Environmental magnetism — the role of magnetic particles in tracing environmental pollution by anthropogenic dust

Abstract

Environmental magnetism, a relatively new field of science, involves the application of standard rockmagnetic
techniques to solve problems arising in palaeoclimatic and provenance studies in various sediments, in studies of
pedogenesis in soils and in the detection of environmental pollution. In this paper the authors are concerned only with the
latter. Anthropogenic pollution sources, such as coal burning plants, steelworks, cement factories and vehicular traffic,
all produce magnetic particles which after travelling some distance in air may be deposited on vegetation and buildings
or fall directly onto the topsoil. With modern equipment it is possible to get useful magnetic signals from the
environmental material even if the magnetic component is just a minute fraction of the sample. Magnetism can thus be
used as a tracer of environmental conditions. For example soil pollution by heavy metals can be detected by susceptibility
measurements given the correlation established of high magnetic susceptibility with elevated concentrations of Cu, Pb
etc. (STRZYSZCZ 1993, STRZYSZCZ et al. 1996, HAY et al. 1997, HANESCH & SCHOLGER 2002, MÁRTON & MÁRTON 2006).
After reviewing the basics of magnetic susceptibility, we proceed to present results of the application of this technique as
outlined below.
The susceptibility of samples from tree trunks were measured (Figure 1) to detect pollution against distance from a
road with heavy traffic along a perpendicular bystreet and found that the pollution could be traced to a distance of about
90 m from the main traffic (Table I). Angular and spherular particles using SEM of Fe-oxide and sulphide as well as of
the metallic Fe composition, these were the sources of the susceptibility signal, were also identified(Figure 2).
The North-Hungarian Inspectorate for Environmental Protection runs monitoring stations in the township of Miskolc
and its environs (Figure 3) and these collect monthly samples of settled dust. Both water soluble and water insoluble
components of the samples are weighed. Their samples of water insoluble dust collected between February, 2005 and
April, 2006 were studied by susceptibility measurement. When all the data were put together, it turnd out that the massspecific susceptibility tends to decrease with increasing sample mass. This suggests that the magnetic pollution can be
high even if the quantity of dust remains low (Figure 4). The highest magnetic signals were obtained for the station of
DAM Rt. (an electrosteel works). This was the main source of industrial pollution, the effect of which (along with
neighbouring ironworks) can at times be detected even as far away as Szent Ferenc Kórház (an hospital and TBsanatorium). However, it is reassuring that housing (e.g. Martintelep) and recreation areas (e. g. Tapolca) are relatively
clean of magnetic pollution (c. f. Figures 3 and 5).
Finally, we studied settled dust samples from Cluj County (Romania) which had collected under the same conditions
as the ones in Miskolc. By courtesy of FARKAS & WEISZBURG (2006) we received samples from nine stations for various
months between March and June, 2003. They had already separated these into seven grainsize fractions from >400 µm to
<1 µm; the second smallest of these fractions (32 µm – 1 µm) contributes most to the mass of the samples (Figure 7). Even
after sampling for mineralogy, all samples were of sufficient mass to measure the magnetic susceptibility, but meaningful
values of mass specific susceptibility were calculated only for samples/month/fractions as shown in Table II. It is
remarkable, that irrespective of the source of pollution (shown in op. cit. above) it is the (32 µm – 1 µm) grainsize fraction
which exhibits significant mass specific susceptibility in almost all samples and also this is the grainsize range in which
the occurrence of the Fe-oxide spherules may be expected.
Pilot samples were subjected to Curie temperature runs both from Miskolc and Cluj County to identify the source of
magnetic signals and this turned out to be magnetite (Figure 8). The concentration of magnetite goes up to 13% of the
mass in the Miskolc samples and but only to 3% in the Cluj county samples.