Mobile colloids in groundwater enhance the transport of chemicals which sorb to particle surfaces. This study addressed the potential for colloid-mediated transport in Wisconsin aquifers by developing fundamental data on colloid concentrations and properties in representative systems. The aquifers examined included those classified as fractured dolomite, sorted sand, and calcareous till, all highly toxic and relatively unperturbed. A broad range of characterization methods were applied to whole groundwater and colloid concentrates prepared by ultracentrifugation and ultrafiltration. Colloid sorption charateristics were probed with a 14C-labeled PCB congener. Colloid concentrations ranged from 200 to 700 µg liter-1, and seasonal variations were noted in at least two systems. Mass-size distributions were typically broad, with shallow maxima in the 30 to 50 and 400 to 1000 nm size intervals. Scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDX) and elemental analysis revealed that larger colloids are domintated by clay and silica particles. Smaller particles, however, were more difficult to categorize. The organic matter component of colloid isolates was high, whereas concentrations of colloidal iron were generally low. Electrophoretic mobilities of colloidal particles were consisitent with a model of colloid particles with a sorbed surface coating of organic matter. Three-phase patitioning experiments with 2,2′,4,4′ -tetrachlorobiphenyl (2,2′,4,4′ -TCBP) revealed that association with colloids can be significant, with up to 23% colloidally bound at natural colloid mass levels. Calculated partition coefficients for colloidal particles ranged from 1.1 x 10 5 to 4.4 x 10 5. Using a retardation factor approach it was estimated that the presence of mobile colloids in the aquifers, on average, increases the mobility of 2,2′,4,4′ -TCBP by 10%.