MAGNETIC PROPERTIES AS A PROXY FOR AIRBORNE SMELTER DUST CONTAMINATION, TACOMA, WA

Heavy metal soil contamination is widespread throughout the South Puget Sound region, due mainly, but not uniquely, to ash from copper smelting operations in Tacoma, WA. Here we take a combined rock magnetic and mineralogical approach to fingerprint likely smelter contamination based on analyses of slag and contaminated soil from near the smelter. Both smelter slag and highly contaminated soil indicate the presence of oxide minerals not observed in uncontaminated soils. Rietveld-type refinement of XRD data is consistent with the presence of near-stoichiometric magnetite in slag (unit cell edge ~8.4Å) along with fayalite and hedenbergite, all of which are also observed in SEM. This is at odds with magnetic analyses of the same slag which suggest some cation substitution (T_c of ~500°C and a suppressed Verwey transition). Low-temperature remanence data from slags suggest a crystallographic transition at ~50K. As might be expected, magnetic oxides in soils appear to contain a mix of a “smelter-related” component and true stoichiometric magnetite based on magnetic properties. The Verwey transition is present (though smoothed) in extracts from contaminated soils, and T_c values of soil samples are ~580°C. XRD of uncontaminated soils does not indicate significant amounts of the minerals present in slag. However, the low-temperature remanence properties of contaminated soils display the same low-temperature transition at ~50K as the slags. Low-temperature magnetic behavior, combined with x-ray diffraction, is a useful tool for identifying Tacoma-area soils contaminated by smelter ash.