COLLOID TRANSPORT IN SATURATED FRACTURED MEDIA: EXPERIMENTAL AND NUMERICAL INVESTIGATIONS USING SYNTHETIC AND NATURAL FRACTURE MATERIALS

The transport of colloids through saturated natural (engineered) and synthetic (Plexiglas) fractured systems is physically and numerically investigated in an attempt to better identify the significant mechanisms governing colloid migration in the subsurface. The colloidal particles employed in this work are 1.0, 0.11, and 0.043 µm diameter fluorescent carboxylate-modified microspheres, injected at various flow rates through parallel-plate fractures with either Plexiglas as both surfaces, or Plexiglas as one surface and a mineral cleavage plane or crystal face as the second surface. Transport experiments conducted using physically and chemically homogeneous synthetic Plexiglas fractures are used to better understand the dispersion of colloids in fractured media. Specifically, the Peclet number of each experiment is varied by changing colloid size, flow rate, and fracture length. Experiments are run under conditions where colloid attachment and remobilization are thought to be negligible. Breakthrough data are fit using a particle-tracking algorithm run inversely with PEST and the effective dispersion rate for each experiment is estimated. As expected from theoretical calculations, results indicate that in these fractures, colloid dispersion due to the velocity gradient is evident, but fully developed Taylor conditions are not realized. Log effective dispersion is found to increase linearly with the log of the Peclet number. These numerical results may be used to differentiate between tailing effects due to dispersive processes and those due to attachment and remobilization when colloids are transported through mineral-Plexiglas fractures.