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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 312 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) 314 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 315 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 316 Environmental Geology 39 (3–4) January 2000 7 Q Springer-Verlag 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. References Borůvka L, Huan-Wei C, Kozák J, Krištoufková S (1996) Heavy contamination of soil with cadmium, lead and zinc in the alluvium of the Litavka river. 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