Real-time gamma imaging of technetium transport through natural and engineered porous materials for radioactive waste disposal.

We present a novel methodology for determining the transport of technetium-99m, a γ-emitting metastable isomer of (99)Tc, through quartz sand and porous media relevant to the disposal of nuclear waste in a geological disposal facility (GDF). Quartz sand is utilized as a model medium, and the applicability of the methodology to determine radionuclide transport in engineered backfill cement is explored using the UK GDF candidate backfill cement, Nirex Reference Vault Backfill (NRVB), in a model system. Two-dimensional distributions in (99m)Tc activity were collected at millimeter-resolution using decay-corrected gamma camera images. Pulse-inputs of ~20 MBq (99m)Tc were introduced into short (<10 cm) water-saturated columns at a constant flow of 0.33 mL min(-1). Changes in calibrated mass distribution of (99m)Tc at 30 s intervals, over a period of several hours, were quantified by spatial moments analysis. Transport parameters were fitted to the experimental data using a one-dimensional convection-dispersion equation, yielding transport properties for this radionuclide in a model GDF environment. These data demonstrate that (99)Tc in the pertechnetate form (Tc(VII)O4(-)) does not sorb to cement backfill during transport under model conditions, resulting in closely conservative transport behavior. This methodology represents a quantitative development of radiotracer imaging and offers the opportunity to conveniently and rapidly characterize transport of gamma-emitting isotopes in opaque media, relevant to the geological disposal of nuclear waste and potentially to a wide variety of other subsurface environments.

Measurement of colloid mobilization and redeposition during drainage in quartz sand.

Movement of wetting and drying fronts through the vadose zone can mobilize and transport colloid particles but the mechanisms are not fully understood. We used mesoscale (mm-dm) fluorescence imaging to measure mobilization of 1.9 microm diameter carboxylate-latex microspheres during drainage in quartz sand. Experiments were performed at ionic strengths of 2-50 mM and drainage rates of 1.0-0.2 mL min(-1). Colloids were mobilized and transported steadily at a sharp decrease in pore saturation marking the drying front. The mobilization rate varied directly with the initial immobile particle concentration. The mobilization rate constant varied inversely with ionic strength and directly with drainage rate. Peak mobile particle concentration at the drying front varied nonmonotonically, and the mobilization efficiency decreased with distance traveled by the drying front, at high ionic strengths. These findings constitute evidence for particle redeposition from the drying front as drainage progresses, which we propose is a key factor in the observed variations with ionic strength and drainage rate in the total number of particles removed during drainage. The measured outcomes of particle mobilization during a drainage event are sensitive to the distributions of immobile particles prior to drainage and dependent on the length scales over which the drainage event is observed.

High-resolution measurement of pore saturation and colloid removal efficiency in quartz sand using fluorescence imaging.

Colloid deposition in unsaturated, nonuniform porous media is poorly explained by current models and difficult to measure using breakthrough curves and retained mass profiles. We present new methods which enable time-lapse fluorescence imaging to quantify variations in pore saturation, theta, and colloid deposition in 2D, nonuniform unsaturated flow fields. Calibration experiments revealed direct proportionality between fluorescence F and theta in 20/30 mesh quartz sand. Analysis of breakthrough data in fluorescence images allows quantification of the mean mobile concentration, mean deposition rate, and hence the colloid removal efficiency eta directly from data at the pixel-scale throughoutthe flow field. We imaged carboxylate-modified latex microspheres from a point source in saturated flow and unsaturated flow across a capillary fringe at 10(-3), 10(-2), and 10(-1) M NaCl. Total numbers of colloids deposited and values of eta increased with ionic strength. We modeled the observed variations in eta with theta to estimate the partitioning of colloid deposition between air-water and solid-water interfaces. In the broad saturation range 0.2 < theta < 1, our results suggest that only at the lowest ionic strength, where deposition at solid-water interfaces was strongly unfavorable, did colloid deposition associated with air-water interfaces significantly influence the total colloid removal.

Noninvasive quantitative measurement of colloid transport in mesoscale porous media using time lapse fluorescence imaging.

We demonstrate noninvasive quantitative imaging of colloid and solute transport at millimeter to decimeter (meso-) scale. Ultraviolet (UV) excited fluorescent solute and colloid tracers were independently measured simultaneously during co-advection through saturated quartz sand. Pulse-input experiments were conducted at constant flow rates and ionic strengths 10(-3), 10(-2) and 10(-1) M NaCl. Tracers were 1.9 microm carboxylate latex microspheres and disodium fluorescein. Spatial moments analysis was used to quantify relative changes in mass distribution of the colloid and solute tracers over time. The solute advected through the sand at a constant velocity proportional to flow rate and was described well by a conservative transport model (CXTFIT). In unfavorable deposition conditions increasing ionic strength produced significant reduction in colloid center of mass transport velocity over time. Velocity trends correlated with the increasing fraction of colloid mass retained along the flowpath. Attachment efficiencies (defined by colloid filtration theory) calculated from nondestructive retained mass data were 0.013 +/- 0.03, 0.09 +/- 0.02, and 0.22 +/- 0.05 at 10(-3), 10(-2), and 10(-1) M ionic strength, respectively, which compared well with previously published data from breakthrough curves and destructive sampling. Mesoscale imaging of colloid mass dynamics can quantify key deposition and transport parameters based on noninvasive, nondestructive, spatially high-resolution data.