Open AccessReview Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites by Clarissa Settimi Clarissa Settimi SciProfiles Scilit Preprints.org Google Scholar , Daniela Zingaretti Daniela Zingaretti SciProfiles Scilit Preprints.org Google Scholar , Renato Baciocchi Renato Baciocchi SciProfiles Scilit Preprints.org Google Scholar and Iason Verginelli Iason Verginelli SciProfiles Scilit Preprints.org Google Scholar * Department of Civil Engineering and Computer Science Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy * Author to whom correspondence should be addressed. Environments 2026, 13(6), 327; https://doi.org/10.3390/environments13060327 (registering DOI) Submission received: 13 May 2026 / Revised: 3 June 2026 / Accepted: 5 June 2026 / Published: 9 June 2026 Abstract This review presents a process-based decision-making framework for chlorinated vapor intrusion (CVI) mitigation. CVI mitigation refers to the set of engineered strategies aimed at interrupting, attenuating or transforming vapor fluxes before they reach indoor environments. Existing literature and technical guidelines typically classify mitigation strategies according to technological configuration (active versus passive), rather than physical and chemical processes governing vapor transport and attenuation, which may lead to suboptimal design choices and reduced system resilience. To address this limitation, this framework proposes a process-based classification of CVI mitigation strategies based on the dominant mechanisms controlling vapor migration in subsurface. Five mechanistic categories are identified: driving-force control through pressure manipulation, dilution via air exchange, diffusive flux control through physical barriers, density-driven attenuation in permeable sub-slab layers, and in situ transformation based on sorption or degradation. By explicitly linking mitigation technologies to transport and transformation processes, the proposed framework provides a structured basis for mechanism-oriented selection, integrating performance, longevity, climate resilience, and lifecycle energy demand. In addition to established mitigation approaches, such as sub-slab depressurization, this work highlights emerging passive strategies, including high permeable granular layers and horizontal reactive or adsorbing barriers, as potential low-energy alternatives for durable management. Overall, the proposed framework supports site-specific, sustainability-oriented decision-making on CVI mitigation. Keywords: chlorinated solvents; vapor intrusion; mitigation techniques; remediation; active mitigation; passive mitigation 1. Introduction Chlorinated solvents contamination is a critical and concerning environmental issue [ 1, 2, 3, 4, 5]. These compounds are widely used in civil and industrial applications like degreasing, dry cleaning, and chemical synthesis [ 3, 6, 7]. Due to their low biodegradability, moderate solubility, and high volatility, chlorinated solvents are persistent and mobile in the environment [ 7]. Therefore, their extensive use and improper management or disposal have led to significant contamination of groundwater and soil [ 1, 7]. In fact, chlorinated solvents like trichloroethylene (TCE) or tetrachloroethylene (PCE) are often present as pollutants at contaminated sites [ 1, 8, 9]. Once released into the environment, chlorinated solvents persist as dense non-aqueous phase liquids (DNAPL), which can migrate below the groundwater table, deposit at low permeable layers and form pools of free product [ 3, 5, 8, 9]. DNAPL contamination is very challenging to manage because of its complex distribution in the subsurface and persistence due to relatively low biodegradability [ 5, 7]. Therefore, this behavior often results in diffuse contamination plumes in groundwater that are challenging to remediate [ 6, 10]. Furthermore, the volatility of chlorinated solvents makes them highly mobile in the unsaturated zone as vapors, leading to potential vapor intrusion (VI) if buildings are present over a contaminated area [ 9, 11, 12, 13]. VI refers to contaminated vapors entering buildings through cracks or openings in foundations or basement walls [ 11, 12, 14, 15]. In the case of chlorinated solvent vapors, the vapor intrusion phenomenon is known as chlorinated vapor intrusion (CVI) [ 16]. In general, contaminated vapors may enter a building due to various factors, including changes in barometric pressure, wind or stack effects, thermal gradients, or depressurization caused by building exhaust systems [ 12, 17, 18]. Pressure and thermal gradients between soil and building drive advective forces that favor vapors entering the building [ 18, 19]. Once inside the building, the soil gas mixes with the indoor air through natural or mechanical ventilation systems [ 17]. CVI represents a critical exposure pathway, posing significant human health risks through the inhalation exposure [ 4, 20] due to the carcinogenicity of many chlorinated solvents [ 1, 4, 20, 21, 22, 23, 24, 25, 26]. To overcome this limitation, a process-oriented framework is required that explicitly links the mitigation technology performance to the governing transport and transformation phenomena of contaminated vapors, thereby supporting more robust and site-specific decision-making. In this context, the present study proposes a mechanistic classification of CVI mitigation systems based on the dominant physical and chemical processes controlling vapor transport and attenuation in the unsaturated zone. Rather than grouping technologies according to their configuration or operational mode, the proposed framework organizes mitigation approaches into five process-based categories: (a) driving-force control through pressure gradient manipulation, (b) concentration reduction via dilution, (c) diffusive flux limitation through physical barriers, (d) density-driven attenuation mechanisms, and (e) subsurface treatment based on transformation and retardation processes (see Figure 1). By framing mitigation options in terms of governing transport and transformation processes, this classification provides a rational basis for strategy selection that is consistent with site-specific conditions and long-term performance objectives, supporting the identification of solutions that minimize energy consumption and reduce lifecycle maintenance requirements. 2. Processes Controlling Chlorinated Vapor Intrusion The assessment and mitigation of chlorinated vapor intrusion require a clear understanding of the physical and chemical processes that control vapor migration in the unsaturated zone [ 18]. Mitigation systems are often described as active or passive, depending on the energy usage [ 12, 17, 23]. However, their effectiveness depends on how they influence the dominant transport and transformation mechanisms at a given site. Vapor-phase chlorinated solvents in the vadose zone are governed by molecular diffusion, advection induced by pressure or density gradients, phase interactions such as sorption, and chemical or biological transformation [ 20, 31, 32, 33, 34]. These processes can occur simultaneously [ 33]. However, their relative importance depends on soil permeability, contaminant concentration, moisture content, building characteristics, and environmental conditions [ 31, 33, 35]. This section summarizes the processes that control CVI and provides the foundation for the process-based classification of mitigation strategies presented in the next section. 2.1. Diffusion-Dominated Vapor Transport 2.2. Pressure-Driven Advection 2.3. Density-Driven Advection 2.4. Sorption and Retardation Processes As vapors migrate through the vadose zone, they interact with soil solid particles and organic matter through sorption [ 6, 33, 42]. This process retards vapor transport and leads to temporary storage of contaminants within the porous medium [ 33]. Sorption depends on soil organic carbon content, mineral surface properties, contaminant characteristics, temperature, and moisture conditions [ 42, 59]. In natural soils, sorption generally provides partial attenuation, but it rarely prevents vapor migration entirely. Engineered systems can enhance this mechanism by introducing materials characterized by high surface area and organic carbon content, such as activated carbon or biochar [ 60, 61, 62]. Sorption-based approaches reduce vapor flux and delay contaminant breakthrough [ 60]. However, these strategies do not eliminate contaminant mass but only retain it. Long-term performance depends on several factors such as adsorption capacity of the material, competitive adsorption, desorption behavior, and material aging [ 60, 63]. These processes form the basis of mitigation strategies that rely on adsorptive barriers. 2.5. Reactive Transformation Processes Chlorinated solvents may undergo chemical or biological transformation in the subsurface [ 1, 2, 3, 6, 7, 9, 31, 33]. Specifically, abiotic reactions include reductive dehalogenation induced by zero-valent iron or iron sulfide and oxidation promoted by strong oxidants [ 9, 64, 65, 66, 67]. Instead, biotic degradation may occur under aerobic or anaerobic conditions, depending on redox state, moisture content, and substrate availability [ 16, 68]. However, in the unsaturated zone its overall contribution is often limited by low water content, reduced microbial activity, and mass transfer constraints, and is therefore typically less significant than in saturated systems [ 10]. Transformation processes reduce contaminant mass through degradation rather than through redirection or dilution of vapor flux [ 15]. Their efficiency depends on several factors, such as reaction kinetics, mass transfer limitations, temperature, moisture, and the persistence of reactive capacity. Engineered reactive layers placed in the unsaturated zone represent an approach in which contaminant attenuation occurs before vapors reach building foundations [ 15, 65, 67, 69]. 3. Process-Based Classification of CVI Mitigation Strategies In this study, the mitigation systems were classified into five categories according to the primary process they affect within the vapor intrusion pathway (see Table 1 and Figure 1). This proposed framework aims to clarify how different approaches modify specific transport and attenuation mechanisms occurring between the subsurface and the building. Driving-force control strategies ( Figure 1a) modify the pressure field to regulate advective soil gas movement. Dilution-based strategies ( Figure 1b) reduce vapor concentrations through controlled air exchange rates. Diffusive flux control strategies ( Figure 1c) are based on the use of physical barriers to limit vapor migration by increasing resistance to diffusive transport and reducing preferential entry pathways. Density-driven attenuation strategies ( Figure 1d) take advantage of vapor density contrasts to promote downward advection within permeable sub-slab layers. Finally, in situ transformation strategies ( Figure 1e) reduce contaminant mass within the subsurface through sorption, retardation, or chemical and biological degradation processes. The following sections describe the techniques associated with each category. 3.1. Driving-Force Control Strategies Driving-force control strategies include remediation and mitigation techniques designed to alter the pressure gradients and advective flow conditions that govern vapor migration from the subsurface into buildings [ 23]. These approaches either induce negative pressure to intercept and extract contaminated vapors or create positive pressure conditions to prevent their inward migration. The category includes both active systems, such as Soil Vapor Extraction (SVE), Sub-Slab Depressurization (SSD) and its variants (DTD, BWD, SMD), and Sub-Slab Pressurization (SSP), as well as passive configurations like Passive Sub-Slab Depressurization (PSSD). Figure 2 illustrates the main driving-force control strategies discussed in this section, highlighting their conceptual configurations and the mechanisms through which they modify subsurface pressure conditions to reduce vapor intrusion. 3.1.1. Soil Vapor Extraction (SVE) Soil Vapor Extraction (SVE) ( Figure 2f) is an in situ remediation technology that removes volatile contaminants from unsaturated soils by applying vacuum to extraction wells, thereby inducing vapor flow toward the extraction system [ 3, 74, 116]. Extracted vapors are collected and treated at the surface prior to discharge into the atmosphere [ 70, 72, 117, 118]. SVE has been widely used for more than three decades to address chlorinated solvents and other volatile organic compounds in the vadose zone [ 13, 72, 73]. 3.1.2. Active Depressurization/Pressurization Systems (SSD, DTD, BWD, SMD and SSP) Active depressurization and pressurization systems represent well-established methods for vapor intrusion mitigation [ 18, 23]. Among these, Sub-Slab Depressurization (SSD) ( Figure 2a) is the most widely applied and is generally considered the most practical solution for addressing chlorinated vapor intrusion [ 18, 53, 77, 122, 123]. SSD uses electric fans to create a negative pressure zone beneath the building slab, reversing the pressure gradient and inducing outward advective flow that prevents vapor entry through cracks and openings [ 76, 122, 124]. Extracted vapors are vented to the atmosphere [ 76, 125, 126], and effective operation typically requires maintaining a pressure differential of 4–10 Pa across the slab [ 17, 23, 98]. SSD can be implemented in both new and existing buildings [ 17, 23, 76], and its performance has been documented at sites impacted by chlorinated solvents, including large residential areas [ 30, 35, 75, 77, 78, 127]. Although capital costs are generally lower than SVE-based mitigation [ 53], preferential pathways may significantly reduce system effectiveness [ 122]. Variants of SSD include Drain Tile Depressurization (DTD) ( Figure 2b) and Block Wall depressurization (BWD) ( Figure 2c) [ 17, 124, 128, 129]. DTD applies negative pressure through existing perimeter drainage systems connected to a fan [ 129], providing a cost-effective alternative where such infrastructure is present [ 23]. Interior drain tiles can enhance sub-slab pressure control near slab–wall joints, whereas exterior systems are generally less effective toward the slab center [ 17]. BWD applies suction to hollow block wall cavities to reduce vapor entry through foundation walls [ 17, 129]. Due to limited pressure propagation, BWD is typically used as a supplement to SSD, particularly in buildings with block wall foundations [ 17, 28]. 3.1.3. Passive Depressurization (PSSD) Passive Sub-Slab depressurization (PSSD) ( Figure 2g) reduces sub-slab pressure relative to indoor air without the use of mechanical fans [ 23]. The system consists of a vent pipe connecting the sub-slab region to the outdoor atmosphere, typically conveyed through conditioned indoor space [ 19, 23]. Airflow is driven by natural thermal and pressure gradients. Temperature differences between the subsurface and the building interior induce upward convective flow in the vent pipe through the stack effect [ 19, 23]. In addition, wind flowing over the roof can create a low-pressure zone that enhances vapor extraction [ 19, 81]. Wind turbines may be installed to support venting [ 23]. 3.2. Dilution-Based Strategies Figure 3 presents the principal dilution-based strategies addressed in this section, illustrating their conceptual configurations and the mechanisms through which increased airflow and ventilation contribute to vapor intrusion mitigation. 3.2.1. Active Ventilation Systems (SSV and CSV) Active ventilation strategies mitigate vapor intrusion by increasing airflow in subsurface or crawlspace zones to dilute contaminant concentrations [ 23, 30, 123]. Sub-Slab Ventilation (SSV) ( Figure 3a) introduces and evacuates air beneath the slab to reduce vapor concentrations and promote removal [ 82, 123]. Its effectiveness depends on sufficient sub-slab permeability, such as in coarse granular fill, to allow adequate airflow [ 52, 123]. SSV is particularly suitable where vapor concentrations are moderate and can be reduced to acceptable levels through dilution rather than strong pressure reversal [ 23]. Crawlspace Ventilation (CSV) ( Figure 3b) similarly mitigates vapor intrusion by increasing the air exchange rate within crawlspaces, typically targeting 1–3 h −1 to dilute contaminant [ 17, 23, 84]. Effective operation may require sealing to isolate the crawlspace [ 84]. In high-risk scenarios, however, submembrane depressurization generally provides more robust control than CSV alone [ 84]. 3.2.2. Passive Ventilation Systems (PSSV and PCSV) 3.2.3. Heating, Ventilating, and Air-Conditioning (HVAC) 3.3. Diffusive Flux Control Strategies 3.3.1. Asphalt–Latex Membranes (ALM) ALMs generally provide low vapor permeability and good chemical resistance to chlorinated solvents, while maintaining flexibility to accommodate minor differential settlement or structural movement [ 92]. Their ability to adhere to materials such as concrete, steel, poly-vinylchloride (PVC), and wood facilitates integration with foundation elements and utility penetrations [ 68]. Commercial products, including Liquid Boot ପ୍ପ (CETCO, Houston, TX, USA) have been widely implemented in vapor intrusion mitigation projects [ 17, 83, 93]. 3.3.2. Thermoplastic Membranes (TM) 3.3.3. Composite Membranes (CM) 3.4. Density-Driven Attenuation Strategies Density-driven attenuation strategies include building-related features that can passively influence vapor transport as a result of density differences between contaminant vapors and ambient soil gas. For chlorinated solvents such as TCE and PCE, whose vapors are denser than ambient soil-gas, specific sub-foundation configurations may favor downward advective flow when permeability and vapor concentrations are sufficiently high, partially counterbalancing upward diffusion toward indoor spaces [ 55]. Unlike active mitigation systems or ventilation-based approaches, these strategies do not rely on mechanical intervention but on the physical characteristics of the contaminant of concern to support natural attenuation mechanisms. Figure 5 presents the main configurations considered within this category, namely high-permeability Granular Fill layers (GF) and aerated floor Void Space Systems (VSS), both of which can facilitate density-driven transport and contribute to reduced sub-slab vapor concentrations under suitable site conditions. 3.4.1. High Permeable Granular Fill Layers (GF) Recent modeling and experimental studies have shown that high-permeability granular layers can promote significant attenuation of chlorinated vapors when source vapor concentrations exceed threshold values [ 55]. Specifically, for GF layers permeabilities exceeding 10 −7 m 2, density-driven attenuation is expected to become relevant at vapor concentrations of 1 mg m −3, while for lower permeabilities (10 −8–10 −10 m 2), attenuation is expected for higher vapor concentrations, exceeding 1 g m −3 [ 55]. In contrast, geometric factors such as layer thickness or groundwater depth appear to exert a limited influence on this mechanism [ 55]. Modeling results on density-driven attenuation aligned with extensive field data for chlorinated solvents vapors from several databases showing attenuation by increasing vapor source concentrations [ 55]. These novel results indicate that granular fill layers characterized by high permeability, beyond their structural and drainage functions, may contribute to passive attenuation of chlorinated vapor intrusion under appropriate site conditions ( Figure 5a). However, granular fill layers used as attenuation strategy due to density-driven advection have been mainly investigated through modeling and controlled experimental studies, therefore dedicated field validation is still limited. 3.4.2. Aerated Floor Void Space System (VSS) Aerated floor Void Space Systems (VSS) ( Figure 5b) create a continuous, highly permeable cavity beneath the slab, providing substantially lower resistance to gas flow than conventional granular layers [ 98, 99]. These systems are typically formed using modular plastic forms installed prior to concrete placement, generating interconnected voids [ 99]. Commercial products such as Ventform (Cordek Ltd., West Sussex, UK) and Cupolex ପ୍ପ (Pontarolo Engineering, San Vito al Tagliamento, Italy) are commonly used [ 86, 98]. The high permeability of the void space promotes uniform airflow distribution and enhances the performance of both active and passive ventilation or depressurization systems [ 98, 99]. In addition, by minimizing resistance to gas movement, VSS may facilitate density-driven transport where sufficient vapor concentration gradients exist, thereby supporting passive attenuation mechanisms under suitable conditions. VSS are primarily implemented in new construction but may also be used during major renovations or slab replacement [ 99]. Increased oxygen availability within the void space may further promote aerobic biodegradation of susceptible compounds [ 99]. 3.5. In Situ Transformation Strategies In situ transformation strategies encompass subsurface systems specifically designed to reduce vapor-phase contaminant mass through chemical, physico-chemical, or biological reactions occurring before vapors reach the building. Rather than primarily controlling pressure gradients or enhancing dilution, these approaches aim to transform chlorinated compounds into less harmful products or to immobilize them within reactive or sorptive media installed in the unsaturated zone [ 60, 104]. This category includes Horizontal Permeable Reactive Barriers (HPRBs), Horizontal Permeable Adsorbing Barriers (HPABs), Aerobic Vapor Migration Barriers (AVMBs), and biological or reactive cover systems, such as biocovers, biofilters or reactive covers. As illustrated in Figure 6, barrier configurations are placed between the contaminant source and the building foundation or atmosphere, where they function as treatment layers that attenuate contaminant fluxes within the vadose zone, while cover configurations are mainly posed as capping to address volatilization and release of contaminated vapors in the atmosphere (i.e., soil or landfill capping). 3.5.1. Horizontal Permeable Reactive Barriers (HPRB) To address this limitation, zero-valent iron (ZVI) has been proposed as a more stable filling material due to its low solubility [ 65, 102]. ZVI promotes reductive dehalogenation of chlorinated vapors, converting them into simple hydrocarbons through hydrogenolysis and β-elimination pathways [ 66, 136]. Laboratory studies demonstrated effective TCE degradation under varying environmental conditions, with performance influenced by humidity, oxygen availability, and material dilution with sand [ 65, 102]. Modified ZVI materials, including bimetallic ZVI–Ni or ZVI–Cu [ 103, 104] and sulfidated ZVI [ 105] have shown enhanced reactivity and improved resistance to passivation. Modeling results indicate that required barrier thickness may range from approximately 1 m for ZVI to less than 20 cm for modified materials [ 65, 103, 104, 105]. Recent 2-D numerical modeling incorporating lateral bypass flow highlighted the importance of barrier configuration, showing that combining horizontal reactive layers with vertical or low-permeability elements improves overall attenuation performance [ 69]. The current level of evidence highlights that HPRBs are promising emerging passive mitigation strategies for addressing chlorinated vapor intrusion, though further higher scale validation is still needed for addressing critical technical aspects and long-term durability under field conditions. 3.5.2. Horizontal Permeable Adsorbing Barriers (HPAB) Generally, carbonaceous materials have been widely investigated for chlorinated vapors adsorption applications [ 60, 140, 141]. While activated carbon is commonly used for VOC treatment [ 72], biochar and coal ash derived from biomass pyrolysis or gasification have emerged as lower-cost and more sustainable alternatives [ 60, 140]. Such materials were involved in HPAB development, and experimental and modeling studies demonstrated effective TCE vapors adsorption under variable environmental conditions [ 60]. For example, based on experiments and modeling, a 0.5 m thick biochar barrier may achieve half-time durations exceeding 15 years at moderate vapor concentrations, although higher source concentrations reduce longevity and may require increased thickness [ 60]. These findings support HPABs as promising emerging passive mitigation options for chlorinated vapors, though further field validation is still limited and needed. Sorbent-based capping systems using biochar have also been proposed to reduce vapor emissions at contaminated sites [ 61]. 3.5.3. Aerobic Vapor Migration Barriers (AVMB) The enhanced oxygen supply stimulates aerobic microbial activity within the porous medium. As contaminated vapors migrate through the oxygenated zone, biodegradable compounds are oxidized by indigenous microorganisms, reducing contaminant mass before vapors reach the building interior [ 109]. In this sense, AVMB systems differ from conventional depressurization systems because their primary objective is not only flow control but also enhancement of biological transformation. Field studies have demonstrated effective attenuation of aerobically degradable hydrocarbons with lower energy demand compared to extraction-only systems [ 109]. For chlorinated solvents, AVMB effectiveness is limited to compounds susceptible to aerobic degradation, such as vinyl chloride. Compounds that require anaerobic reductive dehalogenation, including TCE and PCE, are not effectively treated under aerobic conditions and therefore cannot be controlled by this approach. In this context, the application of anaerobic vapor migration barriers is also challenging in the shallow vadose zone underneath buildings, where atmospheric oxygen diffusion limits the persistence of anaerobic conditions needed for inducing degradation. 3.5.4. Bio/Reactive Covers 4. Longevity, Resilience, and Selection of Mitigation Strategies 4.1. System Longevity and Maintenance Cycles The long-term reliability of vapor intrusion mitigation systems depends largely on the mechanism responsible for attenuation. Because the five mitigation categories differ in operational requirements, material durability, and reactive capacity, they exhibit distinct longevity profiles. Dilution-based strategies similarly rely on continuous airflow to maintain adequate air exchange rates [ 23]. Although typically less pressure-intensive than depressurization systems, their long-term effectiveness remains contingent upon uninterrupted operation and building ventilation control. In contrast, physical barrier and density-driven attenuation strategies are inherently passive [ 55, 92] and therefore not subject to mechanical failure. Their durability depends instead on material integrity and preservation of subsurface hydraulic properties. Polymer membranes may degrade due to aging, puncturing, or prolonged chemical exposure. Density-driven systems require sustained gas-phase permeability within granular layers, which may be affected by moisture redistribution or soil consolidation. In situ transformation systems introduce a distinct longevity constraint. Rather than mechanical wear or structural degradation, their lifespan is governed by depletion of reactive media, passivation of reactive surfaces, or exhaustion of sorption capacity [ 60, 105]. Effective design therefore requires estimating contaminant mass flux so that the reactive lifetime of the treatment system matches the expected persistence of the vapor source. 4.2. Resilience to Climatic Fluctuations and Subsurface Dynamics Beyond intrinsic durability, the resilience of mitigation strategies to environmental variability is essential for long-term risk management. Mechanisms based on pressure manipulation or dilution are directly influenced by atmospheric drivers such as barometric pressure fluctuations, wind loading, and seasonal indoor-outdoor temperature gradients [ 18, 41]. These factors may induce temporal variability in system performance, particularly in passive configurations where pressure fields are not mechanically controlled. Active depressurization systems are less sensitive to climatic variability because they impose a controlled pressure gradient [ 23]. However, their resilience remains dependent on continuous energy supply and mechanical reliability. Density-driven attenuation and in situ transformation strategies are generally less sensitive to short-term atmospheric variability because their performance depends primarily on vapor density, reaction kinetics, and subsurface conditions. Nevertheless, all mechanisms are influenced by subsurface dynamics. Variations in soil moisture, capillary fringe elevation, and gas permeability may alter advective and diffusive transport pathways, affecting both density-driven redistribution and reactive contact efficiency. Physical barriers are largely insensitive to atmospheric fluctuations but may be vulnerable to structural discontinuities or long-term material degradation [ 92]. 4.3. Strategic Transition Toward Lifecycle-Oriented Mitigation The comparative evaluation of longevity, operational dependence, and environmental resilience across the five mechanistic categories highlights the need for a structured decision framework. To support mechanism-based selection, Table 2 summarizes the dominant control processes, representative technologies, key design parameters, applicability constraints, and qualitative operational and carbon implications associated with each category. This table provides a basis for aligning mitigation strategies with site-specific transport dynamics, required response time, and long-term sustainability objectives. The comparison of mitigation mechanisms indicates that no single approach is universally optimal. Mechanisms that provide greater control over vapor transport generally require higher operational effort, energy input, and maintenance. Conversely, approaches with lower operational demands may rely more strongly on favorable site conditions and the long-term preservation of key system properties. Transformation- and retardation-based mechanisms provide an alternative pathway by reducing contaminant mass or mobility, although their long-term effectiveness depends on maintaining sufficient reactive capacity. Consequently, mitigation selection should consider not only immediate risk reduction, but also lifecycle performance, operational requirements, and long-term resilience. Driving-force control systems remain the most reliable option for rapid risk reduction, especially in low-permeability soils and high-concentration settings. Their main limitation is the need for continuous energy input and regular maintenance, which increases lifecycle costs and carbon emissions. Physical barriers and density-driven attenuation offer passive alternatives with minimal operational energy demand. Their effectiveness depends on installation quality and preservation of subsurface gas permeability, making them particularly suitable for new construction or controlled redevelopment settings. In situ transformation and retardation strategies represent a shift from vapor flux redirection to contaminant mass reduction. When designed to accommodate the expected contaminant load, these systems can provide long-term attenuation while requiring little operational energy. As a result, they are well aligned with sustainable remediation principles. Dilution-based strategies are generally appropriate for moderate-risk conditions or as supplementary safety measures. Because their performance depends on building airtightness and climatic variability, they are less suited as primary mechanisms in high-risk contexts. A lifecycle-oriented strategy may combine different mitigation mechanisms over time. Active driving-force control can be used during the initial high-flux phase to achieve rapid risk reduction. As vapor concentrations decline, passive or in situ approaches may become sufficient to maintain long-term protection with lower energy demand. Such hybrid strategies integrate rapid risk reduction with improved long-term resilience and sustainability. 5. Conclusions This work advances the discussion on chlorinated vapor intrusion mitigation by shifting from a technology-centered perspective to a process-based framework grounded in vadose-zone transport and transformation mechanisms. By classifying mitigation systems according to their dominant physical and chemical controls, the proposed scheme clarifies how different strategies modify diffusion, advection, density-driven flow, sorption, and reactive degradation. This mechanistic interpretation enables a more transparent comparison of performance, durability, and climate resilience across system types. The analysis confirms that active driving-force control remains the most reliable solution for rapid risk reduction, whereas emerging passive approaches, such as density-driven attenuation and horizontal reactive or adsorptive barriers offer promising low-energy alternatives for long-term management, although further field validation is required and performance depends on several factors (e.g., installation quality, maintenance and monitoring requirements, material aging, evolving subsurface conditions). Overall, the proposed framework provides a structured basis for lifecycle-oriented and sustainability-informed selection of CVI mitigation strategies. Author Contributions Conceptualization, C.S., D.Z. and I.V.; Methodology, C.S., D.Z. and I.V.; Writing—original draft preparation, C.S. and I.V.; Writing—review and editing, D.Z. and I.V.; Visualization, C.S., D.Z. and I.V.; Supervision, D.Z., R.B. and I.V. All authors have read and agreed to the published version of the manuscript. Funding This research received no external funding. Data Availability Statement No new data was created in this study. The data can be retrieved in the cited references. Conflicts of Interest The authors declare no conflicts of interest. Abbreviations The following abbreviations are used in this manuscript: AC Activated Carbon ALM Asphalt–Latex Membrane AVMB Aerobic Vapor Migration Barrier BWD Block Wall Depressurization CSV Crawlspace Ventilation CM Composite Membrane CVI Chlorinated Vapor Intrusion DCE Dichloroethylene DNAPL Dense Non-Aqueous Phase Liquids DTD Drain Tile Depressurization EDPM Ethylene–Propylene–Diene Monomer EVOH Ethylene Vinyl Alcohol GF Granular Fill layers HDPE High-Density Polyethylene HPAB Horizontal Permeable Adsorbing Barrier HPB Horizontal Permeable Barrier HPRB Horizontal Permeable Reactive Barrier HVAC Heating Ventilating Air Conditioning LLDPE Low-Density Polyethylene PCE Tetrachloroethylene PCSV Passive Crawlspace Ventilation PSSD Passive Sub-Slab Depressurization PSSV Passive Sub-Slab Ventilation PVC Poly Vinylchloride SMD Submembrane Depressurization SSD Sub-Slab Depressurization SSP Sub-Slab Pressurization SSV Sub-Slab Ventilation SVE Soil Vapor Extraction TCE Trichloroethylene TM Thermoplastic Membrane VC Vinyl Chloride VI Vapor Intrusion VOC Volatile Organic Compound VSS Void Space System ZVI Zero-Valent Iron References Figure 1. Processes exploited by CVI mitigation strategies considered in this framework: (a) Driving-Force Control, (b) Dilution-Based Strategies, (c) Diffusive Flux Control, (d) Density-Driven Attenuation, and (e) In Situ Transformation. In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively, and red crosses indicate vapor intrusion interruption. Figure 1. Processes exploited by CVI mitigation strategies considered in this framework: (a) Driving-Force Control, (b) Dilution-Based Strategies, (c) Diffusive Flux Control, (d) Density-Driven Attenuation, and (e) In Situ Transformation. In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively, and red crosses indicate vapor intrusion interruption. Figure 2. Driving-Force Control Strategies. (a) Sub-Slab Depressurization (SSD), (b) Drain Tile Depressurization (DTD), (c) Block Wall Depressurization (BWD), (d) Submembrane Depressurization (SMD), (e) Sub-Slab Pressurization (SSP), (f) Soil Vapor Extraction (SVE), (g) Passive Sub-Slab Depressurization (PSSD). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively. Figure 2. Driving-Force Control Strategies. (a) Sub-Slab Depressurization (SSD), (b) Drain Tile Depressurization (DTD), (c) Block Wall Depressurization (BWD), (d) Submembrane Depressurization (SMD), (e) Sub-Slab Pressurization (SSP), (f) Soil Vapor Extraction (SVE), (g) Passive Sub-Slab Depressurization (PSSD). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively. Figure 3. Dilution-Based Strategies: (a) Sub-Slab Ventilation (SSV), (b) Crawlspace Ventilation (CSV), (c) Passive Sub-slab Ventilation (PSSV), (d) Passive Crawlspace Ventilation (PCSV), (e) Heating, Ventilating, Air-Conditioning (HVAC). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively. Figure 3. Dilution-Based Strategies: (a) Sub-Slab Ventilation (SSV), (b) Crawlspace Ventilation (CSV), (c) Passive Sub-slab Ventilation (PSSV), (d) Passive Crawlspace Ventilation (PCSV), (e) Heating, Ventilating, Air-Conditioning (HVAC). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively. Figure 4. Diffusive Flux Control Strategies: (a) Asphalt–Latex Membrane (ALM), (b) Thermoplastic Membrane (TM), (c) Composite Membrane (CM). In the figure, purple arrows indicate soil vapor fluxes and red crosses indicate vapor intrusion interruption. Figure 4. Diffusive Flux Control Strategies: (a) Asphalt–Latex Membrane (ALM), (b) Thermoplastic Membrane (TM), (c) Composite Membrane (CM). In the figure, purple arrows indicate soil vapor fluxes and red crosses indicate vapor intrusion interruption. Figure 5. Density-Driven Attenuation Strategies: (a) Granular Fill layers (GF), (b) Aerated floor Void Space System (VSS). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively, while orange arrows indicate downward soil vapor fluxes induced by density gradients. Figure 5. Density-Driven Attenuation Strategies: (a) Granular Fill layers (GF), (b) Aerated floor Void Space System (VSS). In the figure, purple and blue arrows indicate soil vapor and air fluxes, respectively, while orange arrows indicate downward soil vapor fluxes induced by density gradients. Figure 6. In Situ Transformation Strategies: (a) Horizontal Permeable Reactive Barriers (HPRB), (b) Horizontal Permeable Adsorbing Barriers (HPAB), (c) Aerobic Vapor Migration Barrier (AVMB), (d) Bio/Reactive covers. In the figure, purple and blue/orange arrows indicate soil vapor and air/oxygen fluxes, respectively. Figure 6. In Situ Transformation Strategies: (a) Horizontal Permeable Reactive Barriers (HPRB), (b) Horizontal Permeable Adsorbing Barriers (HPAB), (c) Aerobic Vapor Migration Barrier (AVMB), (d) Bio/Reactive covers. In the figure, purple and blue/orange arrows indicate soil vapor and air/oxygen fluxes, respectively. Table 1. Overview of mitigation technologies for chlorinated vapor emissions. Table 1. Overview of mitigation technologies for chlorinated vapor emissions. Process Technique References Table 2. Comparative assessment of process-based vapor intrusion mitigation mechanisms. Very Low, Low, Moderate, High, and Very High indicate relative qualitative assessments based on the comparative evaluation presented in this review and should not be interpreted as quantitative performance metrics. Table 2. Comparative assessment of process-based vapor intrusion mitigation mechanisms. Very Low, Low, Moderate, High, and Very High indicate relative qualitative assessments based on the comparative evaluation presented in this review and should not be interpreted as quantitative performance metrics. Attribute Driving-Force Control Dilution-Based Control Diffusive Flux Control Density-Driven Attenuation In Situ Transformation Primary control variable Pressure differential Air exchange rate Diffusion resistance, membrane integrity Permeability Reaction kinetics, sorption capacity Time to effectiveness Immediate Immediate Immediate (if intact) Gradual Gradual (rate-controlled) Durability constraint Mechanical wear, power supply Continuous airflow requirement Material aging, structural bypass Loss of gas permeability Material depletion or passivation Climate sensitivity Low (actively controlled) High (temperature, wind, pressure) Low Moderate (soil moisture) Moderate (temperature, moisture) Maintenance demand High (Active) Moderate (Passive) High (Active) Moderate (Passive) Low Very Low Low Operational energy demand High (Active) None (Passive) High (Active) None (Passive) None None None (except AVMB) Lifecycle carbon intensity High (Active) Moderate (Passive) High (Active) Moderate (Passive) Low Very Low Low Sensitivity to preferential pathways High Moderate Low Moderate Moderate Regulatory maturity Very High High High Low (Emerging) Low (Emerging) Best application context High Risks, rapid control Moderate Risks, supplementary control Moderate-High Risks, New construction Low-Moderate Risks, Dense vapors Low-Moderate Risks, Long-term management Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. Share and Cite MDPI and ACS Style Settimi, C.; Zingaretti, D.; Baciocchi, R.; Verginelli, I. Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites. Environments 2026, 13, 327. https://doi.org/10.3390/environments13060327 AMA Style Settimi C, Zingaretti D, Baciocchi R, Verginelli I. Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites. Environments. 2026; 13(6):327. https://doi.org/10.3390/environments13060327 Chicago/Turabian Style Settimi, Clarissa, Daniela Zingaretti, Renato Baciocchi, and Iason Verginelli. 2026. "Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites" Environments 13, no. 6: 327. https://doi.org/10.3390/environments13060327 APA Style Settimi, C., Zingaretti, D., Baciocchi, R., & Verginelli, I. (2026). Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites. Environments, 13(6), 327. https://doi.org/10.3390/environments13060327 Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here. Article Metrics Article metric data becomes available approximately 24 hours after publication online.
Process-Based Framework for Chlorinated Vapor Intrusion Mitigation Strategies at Contaminated Sites
