Microbeam analyses can be used to determine the morphology, texture, size and abundance of smelter dust trapped in humus. The geographic distribution of dust correlates with the geographic distribution of the geochemical signature of anthropogenic input to humus. Results of the study indicate that the observed dust may be classified into two morphotypes and five classes on the basis of differences in morphology, texture, size and chemistry. Dust observed at sizes larger than the maximum allowable emission size of modern electostatic precipitators suggests that part of the dust accumulation is historical. Most of the observed particles represent a resistant slag component whereas labile dust, often observed in emissions, is rarely present. A mass balance calculated for Fe indicates that smelter dust contributes a small fraction of the Fe in the humus compared to values determined by geochemical methods. Although emission of smelter-derived dust is probably the primary mechanism for delivery of metal to humus, the preserved particles are not the current primary residence site of metal in the humus. This suggests that, soon after deposition, weathering processes have released metals and other elements from the dust to the humus.

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