Groundwater samples collected near a secondary-sewage infiltration site on Cape Cod, Massachusetts, were examined for colloidal materials (10-1000 nm). In two wells the water contained a population of monodisperse 100-nm particles, detected using laser-light scattering and autocorrelation data processing. SEM and SEM-EDAX analysis of these colloidal materials collected on ultrafilters confirmed the laser light scattering result and revealed that these microparticles consisted of primarily iron and phosphorus in a 1.86 Fe to 1.0 P stoichiometric ratio. Chemcial analyses of the water samples, together with equilibrium solubility calculations, strongly suggest that the ion-activity product should exceed the solubility product of a 100-nm diameter predominantly vivianite-type colloidal phase. In light of the results, researchers conclude that these microparticles were formed by sewage-derived phosphate combining with ferrous iron released from the aquifer solids, and that these colloids may be moving in the groundwater flow. Such a subsurface transport process could have major implications regarding the movement of particle-reactive pollutants traditionally viewed as non-mobile in groundwater. (Copyright (c) 1987 Elsevier Science Publishers B.V.)