The retention and release of micron sized particles and organisms in the vadose zone greatly affect the degree of contamination to groundwater systems. Existing models need improvement to more accurately predict the physical conditions under which such particle transportation occur. Recent theory on capillary and friction forces acting at the air/water meniscus/solid (AWmS) interface suggests that grain roughness is an important factor in colloid retention in unsaturated media. The objective of this work is to further investigate the effects of grain surface roughness and size as a primary contaminant transport variable. Unsaturated column experiments with sands of different grain roughness were performed to determine colloidal retention at the AWmS interface. A rectangular 40 mL acrylic vertical flow chamber was built to take advantage of collecting visual and numerical data under the effects of gravity. Colloid behavior was visualized in situ with digital bright field microscopy and breakthrough colloid concentrations were measured with spectrophotometry. Spherical, 5 ìm sized, synthetic colloids were chosen to emulate homogeneous bacteria in shape and size. Two sizes of quartz sand were modified and thoroughly pre-cleaned for tests of unaltered and smoothed (by lapidary rotation) grain surfaces. Completed experimental results demonstrate that smoother grain surfaces retain a lesser amount of colloids than rougher grain surfaces. This effect has shown to be consistent, although in different magnitudes, for both saturated and unsaturated conditions. Current results indicate that the effects of surface roughness diminish with increasing grain size.