Increased production and use of carbon nanotubes (CNTs) may result in the introduction of CNTs to soils and ultimately into groundwater systems. Deposition of CNTs onto surfaces is key to their fate, bioavailability, and reactivity in aquatic systems. This study evaluates the transport and deposition behavior of functionalized single-walled carbon nanotubes (SWNTs) in a well-defined porous media comprising quartz sand. Our results show that increasing solution ionic strength results in increased SWNT deposition (filtration). This observation is consistent with conventional colloidal stability theories, thereby suggesting that physicochemical filtration plays an important role in SWNT transport. However, the relatively insignificant change of SWNT filtration at low ionic strengths (≤3.0 mM KCl) and the incomplete breakthrough of SWNTs in deionized water indicate that physical straining also plays an important role in the capture of SWNTs within the packed sand bed. It is proposed that SWNT shape and structure, particularly the very large aspect ratio and highly bundled (aggregated) state in aqueous solutions, contribute considerably to straining in flow through porous media. We conclude that both physicochemical filtration and straining play a role at low ionic strength (<3.0 mM), while physicochemical filtration is the dominant mechanism of SWNT filtration at higher ionic strengths.