METHODOLOGICAL DEVELOPMENTS IN IN-SITU MEASUREMENT AND VISUALIZATION OF COLLOID TRANSPORT PROCESSES IN THE SUBSURFACE

Measurements that are representative of colloid transport processes in the subsurface are difficult to acquire due both to the small size of the particles and to the inaccessible nature of the soil environment. Most colloid transport studies to date have been conducted in closed columns with destructive sampling where the responses of the system to various stimuli are modeled as the proverbial black box. For mass balance analyses this approach appears to be entirely sufficient, however when questions arise that address the underlying process at play in the system, visualization of the particle movement becomes requisite. In the past decade, developments of non-invasive methods of monitoring have been transformative to the breadth of questions that can be addressed regarding colloid transport phenomena. These approaches include but are not limited to the use of transparent micromodels, epi-fluorescence, magnetic resonance imaging, X-ray computed tomography, and the light-transmission and fluorescence technique. The field of method development in this area of subsurface hydrology is both diverse and dynamic and justifies synthesis as a body of work in its own right. It is this breadth of methodology that is propelling colloid transport experimentation forward into a new era that will reveal new and potentially valuable information about contamination and its remediation in the subsurface. Here we present a review of these methods, including the novel capabilities, advantages, and limitations of each. Particular attention is directed towards the sensitivity of each method and its concomitant signal-to-noise ratio, which is a vital parameter to minimize in a successful visualization system. Advances made in our lab to reduce the signal-to-noise ratio of the light-transmission and fluorescence technique will also be presented. These specific advancements are framed in the context of the capacity of the light-transmission technique to quantify and visualize colloid transport phenomena in variably saturated porous media at the meso-scale under heterogeneous conditions.