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[1] Prediction of colloid transport in the subsurface is relevant to researchers in a variety of fields such as contaminant transport, wastewater treatment, and bioremediation. Investigations have traditionally relied on column studies whereby mechanistic inferences must be drawn on the basis of colloid behavior at the outlet. Over the past decade, development of noninvasive visualization techniques based on visible light, magnetic resonance, and X rays have provided insight into a number of colloid transport mechanisms by enabling direct observation of individual colloids at the pore scale and colloid concentrations at longer length scales. As research focus shifts from transport of ideal colloids in ideal media such as glass beads to natural colloids in natural porous media, these noninvasive techniques will become increasingly useful for studying the collection of mechanisms at work in heterogeneous pore systems. It is useful at this juncture to review recent progress in colloid transport visualization as a starting point for further development of visualization tools to support investigation of colloids in natural systems. We briefly discuss characteristics of visualization systems currently used to study colloid transport in porous media and review representative microscale and mesoscale visualization studies conducted over the past decade, with additional attention given to two optical visualization systems being developed by the authors.

Increasing restriction of agrochemicals is motivating development of ecology-based cropping systems, including green manures, to manage soilborne diseases. Green manures have shown promise in suppressing Verticillium dahliae, but information about effect of different green manures and optimal application rates remains limited. Therefore, we conducted two single-year field experiments comparing effects of Austrian winter pea (Pisum sativum ‘Melrose’), broccoli (Brassica oleracea var. botrytis ‘Excelsior’), and Sudan grass (Sorghum vulgare var. sudanense ‘Monarch’), amended at 6, 12, or 24 Mg ha-1, on soil populations and root infection by V. dahliae, wilt severity, and yield of Russet Burbank potato. Inoculum density was reduced relative to the nonamended, infested control by all broccoli treatments, Austrian winter pea applied at 12 and 24 Mg ha-1, and Sudan grass applied at 12 Mg ha-1. Root infection was not reduced by any green manure treatment. Median wilt severity was reduced approximately 70% by all green manures applied at 24 Mg ha-1 and 74% by Austrian winter pea applied at 12 Mg ha-1. Tuber yield was reduced approximately 20% in nonamended, V. dahliae-infested controls relative to the noninfested controls. No green manure treatment improved yield relative to the nonamended, infested control.

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The transport of micrometer-and submicrometer-sized par cles of bio c and abio c origin through both natural and engineered porous media is relevant to a wide variety of disciplines. While the infl uence on colloid transport of chemical factors such as colloid and media surface chemistry and the ionic strength of the carrying fl uid are fairly well understood, the role of physical factors s ll needs clarifi ca on from a mechanis c standpoint. Along with recent recogni on that pore-scale processes may depend on the coupling of chemistry and local hydrodynamics, which in turn depends on pore topography, there is a growing need for experimental systems that enable visualiza on of individual colloids in pores and pore networks that contain relevant three-dimensional features. We introduce a single camera-epifl uorescence microscope system to track fl uorescent colloids in three dimensions using the apparent diameter of out-of-focus colloids to es mate their z-axis loca on. The current system, using a Plan Achromat 5× objec ve, enables z es ma on of 4.7-μm-diameter fl uorescent microspheres across a range of 1200 μm with accuracy of approximately 34 μm. In addi on to discussion of system limita ons and poten al improvements, we present results from two mini-studies illustra ng the use of three-dimensional par cle tracking to inves gate (i) the behavior of colloids approaching orthogonally oriented grain-to-grain contacts, and (ii) the potenal reten on mechanisms in porous media under chemical condi ons unfavorable to deposi on. The ability to track individual colloids in three dimensions brings experimental capabili es closer in line with computer-modeling capabili es, se ng the stage for synergis c pairing of experimental and theore cal inves ga ons.