**1. Introduction**

In recent years, improper management of non-aqueous phase liquid (NAPL) hydrocarbons such as polyaromatic hydrocarbons, petroleum hydrocarbons, as well as other hazardous substances such as creosote and coal tar, has resulted in the formation of source zone plumes, virtually recalcitrant, in the vadose zone. The impacted vadose zone containing pooled NAPL and its residual are commonly referred to as the source zone. Generally, NAPLs are hydrophobic, low water-soluble liquids with a specific density that can be greater or less than 1. Nonetheless, NAPL chemical constituents that are soluble enough in the vadose source zone architecture may travel downward because of gravitational and capillary forces to contaminate the groundwater [1]. Many NAPL compounds are volatile and their behavior in the vadose zone may cause vapor intrusion concerns. The potential adverse impact of NAPL contamination has engendered significant concerns among the public, policymakers, environmental regulators, and scientists. Even at very low concentrations, NAPL constituents are considered highly toxic, mutagenic, and/or carcinogenic or can pose some other harm to humans and other environmental receptors [2]. Costly site-specific remediation strategies are often warranted and sometimes with limited success for the NAPL source zone and its associated plumes. In many instances,remediation strategies are designed towards partial mass removal, plumes containment, source zone stabilization, relative to a formulated acceptable riskmanagement objective. Surfactant-enhanced soil bioremediation has been proven as a promising technology through both empirical studies and field applications as a result of its low cost and the lack of toxic metabolites. Traditional framework of bioremediating NAPLimpacted soil is a very difficult process because of the mass transfer dissolution limit into the soil solution matrix, sorption onto the soil matrix, toxicity of constituents to soil biota, alteration in soil matrix physical properties. These factors have made the traditional bioremediation design approach at contaminated sites ineffective. Increasing dissolved mass transfer phase is a vital prerequisite towards achieving successful biodegradation of NAPL-impacted soil. Surfactants or surface active agents represent a class of chemicals that has the ability to increase the bioavailability of NAPL constituents by acting as solubilizing agents in the source zone. An *ex situ* remediation design properly strategized will allow exponential optimization of biotreatment process by enhancing the native capability of the soil microorganisms and risk mitigation. This work provides a fundamental review and approach of *ex situ* surfactantenhanced bioremediation of NAPL-contaminated vadose zone as it pertains to an *ex situ* design program.
