Interaction of dissolution, sorption and biodegradation on transport of BTEX in a saturated groundwater system: Numerical modeling and spatial moment analysis

Abstract

Interaction of various physical, chemical and biological transport processes plays an important role in deciding the fate and migration of contaminants in groundwater systems. In this study, a numerical investigation on the interaction of various transport processes of BTEX in a saturated groundwater system is carried out. In addition, the multi-component dissolution from a residual BTEX source under unsteady flow conditions is incorporated in the modeling framework. The model considers Benzene, Toluene, Ethyl Benzene and Xylene dissolving from the residual BTEX source zone to undergo sorption and aerobic biodegradation within the groundwater aquifer. Spatial concentration profiles of dissolved BTEX components under the interaction of various sorption and biodegradation conditions have been studied. Subsequently, a spatial moment analysis is carried out to analyze the effect of interaction of various transport processes on the total dissolved mass and the mobility of dissolved BTEX components. Results from the present numerical study suggest that the interaction of dissolution, sorption and biodegradation significantly influence the spatial distribution of dissolved BTEX components within the saturated groundwater system. Mobility of dissolved BTEX components is also found to be affected by the interaction of these transport processes.

Appendix

1.1 A1. Equilibrium sorption isotherms

Under equilibrium conditions, distribution of adsorbed molecules between the liquid phase and solid phase is indicated by sorption isotherm. But based on the variation in interaction as the flow regime progresses, sorption isotherm can be linear or non-linear. For equilibrium sorption isotherm, the sorption effect is given by the instantaneous retardation factor.

1.2 A2. Linear sorption isotherm

For linear sorption, sorbed concentration, $S\,(\hbox {MM}^{-1})$ and aqueous concentration of solutes, $C \,(\hbox {ML}^{-3})$ are related as expressed in equation (A.1)

$$\begin{aligned} S=K_d C. \end{aligned}$$

(A.1)

The retardation factor for linear sorption can be computed as provided in equation (A.2)

$$\begin{aligned} R_f =1+\frac{\rho _b K_d }{\theta }. \end{aligned}$$

(A.2)

In equation (A.2), $K_d$ represents the distribution coefficient $(\hbox {L}^{3}\hbox {M}^{-1})$, $\rho _b$ represents the mass density of aquifer material $(\hbox {ML}^{-3})$.

1.3 A3. Freundlich non-linear sorption isotherm

Among non-linear sorption isotherms, Freundlich sorption isotherm is the most commonly used isotherm (Weber et al. 1991) to describe sorption in heterogeneous surfaces. In Freundlich isotherm, it is assumed that the concentration of adsorbate increases with the concentration of adsorbate on the adsorbent surface (Freundlich 1906; Quintelas et al. 2008). As per Freundlich isotherm, sorbed concentration and aqueous concentration are related as provided in equation (A.3)

$$\begin{aligned} S=K_f C^{n}. \end{aligned}$$

(A.3)

In equation (A.3), C represents the volume concentration of solute $(\hbox {ML}^{-3}),$ S represents the sorbed concentration $(\hbox {MM}^{-1}),~ K_{f}$ represents the Freundlich coefficient $(\hbox {L}^{3}\hbox {M}^{-1})$ and n represents the Freundlich sorption intensity, n normally varies between 0.7 and 1.2. The retardation factor for Freundlich isotherm in porous media can be computed as provided in equation (A.4)

$$\begin{aligned} R_f =1+\frac{n\rho _b K_f }{\theta }C^{n-1}. \end{aligned}$$

(A.4)

In equation (A.4), $\rho _{b}$ represents the bulk density of aquifer material $(\hbox {ML}^{-3})$.

1.4 A4. Langmuir non-linear sorption isotherm

Another commonly used non-linear isotherm is introduced by Langmuir (1918). Langmuir isotherm assumes a monolayer coverage of adsorbate or homogeneous subsurface. In Langmuir non-linear isotherm, relationship between sorbed concentration and aqueous concentration are given as in equation (A.5)

$$\begin{aligned} S=Q_\mathrm{max} bC/\left( {1+bC} \right) . \end{aligned}$$

(A.5)

In equation (A.5), S represents the sorbed concentration $(\hbox {MM}^{-1}), Q_\mathrm{max}$ represents the maximum Langmuir sorption capacity $(\hbox {MM}^{-1}),~ b$ represents the Langmuir constant $(\hbox {L}^{3}\hbox {M}^{-1})$.

The retardation factor for Langmuir sorption isotherm can be deduced as provided in equation (A.6).

$$\begin{aligned} R_f =1+\frac{\rho _b Q_{\max } b}{\theta (1+bC)^{2}} \end{aligned}$$

(A.6)

where $\rho _{b}$ is the bulk density of aquifer material $(\hbox {ML}^{-3})$ and $\theta$ is the porosity.