The remediation of non-aqueous phase liquids(NAPL) using conventional aquifer treatment technologies islimited by the low solubility of NAPL. Surfactants can promotethe enhanced removal of NAPL through mobilization, a mechanismthat relies on the reduction of interfacial tension (IFT) at theflushing solution / NAPL interface. The conditions governingmobilization can be represented by the total trapping number(NT), a dimensionless quantity relating viscous and buoyancyforces to the capillary forces trapping the NAPL residual. Column studies were conducted with dilute Triton X-100 solutionsdelivered through Ottawa sand spiked with light white mineraloil. At the higher flow rate, the surfactant solutions yieldedNT values greater than the critical NT necessary toinitiate mobilization, thereby promoting greater NAPL recovery asIFT dropped. While the critical NT was not surpassed at thelower flow rate, variations in mineral oil recovery duringflushing clearly indicate a surfactant effect. The surfactant-induced enhancement and retardation of NAPL removal at the lowerflow rate both highlight the limitations of the NT approach. For systems where free product NAPL is present, the totaltrapping number approach requires further refinement to defineits applicability as an indicator for NAPL mobilization.