Natural Source Zone Depletion (NSZD) is typically evaluated for weathered petroleum releases, often after active recovery options have been exhausted. This study addresses a key knowledge gap: when do NSZD rates surpass what can be achieved by recovery methods (here skimming) following a fresh petroleum fuel release? Here we simulate a field case of an intensively investigated fresh release of light non-aqueous phase liquid (LNAPL) diesel into a shallow sandy vadose zone and groundwater, with active efforts to recover LNAPL mass and simultaneously detailed measurements of NSZD rates. To assess uncertainties and key processes we simulated a number of scenarios. Validated multiphase, multicomponent models were applied across three representative mass releases, accounting for full hydrocarbon partitioning, microbial dynamics, and biodegradation pathways. Scenarios with and without residual LNAPL in the vadose zone were compared. Results show that NSZD rates can exceed active recovery (skimming) rates within two years. However, for clean sites with minimal native hydrocarbon-degrading microbes, NSZD dominance was delayed by an additional 1-2 years due to the lag required for microbial community growth. Residual LNAPL in the vadose zone significantly enhanced NSZD, often beyond the range that wells could recover. The timeframes over which active recovery dominated mass removal prior to transitioning to NSZD mass removal being greatest is quantified and compared to that determined from the detailed field investigation. This study offers a quantitative, scenario-based platform to support decision-making on when NSZD can be a viable management strategy for new spills, with potential application to other fuel types and geologic settings.

Globally, recycling of otherwise waste materials into new products is desired. End-of-life tyres are increasingly incorporated into new pavement materials but leaching of entrained chemicals from such products is not well quantified. Chemical concentrations in runoff from pavements may pose environmental and human health risks. High liquid-solid ratio, batch-agitated leaching is standard practice for assessing leachability and hazards of chemicals-of-potential-concern in contaminated soil and wastes but is not reflective of important exposure scenarios and may mislead. A new static surface leaching procedure (SSLP) is introduced that is more representative of chemical leaching from pavement reuse materials whilst in contact with rainfall/runoff water. SSLP was evaluated over 2-14 d intervals against batch-agitated leaching for two rubberised pavement products containing 10-fold different proportions of crumbed end-of-life tyres. Although, batch leaching showed high mass removal of 1,3-diphenylguanidine (1,3-DPG, 34%) and hexamethoxymethylmelamine (HMMM, 30%), both batch- and SSLP-leached concentrations of 1,3-DPG, HMMM and N1-(4-methylpentan-2-yl)-N4-phenylbenzene-1,4-diamine quinone (6PPD-Q) were below ECOSAR-predicted toxicity thresholds for fish and daphnids. SSLP highlighted differences in chemical leachability based on rubber content of pavement products and offers a method applicable to other scenarios, such as PFAS leaching from concrete/asphalt pavements. The SSLP was shown to approximate one-dimensional leaching from the surface of the pavement and to be dominated by diffusive processes, thus yielding a simple repeatable approach.

Petroleum releases remain a major cause of soil and groundwater contamination worldwide, entraining significant resources to remediate. Active remediation methods often face diminishing returns over time, usually reaching an endpoint in a few years, as mass is depleted and petroleum composition changes leaving less mobile and recoverable relative saturations. Prolonged active remediation efforts challenge sustainability metrics based on energy use and greenhouse gas production. Intrinsic biodegradation, volatilization and dissolution collectively deplete petroleum light non aqueous phase liquid (LNAPL) mass, commonly referred to as Natural Source Zone Depletion (NSZD). Natural source zone depletion is typically evaluated for weathered petroleum releases, often after active recovery options have been exhausted. This study focuses on key factors that govern the transition to management of petroleum impacted sites via NSZD - i.e., when NSZD rates (commonly after weathering) adequately surpass what can be achieved by recovery methods. Based on decades of field monitoring, measurement, and modelling, the paper identifies key factors controlling the relative magnitude of active recovery versus NSZD mass removal rates leading to management via NSZD, and offers a conceptual roadmap towards addressing gaps in knowledge and understanding to allow confident transition to NSZD.

Often rates of biodegradation driving natural source zone depletion (NSZD) are estimated for weathered and aged petroleum releases, and often after active recovery is less feasible. But an open question is "When do natural biodegradation rates of petroleum hydrocarbon mass removal exceed what's removable by active engineered approaches for a fresh release of petroleum fuel?". Here we report on intensive site investigations and measurements over 15 months, that quantified active LNAPL mass recovery and NSZD rates implemented shortly after a new release of light non-aqueous phase liquid (LNAPL) diesel into a sandy vadose zone and groundwater. Masses of LNAPL actively recovered via direct pumping, multiphase extraction and primarily skimming were compared to NSZD rates estimated from LiCOR surface chamber measurements of fluxes of carbon dioxide (27 locations) and from soil gas depth profile sampling from nine short-screened recovery wells (RW) and from six multilevel depth samplers (VZ). To maximise recovery of LNAPL up to 13 recovery wells were operating at any one time. On average, LiCOR measured NSZD rates were higher than those estimated by oxygen measured in recovery wells, but those measured by sampling for oxygen in soil gas from VZ samplers were much higher again. Although active recovery was highest initially compared to NSZD rates, at individual paired well locations, the time-averaged LiCOR NSZD rates (5-25 L/m2/y) were higher than averaged skimming rates by up to c. 8 times. Spatial integration across the impacted area showed LNAPL recovery rates (13 locations, up to 82 readings) were substantially higher (5-35 L/m2/y) than NSZD (25 locations and up to 12 readings) rates (2.5-5 L/m2/y) for the first 7-8 months since the release. After that elapsed time, NSZD rates increased and remained at between 2 and 5 times higher (∼ 10 L/m2/y) than skimming based on conservative LiCOR measurements. This was found to be the case whether averaging recovery based on the radius of capture of recovery wells or based on the areal extent of NSZD across the release footprint. This provides an intensively investigated example case of timeframes whereby management of new petroleum releases may be acceptable based on NSZD, and perhaps within an initial 12-18 month period post release. It also provides a dataset that can be used to verify models for assessing the cross over point relevant to other fuel types and site conditions.

Perfluorooctanesulfonic acid (PFOS) is a persistent and toxic compound frequently detected in contaminated soil and groundwater. In this study, we employ density functional theory (DFT) to evaluate the potential of 4H-SiC, a low-cost, environmentally safe, and stable semiconductor, as a candidate sensing material for PFOS via surface adsorption and band gap modulation. Three configurations of deprotonated PFOS (vertical, inclined, and horizontal) were assessed on both Si- and C-terminated 4H-SiC slabs. The implicit effect of solvation (water) was included. In all cases, PFOS exhibited slight but energetically favorable adsorption. Electronic interactions were observed on the C-terminated slab, where PFOS adsorption induced midgap states and considerable band gap reduction. Among the configurations, vertical PFOS produced midgap states closest to the valence band maximum (VBM). In all cases, midgaps resulted from relatively weak interactions between surface carbon atoms and fluorine and carbon atoms in PFOS, as confirmed by analysis of the Kohn-Sham orbitals associated with the midgap energies. Localized surface states were more pronounced on the C-terminated slab compared with the Si-terminated one. Additionally, the planar-averaged potential difference between the two sides of the slab increased (compared to the pristine slab) upon PFOS adsorption on the C-terminated surface. Bader charge analysis revealed minimal electron transfer from PFOS to the surface. The band gap reduction of the C-terminated slab (e.g., from 2.58 eV (pristine) to 1.22 eV upon adsorption of vertically oriented PFOS) may suggest a promising sensing capability on the carbon-terminated surface. This study provides an initial assessment of PFOS interactions with 4H-SiC. Further DFT studies incorporating explicit solvent effects, alternative functionals, surface doping, and complementary experimental work are recommended to support the development of 4H-SiC-based PFOS sensor devices.

Fluorinated pesticides, often referred to as “forever pesticides,” are contaminants of emerging concern due to their persistent and bio-accumulative behaviour in the environment. Gaps in knowledge and technology options and research needs relevant to remediation of such pesticides are identified. Since 1990, the Australian Pesticides and Veterinary Medicines Authority (APVMA) has approved over 90 fluorinated pesticides, many of which are highly persistent and toxic in the soil-water-plant-human-environment continuum. The persistence and lipophilicity of these compounds (pesticides) pose significant risks to environments, with long-term implications to non-targeted plants, animals and human health. Though some studies have reported that metabolites and degradation products are more toxic and persistent in the environment, limited research has been undertaken to quantify the persistence, fate and transport of parent pesticides, metabolites, precursors and intermediates. The detection of ultrashort-chain PFAS (including trifluoroacetic acid) in Australian groundwater further underscores the need for targeted research on the persistence, fate and transport of both parent pesticide compounds and their degradation products in Australian ecosystems. This manuscript collates characteristics and research undertaken on 91 fluorinated pesticides used in Australia, drawing on both local and international research. We summarise current knowledge which brings emphasis to the need for synchronised research on fate and transport of these pesticides and their metabolites, risk assessment, current food chain contamination pathways and innovation in measurement and sensing of these persistent pollutants. The development of advanced analytical methods and the expansion of research into multi-compound contamination scenarios are essential for informed regulatory decisions and effective environmental management.

Environmental decisions increasingly require integration and quantification of numerous processes in models that connect molecular chemistry to landscape scales. Along with constraining data, such models can be used to discern the processes of greatest importance, to evaluate management options and to predict future trends. We elaborate on the concept of environmental science computational labs (ESCLs), namely coordinated virtual infrastructures and capabilities linking multiscale models, data systems and validation standards. We illustrate the relevance of this concept through elaborating on three physically different, but naturally interconnected environmental challenges related to perfluoroalkyl and polyfluoroalkyl substances (PFAS). These include PFAS effects on natural source zone depletion (NSZD) of petroleum hydrocarbons, in-situ sensing in soil and groundwater and thermal destruction in incinerators.

There are a number of key aspects and challenges to the adoption of natural source zone depletion (NSZD) of oil-contaminated soil and groundwater. NSZD involves the interplay of multiple natural partitioning, transport and biodegradation processes that collectively lead to contaminant mass and risk reductions. Here we build an expanded conceptual site model of the NSZD processes that drive and influence NSZD rates, and emphasize the least-understood aspects and major challenges. By elaborating on these least-known topics and remaining challenges, we underscore the need for multidisciplinary research efforts to better address these challenges and to deliver improved management and environmental outcomes. Furthermore, we explore emerging issues and complexities surrounding NSZD, such as additives and emerging contaminants co-mingled with petroleum compounds in source zones, the potential to automate the measurement and quantification of NSZD to gain management efficiencies, and the capability of simulation platforms to capture and predict all key NSZD features. We discuss challenges to its adoption and uptake and how advancements in science can address many of the challenges. Ultimately, our study serves as a roadmap to guide future research endeavours aimed at unravelling the complexities of NSZD, and ensuring its reliable measurement. This will benefit efforts to effectively mitigate environmental risks and enable the broader use of NSZD as a management option for petroleum-impacted sites. This Review explores key aspects and challenges to adoption of the natural source zone depletion (NSZD) of subsurface petroleum hydrocarbons.

Per- and polyfluoroalkyl substances (PFAS) are common soil and groundwater contaminants that persist in the environment. Perfluorooctanesulfonic acid (PFOS) has been a key component in aqueous film-forming foams, extensively used for firefighting in both military and civilian applications, including at oil refineries. During firefighting training, fuels have been historically used to ignite fires, and because of its use in foams, PFOS is often found alongside light nonaqueous phase liquid (LNAPL) petroleum hydrocarbons in subsurface soils and groundwater. The film-forming foams contain supersaturated PFOS, and upon contact with LNAPL and soil fluids, PFOS partitions between the phases. This phenomenon is challenging to investigate by using mesoscale approaches. Here, we use molecular-dynamics simulations to study the behavior of supersaturated PFOS, with a focus on micellization and partitioning at LNAPL-water interfaces. We demonstrate that large quantities of PFOS adsorb at LNAPL-water interfaces, suggesting that such interfaces may serve as major retention sites and long-term sources of PFOS contamination. Moreover, we show that both adsorption and micellization are considerably affected by the counterions used (sodium and hydronium). This may suggest a possible avenue for controlling the partitioning process through gaining a better understanding of the effect of water chemistry on PFOS.

Per- and polyfluoroalkyl substances (PFAS) contamination is widespread in our environment and partitioning at interfaces is a major factor that influences their fate and transport within a soil pore space. To develop better PFAS management and remediation strategies, we need to gain a deeper understanding of PFAS soil-porewater partitioning processes in situ. In this study, we investigated the partitioning of PFAS in a spiked agricultural soil and an Aqueous Film-Forming Foam (AFFF) impacted "aged" soil under various soil moisture conditions and when the soil pores were filled with a non-aqueous phase liquid (NAPL). Sulfur (S) K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to probe S speciation in situ, and spectra were analysed applying a Gaussian Curve Fitting (GCF) approach. Six major S species were identified in both soils and PFAS partitioning behaviour was interpreted based on changes in sulfonate fractions, representing sulfonic acid-based PFAS. In the spiked soil, sulfonate fractions increased the most when soil pores had a water saturation ratio (SW) of 0.25, whereas in the "aged" soil the most significant increase occurred for completely saturated soil pores. These differences were attributed to differing micellar formation behaviours in freshly spiked versus AFFF impacted "aged" soils. A linear correlation was also observed between the ratio of total reduced S and sulfonate fractions against the degree of saturation of "aged" soil. Furthermore, in spiked soil it was observed that a lower percentage NAPL content at SW = 0.25 led to higher partitioning of sulfonic acid-based PFAS.

Natural Source Zone Deletion (NSZD) is a viable long-term management option for sites impacted by petroleum hydrocarbon fuels. NSZD rate estimation methods for petroleum mass losses often use soil gas gradients of oxygen, carbon dioxide, methane or vapour concentrations through the vadose zone. Seeking greater efficiencies, we investigated if existing short-screened wells are reliable for representative sampling of soil gases in a vadose zone undergoing NSZD. At gasoline, diesel, aviation-gasoline, crude oil and background locations, we compared soil gas compositions from three methods: (i) multilevel samplers (VZ) on three occasions over 18 months (three per location); (ii) oxygen sensors (GP) at multiple depths and tracked over 3 years (one per location); and (iii) short-screened monitoring wells (MW) across the water table (five per location) and sampled as per VZ. GP sensors showed seasonal and other variations, yielding variable NSZD estimates. Oxygen depth profiles from GP and VZ measurements showed good correspondence despite fluctuations. Oxygen, carbon dioxide and methane concentrations from the deepest sampling ports of VZ samplers correlated strongly with concentrations from adjacent MW installations, indicating in-well sampling in the field can provide reliable data. The advantages and limitations of using MW data in accelerating NSZD field assessments are outlined.

Porewater in soil vadose zones is an integrator of the fundamental processes governing the transport and partitioning of per and poly-fluoroalkyl substances (PFAS) as they move from source zones to groundwater. Suction lysimeters are being advanced as a method to provide reliable and representative PFAS porewater samples, to inform PFAS leaching and for monitoring remedial approaches. We report outcomes of lysimeter investigations across 3 sites and 18 lysimeters within fine-textured soil profiles. Soil cores were recovered from the same locations, and PFAS concentrations in soils and lysimeter porewater were compared and compared with prior laboratory investigations. Variable concentration distributions with depth of PFAS in soils were found with a maximum sum of PFAS of similar to 56 mg/kg dominated by perfluorooctane sulfonic acid (PFOS). The maximum sum of PFAS in porewater was 13.5 mg/L. Comparison across all collocated soil and porewater concentrations did not provide consistent trends. PFAS mass fractions within lysimeter porewater samples were much higher for most PFAS than mass fractions determined from laboratory investigations, but the fraction was lower for PFOS. The results indicate preferential recovery of individual shorter chain PFAS via leaching at lower liquid: soil ratios such as those experienced under suction during recovery of porewater by lysimeters. Suggestions are offered to advance the use of suction lysimeters in promoting porewater PFAS concentrations as an alternative for regulatory compliance, and in closing the gap between field and laboratory approaches. There is merit in using lysimeters at PFAS field sites with improvements and considerations embraced.

Petroleum releases into the subsurface contribute to global soil carbon emissions. Quantifying releases and changes in releases of carbon from soils over the lifetime of a spill is complex. Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) embodies all key mechanisms for transformation to carbon gases and their release from soils including partitioning, transport and degradation of petroleum components. Quantification of the interconnected behaviours of the soil microbiome, fluid flow, multi-component transport, partitioning, and biodegradation is crucial for understanding NSZD. Volatilization from LNAPL, aerobic biodegradation, methanogenesis, and heat production all lead to release of greenhouse gases to the atmosphere. To estimate carbon emissions, using a validated computational platform, we modelled the long term NSZD of four petroleum hydrocarbon types; crude oil, diesel, jet fuel and gasoline, to span the major products used globally. For two soil types, we estimated 150 years of carbon emissions from annual minor and 25 mostly major petroleum hydrocarbon land release incidents since 1950 - with an estimated released mass of similar to 9 million tonnes across the circumstances considered. Up to 2100 the mass of carbon emitted to the atmosphere is estimated to range from 4 to 6 Teragrams, with nearly 60 % currently released. Nomographs generated help predict the fate of LNAPL plumes and carbon emissions due to NSZD, which is crucially important to management of soil and groundwater contamination. The method provides a basis to include additionally identified and future petroleum releases. It is noted that the petroleum mixture composition, degradation rates, volatilization, and subsurface characteristics all can influence carbon emission estimations.

Petroleum biodegrades and naturally depletes. Natural Source Zone Depletion (NSZD) quantifies this at petroleum affected sites in support of management decisions for cessation of active remediation efforts. Whilst a range of NSZD estimates and methods are available, side by side comparison of NSZD rates across petroleum types in the same soil/groundwater system are lacking, especially linked to the weathering status of petroleum. At a former refinery site near Perth Western Australia, locations contaminated by crude oil, gasoline, diesel and aviation gasoline, have been intensively instrumented to enable (i) measurement of vadose zone major gas (O2, CO2, CH4), volatile organic compounds (VOCs) and temperature depth profiles, (ii) online near-continuous temperature, water level, O2 and VOC concentrations, (iii) depth profiles of groundwater parameters, and (iv) in-well gases, temperatures, water and LNAPL thickness. These measurements were compared to a background location with no history of contamination. Multiple coring events were also undertaken to determine LNAPL mass and its vertical distribution at each location. Additionally, LiCor and Eflux was conducted to measure CO2 fluxes at ground surface. NSZD rates were estimated from the measurement methods across the four petroleum types and the background site. Despite NSZD estimates that differed across some methods at sites (for example at the gasoline and diesel sites Eflux/LiCor estimates were consistently lower by a factor of 3–4 than those obtained using oxygen/temperature depth profile data) the minimum-maximum range of mean NSZD rates showed a distinct decreasing order across the fuel types: highest rates being aviation gasoline (69,000–91,000 L/ha/y), then gasoline, diesel and crude oil (2700–6200 L/ha/y). Reasons for differences are explored. Analysis of LNAPL in cores and from wells, historical data comparisons and simulations over 50 years, show that composition, age and weathering of the releases are critical to current and long-term NSZD mass losses and rate estimates.

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.

Per- and poly-fluoroalkyl substances (PFAS) are a group of organo-fluorine compounds that have been broadly used in consumer and industrial products spanning virtually all sectors. They can be found as surfactants, coatings and liners, polymer additives, fire retardants, adhesives, and many more. The chemical stability of the carbon fluorine bond and amphiphilic nature of PFAS result in their persistence and mobility in the environment via soil porewater, surface water and groundwater, with potential for adverse effects on the environment and human health. There is an emergent and increasing requirement for fast, low-cost, robust, and portable methods to detect PFAS, especially in the field. There may be thousands of PFAS compounds present in soil and water at extremely low concentration (0.01-250 ppb) that require measurement, and traditional technologies for continuous environmental sensing are challenged due to the complexity of soil chemistry. This paper presents a comprehensive review of potentially rapid PFAS measurement methods, focused on techniques for representative sampling of PFAS in porewater from contaminated soil, and approaches for pre-treatment of porewater samples to eliminate these interferences to be ready for PFAS-detecting sensors. The review discusses selectivity, a key factor underlying pre-treatment and sensing performance, and explores the interactions between PFAS and various sensors. PFAS chemical nano-sensors discussed are categorized in terms of the detection mechanism (electrochemical and optical). This review aims to provide guidance and outline the current challenges and implications for future routine PFAS sensing linked to soil porewater collection, to achieve more selective and effective PFAS sensors.

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.

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.

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.

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.