The deposition kinetics of ZnO nanoparticles on silica surfaces and surfaces that pre-coated with Suwannee River humic acid (SRHA) were examined over a wide range of environmentally relevant ionic strengths in both monovalent and divalent solutions by employing a quartz crystal microbalance with dissipation (QCM-D). Deposition efficiencies of ZnO nanoparticles on bare silica surfaces increased with increasing ionic strength in both NaCl and CaCl(2) solutions, which agreed with the trends of interaction forces between nanoparticle and silica surface and thus was consistent with classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The presence of divalent cation Ca(2+) in solutions greatly enhanced the deposition kinetics of ZnO nanoparticles on silica surfaces. Pre-coated SRHA on silica surfaces hindered the deposition of ZnO nanoparticles in both monovalent and divalent solutions, which was not consistent with classic DLVO theory. Steric repulsion, a non-DLVO interaction, was found to be a major mechanism controlling the deposition of ZnO nanoparticle on SRHA-coated silica surfaces.