PINPOINTING TRACE METAL CONCENTRATIONS IN SOIL STRUCTURES

Some land-use practices introduce trace metals and modify soil structure in ways that compromise the soil’s ability to fix trace metals and prevent them from leaching to groundwater. Additionally, some agricultural land-use practices, such as adding manure or biosolids, foster the vertical movement of colloids and associated trace metals. Undocumented land-use activities at New Philadelphia Historical Landmark, Illinois have produced anomalously high levels of the trace metals: copper, zinc, and cadmium that may have detrimental effects to the long-term health of the ecosystem, including the potential to pollute the groundwater. This study aims to identify whether these trace metals are associated with organic matter, clay, or silt coatings on the surfaces of soil aggregates or have been incorporated within the ped interior.

This detailed analysis is not possible with traditional chemical analyses for two reasons: first, most geochemical sample preparation involves homogenization of the soil sample, thus destroying the natural structure of the soil completely; and second, the area sampled by pXRF is much larger (~1.5 cm) than coatings on soil aggregate surfaces, thus producing a somewhat homogenized analysis. In addition, the focus of traditional micromorphological analyses of thin-sections from resin-impregnated soil core samples is primarily qualitative. This study combines this qualitative analysis with (1) the analysis of the resin-impregnated tiles by portable x-ray fluorescence (pXRF) to identify areas of higher trace metal concentrations within the soil sample, and (2) scanning electron microscope – energy dispersive spectrometry (SEM-EDS) to provide semiquantitative trace element data for specific locations of soil structures.