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Article

Impact of Aquifer Heterogeneity on the Migration and Natural Attenuation of Multicomponent Heavy Dense Nonaqueous Phase Liquids (DNAPLs) in a Retired Chemically Polluted Site

by
Wenyi Xie
1,2,
Mei Li
1,2,
Dengdeng Jiang
1,2,
Lingya Kong
1,2,
Mengjie Wang
1,2,
Shaopo Deng
1,2,* and
Xuwei Li
1,2,*
1
Nanjing Institute of Environmental Science, Ministry of Ecology and Environment, Nanjing 210042, China
2
Key Laboratory of Soil Environmental Management and Pollution Control, Ministry of Ecology and Environment, Nanjing 210042, China
*
Authors to whom correspondence should be addressed.
Processes 2025, 13(8), 2338; https://doi.org/10.3390/pr13082338
Submission received: 20 June 2025 / Revised: 19 July 2025 / Accepted: 21 July 2025 / Published: 23 July 2025
(This article belongs to the Section Environmental and Green Processes)
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Abstract

Retired chemically polluted sites in southern Jiangsu Province, China, are characterized by dense nonaqueous phase liquids (DNAPLs) and extremely thick aquifers (>30 m), which pose substantial challenges for determining investigation and remediation depths during redevelopment and exploitation. This study constructed a 2D groundwater transport model using TMVOC to systematically investigate the migration, diffusion, and natural attenuation processes of two typical DNAPLs—1,2-dichloroethane (DCE) and carbon tetrachloride (CTC)—under three scenarios: individual transport, mixed transport, and heterogeneous aquifer conditions, with a simulation period of 35 years. In individual transport scenarios, DCE and CTC showed distinct migration behaviors. DCE achieved a maximum vertical transport distance of 14.01 m and a downstream migration distance of 459.58 m, while CTC reached 13.57 m vertically and 453.51 m downstream. When transported as a mixture, their migration was inhibited: DCE’s vertical and downstream distances decreased to 13.76 m and 440.46 m, respectively; and CTC’s to 13.23 m and 420.32 m, likely due to mutual solvent effects that altered their physicochemical properties such as viscosity and solubility. Under natural attenuation conditions, both DNAPLs ceased downstream transport by the end of the 6th year. DCE concentrations dropped below its risk control value (0.81 mg/L) by the 14th year, and CTC (with a risk control value of 0.23 mg/L) by the 11th year. By the 10th year, DCE’s downstream plume had retreated to 48.65 m, and CTC’s to 0.95 m. In heterogeneous aquifers, vertical upward transport of DCE and CTC increased to 14.82 m and 14.22 m, respectively, due to the partial absence of low-conductivity silt layers, while their downstream distances decreased to 397.99 m and 354.11 m, constrained by low-permeability lenses in the migration path. These quantitative results clarify the dynamic differences in DNAPL transport under varying conditions, highlighting the impacts of multicomponent interactions, natural attenuation, and aquifer heterogeneity. They provide critical references for risk management, scientific determination of remediation depths, and safe exploitation of retired chemically polluted sites with similar hydrogeological characteristics.

1. Introduction

Globally, rapid industrialization and subsequent restructuring have left a legacy of retired pollution sites, posing widespread environmental and public health challenges. For example, the U.S. faces extensive contamination from abandoned mines and orphaned oil/gas wells, with pollutants leaching into groundwater and emitting hazardous gases [1,2,3]. Worldwide, over 500 million potential “brownfields” exist, where industrial pollution exacerbates environmental degradation and social issues like unemployment and community inequality [4]. In low- and middle-income countries, unregulated industrial activities have created thousands of toxic hotspots, with the Toxic Site Identification Program (TSIP) identifying nearly 5000 such sites across 50+ countries [5]. Additionally, historic landfills in coastal and estuarine areas, often lacking proper containment, pose poorly understood risks due to leachate migration [6]. Among the diverse pollutants threatening these retired sites, dense nonaqueous phase liquids (DNAPLs) stand out as a particularly intractable challenge, making their behavior a critical focus for global environmental remediation efforts.
The problem of groundwater pollution by DNAPLs has become increasingly prominent in recent decades, driven by industrialization, agricultural intensification, and urban expansion. This issue is particularly acute in developed regions with complex hydrogeological conditions—such as deltaic areas with thick unconsolidated sediments and dense distributions of chemical and pesticide facilities—where DNAPL contamination threatens groundwater resources and hinders the redevelopment of retired industrial sites [7,8]. DNAPLs, characterized by high density and low solubility, undergo vertical gravity-driven migration, accumulate at the interface of low-permeability layers, and cause long-term groundwater pollution, making their remediation and risk management critical for sustainable land use [9].
DNAPL contaminants in groundwater are frequently a mixture of numerous components with complex pollution distribution characteristics and movement behaviors. They pose serious threats to human health and are difficult to remediate. The physical and chemical characteristics of DNAPLs can be altered by DNPAL combinations, e.g., surface tension, viscosity, density, and boiling point. These physicochemical characteristics can have a significant impact on the residual saturation and migration rate of DNAPLs, creating substantial uncertainty in the remediation and treatment of DNAPL-polluted sites [10]. Due to the wide variety of mixed DNAPL pollutants in groundwater and their relatively complex mechanisms of action, the migration process of multiphase flow in groundwater has become a current research hotspot [10,11].
After DNAPLs enter the groundwater system, their migration is regulated not only by their own physical and chemical properties, but also by the heterogeneity of the groundwater pore medium [2]. Groundwater aquifers often exhibit substantial spatial heterogeneity, which acts as a key control on groundwater dynamic processes—including the migration and transformation of pollutants—thereby complicating groundwater remediation efforts [12]. Related experimental studies have demonstrated that when DNAPL contaminants travel in heterogeneous media, they are impacted by uneven permeability and are intercepted in layers with low permeability [13], which introduces significant uncertainty into groundwater treatment [14]. However, there is currently inadequate research on the characterization of stratigraphic heterogeneity in DNAPL numerical simulations, and further investigations are required.
Researching contaminated locations is more challenging and unpredictable due to the unique nature of DNAPL contaminants. The current relevant standards and technical specifications in China propose that plots contaminated by DNAPL pollutants should be surveyed to the top plate of the first relative waterproof layer, which greatly increases the difficulty of groundwater remediation at (larger than 30 m) DNAPL-polluted sites with thick aquifers [15]. The current depth of research and remediation of polluted land plots cannot generally reach the deep section of a thick aquifer, and the mainstream treatment approach primarily focuses on the investigation and remediation of shallow phreatic aquifers. According to Xie et al. [7], the overall influence of shallow groundwater on human health risk within 70 years is relatively minor due to the presence of DNAPL on a thick aquifer floor. Overall, there has been little discussion regarding the transport processes and mechanisms of DNAPL contaminants below the remediation layer of DNAPL-polluted sites.
The common risk control and remediation technologies for groundwater DNAPL pollution primarily include a permeable reaction barrier (PRB), extraction treatment, multiphase extraction, and monitored natural attenuation (MNA) [16]. Compared to other control and remediation technologies, MNA has the advantages of lower costs and nondestructive effects on the surrounding environment of polluted sites [17,18,19]. This technology is widely used in the world and is currently one of the more effective and feasible technologies for controlling DNAPL in shallow groundwater, especially in chlorinated hydrocarbon-polluted sites [16,20]. Currently, MNA research is gradually being conducted by scholars in China, and these studies have primarily focused more on the natural decay of benzenes (BTEX) or petroleum hydrocarbons and less on the natural degradation process of chlorinated hydrocarbons [21,22,23].
Currently, numerical modeling has become instrumental for understanding DNAPL transport in groundwater, enabling simulations of migration, transformation, and attenuation processes. Among existing frameworks, TMVOC stands out for simulating three-phase (gas-aqueous-NAPL) non-isothermal flow of volatile organic compounds (VOCs), validated in numerous field and laboratory studies [24]. However, its applications remain largely confined to single-component DNAPLs (e.g., trichloroethylene) in homogeneous aquifers [12,25,26], overlooking how multicomponent interactions (e.g., viscosity/solubility alterations) affect transport. For example, Shen et al. [27] modeled natural attenuation of single-component VOCs using TMVOC but did not address multicomponent dynamics. Another tool, MODFLOW-SURFACT, improves accuracy for surfactant-enhanced remediation by simulating solubilization and mobilization [28], yet it neglects natural attenuation processes (e.g., biodegradation) and rarely applies to heterogeneous aquifers [29], limiting its utility for complex field scenarios. Similarly, Transport of Unsaturated Groundwater and Heat (TOUGH2) [30], designed for fractured media, has limited application in thick (>30 m) unconsolidated deltaic sediments with stratified heterogeneity [31]. While recent studies highlight aquifer heterogeneity’s role in retarding DNAPL migration [32,33,34], relatively fewer studies quantify its interaction with natural attenuation (e.g., sorption). Siena and Riva [2] demonstrated DNAPL trapping by low-permeability lenses but excluded attenuation, whereas Kamon et al. [35] explored multicomponent attenuation in shallow aquifers (<15 m) without considering thick deltaic systems. These gaps underscore the need to integrate multicomponent dynamics, heterogeneity, and attenuation in deep aquifer models. Overall, three critical research gaps remain: (1) Multicomponent interactions: Most studies focus on single DNAPL components, but real-world mixtures alter transport via combined properties—prior TMVOC work lacks single vs. mixed comparisons. (2) Coupling heterogeneity and attenuation: Their combined effects on limiting DNAPL migration are rarely quantified. (3) Thick deltaic aquifers: Deltaic retired sites often have >30 m thick, complex aquifers, yet simulations focus on shallow/homogeneous ones, leaving a gap.
In this study, a retired chemically polluted region in China’s Yangtze River Delta is chosen to research DNAPL transport behavior. Based on the TMVOC model, we constructed a multiphase flow model to investigate the transport behavior of multi-component DNAPLs, the natural attenuation process of DNAPLs, and the transport of DNAPLs in heterogeneous aquifers in order to develop more reasonable remediation plans and safety utilization plans for DNAPL-polluted sites. The rest of this paper is organized as follows. The study area is described in Section 2, and in Section 3, the data and methods used in this study are briefly described, e.g., the transport model, heterogeneity model, and DNAPL transport situations. The DNAPL assessments under different transport situations are discussed in Section 4. The conclusions drawn from results and discussions are provided in Section 5.

2. Study Area

The study area is a retired pesticide and chemically polluted site located in Jiangsu Province, China, with a nearly 50-year history of pesticide production [36,37]. Situated in the alluvial plain of the Yangtze River Delta, the site has an average elevation of +3.90 m, typical of the deltaic geomorphic features shaped by long-term fluvial and lacustrine sedimentation [16,38].

2.1. Hydrogeological Setting and Heterogeneity Characteristics

The site’s lithology is dominated by Quaternary sediments, which exhibit significant spatial heterogeneity due to complex depositional processes (e.g., alternating flood events and channel shifts). This heterogeneity is characterized by widespread lenticular bodies and interbedding of multiple lithofacies, including miscellaneous fill, silt, and silt-sand mixtures (Figure 1). Such a stratigraphic structure creates sharp contrasts in hydraulic properties: for instance, the horizontal absolute permeability ranges from 18 mD (silt) to 2851 mD (silt), while vertical permeability varies from 12 mD to 1320 mD (Table 1). This variability is representative of industrialized delta regions, where aquifer heterogeneity strongly influences groundwater flow and solute transport, making the site an ideal case for studying how lithological complexity modulates DNAPL migration.
The total thickness of the upper strata reaches approximately 50 m, forming a thick phreatic aquifer with high water yield. The groundwater table is approximately 1.4 m below ground surface (bgs), and the flow direction is from north to south with a hydraulic gradient of 1‰. The base of the phreatic aquifer is a relatively impermeable, continuous silt layer with a thickness exceeding 15 m, which acts as a barrier to vertical DNAPL migration into deeper formations.

2.2. Selection of Target DNAPLs

A groundwater environmental investigation in 2010 identified over 50 types of DNAPLs at the site. Among these, 1,2-dichloroethane (DCE) and carbon tetrachloride (CTC) were selected as target contaminants for three reasons:
  • Historical relevance: They were primary raw materials and solvents used in the former pesticide production, leading to long-term accumulation in the subsurface.
  • High concentrations: Their maximum detected concentrations were 21.40 mg/L (DCE) and 2.20 mg/L (CTC), exceeding the risk control values for construction land (0.81 mg/L and 0.23 mg/L, respectively; Table 2).
3.
Distinct transport and attenuation behaviors:
(a)
Transport: DCE has higher solubility (8600 mg/L) and lower density (1250 kg/m3) than CTC (solubility: 793 mg/L; density: 1548 kg/m3), leading to more extensive horizontal diffusion. In contrast, CTC’s higher density drives stronger vertical gravity migration, often accumulating at the interface of low-permeability layers (Section 4.1).
(b)
Attenuation: DCE exhibits a higher degradation rate constant (2.67 × 10−7 s−1) than CTC (1.6 × 10−7 s−1; Table 2), but its attenuation depends more on microbial co-metabolism under specific redox conditions. CTC, however, undergoes reductive dechlorination more readily in anaerobic environments, a process less sensitive to substrate availability [27].
These characteristics make DCE and CTC ideal for investigating multicomponent DNAPL dynamics in heterogeneous aquifers.

3. Methods and Data

3.1. Hydrogeological Conceptual Model

The contamination plume of dissolved DNAPLs pollutants in the groundwater primarily exists in the phreatic aquifer. Therefore, the phreatic aquifer was chosen as the target aquifer for this study. The left and right boundaries of this study area were generalized as the boundaries of the fixed water level. The initial groundwater table depth was designed at 1.0 m below ground surface (bgs) on the left and 1.5 m bgs on the right, exhibiting a left-right upward trend with a hydraulic gradient of 1‰. The remaining bottom boundaries were generalized as zero-flow boundaries. The upper boundary of the target aquifer was the unrestricted surface at the top of the submerged aquifer through which the target aquifer exchanges water volume vertically with evaporation discharge and precipitation infiltration. We generalized the water flow of the phreatic aquifer and the transport of DNAPL contaminants into a 2-D multiphase flow transport model. A groundwater temperature of 25 °C was used for the model. Table 1 presents the specific hydrogeological parameters.

3.2. Pollutant Transport Situations

In this study, we assumed that the typical DNAPL contaminants, DCE and CTC, in the phreatic aquifer less than 20 m had been treated and were all below the detection limits. In addition, the DNAPL contaminants in the aquifer at a depth larger than 20 m were still distributed, which was already an unfavorable situation. Figure 1 shows the conceptual model. The physical and chemical parameters of DCE and CTC were obtained from Pruess and Battistelli [24], while the natural attenuation parameters were obtained from the research of Fan et al. [16] and Hu [39]. The physical and chemical property parameters of the typical DNAPLs contaminants, DCE and CTC, are shown in Table 3.
In this study, the boundary concentration of the contaminant plumes was set as the risk control value of the groundwater contaminants for construction land [40]. Drinking groundwater was not considered in the groundwater exposure situations in this study, and only the inhalation of gaseous contaminants from groundwater in outdoor and indoor air was considered. The risk control values of high-risk pollutants in groundwater utilized the lower value between the indoor and outdoor areas. Table 3 shows the calculated results of the risk control value of DCE and CTC.

3.3. TMVOC Model

We used the TMVOC model to solve the process of contaminant transport. TMVOC is one of the subsequent software variants of the TOUGH model for groundwater flow and heat transfer developed by the Lawrence Berkeley National Laboratory [8]. This model can simulate the transport of DNAPLs in saturated and unsaturated porous media, the formation of oil-like lenses on a water’s surface, and the dissolution, transport, volatilization, and adsorption of DNAPLs. In addition, this model can reproduce the transport behavior of organic pollutants in natural environments, as well as explain the distinct transport and distribution patterns of various types of DNAPLs due to their varying physicochemical properties. Refer to the TMVOC specification [8] for a description of the multiphase flow equation, the three-phase conversion equation, and the initial conditions of VOCs.

3.4. T-PROGS Model

In this study, the GMS T-PROGS module was utilized to generate the stratigraphic heterogeneity field for the study area. T-PROGS is a geostatistical model for transition probability [41]. This model has been extensively implemented in groundwater numerical simulations to realize the calculation of porous media probabilistic geo-statistics [2]. Unlike the traditional multivariate indicator geostatistical model based on cross-variogram, the T-PROGS model accounts for the complex continuity of a stratigraphic space, and this facilitates the incorporation of facies-based stratigraphic interpretation data into the model construction procedure. T-PROGS simplifies the anisotropy processing method and can efficiently calculate the transition probability between strata and lithofacies. Additionally, T-PROGS considers the proportion of stratum type allocation, the mutual transformation trend of each stratum type, and the average strata in all directions during the establishment of the model. The stochastic theory and statistical methods are used to interpolate the geological information of the formation space type during the calculation process by analyzing the formations’ existing spatial parameter information. Three stages comprise the majority of the calculation procedure: (1) compute the disparity between the formation’s parameters; (2) simulate the spatial variation in the layer parameters based on the Markov chain; and (3) construct the optimal heterogeneous model according to conditional simulation and error analysis calculated with their respective error functions.

3.5. Model Spatiotemporal Discretization

The site was simulated using 2-D sections, each with a length, width, and height of 500 m, 1 m, and 45 m, respectively. The model was subdivided into 31 layers along the vertical axis and 252 columns along the horizontal axis, with a total of 7812 effective cells that used a Cartesian orthogonal coordinate system. The model domain’s left and right boundary conditions were established with a grid unit of 0.001 m in length. For the top boundary, a grid unit of 0.001 m in height was also used to establish the atmospheric conditions. Notably, layers 1–9, 10–12, 13–19, and 20–30 are miscellaneous fill, silt, silt sand mixed with silt, and silt, respectively. This simulation depicted the initial distribution of the DNAPL below layer 17, after which the DNAPL dissolved and was transported for 35 years. The model layer and grid division are shown in Figure 2. Based on the historical production situation, in this simulation, 1000 kg of DCE and CTC was leaked into three cells of layers 19–20 (20 m beneath the ground) for 3 days, with a leakage rate of 0.0038 kg/s. The DNAPL was then dispersed and transported for 35 years.

4. Results and Discussion

Three situations were simulated in this study: (1) situation 1 compared the transport of the DCE and CTC in the individual state and mixture state; (2) situation 2 simulated the two DNAPLs under natural decay conditions; and (3) situation 3 simulated the transport of DCE under heterogeneous conditions.

4.1. The Transport Process of DNAPL Under Multi-Component Conditions

Figure 2 and Table 4 represent the results of the transport processes of DCE and CTC in the individual and mixture states. In the individual transport state, both contaminant plumes of DCE and CTC remained a certain distance from the land surface during the simulation period. At the end of the simulation period, the plume of DCE obtained the greatest upward transport distance, reaching 14.01 m; however, it was still 5.99 m away from the land surface. The upward transport distance of DCE and CTC was shorter in the mixture transport state than in the individual transport state. The distance of CTC in the mixture state was significantly reduced at the end of the simulation period, which was 1.34 m shorter than that in the individual transport state. The downstream transport processes of DCE and CTC in the individual transport state varied significantly. In the individual transport state, the downstream transport distance of DCE was longer than that of CTC. The mixture state had a shorter transport distance than either substance individually. At the end of the simulation period, the downstream transport distances of DCE and CTC were 459.58 m and 453.51 m, respectively, whereas in the mixed state, these distances were 440.46 m and 420.32 m, respectively.
According to the simulation results, there were significant disparities between the transport processes of DCE and CTC in the different states. First, the migration rate of DCE was considerably faster than that of CTC. Simultaneously, the transport of DCE contaminant plumes exhibited a distinct downward trend, and the concentration center of DCE contaminant plumes was transported far from the initial contaminant plumes. This phenomenon was consistent with the findings of previous research [7]. Distinct physicochemical properties of DNPALs can result in distinct transport processes in porous media, which may explain this phenomenon [27]. Also, the transport of DCE and CTC exhibited a significant stratification phenomenon. The transport in the silty soil mixed with silty sand layer was significantly delayed relative to that in the silty sand layer, resulting in a longer and deeper transport distance of the two DNAPLs in the silty sand layer during the later stage of the simulation. This was primarily because the hydraulic conductivity of the silty soil mixed with silty sand layer was lower than that of the silty sand layer. Due to the increased conductivity of the silty sand layer, the contaminant plume of DCE was further expanded when it penetrated the silty sand layer. In addition, the transport rate of DCE and CTC in the mixture state was significantly slower than that in the individual state. This may have been due to the mutual solvent effect of DCE and CTC that reduces the dissolved concentration of contaminants. In general, there were still spaces from the edge of the contaminants to the land surface. The vertical upward migration distance of pollutants was constrained within the simulation period. Rivett et al. [3,42] also find that the migration rate of the mixture of multicomponent DNAPLs in all directions was significantly reduced compared to their individual migration. This phenomenon is attributed to multicomponent interactions: when mixed DNAPLs exhibit altered physicochemical properties—specifically, increased viscosity and reduced aqueous solubility relative to their individual states, which matches the results of our research.

4.2. Considering the Transport Process of DNAPL Under Natural Decay and Other Processes

Figure 3 and Table 5 show the transport results of DCE and CTC under natural decay conditions. The upward migration distance of the DCE was comparatively longer than that of the CTC, peaking at 3.26 m at the end of the 6th year. However, this distance was only 0.64 m at the end of the tenth year, along with a significant retreat. The CTC plume migrated only 0.50 m upward at the end of the fifth year, after which the subsequent upward migration process was insignificant. In addition, the downstream migration rate of DCE was considerably faster than that of the CTC. At the end of the 6th year, the maximal downstream migration distance of DCE was 52.14 m. At the end of the tenth year, this distance was 48.65 m, indicating a significant retreat of the contaminant plume. In addition, the utmost downstream distance that CTC migrated was only 3.32 m at the end of the fifth year. At the end of the tenth year, this distance underwent a significant retreat with a transport distance of only 0.95 m. In addition, at the end of the fourteenth year, the maximum concentration of the DCE and CTC decreased to 1.94 mg/L and 0.98 mg/L, respectively, both below the risk control values. In general, the migration processes of DCE and CTC under biodegradation conditions differed from each other, while the migration distances of both DNAPL contaminants were limited in all directions. At the end of the simulation, there was still a considerable distance from the edge of the contaminants to the land surface, and the contaminant concentration was below the risk control values.
According to the simulation results, it can be concluded that there were significant differences in the distribution characteristics and degradation rates of DCE and CTC under biodegradation conditions. First, the central point of the DCE-contaminated plume showed an upward migration trend, while the central point of the CTC-contaminated plume had little change in the horizontal direction, only showing a downward migration trend in the vertical direction. The primary cause of this occurrence may have been because DCE has a higher solubility and dissolves more readily in groundwater, and this could cause a more significant process of contaminant plume diffusion and migration. However, the lower solubility and relatively higher density of CTC facilitate density migration towards the deep section of an aquifer and render diffusion and migration in the horizontal direction less likely. Second, DCE degrades more slowly than CTC. Despite having a lower risk control value than CTC, DCE degrades below the risk control value—nearly 3 years slower. Previous studies have demonstrated that perfluorocarbons like CTC have a significant tendency for reduction dechlorination in anaerobic settings, whereas low-chlorine replacements dechlorinate more slowly. Accordingly, the difficulty of degrading chlorinated hydrocarbons increases with decreasing chlorination levels [43], which is consistent with the outcomes of the simulation in this study.

4.3. Considering the Transport Process of DNAPL Under Heterogeneous Conditions

Under the same heterogeneity degree, lithofacies proportion, vertical correlation length, and other conditions, T-PROGS was used to generate the heterogeneous random field of the site to represent real geological bodies with different horizontal spatial continuities. According to the model generalization conditions in Section 3.5, there are four lithofacies in a random field: miscellaneous fill, silt, silt mixed with silt, and silt. The heterogeneous field generated by T-PROGS is shown in Figure 4.
Figure 5 and Table 6 represent the transport simulation results of DCE and CTC in a heterogeneous field. According to the results, the migratory tendencies of DCE and CTC were essentially consistent, with a small vertical upward migration distance and a larger downstream migration distance. In particular, DCE has a significantly long upward migration distance, reaching a height of 14.82 m at the end of the simulation period while remaining 5.18 m away from the land surface. The downstream migration distance of DCE at the end of the simulation was 397.99 m. Compared to DCE, the transport distance of CTC was shorter. At the end of the simulation, the upward migration distance of CTC was 14.22 m, and it was 5.78 m away from the land surface. While the downstream transport distance was 354.11 m at the end of the simulation.
The distribution features of DCE and CTC in the heterogeneous field were more radically altered in the underground environment than they were in a homogeneous field, according to the simulated results in Section 3.1. First, the vertical migration ability of contaminants in a heterogeneous field is enhanced, which is related to a lack of low-conductivity silt layers in some parts of the heterogeneous field. The migration distance of contaminants in the horizontal direction was significantly reduced compared to that in a homogeneous field, and this was related to the presence of low-permeability lenses in the downstream migration pathway. Second, during the contaminant migration processes in various fields, a trace amount of DNAPLs will penetrate the low-permeability silt lens. Silt lenses with low permeability become a barrier to contaminant migration, and the majority of DCE accumulates around them, which would result in a minimal contaminant concentration in silt lenses. In addition, the accumulation phenomenon significantly increases the saturation of pollutants around the silt lens. The stratification of the pollutant migration process in heterogeneous fields is also not obvious due to the presence of more silt with a relatively high permeability coefficient in the silt layer in a heterogeneous field that would cause the dominant flow of pollutants to migrate downstream in the silt layer.
In China, groundwater site investigations often necessitate investigations to the top plate of the first aquifer. However, some sites are impossible to investigate due to limitations such as drilling equipment and extremely thick aquifers. The above results and discussions of this study show that while both DCE and CTC can transport along the upward and downstream directions of groundwater flow, their transport distances did not exceed the simulation area after a groundwater treatment within the reach capability. As a result, while investigating and repairing chemical-polluted sites with thick aquifers, a simulated study of DNAPL transport in the sites should be performed to build a scientifically plausible inquiry and repair depth.

4.4. Broader Implications for DNAPL Risk Management

The findings of this study hold significant implications for the redevelopment of retired chemically polluted sites, particularly in deltaic regions with thick aquifers (>30 m) and heterogeneous lithology.
For site investigation and remediation design: the maximum vertical migration distances of DCE and CTC over 35 years suggest that investigation depths should extend at least 15 m below the initial contamination zone to capture the full extent of DNAPL plumes. This is critical for avoiding underestimation of pollution scope in thick aquifers, where shallow sampling alone may miss deep residual contamination—a common issue in deltaic sites.
For MNA application: the observation that DCE and CTC concentrations drop below risk control values by the 14th and 11th years, respectively, supports MNA as a feasible strategy for these contaminants in similar hydrogeological settings. However, the slower attenuation of DCE highlights the need for site-specific MNA monitoring plans, especially where multicomponent interactions may delay degradation [3,42].
Globally, these results align with challenges faced in other industrialized delta regions, where heterogeneous aquifers and multicomponent DNAPLs complicate remediation. Our quantification of how heterogeneity reduces downstream migration can inform buffer zone design to protect surface ecosystems and groundwater resources.

4.5. Model Limitations and Future Work

This study has several limitations that warrant consideration:
(1) Model dimensionality: The 2D model simplifies the 3D nature of groundwater flow and DNAPL transport, potentially underestimating lateral spreading in directions perpendicular to the model cross-section. Three-dimensional simulations, though computationally intensive, would better capture the influence of lenticular lenses on plume morphology [2].
(2) Boundary conditions: The fixed water level boundaries (left/right) and zero-flow bottom boundary may not fully reflect dynamic field conditions, such as seasonal groundwater table fluctuations or deep recharge. These simplifications could slightly bias migration rates, particularly for long-term (>35 years) predictions.
(3) Parameter uncertainty: Hydrogeological parameters were derived from limited borehole data, introducing spatial averaging biases. Stochastic parameterization using larger datasets would improve representativeness.
(4) Upscaling challenges: Findings from this site, characterized by deltaic sediments, may not directly apply to crystalline or karst aquifers, where fracture-dominated flow governs DNAPL transport. Future studies should validate these trends across diverse hydrogeological settings.

5. Conclusions

This study investigated the migration and natural attenuation of multicomponent DNAPLs (DCE and CTC) in a retired chemically polluted site with thick aquifers using the TMVOC model under scenarios of individual transport, mixed transport, natural attenuation, and heterogeneous conditions. The key findings are as follows:
(1) Migration characteristics under different transport modes: After 35 years, individually transported DCE and CTC reached maximum vertical distances of 14.01 m and 13.57 m, with downstream distances of 459.58 m and 453.51 m, respectively. In mixed transport, their vertical and downstream migration distances decreased by 1.7–7.3% (DCE) and 2.5–7.3% (CTC) due to mutual solvent effects, indicating inhibited multicomponent migration.
(2) Natural attenuation effects: Under natural attenuation, DCE and CTC ceased downstream migration by the 6th year, with concentrations dropping below risk control values (0.81 mg/L for DCE, 0.23 mg/L for CTC) by the 14th and 11th years, respectively. The plume front retreated significantly in the horizontal direction, with DCE’s downstream distance reducing from 52.14 m (6th year) to 48.65 m (10th year).
(3) Influence of aquifer heterogeneity: In heterogeneous fields, vertical upward migration of DCE and CTC increased by 5.8% and 4.8% compared to homogeneous conditions (reaching 14.82 m and 14.22 m, respectively) due to partial silt layer absence. Meanwhile, downstream migration decreased by 13.4% (DCE) and 21.9% (CTC) (to 397.99 m and 354.11 m) due to low-permeability lenses.
These findings quantify the transport dynamics of multicomponent DNAPLs in thick heterogeneous aquifers, providing direct references for determining investigation depths and remediation strategies in similar retired chemically polluted sites.

Author Contributions

Methodology, X.L. and D.J.; software, W.X.; data curation, M.W. and L.K.; writing—original draft preparation, W.X.; writing—review and editing, S.D. and M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonweal Research Institute, grant numbers: GYZX230301, GYZX220303, and GYZX240101, the Jiangsu Innovative and Entrepreneurial Talent Program, grant number: jSSCBS20221648, and the National Key Research and Development Program of China (2024YFC3713002, and 2024YFC3713003).

Data Availability Statement

Data will be provided on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 13 02338 g001
Figure 1. Conceptual model.
Figure 1. Conceptual model.
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Figure 2. Concentration distribution of DNAPLs after 40 years of transport. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride. (c) Mixture of 1,2-Dichloroethane and Carbon Tetrachloride.
Figure 2. Concentration distribution of DNAPLs after 40 years of transport. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride. (c) Mixture of 1,2-Dichloroethane and Carbon Tetrachloride.
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Figure 3. Concentration distribution of DNAPLs after 10 years of natural attenuation. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride.
Figure 3. Concentration distribution of DNAPLs after 10 years of natural attenuation. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride.
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Figure 4. Random heterogeneous field.
Figure 4. Random heterogeneous field.
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Figure 5. Concentration distribution of DNAPLs after 35 a transport in the heterogeneous field. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride.
Figure 5. Concentration distribution of DNAPLs after 35 a transport in the heterogeneous field. (a) 1,2-Dichloroethane. (b) Carbon Tetrachloride.
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Table 1. Hydrogeological parameters.
Table 1. Hydrogeological parameters.
LithologyHorizontal Absolute Permeability (mD)Vertical Absolute Permeability (mD)PorosityDensity
(kg/m3)		Thickness
(m)
Miscellaneous Fill235411580.341876.84
Silt18120.341897.24
Silt Sand Mixed with Silt194910110.341897.219
Silt285113200.341846.223
Table 2. Risk control values of high-risk pollutants in groundwater.
Table 2. Risk control values of high-risk pollutants in groundwater.
DNAPLIndoor Steam Intrusion Risk Control Value
(mg/L)		Outdoor Steam Intrusion Risk Control Value
(mg/L)
DCE0.8118.77
CTC0.2311.07
Table 3. Parameter table of DCE and CTC.
Table 3. Parameter table of DCE and CTC.
Physical and Chemical PropertiesDCECTC
Critical Temperature (K)356.7349.9
Critical Pressure (bar)53.745.6
Critical Compressibility0.2590.272
Viscosity (cP)0.590.90
Pitzer Centrifugal Constant0.2780.193
Electric Dipole Moment (debyes)1.80.0
Boiling Point (K)360.4394.4
Molar Mass (g/mol)98.96153.825
Relative Density (kg/m3)12501548
Temperature of Gas Diffusion (K)289298
Critical Volume (cm3/mole)225275.9
Solubility (mg/L)8600793
Partition Coefficient Koc (m3/kg)0.0140.439
Degradation Decay Constant (s−1)2.67 × 10−71.6 × 10−7
Organic Carbon Fraction in Medium0.0010.001
Table 4. Simulation results of the pollutant transport of typical DNAPLs under Situation 1.
Table 4. Simulation results of the pollutant transport of typical DNAPLs under Situation 1.
PollutantsUpward Transport Distance (m)Downstream Transport Distance (m)
5 a15 a25 a35 a5 a15 a25 a35 a
DCE6.40 9.57 12.20 14.01 81.21 187.51 297.34 459.58
CTC4.99 8.98 11.45 13.57 78.58 180.20 286.40 453.51
DCE (mixed)5.93 9.00 11.28 13.76 80.45 184.41 294.54 440.46
CTC (mixed)4.49 8.41 10.56 13.23 76.66 177.02 283.25 420.32
Table 5. Simulation results of DCE and CTC transport under natural decay conditions.
Table 5. Simulation results of DCE and CTC transport under natural decay conditions.
DNAPLUpward Transport Distance (m)Downstream Transport Distance (m)
1 a4 a6 a10 a1 a4 a6 a10 a
DCE2.612.893.260.6416.1142.9752.1448.65
CTC0.50003.321.682.050.95
Table 6. Simulation results of the DCE transport in a heterogeneous field.
Table 6. Simulation results of the DCE transport in a heterogeneous field.
DNAPLUpstream Transport Distance (m)Downstream Transport Distance (m)
5 a15 a25 a35 a5 a15 a25 a35 a
DCE8.1913.7314.2314.8267.97155.07301.28397.99
CTC5.6212.0013.7414.2260.99144.88257.25354.11
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Xie, W.; Li, M.; Jiang, D.; Kong, L.; Wang, M.; Deng, S.; Li, X. Impact of Aquifer Heterogeneity on the Migration and Natural Attenuation of Multicomponent Heavy Dense Nonaqueous Phase Liquids (DNAPLs) in a Retired Chemically Polluted Site. Processes 2025, 13, 2338. https://doi.org/10.3390/pr13082338

AMA Style

Xie W, Li M, Jiang D, Kong L, Wang M, Deng S, Li X. Impact of Aquifer Heterogeneity on the Migration and Natural Attenuation of Multicomponent Heavy Dense Nonaqueous Phase Liquids (DNAPLs) in a Retired Chemically Polluted Site. Processes. 2025; 13(8):2338. https://doi.org/10.3390/pr13082338

Chicago/Turabian Style

Xie, Wenyi, Mei Li, Dengdeng Jiang, Lingya Kong, Mengjie Wang, Shaopo Deng, and Xuwei Li. 2025. "Impact of Aquifer Heterogeneity on the Migration and Natural Attenuation of Multicomponent Heavy Dense Nonaqueous Phase Liquids (DNAPLs) in a Retired Chemically Polluted Site" Processes 13, no. 8: 2338. https://doi.org/10.3390/pr13082338

APA Style

Xie, W., Li, M., Jiang, D., Kong, L., Wang, M., Deng, S., & Li, X. (2025). Impact of Aquifer Heterogeneity on the Migration and Natural Attenuation of Multicomponent Heavy Dense Nonaqueous Phase Liquids (DNAPLs) in a Retired Chemically Polluted Site. Processes, 13(8), 2338. https://doi.org/10.3390/pr13082338

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