Light Non-aqueous Phase Liquid Migration Research Articles

The Effect Characterization of Lens on LNAPL Migration Based on High-Density Resistivity Imaging Technique

Light non-aqueous phase liquids (LNAPLs), which include various petroleum products, are a significant source of groundwater contamination globally. Once introduced into the subsurface, these contaminants tend to accumulate in the vadose zone, causing chronic soil and water pollution. The vadose zone often contains lens-shaped bodies with diverse properties that can significantly influence the migration and distribution of LNAPLs. Understanding the interaction between LNAPLs and these lens-shaped bodies is crucial for developing effective environmental management and remediation strategies. Prior research has primarily focused on LNAPL behavior in homogeneous media, with less emphasis on the impact of heterogeneous conditions introduced by lens-shaped bodies. To investigate the impact of lens-shaped structures on the migration of LNAPLs and to assess the specific effects of different types of lens-shaped structures on the distribution characteristics of LNAPL migration, this study simulates the LNAPL leakage process using an indoor two-dimensional sandbox. Three distinct test groups were conducted: one with no lens-shaped aquifer, one with a low-permeability lens, and one with a high-permeability lens. This study employs a combination of oil front curve mapping and high-density resistivity imaging techniques to systematically evaluate how the presence of lens-shaped structures affects the migration behavior, distribution patterns, and corresponding resistivity anomalies of LNAPLs. The results indicate that the migration rate and distribution characteristics of LNAPLs are influenced by the presence of a lens in the gas band of the envelope. The maximum vertical migration distances of the LNAPL are as follows: high-permeability lens (45 cm), no lens-shaped aquifer (40 cm), and low-permeability lens (35 cm). Horizontally, the maximum migration distances of the LNAPL to the upper part of the lens body decreases in the order of low-permeability lens, high-permeability lens, and no lens-shaped aquifer. The low-permeability lens impedes the vertical migration of the LNAPL, significantly affecting its migration path. It creates a flow around effect, hindering the downward migration of the LNAPL. In contrast, the high-permeability lens has a weaker retention effect and creates preferential flow paths, promoting the downward migration of the LNAPL. Under conditions with no lens-shaped aquifer and a high-permeability lens, the region of positive resistivity change rate is symmetrical around the axis where the injection point is located. Future research should explore the impact of various LNAPL types, lens geometries, and water table fluctuations on migration patterns. Incorporating numerical simulations could provide deeper insights into the mechanisms controlling LNAPL migration in heterogeneous subsurface environments.

A New Toolbox for Managing Sites Impacted by Light Non-Aqueous Phase Liquids (LNAPLs)

The Concawe LNAPL Toolbox is one of the first completely web-based tools for managing and remediating light non-aqueous phase liquids (LNAPL) impacted sites. An off-line downloadable version is also available for users who do not want to access it via the internet. LNAPLs are typically historic hydrocarbons released into soils and groundwater, such as liquid fuels, crude oil, and condensates. The toolbox was developed by Concawe, the Scientific Division of European Fuel Manufacturers Association, with the support of the engineering and environmental science-consulting agency GSI Environmental. The toolbox, written on the R-Shiny platform, was released in 2022 and includes over 20 different tools, such as, for example, infographics, nomographs, calculators, mobility models, videos, and checklists. These tools are organized into three tiers, with Tier 1 offering basic graphics and essential information, Tier 2 providing simple quantitative models and calculators, and Tier 3 featuring access and explanations of more complex tools. The toolbox is designed to address six key questions that are important for environmental consultants and regulators managing LNAPL sites, including determining the amount of LNAPL, estimating LNAPL migration and persistence, assessing LNAPL risk over time, determining the effectiveness of LNAPL recovery, and estimating Natural Source Zone Depletion (NSZD). The Concawe LNAPL Toolbox is accessible free of charge via a webpage on an internet browser or by downloading the toolbox for use on a personal computer.

Experimental study on migration characteristics of LNAPL in the aquitard under pumping conditions.

Research on the migration behaviors of contaminants in the aquitard has been deficient for an extended period. Clay is commonly employed as an impermeable layer or barrier to stop the migration of contaminants. However, under certain conditions, the clay layer may exhibit permeability to water, thereby allowing contaminants to infiltrate and potentially contaminate adjacent aquifers. Consequently, it holds immense importance to scrutinize and investigate the migration characteristics of light non-aqueous phase liquid (LNAPL) within the aquitard for the purposes of groundwater pollution control and remediation. To evaluate the environmental risk posed by organic contaminants in the aquitard, an experimental model was formulated and devised to monitor the LNAPL concentration in the aquitard under pumping conditions. The correlation between pumping rate and LNAPL concentration was investigated. A self-developed plexiglass sandbox model was used to simulate the migration characteristics of LNAPL in the aquitard under pumping conditions. Four experimental scenarios were designed, varying pumping rates, aquitard thicknesses, and groundwater level changes. The LNAPL concentration curve was derived by systematically tracking and analyzing LNAPL levels at various locations within the aquitard. The results indicated that higher pumping rates corresponded to increased migration of LNAPL, resulting in greater LNAPL ingress into the pumping well during extraction. A thicker aquitard demonstrated a more pronounced inhibitory effect on LNAPL, leading to an extended penetration time of LNAPL within the aquitard. The drawdown within the aquitard exerted a discernible influence on LNAPL migration, with the LNAPL concentration continuing to decrease in tandem with declining water levels during pumping. These research findings can establish a scientific foundation for the control and remediation of contaminants within aquitards.

Evaluation of factors causing lateral migration of light non-aqueous phase liquids (LNAPLs) in onshore oil spill accidents.

Contamination of groundwater by harmful substances poses significant risks to both drinking water sources and aquatic ecosystems, making it a critical environmental concern. Most on-land spill events release organic molecules known as light non-aqueous phase liquids (LNAPLs), which then seep into the ground. Due to their low density and organic composition, they tend to float as they reach the water table. LNAPLs encompass a wide range of non-aqueous phase liquids, including various petroleum products, and can, over time, develop carcinogenic chemicals in water. However, due to frequent changes in hydraulic head, the confinement may fail to contain them, causing them to extend outward. When it contaminates water wells, people cannot reliably consume the water. The removal of dangerous contaminants from groundwater aquifers is made more challenging by LNAPLs. It is imperative to analyze the mechanisms governing LNAPL migration. As a response to this need and the associated dispersion of contaminants into adjacent aquifers, we have conducted a comprehensive qualitative literature review encompassing the years 2000-2022. Groundwater variability, soil structure, and precipitation have been identified as the three primary influential factors, ranked in the following order of significance. The rate of migration is shown to rise dramatically in response to changes in groundwater levels. Different saturation zones and confinement have a major effect on the lateral migration velocity. When the various saturation zones reach a balance, LNAPLs will stop moving. Although higher confinement slows the rate of lateral migration, it speeds up vertical migration. Beyond this, the lateral or vertical movement is also influenced by differences in the permeability of soil strata. Reduced mobility and tighter containment are the outcomes of migrating through fine-grained, low-porosity sand. The gaseous and liquid phases of LNAPLs move more quickly through coarse-grained soils. Due to the complexities and uncertainties associated with LNAPL behavior, accurately foreseeing the future spread of LNAPLs can be challenging. Although studies have utilized modeling techniques to simulate and predict LNAPL migration, the inherent complexities and uncertainties in the subsurface environment make it difficult to precisely predict the extent of LNAPL spread in the future. The granular soil structure considerably affects the porosity and pore pressure.

LNAPL migration processes based on time-lapse electrical resistivity tomography

Contamination from light non-aqueous phase liquids (LNAPLs) and their derivatives, arising from exploration, production, and transportation, has become a prevalent pollution source. This poses direct threats to human health. However, conventional investigative methods face limitations when applied to studying the extent and migration process of LNAPL contamination, as well as the redistribution of LNAPL during groundwater level fluctuations. Conventional methods lack the ability to rapidly, efficiently, and in real-time acquire information about contaminated areas. Therefore, this study utilizes time-lapse electrical resistivity tomography to investigate the migration mechanism of LNAPL under unsaturated conditions, constant groundwater levels, and groundwater level reductions. A relationship between resistivity and water and oil contents was established and used for inverse calculation of LNAPL content via resistivity inversion. Time-lapse electrical resistivity tomography revealed LNAPL migration in a “concave” shape across three conditions. Groundwater presence notably slowed migration, hindering downward movement and leading to a floating oil band. A robust mathematical model was established to derive the relationship between resistivity and water and oil contents. Finally, LNAPL distribution under unsaturated conditions was inversely obtained from resistivity data, showing highest content at the top leak point, obstructed area, and bottom of soil column. Consequently, time-lapse electrical resistivity tomography demonstrates a notable capacity to characterize the LNAPL migration process. This technique constitutes an effective geophysical method for monitoring and describing the characteristics of LNAPL migration. Its significance lies in enhancing our understanding of remediation for LNAPL-induced groundwater and land contamination.

Retention effect and mode of capillary zone on the migration process of LNAPL pollutants based on experimental exploration

Two-dimensional sand tank experiments were designed to investigate the retention process of the capillary zone during the migration of light non-aqueous phase liquid (LNAPL) pollutants. The fine sand and silt media experiments simulated the LNAPL migration process given a shallow point source leakage scenario. The results indicate that the LNAPL was retained in the capillary zone. A retention factor, based on the ratio of the change in the vertical migration velocity of the LNAPL front with time, was proposed to quantitatively characterize the retention effect. The retention factor and time satisfied the function of σ=A×exp(−kt). And the retention factor increased gradually with time, indicating the enhanced retention effect of capillary zone on the vertical migration of LNAPL. The concentration change rate was then used to investigate the LANPL redistribution process, which had a relationship with time of νc=B×ln(t)+C. The capillary zone could be divided from top to bottom into a weak retention zone (B > 0, vc < 0), a strong retention zone (B < 0), and a barrier zone (B > 0, vc > 0). The retention effect of capillary zone on LNAPL migration gradually strengthened during the vertical migration of LNAPL. In addition, the coefficient B had a relationship with the environmental factors (i.e., EC, pH, and ORP) of B=a×sin(b×α×β×γ)c and the fitting coefficient R2 of the function was above 0.913 for both media, indicating a strong correlation between the LNAPL redistribution process and the key environmental factors.

Migration and redistribution of LNAPL in inclined stratified soil media

Leakage of light non-aqueous phase liquid (LNAPL) into soil can cause serious environmental issues. In this study, a two-dimensional device with adjustable dip angles was designed to investigate the migration and redistribution of LNAPL in natural inclined stratified soil media by the light transmission visualization (LTV) technology. The captured experimental images were processed to obtain the diesel distribution based on gray value which could represent the LNAPL saturation distribution. LNAPL may not be able to penetrate through the fine-coarse interface due to the capillary barrier effects. In this case, the vertical and horizontal migration distances (V and H), contaminated area (S), as well as deviation angle (γ) of centroid increased with the dip angle. Increasing the leakage amount to more than 30 mL would result in LNAPL breakthrough at the 10°-inclined interface, leading to much larger V, H, S, and γ than those at 10 mL, while 20-mL LNAPL failed to break through. In the latter case, a lower leakage rate than 10 mL/min would cause larger H and γ but similar V or S in the long term. This study could enrich the understanding of LNAPL contamination in vadose zone, providing reference for the prediction and treatment in realistic inclined contaminated sites.

Novel Insights into the Influence of Soil Microstructure Characteristics on the Migration and Residue of Light Non-Aqueous Phase Liquid.

Understanding the influence of soil microstructure on light non-aqueous phase liquids (LNAPLs) behavior is critical for predicting the formation of residual LNAPLs under spill condition. However, the roles of soil particle and pore on LNAPLs migration and residue remains unclear. Here, the experiment simulated an LNAPLs (diesel) spill that was performed in fourteen types of soils, and the key factors affecting diesel behavior are revealed. There were significant differences between fourteen types of soils, with regard to the soil particle, soil pore, and diesel migration and residue. After 72 h of leakage, the migration distance of diesel ranged from 3.42 cm to 8.82 cm in the soils. Except for sandy soil, diesel was mainly distributed in the 0−3 cm soil layer, and the residual amounts were 7.85−26.66 g/kg. It was further confirmed from microstructure that the consistency of soil particle and volume of soil macropores (0.05−7.5 μm) are important for diesel residue in the 0−1 cm soil layer and migration distance. The large soil particles corresponding to 90% of volume fraction and volume of soil mesopores (<0.05 μm) are key factors affecting diesel residue in the 1−3 cm soil layer. The result helps to further comprehend the formation mechanism of residual LNAPLs in the soil.

Migration behaviour of LNAPL in fractures filled with porous media: Laboratory experiments and numerical simulations

With the increasing requirement of maintaining world energy security and strategic reserves, oil storage and transportation facilities are being built at a large scale. Taking the safe and efficient operation of petroleum storage projects as the goal, a set of experimental apparatus to investigate the migration of contaminants in fractures filled with media was developed to predict and evaluate the environmental risk of oil contaminants leakage. A multiphase numerical flow model based on COMSOL was built based on the laboratory experimental model. Specifically, the migration behaviour of Light Non-aqueous Phase Liquid (LNAPL) through a sand-filled fractured medium was studied by laboratory experiments and numerical simulations. Image and chemical analyses methods were used to monitor and study LNAPL migration behaviour for varying grain sizes of porous medium filling the fractures and varying groundwater table elevations. Laboratory experimental results showed that the LNAPL migration velocity in filled fracture network was significantly faster than that in adjoining porous media during the initial stage of infiltration. The migration velocity increased with the relative permeability of filled sand, which was closely related to the Van Genuchten (VG) model parameters α and n. LNAPL migrated downward with the falling groundwater table and became entrapped with the rising groundwater table, and the amount of entrapment depended on VG model parameters. Hydrogeological parameters were calibrated and LNAPL migration in filled fractured media was predicted using the calibrated numerical model. Simulation results revealed that fracture inclination had an important influence on LNAPL migration in filled fractured media and its migration velocity decreased with a decrease in fracture inclination. These research results can be applied to the control and remediation of oil-contaminated sites in fractured rock settings, such as at underground oil storage tanks and caverns, as well as at underground oil pipelines.

Construction, application and validation of a new algorithm for determining light nonaqueous-phase liquid fluxes in unsaturated zones

An analytical algorithm coupling free-phase migration, precipitation, and natural attenuation through volatilization and biodegradation (FPVB) was developed to calculate the flux of light nonaqueous-phase liquid (LNAPL) leaking from unsaturated zone to groundwater. Sandbox and soil column experiments were performed to identify the LNAPL migration characteristics and states to provide data to establish and verify FPVB algorithm. For free-phase migration, the Kinematic Oily Pollutant Transport (KOPT) model was used to determine LNAPL movement velocity and leakage time. The correlations of water saturation, residual LNAPL saturation and the cumulative dissolution ratio of residual LNAPL were described using an empirical formula for the precipitation leaching process. Equations for diesel volatilization kinetics and first order degradation were used to describe the natural attenuation processes. Coupling the algorithms for the different stages gave the final FPVB algorithm. The FPVB algorithm was used to describe the pollution situation at a real site, and the results were consistent with the actual situation. The FPVB algorithm could be used to quickly assess the scale and degree of pollution with little information on the parameters for the actual LNAPL leakage event.

Upotreba fluorescencije indukovane laserskim zračenjem (LIF) kao metode u Sjedinjenim Američkim Državama

Spatial characterization of the contamination is a fundamental component of any remediation approach. Compared to conventional investigation methods, the laser-induced fluorescence (LIF) is a faster screening tool and provides the detection of hydrocarbon contamination in real-time. This investigation was conducted at a hydrocarbon-contaminated site in Pennsylvania, US. The presence of light nonaqueous-phase liquids (LNAPL) was detected in 6 of 17 LIF borings, with the reference emitter responses ranging between 45% and 225%. The depth of the response was highly accurate and valuable and provided insight into the spatial distribution of contamination. The results indicated that no substantial amount of LNAPL existed along the LIF borings profile, thus excluding this area as a preferential LNAPL migration pathway. The obtained results contributed to the characterization and remediation of this industrial site.

Assessing the impact of water infiltration on LNAPL mobilization in sand column using simplified image analysis method

Laboratory-scale column experiments were carried out to assess the influence of water infiltration on pooled light non-aqueous phase liquid (LNAPL) redistribution in porous media. A simplified image analysis method (SIAM) was used to evaluate the saturation distributions of the LNAPL and water in the entire domain under dynamic conditions. The experiments were conducted for high/low LNAPL volumes LNAPL volumes differentiated as low and high volumes. High resolution SIAM images of the soil column during LNAPL migration and water infiltration events were captured and analyzed. Results indicated that the capillary fringe is about 6–7 cm which was consistent with the capillary height derived from empirical equations. Moreover, SIAM provided an estimate of the field capacity (30%) of the sand. Once the LNAPL infiltration stage was started, the LNAPL was observed to rapidly migrate through the vadose zone. For the case of large LNAPL volume, the LNAPL penetrated further into capillary fringe zone. Analysis of SIAM images showed that the LNAPL redistribution was observed to vary significantly with the rate of infiltration. For higher water infiltration intensity, the injected water exerted a larger hydrodynamic force on the entrapped LNAPL forcing it move further downward into the capillary zone and the saturated zone. Overall, this study demonstrated that the SIAM technique is an accurate and cost-effective tool for the visualization of the time-dependent NAPL/water movement in laboratory-scale experiments and dynamic changes in fluid saturation in porous media.

LNAPL Migration in Vadoze Zone and Its Prevention Using Natural Fiber

ABSTRACT Light non-aqueous phase liquids (LNAPLs) spilt over an area can be a persistent and long-term source of groundwater contamination. Once the groundwater gets contaminated, it is difficult to remediate. Migration of the spilt LNAPL should be contained within the unsaturated zone of the soil such that the LNAPL does not reach the groundwater source. Although the presence of LNAPLs in the unsaturated zone is also a long-term source, the provision of proper liners can contain the LNAPL within this region. The use of natural fiber as a liner in the unsaturated zone can be a sustainable solution to this problem if it can effectively contain the LNAPL. The objective of the present study was to investigate whether the use of natural fiber (coir geotextile) has supplemented the inherent attenuation characteristics of soils in the unsaturated zone thereby controlling the LNAPL migration. The study reveals that coir geotextiles were more effective in reducing the LNAPL migration in laterite soil than in sandy soil. Coir geotextiles are effective in controlling the migration of LNAPL if these are embedded in the soil with permeability less than 2.30 × 10−05 m/s.

Comparative investigation of vapor expulsion technique (VET: Via N2-treatment) and carbonate–clay based adsorbents as sustainable approaches for oil-leak remediation

Abstract Immobilization of light non-aqueous phase liquids (LNAPLs) in porous sediment materials is directly associated with the physicochemical properties of both the sediment materials and the fluids. This study investigated the efficiency of carbonate (calcite) and clay (kaolinite) minerals in immobilizing the migration of LNAPL (e.g. crude-oil and diesel), through a blend of the materials with sand without chemical additives. In this study, BET (Brunauer, Emmett, and Teller) method was used to obtain the surface area of sand, carbonate, and clay materials. Fluid properties (surface tension, and density) were measured with Kruss K11 force tensiometer while viscosity was determined with the Anton Paar MCR 302 Rheometer. Secondly, natural materials known to possess adsorptive capacities were characterized, tested, and compared. In this work, adsorbent-blends were created and mixed with sand material to achieve a comparable textural distribution. From the LNAPL migration experiment, both Adsorbent and Cl-Adsorbent showed high adsorptive efficiency for crude-oil and diesel, respectively. In comparison, Absorbent-1&2 exhibited stronger adsorption than Cl-Adsorbent1&2 under same conditions. The results demonstrate that Adsorbent-1&2 is most efficient in immobilizing LNAPL. Moreover, increasing the weight of adsorbent and combining mineral adsorbents from different materials, significantly reduced LNAPL distribution. Thirdly, a process termed “Vapor Expulsion Technique (VET)”, through which LNAPL-contaminated model-sand was treated with N 2 gas was developed. VET is a method for immobilizing LNAPL subsurface migration in oil-contaminated soil. Based on decrease in LNAPL penetration-depths, the study observed that Adsorbent-2 and the VET yielded 36%, and 64% LNAPL immobilization efficiencies (IE, %), respectively.

Hydrocarbon Penetration in Multi-Porosity Soil

Groundwater contamination is crucial, particularly due to hydrocarbon liquid leakages. Additionally, the vibration impact affects the migration of hydrocarbon through the subsurface soil to the groundwater surface which is causes by the dynamic stress that is imposed on the soils. Therefore, this paper presents the investigation and discussions on the penetration of Light Non-Aqueous Phase Liquid (LNAPL) in vibrated unsaturated double-porosity laterite soil with 30% moisture content. The apparatus used for this experiment are a vibrating table, acrylic soil column, mirror, LNAPL and a Nikon D90 digital camera. The LNAPL migration pattern in laterite soil was monitored and recorded using a digital image processing technique (DIPT) at certain time intervals. The images were processed using Surfer software and Matlab routine to plot the LNAPL hue-saturation-intensity (HSI) values. As a result, the migration rate of LNAPL decreased with the impact of vibration towards the soil due to the rearrangement and porosity of the soil structure and this was found to affect the speed of liquid penetration.

Migration of hydrocarbon liquid in unsaturated laterite soil with dissimilar volume and moisture content

Groundwater contaminations due to hydrocarbon or organic solvent spills in saturated or unsaturated zones affect the water resources. The quantity of hydrocarbon spills in the groundwater might influence the hydrocarbon migration. In addition, the soil moisture content in influencing the rate of migration is also unknown. Therefore, this paper presents the investigation migration of Light Non-Aqueous Phase Liquid (LNAPL) in unsaturated laterite soil with two dissimilar moisture contents and different of LNAPL volume. These lab-scale studies discuss the results of migration phenomena by employing image analysis by using soil column, mirror, LNAPL, and Nikon D90 digital camera. Aggregated soil was poured and compressed in an acrylic column until 100 mm height. Then LNAPL was poured onto the soil column surface instantaneously. The LNAPL migration pattern in laterite soil was monitored and recorded using digital image processing technique (DIPT) at certain time intervals then were processed with Surfer software and Matlab routine for plotting the LNAPL migration pattern. As a result, the analysis displayed the higher rate migration of LNAPL with high moisture content and bigger amount of LNAPL volume. However, the rates of LNAPL migration decrease with lower moisture content and small amount of LNAPL volume. The migration time required to reach bottom of soil was longer for low moisture content with smaller amount of LNAPL volume as compared to the high moisture content with bigger amount of LNAPL volume.

Experimental and numerical research on migration of LNAPL contaminants in fractured porous media

The contamination of groundwater by organic compounds is becoming more common and severe. A diesel tracer, a light non-aqueous phase liquid (LNAPL), was selected to carry out experiments of contaminant migration in fractured porous media. At the same time, trials of contaminant remediation methods were conducted using distilled water. The experimental results showed that the dominant flow was more obvious in fractured porous media than in porous media owing to the effect of the fractures on contaminant migration. The mechanism of contaminant migration and remediation was revealed by the movement of oil-driven air and water-driven oil. The dispersion parameters of these media were calibrated through the calculated areas and the measured contamination and remediation areas, using a numerical method. The predicted results indicate that the greater the injection rate, the shorter the time to penetrate the fractured porous media. However, relative to the LNAPL migration experiments, the water-driven oil processes were slower and the remediation time was longer at the same injection rate.

LNAPL saturation distribution under the influence of water table fluctuations using simplified image analysis method

Laboratory-scale column experiments were conducted to investigate the influence of water table fluctuation on light non-aqueous phase liquid (LNAPL) migration and redistribution in porous media. A simplified image analysis method (SIAM) was used to evaluate the saturation distributions of the LNAPL and water in the entire domain under dynamic conditions. Two LNAPL volumes differentiated as low and high volumes were exposed to two patterns of water table fluctuations. High-resolution SIAM images of the soil column during LNAPL migration and subsequent water table fluctuations were collected and analyzed. Results show that when the water table was lowered in the experiment of low LNAPL volume by 10 cm (from 28 to 18 cm) and then raised back to its initial location (28 cm), the water saturation dropped from 100% initially to around 30% while the LNAPL saturation increased and remained at 7% to 25% at 36 cm. By doubling the LNAPL volume to 50 ml, the initial LNAPL saturation above the water table increased and the capillary fringe height increased from 36 to 38 cm. Comparison between the two LNAPL volumes showed that the LNAPL covered a larger area for the case of high LNAPL. The LNAPL saturation extended up to around 53 cm while for the case of low LNAPL, the LNAPL extended up to height of 50 cm. Moreover, water saturation below was lesser for the case of high LNAPL. This was due to the increasing volume of LNAPL which caused the water to be pushed downward. However, similar downward migration of water was observed in both sets of experiments. This indicates that doubling the volume of LNAPL while maintaining the same groundwater fluctuation pattern did not affect the general LNAPL behavior. Overall, this study demonstrates that the SIAM technique is an accurate and cost-effective tool for the visualization of the time-dependent NAPL/water movement in laboratory-scale experiments and dynamic changes in fluid saturation in porous media.

ASSESSMENT AND REMEDIATION OF A LIGHT NON-AQUEOUS PHASE LIQUID HYDROCARBON PLUME IN FRACTURED BEDROCK

INTRODUCTION During the extension of an access road on the Canadian Forces Base (CFB) Esquimalt in Colwood, British Columbia, a black, viscous, liquid hydrocarbon product was observed oozing from newly exposed bedrock fractures in the roadside. Road excavation was subsequently halted to undertake assessment and remediation of the hydrocarbon product. The exposed bedrock was dammed with sawdust, a geomembrane barrier was installed and the area was backfilled until an appropriate course of action could be determined. The site location is shown in Figure 1. The property boundary and key site features are shown in Figure 2; the hydrocarbon seep is shown in Figure 3. Bunker C oil is a heavy-end (high molecular weight) hydrocarbon product that has a specific gravity slightly less than water and is therefore a light non-aqueous phase liquid (LNAPL). The source of the LNAPL was inferred to originate from a decommissioned fuel depot located approximately 100 m distance uphill from the road, where 40,000 barrels of Bunker C fuel oil were historically stored in one of three, large above-ground storage tanks (ASTs). A Bunker C oil spill reportedly took place at the tank farm more than two decades prior; however, the spill volume was unknown and initial investigations found no evidence of contamination between the roadside LNAPL occurrence and the former AST. Furthermore, there was also anecdotal evidence that an historic asphalt manufacturing facility may have operated in the vicinity of the access road. A hydrocarbon product similar to Bunker C is used in the manufacture of asphalt. The source and extent of the LNAPL and the potential migration pathways to the roadside location were therefore unknown. When an LNAPL spill occurs in the subsurface, the LNAPL can migrate downward under gravity through the soil pore space in the unsaturated zone. When LNAPL encounters the ground water table (the top of the saturated zone), it tends to spread out laterally because it is less dense than water and will migrate primarily in the direction of the water table gradient (water table slope). However, when LNAPL encounters bedrock, the direction of LNAPL migration can become much more complicated depending on the degree and orientation of bedrock fractures that control its movement. When fracture density is sufficiently high and the fractures are interconnected, contamination is able to migrate down-gradient through the bedrock in the same manner as through unconsolidated materials. However, when dominant structural features are prevalent that favour specific orientations, preferential pathways are created that can result in the cross-gradient migration of LNAPL. This paper presents a case study for the assessment and remediation of LNAPL in bedrock at the Canadian Forces Base (CFB) Esquimalt (the site). Geological mapping of bedrock structural features has long been used by the mining industry to identify key structures associated with economic zones of mineralization and to predict the location and extent of mineralized targets. In a similar regard, to effectively remediate LNAPL within fractured bedrock requires the identification and characterization of any structural features that might be controlling the preferential migration of LNAPL within the subsurface to other areas of the site. A significant amount of surface outcrop is present at the site and this was recognized as a cost-effective opportunity to complete a geological assessment of the bedrock. A geological mapping program was subsequently undertaken to assess bedrock outcrops for fracture density, fracture aperture, the orientation of primary fracture sets and lithologic contacts. The area was also inspected for larger scale structural features such as faults, deformation and erosional features that might influence contaminant migration. Fracture sets and lithologic contacts were mapped by outcrop location, and fractures with visible LNAPL were mapped separately from those without LNAPL. The geological data collected was used to construct stereographic projections of structural planes on a stereonet. Poles to structural planes were plotted and colour-coded by area and by presence/absence of LNAPL. The plots were then analyzed individually, and as a composite plot, to identify the dominant preferential pathways controlling LNAPL migration at the site. By superimposing these features on areas where LNAPL was observed, LNAPL delineation targets were effectively identified and the plume was subsequently delineated with confidence and remediated.

Assessing the influence of infiltration on the migration of light non-aqueous phase liquid in double-porosity soil media using a light transmission visualization method

The influence of infiltration on the migration of a light non-aqueous phase liquid (LNAPL) in double-porosity soil using the light transmission visualization (LTV) technique is investigated. Two LNAPL volumes (low and high volumes) were exposed to two rainfall intensities (light and heavy infiltration). For comparison purposes, the experiments were also repeated by compacting the flow chamber with silica sand only to represent the single-porosity medium and to investigate the influence of double-porosity on LNAPL migration. High-resolution LTV images of the flow chamber during LNAPL injection and subsequent water infiltration events were collected. Results show that the LNAPL migration depth during injection and its migration velocity were both correlated to the LNAPL volume. Subsequent water infiltration events caused the LNAPL that was entrapped in the porous media to be pushed further downward in all the experiments. The LNAPL migration velocity was 1.1 and 1.6 cm/h for the low and high LNAPL spillage volumes for double-porosity experiments, respectively, a reduction rate of 64.7 and 70% compared to the LNAPL migration velocity during LNAPL injection, respectively. However, for single-porosity experiments, the LNAPL migration velocity was 0.7 and 1.2 cm/h for the low and high LNAPL volumes, respectively. Furthermore, it was observed that the capillary fringe level was depressed in the saturated zone due to the influence of both infiltration and LNAPL volume. This study demonstrates that the LTV technique is an accurate and cost-effective laboratory tool for the visualization of the time-dependent influence of infiltration on LNAPL migration in porous media.