Process Monitoring of tracer transport through artificial soil with GeoPET

Positron-Emission-Tomography (PET) enables direct and quantitative monitoring of the spatiotemporal distributions of dissolved inert and/or reactive PET-nuclides and PET-nuclide-labelled compounds during their passage through decimeter-scaled material samples. We apply our biomedical PET-scanner (ClearPET, Raytest) exclusively to geomaterial samples and reach the physical limit of spatial resolution of nearly 1 mm with pico-molar sensitivity.
Artificial soil columns (68.5% fine-medium sand, 25.5% coarse silt, 5% Illit, 1% Goethit) have been prepared in Jena. µCT-images have been produced at the Bundesanstalt für Materialforschung und –prüfung (BAM) and analysed in Mainz. They show layering and inhomogeneous water saturation, yet a roughly homogeneous grain distribution. PET-imaging was conducted during flow-through experiments into the upright column, which is perpendicular to the scanner axis.
First flow experiments have been conducted with – presumably – conservative tracers ([¹⁸F]KF (decay time 109.77 min) and Eosin). Conventional break-through curves (BTCs) have been recorded both by fluorescence detection of Eosin and with a Gamma Radioactivity Flow-Through Monitor (Gabi Star, Raytest). From the transport process observation by sequential PET-imaging we could derive longitudinal BTCs during the tracer propagation through the column. Further, it could be shown that ¹⁸F- was slightly retarded and - more significantly - the effective pore volume was inhomogeneous (preferential transport through the sample).
These experiments will be repeated on newly constructed columns with plastic endplates (instead of the original stainless steel ones) and an optimized dead volume.
To preliminarily conclude, we demonstrated that the combined tomographic information on pore structure and spatio-temporal transport process observation in a soil model column yields an essential gain in information for describing and parameterizing transport processes, compared to input-output experiments. Such measurements represent the experimental control of ab-initio model simulations of conservative and reactive transport, e.g. [⁶⁴Cu]MCPA), and of the interpretation of conventional input-output transport experiments, conducted by the partners.