of changes in water depth

(i.e. Fig. 6A) but require the presence of a highly

magnetized upper sediment layer.

5.2.2. Dredge basin high-resolution survey

The results of the detailed survey of the dredge

basin are shown in Fig. 7. The deep rectangular basin

(ca. 1000�600 m) is the result of systematic

dredging of an average of 10 m of sediment from

the harbour bottom. The basin has a rugged bottom

topography (8–23 m below lake level) made up of

linear ridges and excavated troughs that are oriented

in the direction of dredging to the northeast. The

centre of the basin is crossed by a prominent ridge of

sediment that rises more than 10 m above the

excavated areas (Fig. 7A). The sediment ridge stands

dle Reef and submerged meander channel (location shown in Fig

llow evaluation of effects of water depth changes. (A) Single-laye

. (B) Modelled profile for case of upper layer of more magnetized

e-colonial sediments (j=1�10�5). Depth of lower layer extends to

.

r

Fig. 7. (A) High-resolution bathymetry map for dredged basin. (B) Magnetic residual field maps for same area. Magnetic lows correspond with

dredged areas and ridge-like magnetic highs with unexcavated ridges of contaminated sediment. Point magnetic targets indicate the location of

ferrous refuse.


out clearly as a sinuous magnetic high on the

magnetic residual map, while the surrounding

dredged areas correspond with lower magnetic

intensities (Fig. 7B). The magnetic contrast between

the ridge and the excavated area is substantial (up to

30 nT) and cannot be attributed to changes in water

depth, since these have been compensated with drape

corrections during processing.

The measurements from a single core taken at the

eastern edge of the magnetic high show high

susceptibility values (N2�10�4 cgs, Fig. 2B) and

identify the ridge as a mound of contaminant-

impacted sediment (Versteeg et al., 1997). The sedi-

ment ridge was apparently left intact during dredging

operations. The lower magnetic intensity of excavated

areas thus reflects the removal of the more magnetized

upper sediment layer. It is also noted that the magnetic

high defining the ridge is much broader than its

corresponding bathymetric ridge (Fig. 7A,B); this

may indicate a dispersion effect, whereby ridge

sediments are being eroded and transported into the

deeper excavated areas by bottom currents (Brassard

and Morris, 1997) or by slumping of steep side slopes.

The site is of interest from a sediment remediation

standpoint, because it demonstrates that magnetic

surveying together with detailed bathymetric mapping

can be used as a tool to monitor the progress and

effectiveness of dredging activities.

6. Apparent susceptibility mapping

As a further aid to evaluating the magnetic survey

results, the apparent magnetic susceptibility ja was

Fig. 8. (A) Apparent susceptibility map calculated from the IGRF-corrected total field data. Susceptibility data are draped on sun-shaded

bathymetric surface from Fig. 4A. (B) Qualitative sediment classification map showing relative contaminant impact levels across harbour.

Sediment impact levels defined on basis of core data from Randle Reef (Versteeg et al., 1997).



calculated from the total field data using Eq. (1.2)

(Fig. 8A). As noted above, the apparent susceptibility

filter does not take into consideration the sediment

magnetic remanence and is used here only as a

qualitative estimator of the spatial variations in bottom

sediment magnetization and pollutant impacts. The

range of the calculated apparent susceptibilities (10�6

to 5�10�4 cgs, Fig. 8A) is comparable with the core

susceptibility values and shows good spatial corre-

spondence with the previously mapped j distribution

(Fig. 2B). As with the residual field map, the localized

detail is much enhanced due to the much greater

spatial sampling resolution of the ja map. The

contaminated areas at Randle Reef, the steel works

outfall and the dredged basin are all clearly identified

as areas of high magnetic susceptibility (N2�10�4

cgs). The map also indicates several localized areas of

elevated ja values and areas of possible bottom

contamination along the north shore of the harbour

and within the central deep part of the harbour basin.

A further potential application of the apparent

susceptibility map is that it can be used as a rapid

means for qualitatively classifying the relative con-

taminant impact levels in bottom sediments. For

example, by establishing a dbackgroundT threshold

level for the magnetic susceptibility of uncontami-

nated sediments (i.e. based on core data; Fig. 2A), it is

possible to use the magnetic survey results to classify

contaminated areas. The core data of Versteeg et al.

(1997) indicate a background value of b10�5 cgs is

typical for uncontaminated pre-colonial sediments

within the harbour (Fig. 2A). Fig. 8B is based on a

simple classification scheme that identifies areas of

low (N1�10�5–5�10�5 cgs), moderate (N5�10�5–

1�10�4 cgs) and high contaminant impact levels

(N1�10�4 cgs). The moderate and high impact classes

were defined with reference to the core susceptibility

data from Randle Reef, the most heavily contaminated

site in the harbour. The exercise shows that a large

area of the harbour (N60%) is significantly above the

background threshold and more than 30% of the

bottom sediments have probable contaminant impacts

at the moderate or severe levels. Although the scheme

is qualitative and is not intended be used to predict

actual pollutant concentrations, it serves to demon-

strate that magnetic survey data can be used as a basis

for classifying probable sediment impact levels. Such

a classification could be employed for example, to

design a core sampling strategy during the reconnais-

sance phases of mapping a contaminated waterway.

7. Discussion

This study shows that strong contrasts in magnetic

susceptibility associated with the presence of urban-

source magnetic oxides in lake sediment may be

detected using a towed marine magnetometer. System-

atic magnetic mapping of Hamilton Harbour identifies

a number of positive magnetic anomalies on the

harbour bottom that can be related to discrete point

source inputs of urban and industrial effluents

identified in previous coring work (Morris et al.,

1994; Versteeg et al., 1995, 1997). Comparison of

survey data with core-derived magnetic susceptibility

maps and the results of forward modelling indicate

that the positive anomalies are generated by a thin

upper layer of high susceptibility contaminated sedi-

ment. The calculated apparent susceptibility (Fig. 8A)

and the core-derived susceptibility maps (Fig. 2A)

show similar magnetic anomaly pattern, which can be

attributed to changes in the thickness of the contami-

nated layer across the harbour. These results indicate

that marine magnetic surveys are a viable and

complimentary approach to core-based geochemical

sampling in situations where there is a need to map

large basins for reconnaissance purposes.

The major advantage over core-based methods is

the increased sampling density that can be achieved

when the survey is acquired as a grid-work of closely

spaced survey lines that systematically cover the lake

bottom (Fig. 3). In this study, we conducted surveys

with line spacings as small as 50 m and a magneto-

meter sampling rate of 4 Hz. This yields approx-

imately one measurement every metre in the inline

direction at boat speeds of ~15 km/h. The high density

of data obtained allows for greatly enhanced spatial

resolution and recording of magnetic anomalies with

inline spatial frequencies as small as 2 m. This

translates into highly detailed images of bottom

sediment magnetic response (e.g. Figs. 5 and 7) that

can be used to map the post-industrial sediment

distribution. A further advantage is that magnetic

surveying is non-invasive and avoids sediment dis-

turbance and the resuspension of contaminants. The

application of magnetic survey methods during the


initial stages of a remediation program could also

significantly reduce the total costs of a clean up

program by focussing efforts on the most heavily

impacted areas (e.g. Fig. 8B), thereby reducing the

total number of boreholes required. In the case of

Hamilton Harbour, which was chosen primarily as a

test case in this study, the distribution of contaminated

sediment has been reasonably well characterized

based on previous coring work. The magnetic map-

ping in this study serves to verify previous work and

also provides much higher resolution images of the

spatial distribution of urban sediments within the

harbour. It should also be emphasized that while

Hamilton Harbour is relatively well understood,

significantly less is known about contaminated sedi-

ment distribution in the other areas of concern within

the Great Lakes basins.

The detection of shallow-sourced magnetic anoma-

lies is dependent upon collection of closely spaced

survey lines (Fig. 3) and careful post-cruise process-

ing to remove diurnal and other systematic errors.

Simultaneous collection of high resolution bathymetry

data is also critical for removal of water depth-related

variations in the magnetic field intensity that would

otherwise mask subtle magnetic anomaly patterns.

The high-resolution bathymetry data also aids in the

interpretation of contaminated sediment accumulation

patterns. Figs. 5 and 7 demonstrate that bathymetric

lows are commonly areas of contaminated sediment

accumulation in Hamilton Harbour, most likely as a

result of the down gradient transport of sediment by

wave resuspension and bottom currents (Brassard and

Morris, 1997). Other areas of contaminated sediment

are associated with positive bathymetric features such

as mounds and ridges formed at the effluent discharge

points into the harbour (e.g. Randle Reef, steel works

outfall; Fig. 5B) or where bottom dredging has taken

place (Fig. 7B).

7.1. Limitations of method

A primary limitation of all magnetic proxy

methods is that they cannot directly determine the

actual pollutant concentrations in sediment. It is

unlikely, therefore, that magnetic surveying will

replace conventional geochemical analysis, but it

does offer an effective and rapid approach for

reconnaissance mapping of large contaminated water-

ways. Magnetic surveys could be employed, for

example, during the early stages of remediation to

map the location of impacted bottom sediments prior

to detailed coring and geochemical sampling. This in

turn would increase the efficiency and reduce the cost

of remediation activities by allowing sampling efforts

to be focussed on the impacted areas.

Another important limitation is that the method is

only applicable in areas where the natural (pre-

colonial) bottom sediments have a low magnetite

content and low magnetic susceptibilities when

compared to urban-sourced sediments. The method

is likely to be unsuccessful, for example, in Precam-

brian shield areas where high susceptibility crystalline

bedrock is at or near the surface. Under these

conditions, the measured magnetic field strength will

be dominated by the bedrock response. Areas that

have favourable geology for shallow magnetic map-

ping include much of the lower Great Lakes basins

and other continental and coastal areas that are

underlain by a substantial thickness of low suscept-

ibility sedimentary cover rocks. Even in these areas,

an understanding of the availability and distribution of

naturally occurring magnetic minerals in sediments is

critical for interpreting the magnetic survey results.

This requires that some core data will need to be

collected prior to the survey, to allow determination of

the contrast in susceptibilities of natural and urban

sediments.

Areas of extreme magnetic gradients (i.e. N100 nT/

m), for example near the steel works docks in

Hamilton Harbour, render total field data unusable

for the purposes of mapping sediment response. This

is an important restriction since many contaminated

waterways lie within close proximity of steel-con-

structed docks and other ferrous infrastructure. The

presence of submerged or buried ferrous materials

such as underwater pipelines and refuse (e.g. Figs. 5B

and 7B) may also limit surveying in some urban

waterways. In most cases, however, ferrous objects

can easily be recognised by their characteristic dipole

form and can be removed during data editing.

While magnetic surveys can provide excellent

discrimination of the areal distribution of contami-

nated sediments (Fig. 8A), they provide less direct

information about the depth extent and sub-bottom

continuity of contaminated layers. Forward and also

inverse modelling of magnetic data (e.g. Fig. 6) can


provide estimates of layer thickness but require that

sediment magnetic susceptibilities are well con-

strained from core data. Other approaches involve

integrating magnetic surveys and core-data with

acoustical methods that directly image the sub-

bottom stratigraphy. In recent work, we have

acquired magnetics simultaneously with broad-band

(5–15 kHz) chirp sonar (Eyles et al., 2003).

Typically, the sub-bottom sonar profiler achieves

decimetre-scale resolution of sediment layering and

also permits quantitative classification of bottom

sediment type and geotechnical properties based on

analysis of the reflected acoustic impulse (Leblanc

et al., 1992; Caufield and Filkins, 1999). Ongoing

work is aimed at dfusingT magnetic images with

sediment type and thickness maps generated from

chirp sonar data. It is anticipated that the integra-

tion of magnetic, bathymetric and acoustic sub-

bottom data will ultimately lead to an improved

and more rapid means of estimating the thickness

and total volumes of contaminated sediments

requiring clean-up.

8. Summary

Magnetic surveys have been employed in Hamilton

Harbour to map the extent of a contaminated upper

layer of urban-sourced sediment. The resulting mag-

netic anomaly maps ((Figs. 4B, 5B and 7B)) corre-

spond closely with core-derived data from previous

studies (Fig. 2B) and serve to better resolve the

location of contaminant outfalls and sediment accu-

mulation areas within the harbour. Forward modelling

of the magnetic results shows that the maximum

thickness of contaminated sediments within Randle

Reef, the most heavily impacted area of the basin, is

about 5 m. The pattern of anomalies surrounding

Randle Reef indicates that bathymetric lows in the

harbour bottom are primary accumulation sites for

contaminated sediments. Apparent susceptibility maps

calculated from magnetic survey data provide a rapid

means for classifying sediment impact levels prior to

the collection of detailed core and geochemical data.

This approach is likely to valuable in the initial stages

of mapping large contaminated waterways where

coring on a grid basis is prohibitively expensive and

time consuming. Mapping of a dredged area in the

eastern harbour has shown that magnetic surveying

also has some potential as a method for monitoring the

progress and effectiveness of dredging operations once

remediation work has begun.

Acknowledgements

This project was supported through a Natural

Science and Engineering Research Council of Canada

grants to Boyce and a Centre for Research in Earth

and Space Technology grant to Morris and Boyce.

The authors thank K. Versteeg for access to core data,

D. Hrvoic and M. Marlowe (Marine Magnetics) for

technical support, and C. Clark for assistance in the

field. The processing of magnetic survey data was

facilitated by an academic software grant from

Geosoft.

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steel works outfall

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