Download Low-field magnetic susceptibility: a proxy method of estimating

Cases and solutions
Low-field magnetic susceptibility:
a proxy method of estimating
increased pollution of different
environmental systems
E. Petrovský 7 A. Kapička 7 N. Jordanova 7 M. Knab 7 V. Hoffmann
Abstract A need for rapid and inexpensive (proxy)
methods of outlining areas exposed to increased
pollution by atmospheric particulates of industrial
origin caused scientists in various fields to use and
validate different non-traditional (or non-chemical)
techniques. Among them, soil magnetometry seems
to be a suitable tool, at least in some cases. This
method is based on the knowledge that ferrimagnetic particles, namely magnetite, are produced
from pyrite during combustion of fossil fuel. Besides the combustion processes, magnetic particles
can also originate from road traffic, for example, or
can be included in various waste-water outlets.
Magnetite, Fe3O4, ranks among the main ferrimagnetic minerals and its identification in various ecosystems can contribute to fast and simple outlining
of areas and sites exposed to higher pollution impact. Here the method of magnetic mapping of anthropogenic pollution will be introduced using a review of our recent case studies on different ecosystems in the Czech Republic and Germany.
Introduction
Deposition of atmospheric particulates represents one of
the most important contributions to environmental
stresses. Apart from a gaseous phase, heavy metals in the
atmosphere are associated with dust particles, which are
comprised mainly of wind-blown soil and fly-ash particles. In general, natural mechanical processes, such as
Received: 20 June 1998 7 Accepted: 9 November 1998
E. Petrovský (Y) 7 A. Kapička 7 N. Jordanova 1
Geophysical Institute, Acad. Sci. Czech Republic, Bočni II/1401,
141 31 Prague 4, Czech Republic
Fax: c420-2-72761549
e-mail: [email protected]
M. Knab 7 V. Hoffmann
Institute of Geology and Paleontology, University of
Tuebingen, Sigwartstr. 10, 72076 Tuebingen, Germany
1
Permanent address: Geophysical Institute BAN, Acad.
Bonchev str., bl. 3, Sofia, Bulgaria
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Environmental Geology 39 (3–4) January 2000 7 Q Springer-Verlag
wind abrasion, produce coarser particles than do combustion processes. Following Godbeer and Swaine (1995),
dust particles are commonly larger than 5 mm, but windblown clay particles of sub-micron size are also common.
The average size range of atmospheric particles varies
from 0.01 mm (limited by agglomeration induced by
Brownian motion) to 20 mm (limited by gravitational setting). In fly ash, the particle size depends on the equipment employed to attenuate the pollution emission from
the source.
Magnetite, Fe 2cFe23cO4, is the most important magnetic
mineral on the earth. It is a cubic mineral with a spinel
structure and is described in detail in Dunlop and Özdemir (1997). Magnetite occurs on the continents and in
the ocean crust as a primary or secondary mineral in igneous, sedimentary and metamorphic rocks. In soils and
sediments it can also result from bacterial activity. In the
atmosphere, it can also originate from combustion (and
other industrial) processes. The cubic structure of magnetite can incorporate various toxic elements (Georgeaud
and others 1997a, b). Hence, industrially produced magnetite can be associated with these toxic elements and
early studies proving a significant relationship between
magnetite concentration in atmospheric dust and/or urban sediments on one side, and lead (and other toxic elements) on the other, are dated back to mid-eighties (e.g.
Hunt and others 1984). The whole effort has been focused on potential employment of magnetic methods in
estimating increased heavy metal concentrations.
Since magnetic iron oxides are one of the constituents of
industrial fly ash, rock-magnetic methods, as a proxy for
more time-consuming chemical methods can provide a
picture of pollution sources and their spatial distribution.
The mineralogical phases of solid waste products, including fly-ash, from coal-burning power plants have been
described for instance by Vassilev (1992). The inorganic
component of the waste material studied consisted of
amorphous glassy spherules as well as crystalline minerals. These solid particulates are found in various spherical forms, termed cenospheres, plerospheres, dermaspheres, ferrospheres (typically hollow spheres filled with
smaller spherules, differing in the mechanisms of origin)
and solid spheres (see Fisher and others 1976; Matzka
1997; Hoffmann and others 1999), varying in size from 1
to 50 mm. Ferrimagnetic metal aerosols originating from
the processing of various steels, studied by Kalliomäki
and others (1982), indicate that Fe in the form of magne-
Cases and solutions
tite is pelletized with the aid of fine-ground coke and limestone. During sintering at ;1000 7C silicates are partly
smelted and magnetite is oxidised to hematite. In the
blast furnace, reduction of hematite to metallic iron takes
place with the aid of carbon monoxide and direct reaction between hematite and coke. In this case, air-borne
dust originates both from molten steel and partly congealed slag. Concentrations of Fe in dusts and fumes originating from steel production vary in the range of tens
weight percent. Cement production can also be considered as a major source of air pollution. Strzyszcz (1995)
reported on concentrations of ferrimagnetics, namely
magnetite, in soils exposed to emissions from cement
plants and found concentrations of magnetite between
0.24–0.89% at distances of 500 m away from plants along
the prevailing wind direction. The concentration decreased with increasing distance, reaching 0.02–0.16% at
a distance of 1 3 km.
Most unburned fossil fuel is basically non-magnetic, with
the magnetic moment corresponding to F10 ppm of
magnetite by weight. However, the products of the combustion of fossil fuels can be rich in magnetite, with an
estimated magnetite content of 500–10 000 ppm by weight
(Flanders 1994). In the case of coal-ash the magnetite
content may reach 160 000 ppm. It was shown that pyrite
at temperatures of 1000 7C and higher, in the absence of
air, dissociates and forms pyrrhotite and S gas (Flanders
1994). At even higher temperatures, this pyrrhotite then
decomposes into iron and S ions, with the iron oxidizing
to form spherical magnetite particles. For every 1% increase in weight percent of sulphur in coal, a 7% increase
in iron oxide in the coal-ash is produced. Flanders (1994)
estimated that if coal-burning power plants in the U.S.
were spread uniformly across the country, and 1% of fly
ash became air borne and settled to the ground, one
would observe an increase in magnetization of 700 Am 2
per cm 2 of exposed soil surface per day.
Evidently, industrial fly ashes are rich in magnetite with
grain sizes ranging from sub-micron single-domain particles, to coarse multi-domain grains several tens of microns in size. Following the granulometric analysis
(Strzyszcz and others 1996), most of the magnetic fraction in coal-fired fly ash is present in the grain-size fraction from 2 to 50 mm. The magnetite in these samples
has a typical morphology, containing mostly spherules
with a rough “orange-peel” surface, while magnetic parameters are quite different from those found in similar
natural or synthetic magnetites.
Another important source of atmospheric pollution, although limited in extent, is road traffic. Gradient density
and magnetic separation methods were used in order to
increase the concentration of Pb compounds from automotive sources in soil samples (Olson and Skogerboe
1975). In addition to Pb compounds (lead sulphate), in
their samples they consistently found hematite, magnetite
and an iron aluminum silicate. Using microscopic observations and emission spectrography, they identified several distinct types of agglomerates and crystalline materials. One of these, a dark red crystalline material, was
usually agglomerated with magnetite. Magnetic separation was also used in another study (Hopke and others
1980). They showed that Pb, the primary constituent of
automobile exhaust before the recent conversion to the
use of unleaded gasoline, was found mainly in association
with high-density magnetic particles (presumably magnetite), while only 30% of the Pb was found in close association with soil and cement particles. The iron particles
probably result from car-body rusting or ablation from
the interior of exhaust systems and breaks. Zn, usually
occurring together with large, high-density non-magnetic
particles, is most probably derived from soils and cement. Finally, they showed that As, Cd, Cr and Co are
derived from the exhaust of automobiles and from tires
as particles from tire ware. Furthermore, road surface
(asphalt additives) represents another significant source
of pollutants. Various sources of industrial pollution produce significant amounts of source-specific magnetic
minerals, in particular magnetite. Therefore, simple and
fast magnetic measurements, related to concentration of
ferrimagnetic minerals, can be used as a proxy for spatial
distribution of pollution. For instance, in-situ field mapping of soil low-field magnetic susceptibility (MS) proved
to be a suitable method (e.g., Strzyszcz and others 1996;
Hay and others 1997; Hoffmann and others 1999; Scholger 1996, 1997a, b, 1998; Heller and others 1998). This
method is fast, cheap and enables collection of large data
sets necessary for statistical and graphical interpretation
of spatial distribution patterns of magnetic parameters
related to man-made pollutants. Moreover, history of pollution can be revealed by studying soil and sediment
depth profiles. Soil is not the only suitable carrier of
magnetic signature of pollution. For instance, concentrations of iron oxides in the upper layers of ombrotrophic
peat profiles from sites at varying distances from major
urban sources were studied by Oldfield and others (1978).
Regional heavy metal deposition in pine-tree barks was
evaluated by Huhn and others (1995). In the latter study,
MS was used as an additional tool in order to clarify the
exchange of mobile heavy metals between the bark and
wood. Finally, two-year-old needles from pines at 31 sites
in the industrial region of Leipzig-Halle in Germany were
analysed and enhanced MS of the needles was linked to
fly ash deposition from power plants (Schädlich and others 1995).
Magnetic susceptibility can be measured both in situ using portable susceptometers, or in laboratory using more
sensitive meters. The portable meter represents a loop,
supplied with a power generating low-frequency magnetic
field. When sample material is placed within the influence of this field, a change in the frequency results,
which is converted to a value of magnetic susceptibility.
Laboratory instruments are more sophisticated and
usually based on a bridge system of compensated coils.
The sample is inserted into the cavity of a pick-up coil
and the current necessary to re-balance the bridge is then
the measure of magnetic susceptibility of the sample.
The above introduction does not represent a complete review of all studies dealing with magnetic identification
Environmental Geology 39 (3–4) January 2000 7 Q Springer-Verlag
313
Cases and solutions
and mapping of pollution, which can be found elsewhere
(Petrovský and Elwood 1999), but should only provide
basic knowledge on the background of present environmental magnetism. The aim of this paper is to demonstrate this method by reviewing the results of our several
case studies, including atmospheric deposition as well as
pollution of river sediments, carried out recently on territories in the Czech Republic and Germany and thus to
introduce the method to scientists dealing with environmental pollution as a potential method of estimating areas exposed to high pollution impact. Physical or chemical bounds between anthropogenic magnetite and heavy
metals are not a subject of this study.
Results
Pollution mapping around a power plant
Soil MS in the vicinity (to ;20 km) of a brown-coal
burning power plant in the Czech Republic has been examined recently by Kapička and others (1997, 1999). In
this study, field measurements using a Bartington MS2
system were verified by laboratory data obtained on soil
samples using a KLY-2 kappabridge (Kapička and others
1997). The power plant of concern produced several
thousands of tons of fly ash per year. Using laboratory
separation by magnetic field of 300 mT, some 40wt% of
the original raw material can be extracted. SEM observations revealed spherules typical for air borne particles,
found also by Matzka (1997). Measurements of Curie
temperature proved that magnetite is the main ferrimagnetic phase present here. If the annually emitted amount
of fly ash was to be distributed evenly over the whole
studied area, the average MS value would be ten times
higher than the sensitivity of the used kappameter. Maps
of surface soil MS show unpolluted southward direction
and more polluted north-eastward direction (Fig. 1a).
However in this case, correlation with concentrations of
heavy metals (Fig. 1b) was not that convincing, probably
Fig. 1
a Contours of magnetic susceptibility of soil around Počerady
power plant (black dot) in the Czech Republic; b concentration
of Co over the same area (Kapička and others 1997, 1999)
due to the high levels and variability of natural background contents of heavy metals and the presence of several major pollution sources to the north of the study
area. Provided such mapping is carried out repeatedly,
after a certain time period (or before industrialization of
the area), development of the pollution impact can be di-
Fig. 2
Concentration of Zn in contaminated
fluvisols on the left bank of Litavka river
in a depth of 60 cm. Zero point on the
transect distance axis indicates the
contamination source (Petrovský and
others 1997)
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Environmental Geology 39 (3–4) January 2000 7 Q Springer-Verlag
Cases and solutions
Fig. 3
Magnetic susceptibility of the same
samples as in Fig. 2 (Petrovský and
others 1997)
Fig. 4
High-resolution magnetic map in a
vicinity of a road, showing clearly spatial
distribution of MS affected by a road
traffic (Knab 1997; Hoffmann and others
1999)
rectly estimated. In this case, more distant areas to the
south obviously represent relatively clean, unpolluted region.
Measurements on contaminated fluvisols
Fluvisol formation, created by the breakdown of a pit
containing ashes from a Pb smelter in the town of Příbram near Prague, were examined for magnetic susceptibility in relation to high heavy metal concentrations (Petrovský and others 1997). These soils have been analysed
chemically in detail by Kozák and others (1995) and Borůvka and others (1996, 1997). The spatial concentration
of Zn at a depth of 60 cm in the fluvisol located on the
left bank of the Litavka river is shown in Fig. 2. Distribution of MS for the same samples show a very similar pattern, as shown in Fig. 3. Qualitatively the same results
were obtained for Pb and Cd. In this case coherence is
due to a common origin of the pollutants. Detailed results will be published elsewhere.
Mapping of the effect of road traffic
Pollution due to road traffic was studied intensively by
Knab (1997) and Hoffmann and others (1999). Optical
microscope observations revealed typical spherules interpreted as magnetite-like phase. Spatial distribution, as
well as depth profiles of MS along a German motorway
Environmental Geology 39 (3–4) January 2000 7 Q Springer-Verlag
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Cases and solutions
Fig. 5
Profiles of soil-surface
magnetic susceptibility
measured along a motorway
(Knab 1997; Hoffmann and
others 1999)
Fig. 6
Average values of magnetic
susceptibility of carriers of
anthropogenic magnetic
particles (bars) and lithogenic
background (line) along the
Vltava river
have been analysed. Parallel MS profiles, measured perpendicular to the road surface to a distance of some 20 m
on each side of the road reflect a clear asymmetry in the
prevailing wind direction and a sharp maximum observed within 2–5 m of the road edge. Moreover, a small
increase of the MS values was observed as a result of
continuous atmospheric deposition during a winter period of some 3 weeks with no snowfall (not shown here).
Figure 4 shows results of a high-resolution MS mapping
on the same site using a 20-by-20-cm grid. The highest
values might be due to single iron/steel parts. In general,
the maximum susceptibility values are located directly on
the edge of the asphalt surface. The polluted area, as well
as the smaller track running nearly parallel to the road is
clearly reflected by the magnetic data. It is interesting to
note that the width of the contaminated area in this case
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does not change within the investigated area and the
highest signal is always obtained directly on the tar-surface edge.
In order to highlight the trend of the susceptibility versus
distance on the road side, all 51 parallel profiles measured of the high-resolution mapping are plotted in Fig. 5.
Starting from the tar-surface (left side), several anomalies
in the MS data were observed which could be interpreted
as the effect of various particle-transport mechanisms
due to natural processes and vegetation treatment.
Tracing minor pollution in stream sediments
Pollution produced by minor isolated sources in small
villages and towns was traced magnetically by MS measurements of stream sediments of the Vltava (Moldau)
river in the southern Czech Republic. This area is suita-
Cases and solutions
Fig. 7
Average MS values of stream
sediments and average
deviation of the data.
Magnetic enhancement and
more pronounced data scatter
can be linked to the close
sources of water pollution. The
dotted line can be interpreted
in terms of downstream
accummulation of both
anthropogenic and natural
contributions. It is constructed
as a line connecting minimum
average MS values with
minimum data scatter before
the next enhancement
ble for detecting magnetic enhancement due to human
activity because lithogenic contribution can be neglected
(mostly orthogneiss and few granitoids). Hence, constructions, gutter outlets, sawmills, paper works, boilers, etc.,
represent pollution sources isolated from each other and
contributing dominant part of magnetic particles. As a
result, enhancement in MS values of stream sediments,
measured directly on the river floor, can be attributed to
the close pollution sources, as shown in Fig. 6. Moreover,
the closer an isolated single source is, the more scattered
around the average are the data (Fig. 7), as shown by the
error bars representing the average of the absolute deviations of data points from their mean (a measure of the
variability in a data set).
Conclusion
Magnetic mapping of air-borne solid particulates of anthropogenic origin has recently become a method used
by several rock-magnetic laboratories in environmental
studies.
Measurements of low-field magnetic susceptibility of various carriers of deposited solid particulates can serve as a
proxy tool for mapping areas exposed to different pollution intensity and for tracing the pollution transport and
can be beneficially applied to various ecosystems. Moreover, the pollution development can be estimated by
measurements repeated after longer time period. Pollution history (back in time) can be revealed from measurements of depth profiles of soils and/or sediments. The
method is fast, cheap, and enables acquisition of large
data sets. However, results of magnetic mapping have to
be interpreted with respect to specific conditions of individual study sites, and special attention has to be paid to
variances in natural contents of (ferri)magnetic particles,
as well as to diagenetic processes in soils and sediments.
Despite that, it can provide a preliminary figure of pollution distribution and pathways.
Acknowledgements This study was supported by Grant Agency
of the Czech Republic through grant nr 205/96/0260, by Grant
Agency of the Academy of Sciences of the Czech Republic
through grant nr A3012605, and by a joint project between the
Academy of Sciences of the Czech Republic and Deutsche Forschungs Gemeinschaft grant to V.H.
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