In the interest of fully assessing the potential environmental risks linked to "nanolitter," we need to be able to predict the persistence, toxicity, and mobility of engineered nanomaterials. in the natural subsurface environment. To examine the effects of particle size and natural organic matter on nanoparticle mobility, laboratory-scale filtration experiments were performed using different sized model nanomaterials (i.e., latex colloids having diameters of 50, 110, and 1500 nm) in the presence and absence of 5.0 mg/L Suwannee River humic acid (SRHA). At low ionic strengths (1-10 mM KCI), an increase in attachment efficiency (alpha) with increasing particle size was observed. This result contrasts with predictions of particle filtration based on attachment in the primary energy minimum of the particle-grain interaction energy profile evaluated using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The presence of SRHA generally resulted in a decrease in alpha over the range of experimental conditions investigated. Results of particle transport experiments combined with particle characterization measurements suggest that the decrease in colloid attachment in the presence of SRHA is related to the combined influence of the mechanisms of charge stabilization and steric stabilization. (C) 2008 Published by Elsevier Inc.