The transport of groundwater colloids (e.g., bacteria, viruses, clay particles) is governed, in part, by the rate at which the colloids strike and adhere to aquifer mineral grains. We report results of coupled flow and transport simulations that reveal how changes in mineral-grain shape and surface roughness influence the single-collector contact efficiency (ηT) -- a parameter of colloid-filtration theory that quantifies the frequency of colloid collisions with the surface of a mineral grain (collector). Our analyses indicate that the sensitivity of ηT to variations in collector shape and roughness depends on colloid size and suggests that quantitative descriptions of colloid transport through real aquifers will, in some cases, require nontraditional approaches that are not limited by the simplifying assumptions of smooth, spherical collectors.