Advective and diffusive gas phase transport in vadose zones: Importance for defining vapour risks and natural source zone depletion of petroleum hydrocarbons.

Quantifying the interlinked behaviour of the soil microbiome, fluid flow, multi-component transport and partitioning, and biodegradation is key to characterising vapour risks and natural source zone depletion (NSZD) of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. Critical to vapour transport and NSZD is transport of gases through the vadose zone (oxygen from the atmosphere, volatile organic compounds (VOCs), methane and carbon dioxide from the zone of LNAPL biodegradation). Volatilisation of VOCs from LNAPL, aerobic biodegradation, methanogenesis and heat production all generate gas pressure changes that may lead to enhanced gas fluxes apart from diffusion. Despite the importance of the gaseous phase dynamics in the vadose zone processes, the relative pressure changes and consequent scales of advective (buoyancy and pressure driven) / diffusive transport is less studied. We use a validated multi-phase multi-component non-isothermal modelling framework to differentiate gas transport mechanisms. We simulate a multicomponent unweathered gasoline LNAPL with high VOC content to maximise the potential for pressure changes due to volatilisation and to enable the joint effects of methanogenesis and shallower aerobic biodegradation of vapours to be assessed, along with heat production. Considering a uniform fine sand profile with LNAPL resident in the water table capillary zone, results suggest that biodegradation plays the key role in gas phase formation and consequent pressure build-up. Results suggest that advection is the main transport mechanism over a thin zone inside the LNAPL/capillary region, where the effective gaseous diffusion is very low. In the bulk of the vadose zone above the LNAPL region, the pressure change is minimal, and gaseous diffusion is dominant. Even for high biodegradation rate cases, pressure build-up due to heat generation (inducing buoyancy effects) is smaller than the contribution of gas formation due to biodegradation. The findings are critical to support broader assumptions of diffusive transport being dominant in vapour transport and NSZD assessments.

The Dynamics of Per- and Polyfluoroalkyl Substances (PFAS) at Interfaces in Porous Media: A Computational Roadmap from Nanoscale Molecular Dynamics Simulation to Macroscale Modeling.

Managing and remediating perfluoroalkyl and polyfluoroalkyl substance (PFAS) contaminated sites remains challenging. The major reasons are the complexity of geological media, partly unknown dynamics of the PFAS in different phases and at fluid-fluid and fluid-solid interfaces, and the presence of cocontaminants such as nonaqueous phase liquids (NAPLs). Critical knowledge gaps exist in understanding the behavior and fate of PFAS in vadose and saturated zones and in other porous media such as concrete and asphalt. The complexity of PFAS-surface interactions warrants the use of advanced characterization and computational tools to understand and quantify nanoscale behavior of the molecules. This can then be upscaled to the microscale to develop a constitutive relationship, in particular to distinguish between surface and bulk diffusion. The dominance of surface diffusion compared to bulk diffusion results in the solutocapillary Marangoni effect, which has not been considered while investigating the fate of PFAS. Without a deep understanding of these phenomena, derivation of constitutive relationships is challenging. The current Darcy scale mass-transfer models use constitutive relationships derived from either experiments or field measurements, which makes their applicability potentially limited. Here we review current efforts and propose a roadmap for developing Darcy scale transport equations for PFAS. We find that this needs to be based on systematic upscaling of both experimental and computational studies from nano- to microscales. We highlight recent efforts to undertake molecular dynamics simulations on problems with similar levels of complexity and explore the feasibility of conducting nanoscale simulations on PFAS dynamics at the interface of fluid pairs.

Reviewing the Bioremediation of Contaminants in Groundwater: Investigations over 40 Years Provide Insights into What's Achievable.

Biodegradation and biotransformation of contaminants in groundwater commonly occurs naturally. However, natural biodegradation rates can be slow leading to elongated contaminant plumes and prolonged risks that demand greater remedial intervention. Enhancement of the biodegradation of contaminants in groundwater can be induced by the addition of amendments to change the geochemical conditions to those that are more favorable for indigenous or added biota. Enhancing biodegradation requires collocation of the contaminant of concern with the 'right' microbial communities under the 'right' geochemical conditions, so that the microbiota thrive and bio-transform, degrade or lock up the contaminant of interest. This is most easily achievable at laboratory or bench scale where mixing is easily performed, and mass transfer limitations are minimized. However, inducing such changes at field scale in aquifers is non-trivial - amendments do not easily mix into groundwater because it is a laminar (non-turbulent) and low-energy flow environment. Bioaugmentation of cultured or genetically modified organisms have also been considered to add to groundwater to enhance contaminant degradation rates. Here we provide an overview of research studies over approximately 40 years that highlight the progression of understanding from natural biodegradation of plumes in groundwater to active bioremediation efforts that have been variably successful at field scale. Investigated contaminants providing insights include petroleum hydrocarbons, chlorinated and brominated hydrocarbons, ammonium, metals, munition compounds, atrazine and per- and polyfluorinated alkyl substances. The redox and electron acceptor/donor conditions that are inducive to biodegradation for a range of contaminants are highlighted. Biodegradation is challenged by the availability of electron donors/acceptors in the core of plumes and on plume fringes. Cases for bioaugmentation are identified. A long history of investigations provides examples of the importance of amendment delivery mechanisms, scale-up from laboratory to field, and field-scale demonstration of the effectiveness of groundwater bioremediation technologies. Advantages and disadvantages of remedial approaches are tabulated. The value and contributions of integrative modelling advances are identified. The literature review and example cases provide a deep understanding of what scale of bioremediation might be achievable for groundwater plumes. Limitations to bioremediation strategies outlined here will help direct future efforts. Addressing the sources of groundwater plumes as well as bioremediation of the plume itself will achieve more effective outcomes. Twelve 'lessons learnt' are synthesized from the review.

Advancing "Autonomous" sensing and prediction of the subsurface environment: a review and exploration of the challenges for soil and groundwater contamination.

Can we hope for autonomous (self-contained in situ) sensing of subsurface soil and groundwater pollutants to satisfy relevant regulatory criteria? Global advances in sensors, communications, digital technologies, and computational capacity offer this potential. Here we review past efforts to advance subsurface investigation techniques and technologies, and computational efforts to create a digital twin (representation) of subsurface processes. In the context of the potential to link measurement and sensing to a digital twin computation platform, we outline five criteria that might make it possible. Significant advances in sensors based on passive measurement devices are proposed. As an example of what might be achievable, using the five criteria, we describe the deployment of online real-time sensors and simulations for a case study of a petroleum site where natural source zone depletion (NSZD) is underway as a potential biodegradation management option, and where a high-quality conceptual site model is available. Multiple sensors targeting parameters (major gases and temperature influenced by soil moisture) relevant to the subsurface NSZD biodegradation processes are shown to offer the potential to map subsurface processes spatially and temporally and provide continuous estimates of degradation rates for management decisions, constrained by a computational platform of the key processes. Current limitations and gaps in technologies and knowledge are highlighted specific to the case study. More generally, additional key advances required to achieve autonomous sensing of subsurface soil and groundwater pollutants are outlined.

Volatilization Potential of Per- and Poly-fluoroalkyl Substances from Airfield Pavements and during Recycling of Asphalt.

Per- and poly-fluoroalkyl substances (PFAS) in water are typically present in their ionic (nonvolatile) forms; however, these can transition to their nonionic (volatile) forms when in contact with organic solvents and organic matrices. In particular, when PFAS are dissolved in organic solvents such as residues left from firefighting foams, fuels, and bitumen present in asphalt, the equilibrium between ionic and nonionic forms can trend toward more volatile nonionic forms of PFAS. We assessed the volatility of common PFAS based on calculated and available experimental data across ambient temperature ranges experienced by airfield pavements and at elevated temperatures associated with reworking asphalts for reuse. Volatilities are shown to be comparable to hydrocarbons in the semivolatile range, suggesting that volatilization is a viable loss mechanism for some PFAS that are nonvolatile in water. The present study points to future investigative needs for this unexplored mass loss mechanism and potential exposure pathway. Environ Toxicol Chem 2022;41:2202-2208. © 2022 Commonwealth of Australia. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Laboratory batch representation of PFAS leaching from aged field soils: Intercomparison across new and standard approaches.

Relating laboratory leaching methods to partitioning and transport of per- and poly-fluoroalkyl substances (PFAS) in field soils is challenging, making estimation of fluxes to groundwater and surface water uncertain. Existing laboratory leaching methods have limitations when assessing field leaching. For 37 aged field soils from five sites historically contaminated with PFAS over decades, we assess PFAS leaching using new and existing laboratory leaching methods to provide alternative methods better reflecting PFAS risks posed by its leaching and movement. Dominant PFAS in the soils were perfluorooctane sulfonic acid, perfluorohexane sulfonic acid, and perfluorohexanoic acid and to a lesser extent perfluorooctanoic acid. Leaching from intact soil cores (Exp 1) was taken to reflect field conditions. These were compared to two new laboratory batch tests, saturate-spin (Exp 2) and saturate-tumble-spin (Exp 3), and two standard approaches; Australian Standard Leaching Procedure (ASLP, Exp 4) and the Leaching Environmental Assessment Framework (LEAF, Exp 5). The tests varied in terms of liquid:soil ratio, tumbling time and pH of the starting solution, with LEAF-1313 conducted across seven pHs (2-12). Correlations between leachate and soil concentrations were highest for Exp 4 and Exp 5 (R2 = 0.72-0.98) and lowest for Exp 3 (R2 = 0.53). The PFAS mass leached as a fraction of the total increased such that: soil core leaching (27 %) < saturate-spin (30 %) < saturate-tumble-spin (65 %) ≤ LEAF-1313 (65 to 88 % at pH 5-9) < ASLP (90 %). As a fraction of individual PFAS compounds in leachate compared with soil, the shorter chain PFAS (e.g., perfluorobutanoic acid) were higher in the leachate in all tests. Across all tests, the saturate-spin batch test most closely represented intact soil core leaching and therefore potentially provides a measure more analogous of in situ soil leaching at field sites. Other methods would apply to broader applications such as landfill disposal.

The role of predicted chemotactic and hydrocarbon degrading taxa in natural source zone depletion at a legacy petroleum hydrocarbon site.

Petroleum hydrocarbon contamination is a global problem which can cause long-term environmental damage and impacts water security. Natural source zone depletion (NSZD) is the natural degradation of such contaminants. Chemotaxis is an aspect of NSZD which is not fully understood, but one that grants microorganisms the ability to alter their motion in response to a chemical concentration gradient potentially enhancing petroleum NSZD mass removal rates. This study investigates the distribution of potentially chemotactic and hydrocarbon degrading microbes (CD) across the water table of a legacy petroleum hydrocarbon site near Perth, Western Australia in areas impacted by crude oil, diesel and jet fuel. Core samples were recovered and analysed for hydrocarbon contamination using gas chromatography. Predictive metagenomic profiling was undertaken to infer functionality using a combination of 16 S rRNA sequencing and PICRUSt2 analysis. Naphthalene contamination was found to significantly increase the occurrence of potential CD microbes, including members of the Comamonadaceae and Geobacteraceae families, which may enhance NSZD. Further work to explore and define this link is important for reliable estimation of biodegradation of petroleum hydrocarbon fuels. Furthermore, the outcomes suggest that the chemotactic parameter within existing NSZD models should be reviewed to accommodate CD accumulation in areas of naphthalene contamination, thereby providing a more accurate quantification of risk from petroleum impacts in subsurface environments, and the scale of risk mitigation due to NSZD.

Towards a digital twin for characterising natural source zone depletion: A feasibility study based on the Bemidji site.

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) may be a valid long-term management option at petroleum impacted sites. However, its future long-term reliability needs to be established. NSZD includes partitioning, biotic and abiotic degradation of LNAPL components plus multiphase fluid dynamics in the subsurface. Over time, LNAPL components are depleted and those partitioning to various phases change, as do those available for biodegradation. To accommodate these processes and predict trends and NSZD over decades to centuries, for the first time, we incorporated a multi-phase multi-component multi-microbe non-isothermal approach to representatively simulate NSZD at field scale. To validate the approach we successfully mimic data from the LNAPL release at the Bemidji site. We simulate the entire depth of saturated and unsaturated zones over the 27 years of post-release measurements. The study progresses the idea of creating a generic digital twin of NSZD processes and future trends. Outcomes show the feasibility and affordability of such detailed computational approaches to improve decision-making for site management and restoration strategies. The study provided a basis to progress a computational digital twin for complex subsurface systems.

Quantifying the benefits of in-time and in-place responses to remediate acute LNAPL release incidents.

Acute large volume spills from storage tanks of petroleum hydrocarbons as light non aqueous phase liquids (LNAPLs) can contaminate soil and groundwater and may have the potential to pose explosive and other risks. In consideration of an acute LNAPL release scenario, we explore the value of a rapid remediation response, and the value of installing remediation infrastructure in close proximity to the spill location, in effecting greater recovery of LNAPL mass from the subsurface. For the first time, a verified three-dimensional multi-phase numerical framework and supercomputing resources was applied to explore the significance of in-time and in-place remediation actions. A sand aquifer, two release volumes and a low viscosity LNAPL were considered in key scenarios. The time of commencement of LNAPL remediation activities and the location of recovery wells were assessed requiring asymmetric computational considerations. The volume of LNAPL released considerably affected the depth of LNAPL penetration below the groundwater table, the radius of the plume over time and the recoverable LNAPL mass. The remediation efficiency was almost linearly correlated with the commencement time, but was a non-linear function of the distance of an extraction well from the spill release point. The ratio of the recovered LNAPL in a well located at the centre of the spill/release compared to a well located 5 m away was more than 3.5, for recovery starting only 7 days after the release. Early commencement of remediation with a recovery well located at the centre of the plume was estimated to recover 190 times more LNAPL mass than a one-month delayed commencement through a well 15 m away from the centre of the LNAPL plume. Optimally, nearly 40% of the initially released LNAPL could be recovered within two months of commencing LNAPL recovery actions.

Laser-induced fluorescence logging as a high-resolution characterisation tool to assess LNAPL mobility.

High-resolution characterisation tools such as Laser-Induced Fluorescence (LIF) logging represent a step forward towards a more effective management of sites contaminated by light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. In this paper, the applicability of LIF response as an indicator of LNAPL mobility at one site with an unconsolidated aquifer was investigated. LIF profiles were logged adjacent to twin coring locations and wells with LNAPL transmissivity (Tn) measurements in a heterogeneous gasoline contaminated site in Western Australia. LIF response was correlated to Tn to a greater extent than LNAPL saturation (Sn) measurements from coring. In particular, LIF signal maxima were a better indicator of Tn than the integral LIF signal. Furthermore, LIF allowed rapid identification of areas with long-term near-immobile LNAPL (entrapped and residual) because of the multi-wavelength waveforms associated with distinct subsurface characteristics. It was also demonstrated that the delineation of presumably less-mobile intervals could be enhanced by using the relative LIF response in the 350 nm wavelength channel. Thus, this work gave evidence that LIF logging provides valuable information about LNAPL distribution and mobility in commonly found subsurface settings, despite generally poor correlations with Sn measurements. LIF probes can be successfully used to guide the installation and application of more costly conventional methods in addition to the development of existing site models.

Investigation into the microbial communities and associated crude oil-contamination along a Gulf War impacted groundwater system in Kuwait.

During the First Gulf War (1991) a large number of oil wells were destroyed and oil fires subsequently extinguished with seawater. As a result Kuwait's sparse fresh groundwater resources were severely contaminated with crude oil. Since then limited research has focused on the microbial community ecology of the groundwater and their impact on the associated contamination. Here, the microbial community ecology (bacterial, archaeal and eukaryotic) and how it relates to the characteristics of the hydrocarbon contaminants were examined for the first time since the 1991 event. This study was conducted using 15 wells along the main groundwater flow direction and detected several potential hydrocarbon degrading microorganisms such as Hyphomicrobiaceae, Porphyromonadaceae and Eurotiomycetes. The beta diversity of the microbial communities correlated significantly with total petroleum hydrocarbon (TPH) concentrations and salinity. The TPH consisted mainly of polar compounds present as an unresolved complex mixture (UCM) of a highly recalcitrant nature. Based on the proportions of TPH to dissolved organic carbon (DOC), the results indicate that some minor biodegradation has occurred within highly contaminated aquifer zones. However, overall the results from this study suggest that the observed variations in TPH concentrations among the sampled wells are mainly induced by mixing/dilution with pristine groundwater rather than by biodegradation of the contaminants. The findings make an important contribution to better understand the fate of the groundwater pollution in Kuwait, with important implications for the design of future remediation efforts.

Natural source zone depletion of LNAPL: A critical review supporting modelling approaches.

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) includes partitioning, transport and degradation of LNAPL components. NSZD is being considered as a site closure option during later stages of active remediation of LNAPL contaminated sites, and where LNAPL mass removal is limiting. To ensure NSZD meets compliance criteria and to design enhanced NSZD actions if required, residual risks posed by LNAPL and its long term behaviour require estimation. Prediction of long-term NSZD trends requires linking physicochemical partitioning and transport processes with bioprocesses at multiple scales within a modelling framework. Here we expand and build on the knowledge base of a recent review of NSZD, to establish the key processes and understanding required to model NSZD long term. We describe key challenges to our understanding, inclusive of the dominance of methanogenic or aerobic biodegradation processes, the potentially changeability of rates due to the weathering profile of LNAPL product types and ages, and linkages to underlying bioprocesses. We critically discuss different scales in subsurface simulation and modelling of NSZD. Focusing on processes at Darcy scale, 36 models addressing processes of importance to NSZD are investigated. We investigate the capabilities of models to accommodate more than 20 subsurface transport and transformation phenomena and present comparisons in several tables. We discuss the applicability of each group of models for specific site conditions.

Towards characterizing LNAPL remediation endpoints.

Remediating sites contaminated with light non-aqueous phase liquids (LNAPLs) is a demanding and often prolonged task. It is vital to determine when it is appropriate to cease engineered remedial efforts based on the long-term effectiveness of remediation technology options. For the first time, the long term effectiveness of a range of LNAPL remediation approaches including skimming and vacuum-enhanced skimming each with and without water table drawdown was simulated through a multi-phase and multi-component approach. LNAPL components of gasoline were simulated to show how component changes affect the LNAPL's multi-phase behaviour and to inform the risk profile of the LNAPL. The four remediation approaches along with five types of soils, two states of the LNAPL specific mass and finite and infinite LNAPL plumes resulted in 80 simulation scenarios. Effective conservative mass removal endpoints for all the simulations were determined. As a key driver of risk, the persistence and mass removal of benzene was investigated across the scenarios. The time to effectively achieve a technology endpoint varied from 2 to 6 years. The recovered LNAPL in the liquid phase varied from 5% to 53% of the initial mass. The recovered LNAPL mass as extracted vapour was also quantified. Additional mass loss through induced biodegradation was not determined. Across numerous field conditions and release incidents, graphical outcomes provide conservative (i.e. more prolonged or greater mass recovery potential) LNAPL remediation endpoints for use in discussing the halting or continuance of engineered remedial efforts.

LNAPL transmissivity as a remediation metric in complex sites under water table fluctuations.

Water table fluctuations affect the recoverability of light non-aqueous phase liquid (LNAPL) petroleum hydrocarbons. LNAPL transmissivity (Tn) is being applied as an improved metric for LNAPL recoverability. In this paper, the applicability of Tn as a lagging and leading metric in unconsolidated aquifers under variable water table conditions was investigated. Tn values obtained through baildown testing and recovery data-based methods (skimming) were compared in three areas of a heterogeneous gasoline contaminated site in Western Australia. High-resolution characterisation methods were applied to account for differences in the stratigraphic profile and LNAPL distribution. The results showed a range of Tn from 0 m2/day to 2.13 m2/day, exhibiting a strong spatial and temporal variability. Additionally, observations indicated that Tn reductions may be more affected by the potentiometric surface elevation (Zaw) than by the application of mass recovery technologies. These observations reflected limitations of Tn as a lagging metric and a Remedial Endpoint. On the other hand, the consistency and accuracy of Tn as a leading metric was affected by the subsurface conditions. For instance, the area with a larger vertical LNAPL distribution and higher LNAPL saturations found Tn to be less sensitive to changes in Zaw than the other two areas during the skimming trials. Tn values from baildown and skimming tests were generally in a close agreement (less than a factor of 2 difference), although higher discrepancies (by a factor up to 7.3) were found, probably linked to a preferential migration pathway and Zaw. Under stable Zaw, Tn was found to be a relatively reliable metric. However, variable water table conditions affected Tn and caution should be exercised in such scenarios. Consequently, remediation practitioners, researchers and regulators should account for the nexus between Tn, LNAPL distribution, geological setting and temporal effects for a more efficient and sustainable management of complex contaminated sites.

Field-scale multi-phase LNAPL remediation: Validating a new computational framework against sequential field pilot trials.

Remediation of subsurface systems, including groundwater, soil and soil gas, contaminated with light non-aqueous phase liquids (LNAPLs) is challenging. Field-scale pilot trials of multi-phase remediation were undertaken at a site to determine the effectiveness of recovery options. Sequential LNAPL skimming and vacuum-enhanced skimming, with and without water table drawdown were trialled over 78days; in total extracting over 5m3 of LNAPL. For the first time, a multi-component simulation framework (including the multi-phase multi-component code TMVOC-MP and processing codes) was developed and applied to simulate the broad range of multi-phase remediation and recovery methods used in the field trials. This framework was validated against the sequential pilot trials by comparing predicted and measured LNAPL mass removal rates and compositional changes. The framework was tested on both a Cray supercomputer and a cluster. Simulations mimicked trends in LNAPL recovery rates (from 0.14 to 3mL/s) across all remediation techniques each operating over periods of 4-14days over the 78day trial. The code also approximated order of magnitude compositional changes of hazardous chemical concentrations in extracted gas during vacuum-enhanced recovery. The verified framework enables longer term prediction of the effectiveness of remediation approaches allowing better determination of remediation endpoints and long-term risks.

A computational assessment of representative sampling of soil gas using existing groundwater monitoring wells screened across the water table.

Representative sampling is of great importance to decision making regarding contaminant site risks and remedial effectiveness. A focus here is whether existing groundwater monitoring wells screened across the water table can be sampled to yield representative indicators of soil gas composition. For the first time, we provide multi-phase, multi-component computational simulations to address this. We simulated high and low gas extractions rate strategies to sample the gas phase over short and extended screening intervals across the water table. We investigated the options against a field data set representative of typical hydrocarbon vapour profiles, inclusive of major gases, oxygen and carbon dioxide. We also evaluated the sampling options for uniform and non-uniform multi-component gasoline LNAPL distributions, including hazardous chemicals. Less sensitivity to the sampling option was observed for depth-wise increasing concentration profiles with a near-constant concentration across the screen. Significant discrepancy between the ratio of different compounds in the sample and in-situ (real) values was observed for high-rate gas extraction (particularly for an extended-screen). Low-rate gas extraction provided satisfactory results for all the scenarios. Shorter screening slightly improved the accuracy of this option. Graphical representations are provided to allow assessment of the applicability of each sampling option for various site conditions.

Evaluating the reliability of equilibrium dissolution assumption from residual gasoline in contact with water saturated sands.

Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.

Dissolution of multi-component LNAPL gasolines: the effects of weathering and composition.

The composition of light non-aqueous phase liquid (LNAPL) gasoline and other petroleum products changes profoundly over their life once released into aquifers. However limited attention has been given to how such changes affect key parameters such as the activity coefficients which control partitioning of components of petroleum fuel into groundwater and are used to predict long-term risk from fuel releases. Laboratory experiments were conducted on a range of fresh, weathered and synthetic gasoline mixtures designed to mimic the expected changes in composition in an aquifer. Weathered gasoline created under controlled evaporation and water washing, and naturally weathered gasoline, were investigated. Equilibrium concentrations in water and molar fractions in the gasoline mixtures were compared with equilibrium concentrations predicted by Raoult's law assuming ideal behaviour of the solutions. The experiments carried out allowed the relative sensitivity of the activity coefficients of key risk drivers such as benzene, toluene, ethylbenzene and xylene (BTEX) compounds to be quantified with respect to the presence of other types of compounds and where the source LNAPL had undergone different types of weathering. Results differed for the mixtures examined but in some cases higher than predicted dissolved equilibrium concentrations showed non-ideal behaviour for toluene, benzene and xylenes. Comparison of the activity coefficients showed that the naturally weathered gasoline and a 50% evaporated unleaded gasoline present a similar range of values varying between 1.0 and 1.2, suggesting close to ideal partitioning between the LNAPL and water. The fresh and water-washed gasoline had higher values for the activity coefficient, from 1.2 to 1.4, indicating non-ideal partitioning. Results from synthetic mixtures demonstrated that these differences could be due to the different molar fractions of the nC5 and nC6 aliphatic hydrocarbons acting on the molecular interactions, while differences in molar volumes seemed to have less of an influence on ideality.