CHEMICAL AND MINERALOGICAL ANALYSES OF CONTAMINATED SOILS AT THE ANACONDA SMELTER SITE, MONTANA, USA
Contaminated soils proximal to the smelter tailings storage ponds were collected along the local hydraulic gradient. A suite of four soil cores were collected, from the surface to depths between 0.63 and 2.5 m. Samples were taken from the cores, dried and sieved to ≤ 0.250 mm; then used in two chemical leaching procedures. The leaching procedures utilized included the Tessier sequential extraction procedure (Analytical Chemistry, 1979) and EPA Method 3050B. In addition, microscope thin-sections were prepared from the sieved untreated soils and residue material from the leaching procedures. Scanning Electron Microscopy (HR-SEM) with energy dispersive X-ray spectrometry (EDS) was used to further characterizes the soils.
Data from the Tessier extraction procedure shows that the highest concentrations of all 5 contaminant metals occur at a depth between 15 and 30 cm. Moreover, trends in metal concentrations appear to be controlled principally by the redox environment and infiltration of surface precipitation. Surface precipitation is limited, as the local climate is arid with an annual average precipitation of 35 cm (NOAA database). In the upper-most samples, above a visible redox boundary, the heavy metals are associated with iron-oxide phases, as defined by the Tessier procedure. From the microscope thin-sections and SEM characterization it was observed that the iron-oxide phases coat large framework grains (e.g., plagioclase and quartz) or occur as fine soil matrix. HR-SEM with EDS analyses is being used to further examine the iron-oxide grain coatings.