Physical and chemical determinants of colloid transport and deposition in water-unsaturated sand and Yucca Mountain tuff material

Colloid mobility was determined in a system consisting of quartz sand or crushed Yucca Mountain tuff, simulated groundwater (J-13), and hydrophilic latex particles. Water content (theta) and ionic strength (l; Dl water, 0.1 x, Ix, 10x groundwater dilution) were manipulated to define limiting conditions for colloid transport at the Yucca Mountain site. Colloid transport, measured with a centrifuge method at relatively high theta (saturation > 36% for sand, > 62% for crushed tuff) in D1 water, was equivalent to transport at 100% saturation measured with conventional columns. When variables were isolated, increasing I and decreasing theta resulted in a greater extent of colloid deposition; I was more important at higher theta; physical properties were more important at lower theta. l and theta had an interactive effect on colloid deposition whereby synergism was generally observed, especially for simulated groundwater (1x); antagonism was observed at 10x groundwater dilution. At 19% moisture saturation on the crushed tuff, a decreasing rate of colloid deposition was observed. This corresponded to a hydrodynamic condition of 79% immobile water where solute tracers were excluded from a fraction of the pore volume. This suggests that a portion of the favorable sites for deposition were associated with the excluded or immobile water domain and were not accessible to colloids.