Increasing use of nanoparticles (NPs) in commercial products has raised regulatory and public concerns about the potential environmental and societal impacts of nanotechnologies. Environmental concerns include the fate and transport behavior of NP themselves as well as their potential to serve as carriers of heavy metals or other contaminants because of their large surface area and high reactivity. Studies on the fate, transport, and transformation of NPs once they are released into the environment are emerging. In this work, magnetite (Fe3O4) was chosen as the model metal oxide nanoparticle to study the stability, agglomeration and transport behavior of engineered NPs under different environmental conditions, including solution pH and ionic strength, and the presence of humic acid (HA). We measured the size distribution of magnetite NPs as a function of time at different values of ionic strength and pH in the presence and absence of HA via dynamic light scattering. We found that the NPs were stable at low ionic strength and as ionic strength increased, the initial rate of particle agglomeration hence particle size increased. Agglomeration was also pH-dependent at a low electrolyte concentration examined. When the pH of the electrolyte solution was close to the particles' point zero of charge (PZC at pH7.5), agglomeration took place more quickly and the suspension remained stable when the pH was below or above the PZC. We also found that presence of HA could both promote NPs' stability (as dispersed particles) and Agglomeration, depending on HA concentration. A negative shift of zeta-potential caused by HA was also observed. Images from atomic force microscope (AFM) show formation of NP-HA complexes and this process is affected by solution pH and ionic strength. The quick attachment of HA on the surface of particles, forming a “mask” or “wrap” around the NPs, changes the surface charge state and the aggregation behavior of the NPs. Based on results from the batch experiments, we ran column experiments at four different ionic strengths to examine the transport and retention of magnetite NPs under saturated flow conditions. The results are consistent with the stability information from the batch experiments. These results are being analyzed with the DLVO energy calculations, which describe the interaction potential between nanoparticles as well as that between NPs and collector surfaces.