The Combination of EMP Compositional Maps With Other Image Data to Investigate Microscale Contaminant Geochemistry

EMP compositional imaging is limited by collection times to relatively high detection levels (100+ ppm). The mobility of environmental contaminants is often controlled by sorption or ion exchange processes so that they are present at levels below the detection limit. The phases in spatial association with the contaminants can be determined by combining EMP images (including elemental abundance and backscattered electron images) with compositional images produced by more sensitive methods at lower optical resolution. The mineral-specific processes responsible for contaminant immobilization may then be inferred from phase and contaminant distributions to provide a basis for the interpretation of macroscale data from leaching or cation exchange experiments. The association of sediment phases with 90Sr and 137Cs in separate field samples was investigated using EMP images matched to autoradiographs collected with a resolution of 25 μm and sensitive to sub-Bq levels of radiation. 90Sr was associated with smectites within weathered basalt clasts, and controlled by cation exchange in sediments that were coarse-grained and contained little or no gravitationally measurable clay fraction. 137Cs was confined to micaceous minerals. Subsequent experimentation with micas and combined X-ray microprobe (XMP), TEM, and EMP analysis showed that the Cs was bound to frayed edge sites on and within the micas. XMP methods allow valence determinations with a detection limit of 1 ppm at a resolution of 10 μm, and were used along with EMP analysis to show that U contamination in some contaminated sediments was unreduced and precipitated as a uranyl silicate in microfractures in granitic sediment clasts. To investigate the indirect effects of microbiota on 99Tc, pertechnetate (99TcO4-) was exposed to bioreduced sediments, which were subsequently examined using EMP and XMP. The pertechnetate was reduced to 99TcO2 by sediment-bound Fe(II), and was preferentially bound to iron-rich phyllosilicate clasts. The combination of sensitive and high-resolution images yielded valuable data for the management of trace contaminants and extended the utility of EMP analyses.