Rock-magnetic indicators for soil pollution

The preservation of our natural resources -soil, water and air - is an issue of growing importance. In soils, industrial contaminants and exhaust gases lead to an accumulation of pollutants over many years. Therefore soil surveys are carried out by the Austrian provinces and samples are analysed chemically to identify the amount of heavy metals in the soils. Due to the high costs of the chemical measurements, the number of measured samples is limited. During recent years, a new method has been developed: the application of measurements of magnetic susceptibility as indicator for heavy metal pollution. It has been shown that a close correlation exists between the magnetic susceptibility and the concentration of potentially hazardous heavy metals in a sample. Magnetic susceptibility is fast to measure and cost effective. Therefore it is possible both to use denser grids and to repeat measurements within shorter time intervals. Thus, the magnetic method complements the chemical method, minimizing the expenses. The magnetic method has successfully been employed to delineate polluted areas. But the basis of the correlation between heavy metals and magnetic minerals is not yet fully understood. The nature and the amount of the pollutants can therefore not be derived from the magnetic measurements. The proposed study aims at closing this gap in our knowledge. First, the soil database compiled during previous projects will be analysed according to the different soil types. For each soil type, threshold values will be defined which distinguish between polluted and unpolluted soils. Additional magnetic parameters (remanence, hysteresis, low temperature measurements) will allow to divide the magnetic minerals in those originating from the parent material, minerals formed within the soil and minerals produced during different industrial processes or by vehicles. The goal is to compile a set of parameters which differentiates quickly between magnetic material from different sources. For each of these sources, the relation between magnetic minerals and heavy metals will be analysed in detail. Mineralogical methods like electron microscopy and X-ray diffraction will help to fulfil this task. During the proposed project, a database will be compiled containing information about the magnetic signature of contaminants from different sources and the relation between this signature and the heavy metal load of anthropogenic dusts. Thus, monitoring of magnetic susceptibility can further be developed from a more qualitative to a quantitative method.

One possibility to delineate areas polluted with heavy metals are measurements of magnetic susceptibility of soils. It is based on the fact that, during combustion processes, heavy metals and magnetic particles are produced together. Field measurements of susceptibility are the fastest and least expensive method to get an overview of the pollution in a region. We developed new tools for anomaly detection to recognize possibly problematic regions and "hot spots" of soil pollution. Magnetic measurements of soil samples from the Austrian Soil Inventory Programmes were used to produce magnetic susceptibility maps for the upper 20 cm of soil and for the layer between 20 and 50 cm in a study area covering the provinces Carinthia, Styria, Burgenland, Lower and Upper Austria. The interpretation of such maps was until now only possible by experts with detailed knowledge about the geology and soil types in a study area. Susceptibility does not only depend on the input of anthropogenic dusts. There are also natural contributions. Therefore a major requirement for evaluating the maps is the definition of respective natural background values. The specific mixture of heavy metals which correlate with susceptibility depends on their source and is therefore different from region to region, e.g. in soils near steelworks chromium was found in flakes attached on the surface of technogenic spherules smaller than 0,1 mm. Once the specific heavy metal signature of a region is identified, heavy metal contents can be estimated from susceptibility values.