Transient migration behavior of VOC vapor in layered unsaturated soils subjected to multiple time-dependent point pollution sources: Analytical study

Highlights

• A novel prediction model for VOC vapor migration in heterogeneous unsaturated soil

• Time-dependent release characteristic and interaction of point pollution sources

• Contaminant degradation and water-level fluctuation

• An analytical method for estimating the potential risk assessment of VOC vapor

• Some advice for the remediation of subsurface VOC vapor

Abstract

Predicting the migration behavior of volatile organic compounds (VOCs) vapor is essential for the remediation of subsurface contamination such as soil vapor extraction. Previous analytical prediction models of VOCs migration are mostly limited to constant-concentration nonpoint sources in homogeneous soil. Thus, this study presents a novel analytical model for two-dimensional transport of VOCs vapor subjected to multiple time-dependent point sources involving transient diffusion, sorption and degradation in layered unsaturated soils. Two representative time-dependent sources, i.e., an instantaneous source and a finite pulse source, are used to describe the short-term and long-term leakage. Results reveal that soil heterogeneity can cause pollution accumulation, especially in low-diffusivity capillary fringe. The assumption of an equivalent plane source from multiple point sources would significantly overestimate the vapor concentration and the contaminated range. The previous single point source model is no longer inapplicable when the relative distance and/or the release interval between sources is small, giving a strong interaction between multiple sources. Moreover, a faster vapor degradation rate or a higher groundwater level will reduce the area of vapor plume linearly. Hence, close attention should be paid to the time-variation characteristics of multiple sources, the vapor degradation and the groundwater level fluctuation in practice to facilitate soil remediation. The proposed model is a promising tool for addressing the above issue.

Introduction

The fate and transport of volatile hazardous substances in the subsurface has drawn much attention because they can escape into the atmosphere and/or indoor air and pose a potential threat to the health of human beings on the ground. For example, the leaked volatile organic compounds (VOCs) vapor from buried gasoline tanks intruded to the public office buildings in Morgantown, West Virginia, and caused the eye irritation, headache, and nausea of employees (Kullman and Hill, 1990). Similar contamination risks are widespread all over the world such as the leakage of broken/abandoned underground storage tanks, gas pipelines and sewers (Chang and Lin, 2006; Guo et al., 2020; McHugh et al., 2017; McWatters et al., 2019; Nordbotten et al., 2009; Rui et al., 2017). Up to now, various numerical models (Falta et al., 1989; Jang and Aral, 2007; Shen et al., 2012) or analytical models (Feng et al., 2019; McAlary et al., 2020; Shan and Stephens, 1995; Troldborg et al., 2009; Verginelli et al., 2016; Yao et al., 2012) have been developed to predict VOC migration behavior in the subsurface. This study will focus on analytical methods since they can reveal the basic mechanism of VOC migration and serve as a benchmark for numerical simulation.

There are two main categories of contaminant sources in the subsurface: non-point sources and point sources (Loague and Corwin, 2005). Non-point vapor sources are generally modeled as a boundary condition of a studied domain to reflect the volatilization of dissolved VOCs or light non-aqueous phase liquid (LNAPL) near groundwater level (Feng et al., 2020). Many scholars have developed one-dimensional (1D) or two-dimensional (2D) vapor transport models with a non-point source boundary to study the migration behavior and intrusion risk of vapor (DeVaull, 2007; Shen et al., 2014; Shen and Suuberg, 2014; Verginelli et al., 2016; Wang et al., 2021; Xie et al., 2018; Xie et al., 2017; Yao et al., 2015). However, these models generally ignored the release of vapor from actual point pollution sources and the subsequent migration in soils.

Internal point pollution sources can be applied to describe the leakage or release of vapor from gas pipelines, storage tanks, sewers, and/or the in-situ point injection of pesticides and tracer (Armon and Hänninen, 2015; Liang et al., 2016; Rui et al., 2017; Yates, 2009). Based on classic 1D advection-dispersion equations, Basha and El-Habel (1993) developed a single point-source analytical solution. Aral and Liao (1996) and Yadav and Jaiswal (2011) extended this model to a 2D infinite domain considering transverse dispersion. Yates (2009) adopted a constant-concentration point source to describe pesticide injection and successfully predicted the VOC vapor migration in semi-infinite unsaturated soil. However, it cannot be applied to some multi-point leak scenarios with a time-variation release rate, such as a fixed mass of VOCs escaping from multiple holes or defects in tanks/pipelines (Chang and Lin, 2006; Nordbotten et al., 2009). Moreover, an infinite/semi-infinite homogeneous domain was generally adopted in the previous point source models to facilitate the solving for analytical solutions (Troldborg et al., 2009; Yates, 2009), and thus the soil stratification due to sedimentation, and/or groundwater table fluctuation due to rainfall and transpiration cannot be reflected (Ding et al., 2020). However, multiple point sources, time-dependent release/leak, soil stratification and water-level fluctuation that have seldom been addressed by previous point source models to the best of the authors' knowledge would play an important role in affecting the migration behavior of VOCs vapor in the subsurface.

The aim of this paper is (1) to develop a general analytical model that is applicable for VOC vapor transient migration involving diffusion, adsorption and degradation in layered unsaturated soils with multiple time-dependent point sources, (2) to investigate influences of the interaction between point pollution sources, the time-variation characteristics of sources, contaminant degradation and water-level fluctuation using this model, and (3) to provide some valuable proposals for the remediation of subsurface VOCs vapor.