**2. Architecture of NAPL in the vadose zone**

Once accidentally released in the vadose zone, a NAPL will begin to create a dynamic source zone as it is contacting the soil matrix. A simplified conceptual site model (CSM) of a NAPL release in the vadose zone is shown in **Figure 1.** A NAPL heavier than water is defined as a dense NAPL (DNAPL), and if the NAPL has a density less than water, it is referred to as a light NAPL (LNAPL). In some cases, the source release may be single or a mixture of both types of

**Figure 1.** Simplified CSM of a NAPL release in the vadose zone.

**1. Introduction**

308 Soil Contamination - Current Consequences and Further Solutions

program.

**2. Architecture of NAPL in the vadose zone**

Once accidentally released in the vadose zone, a NAPL will begin to create a dynamic source zone as it is contacting the soil matrix. A simplified conceptual site model (CSM) of a NAPL release in the vadose zone is shown in **Figure 1.** A NAPL heavier than water is defined as a dense NAPL (DNAPL), and if the NAPL has a density less than water, it is referred to as a light NAPL (LNAPL). In some cases, the source release may be single or a mixture of both types of

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

NAPL. Irrespective, the NAPL will typically consist of multi-component of chemical compounds with varying degree of solubility. **Table 1** provides examples of characteristics for DNAPL and LNAPL compounds commonly encountered at contaminated sites. When released in significant quantity, the presence of air in soil pores in the vadose zone allows a NAPL to move downward under the force of gravity expressed in the capillary and bond number without overcoming a displacement pressure [3]. Soil NAPL saturation [4] is defined as Eq. (1):

$$\text{CS}\_s = \text{NAPL Volume released/Volume of Open Power Space} \tag{1}$$

Fraction of the NAPL is held in place by capillary forces in the soil open pores space through which it flows. This immobile fraction under static conditions is termed residual saturation or globules. As a result, this creates a persistent source of contamination for groundwater. The relative fraction of a NAPL fluid immobilized and a continuous NAPL becomes discontinuous in a given volume of soil is termed residual saturation, Rs, which is expressed as Eq. (2):

$$\mathcal{R}\_s = \text{(Volume ofNAPL/Volume of voids)}100\tag{2}$$

In addition, retention capacity (Rc) [5] has also been used to describe residual saturation of the non-wetting phase in the vadose zone as in Eq. (3):

$$\mathbf{R}\_{\mathfrak{e}} = \mathbf{R}\_{\mathfrak{s}} \times \text{soil porosity} \tag{3}$$


Source: PubChem, Properties are typically at 20°C;

\* D =Dense; \* L = Light.

**Table 1.** Properties of select NAPL common pollutants.

Depending on the vadose zone NAPL-related characteristics and volume released, several distinct plumes may emerge. As the system strives to maintain a locale-scale equilibrium, contaminants may be transferred between phase media as environmental conditions change in accordance with equilibrium constants (**Figure 2**). In the vadose zone, the vapor and dissolved phases are significant in terms of mass transfer and transport as well as further spreading of contamination. Under most conditions in low conductivity areas into which diffusion and migration of a NAPL plume have occurred, these migration pathways can become intermittent sources of low-level contamination after the NAPL source mass has disappeared [4]. If the source zone and/or pooled NAPL is not timely and effectively risk managed, downward migration of NAPL constituents will eventually enter the phreatic zone resulting to further spreading of contamination at the site and significant additional remediation costs. The presence of moisture in the soil as well as infiltrating precipitation is required for downward movement of dissolved NAPL contaminants. The fundamental mass transport equation for the vadose zone can be applied according to Eq. (4):

$$\mathcal{R}\left\{\widehat{\mathsf{S}}\mathsf{C}/\mathsf{S}\mathsf{t}\right\} = \mathsf{D}\_{\mathsf{i}}\left\langle \widehat{\mathsf{S}}^{\mathsf{I}}\mathsf{C}/\mathsf{S}\mathsf{t}\mathsf{z}^{\mathsf{I}} \right\rangle - \mathsf{V}\left\langle \widehat{\mathsf{S}}\mathsf{C}/\mathsf{S}\mathsf{t}\mathsf{z} \right\rangle - \eta \mathsf{C} + \mathsf{J} \tag{4}$$

where

**Compounds NAPL**

Xylenes: 0-Xylenes m-Xylene p-Xylene

\* D =Dense; \* **type (D or L)\***

310 Soil Contamination - Current Consequences and Further Solutions

Source: PubChem, Properties are typically at 20°C;

**Table 1.** Properties of select NAPL common pollutants.

L = Light.

**Molecular formula**

**Molecular weight (g/mole)**

Chloroform D CHCl3 119.38 8000 1483 160 0.58 Perchloroethylene D C2Cl4 165.83 1100 1623 14 0.89 Aroclor 1254 D C12H5Cl15 326.43 0.057 1540 7.71E−05 1800 Aroclor 1242 D C12H6Cl4 261 0.200 1381 1.00 E−03 1350 Carbon tetrachloride D CCl4 153.82 8000 1590 90 0.91 Methylene chloride D CH2Cl2 84.93 13,000 1330 435 0.44

Naphthalene D C10H8 128.17 30 1140 9.44E−02 0.9684 @80°C

Nitrobenzene D C6H5NO2 123.11 2090 1204 0.245 1.863 Anthracene D C14H10 178.23 1.29 1250 6.56E−06 3.00−01 Nitrobenzene D C6H5NO2 123.11 2090 1204 0.245 1.863 Benzene L C6H6 78.11 1840 876 95 0.75 Ethylbenzene L C6H5CH2CH3 106.17 152 866 9.998 0.669 Toluene L C7H8 92.14 520 862 21 0.59

L C8H10 106.16 178

MTBE L C5H12O 88.15 55,000 740 250 0.35 @15°C Phenol L C6H6O 94.11 82,800 1060 0.40 9.7 @20 oC

Depending on the vadose zone NAPL-related characteristics and volume released, several distinct plumes may emerge. As the system strives to maintain a locale-scale equilibrium, contaminants may be transferred between phase media as environmental conditions change in accordance with equilibrium constants (**Figure 2**). In the vadose zone, the vapor and dissolved phases are significant in terms of mass transfer and transport as well as further spreading of contamination. Under most conditions in low conductivity areas into which diffusion and migration of a NAPL plume have occurred, these migration pathways can become intermittent sources of low-level contamination after the NAPL source mass has disappeared [4]. If the source zone and/or pooled NAPL is not timely and effectively risk managed, downward migration of NAPL constituents will eventually enter the phreatic zone resulting to further spreading of contamination at the site and significant additional remediation costs. The presence of moisture in the soil as well as infiltrating precipitation is required

**Aqueous solubility (mg/L)**

161 162 880 860 860 7 8.29 9

0.812 0.62 0.61

**Density (kg/m3 )**

**Vapor pressure (mmHg) @ 25°C**

**Viscosity (cP)**


**Figure 2.** Dynamic of chemical phases in mass distribution of NAPL in the vadose zone.

NAPL movement once it reaches the saturated zone will be a function of its density. Evidence suggests that Darcy's equation used to describe fluid movement through a permeable bed can be equally applied. Numerical models have been used to predict movement of NAPLs in porous media [5–7]. In a one-dimensional model, hydraulic conductivity variable, K, is replaced by intrinsic permeability, κ to take into consideration the varying hydraulic characteristics pertaining to a NAPL fluid [8]. The negative sign in Eq. (5) is to indicate that flow is in the direction of decreasing head:

$$V = - (\kappa \text{ pg} \mid \alpha) \, dh \, \text{d}L \tag{5}$$

where

*V* = Darcy velocity (cm/s)

*κ* = intrinsic permeability (1 darcy = 1 × 10−8 cm2 )

p = density of NAPL (g/cm3 )

g = force of gravity (980 cm/s2 )

ω = dynamic viscosity (cp) of NAPL

dh/dL = hydraulic gradient of NAPL mass in Eq. (5), the hydraulic gradient is derived as described in Eq. (4), then Eq. (6) is expressed as:

$$
\Box \mathsf{lp} / \mathsf{cl} = \langle \mathsf{B} + \mathsf{Q} \, / \mathsf{p} \mathbf{g} \rangle \, \mathsf{cl} \tag{6}
$$

where

β = reference elevation

Q = atmospheric pressure.
