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.

On quantifying global carbon emission from oil contaminated lands over centuries

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 ~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.

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.

Towards predicting DNAPL source zone formation to improve plume assessment: Using robust laboratory and numerical experiments to evaluate the relevance of retention curve characteristics

We conducted multiple laboratory trials in a robust and repeatable experimental layout to study dense non-aqueous phase liquid (DNAPL) source zone formation. We extended an image processing and analysis framework to derive DNAPL saturation distributions from reflective optical imaging data, with volume balance deviations < 5.07%. We used a multiphase flow model to simulate source zone formation in a Monte Carlo approach, where the parameter space was defined by the variation of retention curve parameters. Integral and geometric measures were used to characterize the source zones and implemented into a multi-criteria objective function. The latter showed good agreement between observation data and simulation results for effective DNAPL saturation values > 0.04, especially for early stages of DNAPL migration. The common hypothesis that parameters defining the DNAPL-water retention curves are constant over time was not confirmed. Once DNAPL pooling started, the optimal fit in the parameter space was significantly different compared to the earlier DNAPL migration stages. We suspect more complex processes (e.g., capillary hysteresis, adsorption) to become relevant during pool formation. Our results reveal deficits in the grayscale-DNAPL saturation relationship definition and laboratory estimation of DNAPL-water retention curve parameters to overcome current limitations to describe DNAPL source zone formation.

Comparison and verification of turbulence Reynolds-averaged Navier-Stokes closures to model spatially varied flows

The robustness and accuracy of Reynolds-averaged Navier–Stokes (RANS) models was investigated for complex turbulent flow in an open channel receiving lateral inflow, also known as spatially varied flow with increasing discharge (SVF). The three RANS turbulence models tested include realizable k–ε, shear stress transport k–ω and Reynolds stress model based on their prominence to model jets in crossflows. Results were compared to experimental laser Doppler velocimetry measurements from a previous study. RANS results in the uniform flow region and farther from the jet centreline were more accurate than within the lateral inflow region. On the leeward side of the jet, RANS models failed to capture the downward velocity vectors resulting in major deviations in vertical velocity. Among RANS models minor variations were noted at impingement and near the water surface. Regardless of inadequately predicting complex characteristics of SVF, RANS models matched experimental water surface profiles and proved more superior to the theoretical approach currently used for design purposes.

Large eddy simulation of turbidity currents in a narrow channel with different obstacle configurations

Turbidity currents are frequently observed in natural and man-made environments, with the potential of adversely impacting the performance and functionality of hydraulic structures through sedimentation and reduction in storage capacity and an increased erosion. Construction of obstacles upstream of hydraulic structures is a common method of tackling adverse effects of turbidity currents. This paper numerically investigates the impacts of obstacle's height and geometrical shape on the settling of sediments and hydrodynamics of turbidity currents in a narrow channel. A robust numerical model based on LES method was developed and successfully validated against physical modelling measurements. This study modelled the effects of discretization of particles size distribution on sediment deposition and propagation in the channel. Two obstacles geometry including rectangle and triangle were studied with varying heights of 0.06, 0.10 and 0.15 m. The results show that increasing the obstacle height will reduce the magnitude of dense current velocity and sediment transport in narrow channels. It was also observed that the rectangular obstacles have more pronounced effects on obstructing the flow of turbidity current, leading to an increase in the sediment deposition and mitigating the impacts of turbidity currents.

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.

A large eddy simulation study to assess low-speed wind and baffle orientation effects in a water treatment sedimentation basin

Hydraulic performance of clarifiers in water and wastewater treatment plants significantly affects the settling efficiency of suspended particles. Structural and ambient parameters can deteriorate this performance. Through a verified three dimensional numerical study, we evaluated hydraulic performance and settling efficiency in a rectangular clarifier with a nominal hydraulic retention time (HRT) of 1 h and options for structural baffles with angles of 20°, 30°, 45° and 70°. Large eddy simulation and Lagrangian particle tracing were used to trace particles 80 to 850 μm in diameter. A passive scalar tracer study was conducted to reveal discrepancies in nominal and real HRT. By posing a 5 m/s wind, ten different scenarios were simulated. The wind caused 17% and 6% reduction in HRT and settling efficiency, respectively. Baffles improved these indicators with the 45° baffle showing the best performance with an approximate settling efficiency of 93%. The study highlighted the importance of using baffles, in particular for small size particles for which influencing factors such as wind deteriorate their settling efficiency.

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.

Asymptotic solutions for turbulent mass transfer at high Schmidt number

We present closed-form solutions for high Schmidt number mass transfer in a hydrodynamically fully developed turbulent flow. Governing equations for the near- and far-field are developed for a large class of boundary conditions (BCs) for which the mass flux is a function of the concentration at the wall. We show that for this class of BCs, which includes nonlinear wall reactions, the mass transfer coefficient is independent of the BC and the Sherwood correlation is therefore universal. For Dirichlet, Neumann and Robin BCs, the far-field solutions are in good correspondence with the method of separating variables and near-field solutions are in good agreement with numerical simulations. However, in contrast with the far-field solutions, the Sherwood correlation in the near-field depends on the specific BC. As an example of nonlinear BCs, solutions for a second-order wall reaction are derived which are compared with numerical simulations and found to be in excellent agreement.