Saturday, 15 July 2006

Using Magnetic Spherules for Measuring Soil Erosion and Pollutant Transfer.

Alexander N. Gennadiyev1, Kenneth Olson2, and Serge S. Chernyanskii1. (1) Moscow State Univ, Faculty of Geography, 1 Leninskiye Gory, Moscow, 119992, Russia, (2) Univ of Illinois, Dept of Natural Resources and Environmental Sciences, W-503 Turner Hall, 1102 South Goodwin Ave, Urbana, IL 61801

Among the methods to measure soil erosion, the uses of soil microcomponents as markers of soil mass transfer stand out. Most prominent one is radio-cesium methodology. Recently, attention has been focused on the method of magnetic tracer based on evaluating both spatial and temporal changes in supplies of magnetic spherules (MS) in soil catenas. This method was developed in USA during the 1990s. In the last 5 years it has had widespread applications in Russia. MS are derived from pyrolytic processes, such as coal burning. With the advent of railroads (about 150 years ago), technogenic spherules began to fall out intensively on the soil cover. As was registered in Yaroslavl, one of the main industrial centers of the European Russia, the current rate of MS accretion on soil cover may reach 0.1 to 1.2 grams per square meter. The procedure of MS extracting and calculating is quite simple and cost-effective, but somewhat time consuming. Following chemical (or ultrasonic) destruction of soil organic matter (aggregates) and wet magnetic separation, polarizing microscope equipped with digital camera and appropriate software can be used to identify spherules in magnetic fractions and calculate their total share. Duplicating transects and appropriately repeated sampling provides statistical validity of the results. Since 1999, some 1500 soil samples have been analyzed according to the described procedure. This enabled the estimation of the rates of soil erosion over the various regions both in the USA and in Russia. In most cases, soils contained 5 to 160 grams of MS per square meter, with 50-95 % of spherules falling within the upper 5-20 cm. From the 1 to 50 micrometers of the observed range limits, the spherules having 5 to 35 micrometers in diameter are the most abundant. Loamy chernozems, sod-podzolic and grey wooded soils occupied semi-flat interfluves demonstrate moderate degree of MS supplies spatial variability, with coefficient of variation of 25-27 %. Since vegetation serves as a filter for air flows, soils under mature forests normally contain 1,2 to 3 times more MS as compared to soils of non-cultivated grasslands. Cultivated soils show stronger depletion in MS and their profile and lateral redistribution as a result of tillage-induced erosion, translocation and soil removal with harvest and on wheels of agricultural equipment. The forested sod-podzolic soils of the Moscow region developed on slopes of up to 8 degrees demonstrate MS losses equivalent to 10 to 15 t ha-1 of the annual removal of topsoil material (as averaged for the 150 yr period of intensive fly ash fall out). In Tula region, cultivated chernozems of gentle slopes have lost 5 to 70 % of MS or 18 cm thick topsoil, with mean annual losses being equal to 8 t ha-1. Since the majority of soil landscapes are located within open river basins, many of magnetic spherules are transported to valleys with surface run-off. The share of this MS fraction can range from 5-7 % to 40-50 % depending on soil properties, topography conditions, land use regime and other factors. The magnetic tracer methodology was also successfully applied to assessing rates of archaeological monument destruction. Cahokia mounds (USA, IL, and Mississippi Valley) including the 30 m high Monks mound, one of the highest North-American earthen monuments, have been the subject of this research. Different mounds varied in land use history, so it was possible not only to estimate their physical destruction, but to find links between changes of MS soil supplies and periods of human activities in this area. Since technogenic magnetic spherules are chemically inert, they do not have a place in a list of high-priority pollutants. At the same time, MS have a potential to be used as effective tracer for toxic pollutants associated with fly ash. For example, coal burning results in emission of magnetic spherules along with polycyclic aromatic hydrocarbons (PAH) which can produce toxic, carcinogenic and mutagenic effects. Unlike MS, PAH usually demonstrate instability in atmosphere and upper soil layers where they are exposed to relatively intensive physical, chemical and biochemical destruction. Ratios between MS and PAH in soils and atmospheric fallouts have been studied in detail and quantified by the example of a highly polluted area. Based on these data, flows of PAH translocations in soil cover have been revealed and rates of their destruction determined.

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