Preliminary Results of Magnetic Characterisation of

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Soils in the Tandil Region (Argentina) Affected by the Pollution of .... The soil profile at Na 2 presents common rock fragments less than 3mm in diameter.
EUROPEAN JOURNAL OF ENVIRONMENTAL AND ENGINEERING GEOPHYSICS, 7, 35-58 (2002)

Preliminary Results of Magnetic Characterisation of Different Soils in the Tandil Region (Argentina) Affected by the Pollution of Metallurgical Factory

M. A. E. Chaparro1,2, C. S. G. Gogorza1,3, A. Lavat4, S. Pazos5, A. M. Sinito1,3 (Received May, 2000; revised version accepted July, 2001)

Abstract. Magnetic measurements of soils exposed to pollution became one auxiliary tool for estimating contamination. In this work, preliminary studies of magnetic properties of recent soils from the Tandil region, Buenos Aires Province (Argentina) are presented, paying special attention to ‘magnetic enhancement’ on topsoil’s layers, this enhancement may be explained by the fallout from the atmosphere of pollutant particles generated by metallurgical factories. An integrated analysis of magnetic parameters was carried out: susceptibility (in situ, laboratory, frequency dependence, temperature dependence), Isothermal Remanent Magnetisation (IRM curves, SIRM, BCR, S ratio, χ vs. SIRM), in order to know, in a qualitative way, the magnetic mineral composition, grain size, and characteristics of the main carriers. X-Ray Diffraction (XRD) analyses support the magnetic results. The comparison of the results showed significant differences between the concentration and grain sizes of ferrimagnetic oxides present in polluted and unpolluted or less polluted soils. Keywords: environmental magnetism, pollution, soils. ________________________ 1

IFAS, Universidad Nacional del Centro de la Provincia de Buenos Aires (UNCPBA), Pinto

399, 7000 Tandil (Argentina) e-mail: [email protected] 2

Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CICPBA)

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Comisión Nacional de Investigaciones Científicas y Técnicas (CONICET)

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Facultad de Ingeniería, UNCPBA, Av. Del Valle 5737, 7400 Olavarría (Argentina)

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Facultad de Agronomía, UNCPBA, C.C. 47 - 7300 Azul (Argentina)

ISSN: 1359-8155. ©2002 Geophysical Press Ltd.

CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

Introduction

The ‘magnetic enhancement’ has been greatly studied, its origin can be related to several phenomena (Tite and Linington, 1975). Many authors (Mullins, 1977; Thompson et al, 1980; Maher, 1986) studied different mechanisms responsible for the enhancement, such as, burning, chemical processes, magnetised particles emission, etc. It has been known for many years that the development of a soil profile is accompanied by an increase in the magnetic susceptibility of the A horizon. This enhancement has been attributed to a variety of processes, including fire, biological activity and oxidation/reduction reactions. Comparisons of magnetic susceptibility profiles through the different soils in a chronosequence have shown that the enhancement extends as well into the B horizon of a soil and that the mechanism of enhancement involves both preferential accumulation of inherited (lithogenic) magnetic minerals and formation of new (pedogenic) magnetic minerals by soil-forming processes (Verosub and Roberts, 1995). Many authors have related the amount of susceptibility enhacement to the quantity of organic matter in the soil (Mullins, 1977). The main role of organic matter seems to be providing a substrate for heterotrophic micro-organisms which give the reducing conditions required to get iron into solution in the first place. The power and persistence of these reducing conditions will be a function of the quantity and type of organic matter and associated micro-organisms, as well as of the soil temperature and water regime. Mechanical and chemical weathering processes produce major changes in sediment source type, especially through glacial-interglacial sequences, but during Holocene their

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main effect is in terms of soil development which in temperature zones may include magnetic enhancement of surface horizons. Other mentioned mechanisms causing enhancement is burning; fire gives rise to secondary

magnetic

minerals

in

the

soil

by

converting

paramagnetic

or

antiferromagnetic forms of iron to predominantly ferrimagnetic oxides. The degree of conversion and hence the amount and type of magnetic mineral thus formed is related largely to the initial concentration of potentially convertible iron in the surface soil, the atmosphere during combustion and the maximum temperature reached. The dust particles that are richest in heavy metals are fly-ashes, which are emitted from coal-burning power plants and industrial smelters. The heavy metal content of aerosol and fly-ash samples is significant with regard to air quality. To evaluate air quality, rapid means for assesing levels of industrial pollution are needed. A number of studies have indicated potential relationships between heavy metals and magnetic mineral concentration dependent parameters for both aquatic and atmospheric environments. (Hunt et al, 1984; Beckwith et al, 1986; Strzyszcz et al, 1996; Heller et al, 1998; Durza, 1999, Hoffmann et al., 1999, Petrovsky et al., 2000). These studies, among others, indicate the possible existence of linear correlations between metals and magnetic parameters. Some authors note that most metals in coal-fired fly-ash samples were associated with specific surface oxides of iron, manganese or aluminium. They record copper, chromium, arsenic and zinc, as being associated with iron oxides in almost all cases, cadmium and nickel mostly with manganese, and lead with either.(Thompson and Oldfield, 1986). Petrovsky and Ellwood (1999) present a complete review of recent applications of magnetic methods to pollution studies and analyse the advantages and disadvantages of these methods. In this work different examples are mentioned, in some

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CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

of them high correlation coefficients were found between the magnetic susceptibility of soils and Cu, Pb, Zn and Ni content, in another there is not convincing correlation between magnetic susceptibility and specific concentrations of individual metals, but there is a good correlation with the sum of the heavy metals. In other studies the correlation is not so strong, probably due to the high levels and variability of natural background heavy metals. Magnetic methods, as proxy for more time-consuming chemical methods, can provide information concerning pollution sources and their spatial distribution. Magnetite and hematite are the iron oxide minerals that are most found in industrial fly-ashes. Also, during crystal growth in melts, magnetite and hematite are produced from iron-bearing clay. Pyrite is one of the most important iron containing minerals in coal. The aim of our research is the preliminary study of the influence of fallout from the atmosphere of pollutants, derived from metallurgical factory combustion. Magnetic studies would allow determining the pollution condition in affected zones, through fast and inexpensive methods carried out in situ. These methods are nondestructive and the sampling process does not affect the environment. In this work, we chose soils from two different areas: soils on urban centre, that is, near metallurgical factories, named “factory”, and soils from zones far from the urban centre, named “natural”. The goal was to analyse the characteristics of these soils using magnetic measurement techniques, to identify magnetic enhancement due to the influences mentioned above and to know, in a qualitative way, the magnetic composition and features of the main carriers.

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MAGNETIC CHARACTERISATION

The magnetic results may be supported by studies on mineralogical composition of soil samples in order to carry out a systematic correlation between the phases present and the magnetic behaviour. X-Ray Diffraction (XRD) is the most widely used technique for characterisation of ceramic raw materials, soils and sediments. More detailed information about the chemical bonds and structural units present in each component may be obtained with the aid of spectroscopic tools. Geochemical analyses allow to compare magnetic results with heavy metals content.

Industrial Environment, Sampling Site and Geological Setting

Although there is a general perception that pollution is a problem in the major urban and industrial centres of Latin America, data on industrial pollution in the region is fragmentary and lacking in comparability both over time and between countries. Data air pollution indicates that in a number of Latin American cities, concentrations of pollutants are above the World Health Organization guidelines for air quality standards. But it is very difficult to obtain estimates of industrial emissions and effluent, particularly over time. This partly reflects the lack of monitoring of pollution in the past in Latin America. (Jenkins, 1998). The carbon dioxide emissions in Argentina are available and show a moderate growth between 1970 and 1995. In 1997, carbon emissions from the industrial sector (15.1 million metric tons of carbon) comprised 43% of all carbon emissions in Argentina. The majority of industrial carbon emissions in Argentina is the result of coal-fired iron and steel plant operations (data from the Energy Information Administration of USA).

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CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

Unfortunately similar estimates are not available for other industrial emissions. Therefore in order to get estimates of industrial emissions for a much wider range of pollutants Jenkins (1998) calculated another set of indicators using the World Bank’s Industrial Pollution System (IPPS). The IPPS consists of a set of coefficients, which relate emissions of pollutants to value added, output or employment. These have been calculated by the World Bank from US data on emissions and industrial production and they can be applied to industrial data for other countries to obtain pollution estimates. Using IPPS, estimations of particulate concentration in water body, total suspended solids, PM10, sulfur dioxide, carbon monoxide, volatile organic compounds, etc. were carried out. The rate of growth of the pollution intensity of Argentinian manufacturing production in three five year periods from 1980 to 1995 (Jenkins, 1998) show an increase of most of pollutants in the first two periods, and a decline in overall pollution intensity, with the exception of metals and total suspended solids, in the last period. The sampling sites are in the city of Tandil, Buenos Aires Province (Argentina), located near the Tandilia belt. Tandil is a relative small city (about 100,000 inhabitants) with few factories (about 0.2 per Km-2) located in the urban area, where the pollution problem is even young and it is not of long-range. Small industries already had settled in Tandil in XIX century (that of the leather, milky, breweries, etc.). Some of these like the forges -with their products: plows, cars, economic kitchens and carriages- and the flour mills had a strong impulse beginning in 1883 (Dipaola, 1998; UNCPBA, 1977). Since the elaboration overcame, as for quantity, to the population's necessities and the demands of the County, Tandil became the industrial center of the center of the State of Buenos Aires.

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MAGNETIC CHARACTERISATION

In 1918 the metallurgical industry arose in gravitate form in Tandil, with a factory which manufactured kitchens, stoves, top of cylinders, among other products. In 1948, another metallurgical factories and numerous foundries were installed. The Tandilia belt (Teruggi et al., 1968, Teruggi et al., 1973, Teruggi and Kilmurray, 1975) represents a physiographic province made up by hills, which rise up between 50 and 250m from the Pampean plain. The belt is not continuous, and its elevations are separated by valleys and undulated plains. This area consist (Teruggi and Kilmurray, 1975) of a Precambrian metamorphic basement, a Lower Palaeozoic or Precambrian sedimentary cover and quaternary deposits. Tertiary terrain is only probable in the subsoil. The crystalline basement is composed

by

granitoids,

heterogeneous

and

homogeneous

migmatites

and

metamorphites. The Lower Palaeozoic or Precambrian sedimentary cover by orthoquartzites, dolomites, clays and limestones. The quaternary deposits are composed by pebbles, eolianites, lacustrian clays and alluvial deposits. The opaque mineral associations found in rocks from the basement in the vicinities of Tandil city were separated in four groups: 1) Titano-Ilmenite, 2) Ilmenite-Rutile-Pyrite, 3) Pyrrothite-Calcopyrite-Co and 4) Ni minerals (Echeveste, H. and Fernandez, R., pers. com.).

Soil Sampling

Most of the soils in the Humid Pampa are developed on Pampean loess under herbaceous vegetation which also applies to the soils in this study. All the sites in this

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CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

study are located in the piedmont of the Tandil Sierras. Soils are developed in a Quaternary loess mantle of variable thickness covering the Pre-Cambrian granitic basement (Teruggi and Kilmurray, 1975). The soils classify as Argiudolls (SAGyP – INTA, 1989) according to the Soil Taxonomy (Soil Survey Staff, 1998) with a horizon sequence A – Bt – C – R. All studied soil profiles are rather young soils, and correspond to the upper A horizon or mollic epipedon, with about 5 to 6% organic matter content. Six different sites were chosen. We chose four sampling sites, each one located about 100m far from four small metallurgic factories sited in the urban area, and two “natural” sites, out of the urban area, taking care of excluding cultivated soils. The “factory” and “natural” sites are labelled Fa and Na respectively (Fig.1). The soil profile at Na 2 presents common rock fragments less than 3mm in diameter throughout its depth as a consequence of the proximity to the outcropping granite. This feature is common in the area and corresponds to the synchronous deposition of loess and erosion debris of the granite outcrops. We considered three layers at each site, a surface layer (S), another at a depth of 10 cm (1) and the last one at a depth of 20 cm (2), all of them within the mollic epipedon. On every layer, susceptibility in situ was measured in a continuous way, along two susceptibility profiles (A and B) about 2 m long (see Fig. 2). Hand samples (about 10 cm3) were taken out in each site; about 150 samples were collected to analyse at the laboratory.

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MAGNETIC CHARACTERISATION

Fig. 1. Map of Tandil, Buenos Aires Province (Argentina). Six different selected sites have been marked in the sampling region. We named “factory” soils Fa.1 to Fa.4 and natural zones Na.1 and Na.2.

Layer S

Layer 1

Layer 2

Profile SA

10 cm Profile 1A

Profile SB

10 cm

Profile 2A Profile 1B

Profile 2B 200 cm Fig. 2. Schematic diagram of field work for the soil sampling.

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17 cm

CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

Measurement techniques

Magnetic susceptibility was measured using a Bartington MS2 Magnetic Susceptibility Meter, linked to MS2F Field Sensor or MS2B Dual Frequency Sensor (high and low frequency, 470 and 4700 Hz), for measurements in situ or at the laboratory, respectively. For the laboratory measurements, two readings were taken for each sample and an average was calculated. The accuracy is about 1%. Using the first sensor, magnetic susceptibility in situ (κ) was measured along the soil profiles. With the second sensor several magnetic parameters were researched, such as, specific susceptibility (χ), frequency-dependent susceptibility (Dearing et al., 1996) ( χ FD % =

χ LF − χ HF * 100 ). Temperature dependence of magnetic susceptibility was χ LF

measured using a devise for the Bartington Susceptibilimeter, developed in our laboratory. The latter measurements were carried out using complementary equipment: furnace (the range was between 20oC to 700oC), digital thermometer, and a suitable water-jacket that isolates thermally the MS2B sensor from hot specimen. The heating was carried out in air. The heating rate was about 10°C/min; the cooling rate was approximately exponential, the average values were 135°C/min for temperatures above 350°C and 42°C/min for temperatures below 350°C. Unfortunately the accuracy equipment does not allow a more detailed monitoring of the sample behaviour at low values of susceptibility, i.e. at very high temperatures. Isothermal Remanent Magnetisation (IRM) was carried out. A Pulse Magnetiser PM-4 (prototype built by Dr. H Bohnel) and a Digico Fluxgate Spinner Rock Magnetometer were used.

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In order to carry out these measurements, the samples were appropriately prepared at the laboratory. They were hardened using a Sodium Silicate solution. The specimens were magnetised in growing steps, reaching the Saturation IRM (SIRM) and backfield was applied afterwards; about twenty four (24) steps with direct and back field were performed. The range of applied field was between 0 to 1.1 T. IRM acquisition curves; SIRM, remanent coercivity field (the reverse field needed to remove remanent magnetisation, BCR), S ratio (IRM-100mT/SIRM), and SIRM/χ ratio was obtained. The major and minor mineral phases in some soil samples were identified by X-ray diffraction, using an automatic Philips PW1710 X-ray diffractometer with graphite monochromated Cu-Kα radiation, automatic divergence slit and scan speed of 0.005°2θ s-1. Also the spectroscopic behaviour of some samples was analysed by means of Fourier Transform infrared spectroscopy (FTIR). These studies were carried out in order to confirm the diffractometric results and get additional information about the mineralogical composition. The samples were previously dried at 100°C within a few hours to eliminate the interference of moisture. Infrared spectra were recorded on a Nicolet-Magna 550 FTIR instrument, using the KBr pellet technique. Chemical analyses were carried out in order to determine heavy metals (Pb, Cd, Fe, Ni and Zn) contain in the samples. The used method was Atomic Absorption Spectrophotometry with direct aspiration in air-acetylene flame. (Standard Methods for the Examination of Water and Wastewater, APHA, AWWA, WEF. 20th. Edition, 1998).

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CHAPARRO, GOGORZA, LAVAT, PAZOS & SINITO

Results

The κ measurements were taken along rows of 2m. κ curves for every site at layers S and 1 are shown in Fig. 3. The κ average values for polluted soils (Fa.1-4) range from 467x 10-5 SI to 648x 10-5 SI for layers S, from 596x 10-5 SI to 743x 10-5 SI for layers 1 and from 643x 10-5 SI to 760x 10-5 SI for layers 2 (Table 1). The κ average values for natural soils are significantly lower (a third or lower than the values from polluted sites), they range from 39x 10-5 SI to 167x 10-5 SI for layers S, from 52x 10-5 SI to 171x 10-5 SI for layers 1 and from 65x 10-5 SI to 192x 10-5 SI for layers 2 (Table 1). The χ and χFD% values were listed in Table 1. In “factory” soils χFD% values between 0.5 and 3.7% for layers S, between 1.7 and 4.2% for layers 1 and between 1.4 and 5.7% for layers 2, were found. Higher values (0-9.2 %for layers S, 3.2-8.3% for layers 1 and 5.7-9.4% for layers 2) correspond to natural areas, especially in the Na.2 zone, where the mean value was about 7.6 %. In order to take into account the potential lithogenic contribution some samples were sieved (2mm sieve). The χ of the coarser fraction is slightly lower than the χ of the finer fraction. The difference between the χ of raw samples and sieved samples (fraction