Light Non-aqueous Phase Liquid Saturation Research Articles

Influence of advection on the soil gas radon deficit technique for the quantification of LNAPL

The Radon (Rn) deficit technique is a rapid, low-cost, and non-invasive method to identify and quantify light non-aqueous phase liquids (LNAPL) in the soil. LNAPL saturation is typically estimated from Rn deficit using Rn partition coefficients, assuming equilibrium conditions. This work examines the applicability of this method in the presence of local advective fluxes that can be generated by groundwater fluctuations or biodegradation processes in the source zone. To this end, a one-dimensional analytical model was developed to simulate the steady-state diffusive-advective transport of soil gas Rn in the presence of LNAPL. The analytical solution was first validated against an existing numerical model adapted to include advection. Then a series of simulations to study the effect of advection on Rn profiles were carried out. It was found that in high-permeability soils (such as sandy soils), advective phenomena can significantly affect Rn deficit curves in the subsurface compared with those expected, assuming either equilibrium conditions or a diffusion-dominated transport. Namely, in the presence of pressure gradients generated by groundwater fluctuations, applying the traditional Rn deficit technique (assuming equilibrium conditions) can lead to an underestimation of LNAPL saturation. Furthermore, in the presence of methanogenesis processes (e.g., in the case of a fresh LNAPL of petroleum hydrocarbons), local advective fluxes can be expected above the source zone. In such cases, Rn concentrations above the source zone can be higher than those above background areas without advective phenomena, resulting in Rn deficits higher than 1 (i.e., Rn excess), and thus leading to a wrong interpretation regarding the presence of LNAPL in the subsurface if advection is not considered. Overall, the results obtained suggest that advection should be considered in the presence of pressure gradients in the subsurface to ensure an effective application of the soil gas Rn-deficit technique for quantitative estimation of LNAPL saturation.

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.

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.

LNAPL Recovery Endpoints: Lessons Learnt Through Modeling, Experiments, and Field Trials

AbstractIt is important to estimate what light nonaqueous phase liquid (LNAPL) recovery can be practicably achieved from subsurface environments. Over the last decade, research to address this included a broad field program, laboratory measurements and experimentation, and modeling approaches. Here, we consolidate key findings from the research in the context of current literature and understanding, with a focus on a well‐validated, multiphase multicomponent modeling approach to achieve estimates of reasonable endpoints for LNAPL recovery. Simple analytical models can provide approximate saturation distributions and estimates of LNAPL recoverability via transmissivity approximation, but are insufficient to predict LNAPL saturation‐ and composition‐based recovery endpoints for various recovery technologies. This is because they cannot account for multiphase, multicomponent fate and transport and key processes such as hysteresis. Recent advances to improve estimates of the fraction of recoverable LNAPL and its transmissivity are summarized. These advances include further development and application of a well‐validated model to characterize active LNAPL recovery endpoints. We present key factors that affect the determination of LNAPL recovery endpoints, and outline how recovery endpoints are affected by natural source zone depletion (NSZD—currently gaining acceptance as a LNAPL remediation option). Major factors include geo‐physical characteristics of the formation, magnitude of an LNAPL release and partitioning properties of the key LNAPL constituents of concern. Based on the capabilities of the validated model, the paper also provides a basis to optimize LNAPL recovery efforts.

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

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

A Screening Model to Predict Entrapped LNAPL Depletion

Accidental leakage of hydrocarbons is a common subsurface contamination scenario. Once released, the hydrocarbons migrate until they reach the vicinity of the uppermost portion of the saturated zone, where it accumulates. Whenever the amplitude of the water table fluctuation is high, the light non-aqueous phase liquid (LNAPL) may be completely entrapped in the saturated zone. The entrapped LNAPL, comprised of multicomponent products (e.g., gasoline, jet fuel, diesel), is responsible for the release of benzene, toluene, ethylbenzene, and xylenes (BTEX) into the water, thus generating the dissolved phase plumes of these compounds. In order to estimate the time required for source-zone depletion, we developed an algorithm that calculates the mass loss of BTEX compounds in LNAPL over time. The simulations performed with our algorithm provided results akin to those observed in the field and demonstrated that the depletion rate will be more pronounced in regions with high LNAPL saturation. Further, the LNAPL depletion rate is mostly controlled by flow rate and is less sensible to the biodegradation rate in the aqueous phase.

Testing an Analytical Model for Predicting Subsurface LNAPL Distributions from Current and Historic Fluid Levels in Monitoring Wells: A Preliminary Test Considering Hysteresis

Knowledge of subsurface light nonaqueous phase liquid (LNAPL) saturation is important for developing a conceptual model and a plan for addressing LNAPL contaminated sites. Investigators commonly predict LNAPL mobility and potential recoverability using information such as LNAPL physical properties, subsurface characteristics, and LNAPL saturations. Several models exist that estimate the LNAPL specific volume and transmissivity from fluid levels in monitoring wells. Commonly, investigators use main drainage capillary pressure–saturation relations because they are more frequently measured and available in the literature. However, main drainage capillary pressure–saturation relations may not reflect field conditions due to capillary pressure–saturation hysteresis. In this paper, we conduct a preliminary test of a recent analytical model that predicts subsurface LNAPL saturations, specific volume, and transmissivity against data measured at a LNAPL contaminated site. We call our test preliminary because we compare only measured and predicted vertical LNAPL saturations at a single site. Our results show there is better agreement between measured and predicted LNAPL saturations when imbibition capillary pressure–saturation relations are employed versus main drainage capillary pressure–saturation relations. Although further testing of the model for different conditions and sites is warranted, the preliminary test of the model was positive when consideration was given to capillary pressure–saturation hysteresis, which suggests the model can yield reasonable predictions that can help develop and update conceptual site models for addressing subsurface LNAPL contamination. Parameters describing capillary pressure–saturation relations need to reflect conditions existing at the time when the fluid levels in a well are measured.

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.

Numerical Modeling of Multiphase Extraction (MPE) Aiming at LNAPL Recovery in Tropical Soils

Subsurface contamination by light non-aqueous phase liquids (LNAPL) is a widespread global problem that requires appropriate techniques to remediate soil and groundwater. In this paper, the subsurface transport over multiple phases (STOMP) model was used to simulate LNAPL multiphase flow and transport during multiphase extraction (MPE) application in two Brazilian tropical soils (silty sand and oxisol) contaminated by diesel. The model was applied to a hypothetical contamination site, with the initial LNAPL thickness observed in well extraction. The first part consisted of the MPE system sensitivity analysis, varying the applied vacuum and tip tube position. The Van Genuchten α parameter and hydraulic conductivity were the properties that most affected LNAPL saturation and fluid extraction volumes. Suitable applied vacuum and tip tube position parametrization was imperative for the efficiency of LNAPL extraction. After the definition of an appropriate MPE system configuration, simulations demonstrated that the immobile LNAPL saturation affected fluid extraction and diesel oil concentrations in aqueous and gas saturation. The model applied is able to predict LNAPL contaminant behavior in porous media during MPE technique application.

LNAPL saturation derived from laser induced fluorescence method

Light non-aqueous-phase liquid (LNAPL) spills are a widespread source of contamination in shallow aquifers. Owing to their human health risks, remediation actions should be undertaken to recover the contaminants from the subsurface. However, traditional investigation techniques do not assess the actual volume of residual hydrocarbon in the pore space, hindering the effectiveness of remediation predictions. The emergence of the high-resolution laser-induced fluorescence (LIF) technique has allowed the extent of NAPL migration and distribution to be determined in the field. Despite the good potential of LIF, this technique has not yet been used to quantify the volume or saturation of NAPL in porous media. By conducting medium-scale spill experiments, efforts have been undertaken to identify the empirical fluorescence signal relationship between LIF and LNAPL saturation. The comparison of both parameters indicates that LIF can predict the LNAPL saturation following an exponential function. However, owing to the high variability of the composition of LNAPL and the weathering stage, empirical coefficients to predict the saturation of LNAPL by fluorescence intensity are site-dependent. The measurement of saturation by LIF opens the possibility of more precise LNAPL volume estimation, including complex NAPL distribution scenarios.

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

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

Free LNAPL Volume Estimation by Pancake Model and Vertical Equilibrium Model: Comparison of Results, Limitations, and Critical Points

Light nonaqueous phase liquids (LNAPLs), due to their low solubility, dissolve slowly, acting as a long-term source of water contamination, and consequently they represent an important environmental issue. In the subsoil, more than 99% of spilled LNAPL remains as adsorbed and free phase; therefore, the volume estimation of free phase, obtained in this case through two different conceptual models (Pancake Model and Vertical Equilibrium Model), is considered a fundamental step for a correct site remediation. According to the first model, the LNAPL floating on the water table and its saturation is up to 100%; instead, according to the second one, the LNAPL can penetrate below the water table and the coexistence of LNAPL, water, and air in the pore fraction, leads to a lower LNAPL saturation, variable with the depth. Actually, in subsoil LNAPL and water saturations vary with depth due to the influence of capillarity, leading to the inaccuracy of Pancake Model assumption. Despite the evident limitation of Pancake Model, both models were applied, coupled with area calculations with Thiessen polygons and grid at regular mesh, to roughly estimate the free LNAPL volume existing in a contaminated site. The volume estimation carried out, considering the LNAPL type and its features, the soil type, and relative effective porosity, provides estimates of volumes having differences up to thousands of cubic meters. The results analysis shows that this estimation has several critical points such as area definition and the lack of site-specific data (e.g., porosity). Indeed, the sensitivity analysis for porosity shows that a reduction of this parameter provides a 20% reduction of estimated volume.

Impact of heterogeneous properties of soil and LNAPL on surfactant-enhanced capillary desaturation

This paper investigates low-concentration (<1wt%) surfactant flushing when used as a follow-up technology for multiphase vacuum extraction on heterogeneous sites. Challenges posed by soil permeability, pore-size distribution, mineralogy, light non-aqueous phase liquid (LNAPL) weathering and groundwater hardness were quantified through batch and soil column tests. Compatibility issues between the mixed mineralogy soils, hard groundwater, mixed LNAPL and usual anionic surfactants were observed. The selected solution was a Winsor type I system promoting an interfacial tension of 0.06mN/m between the site LNAPL and the amphoteric surfactant CAS in aqueous solution at pH12. Surfactant loses to adsorption and pore media plugging were observed in the fine soil fraction. The capillary desaturation curves (CDC) obtained with the column tests suggested mixed-wettability behavior. The soil permeability strongly influenced LNAPL recovery, as expressed by the relationship obtained between capillary numbers (NCa) and hydraulic gradients. In this case, the critical NCa, marking the onset of capillary desaturation, could only be obtained with realistic hydraulic gradients in the coarse soil fraction. At those gradients, potential LNAPL recovery was 30% at the most. Unlike previously published CDCs, the relationship between NCa (log-scale) and LNAPL recovery was not linear but dependant on residual LNAPL saturation.

Field data and numerical simulation of btex concentration trends under water table fluctuations: Example of a jet fuel-contaminated site in Brazil

Mass transfer of light non-aqueous phase liquids (LNAPLs) trapped in porous media is a complex phenomenon. Water table fluctuations have been identified as responsible for generating significant variations in the concentration of dissolved hydrocarbons. Based on field evidence, this work presents a conceptual model and a numerical solution for mass transfer from entrapped LNAPL to groundwater controlled by both LNAPL saturation and seasonal water table fluctuations within the LNAPL smear zone. The numerical approach is capable of reproducing aqueous BTEX concentration trends under three different scenarios – water table fluctuating within smear zone, above the smear zone and partially within smear zone, resulting in in-phase, out-of-phase and alternating in-phase and out-of-phase BTEX concentration trend with respect to water table oscillation, respectively. The results demonstrate the model's applicability under observed field conditions and its ability to predict source zone depletion.

Numerical modelling on rate-limited dissolution mass transfer of entrapped petroleum hydrocarbons in a saturated sub-surface system

Petroleum hydrocarbons generally referred to as light non-aqueous phase liquid (LNAPL) entrapped within a saturated subsurface geological formation can result in longer duration of contamination due to mass transfer limitations associated with heterogeneity of the system. Considering a part of the total pore volume as immobile is significant in capturing the behaviour of extended tailing by solute breakthrough profiles. In this context, the present numerical model investigates dissolution characteristics of residual petroleum hydrocarbons (LNAPL) resulting from both mobile and immobile domains under varying conditions of rate-limited diffusive mass transfer, longitudinal dispersivity and mean mobile fluid velocity in a saturated groundwater system. The numerical results suggest that the influence of residual LNAPL saturation on dissolution mass transfer coefficient is critical in comparison with mobile flow velocity and dispersivity. In addition, the rate-limited effect becomes predominant with significant increase in advective velocity and reduction in residual LNAPL saturation for structured porous media. It is concluded from this study that the transport of dissolved contaminant is dominated by advection at the mobile pores, while the diffusive equilibrium at the immobile volume is significant in characterizing the extended tailing and subsequently determines the overall mass transfer resistance.

222 Rn as Natural Tracer for LNAPL Recovery in a Crude Oil-Contaminated Aquifer

The objective of this study was to investigate whether 222Rn in groundwater can be used as a tracer for light non-aqueous phase liquid (LNAPL) quantification at a field site treated by dual-phase LNAPL removal. After the break of a pipeline, 5 ha of soil in the nature reserve Coussouls de Crau in southern France was contaminated by 5100 m3 of crude oil. Part of this oil seeped into the underlying gravel aquifer and formed a floating oil body of about 3.9 ha. The remediation consists of plume management by hydraulic groundwater barriers and LNAPL extraction in the source zone. 222Rn measurements were performed in 21 wells in and outside the source zone during 15 months. In uncontaminated groundwater, the radon activity was relatively constant and remained always >11 Bq/L. The variability of radon activity measurements in wells affected by the pump-and-skim system was consistent with the measurements in wells that were not impacted by the system. The mean activities in wells in the source zone were, in general, significantly lower than in wells upgradient of the source zone, owing to partitioning of 222Rn into the oil phase. The lowest activities were found in zones with high non-aqueous phase liquid (NAPL) recovery. LNAPL saturations around each recovery well were furthermore calculated during a period of high groundwater level, using a laboratory-determined crude oil–water partitioning coefficient of 38.5 ± 2.9. This yielded an estimated volume of residual crude oil of 309 ± 93 m3 below the capillary fringe. We find that 222Rn is a useful and cheap groundwater tracer for finding zones of good LNAPL recovery in an aquifer treated by dual-phase LNAPL removal, but that quantification of NAPL saturation using Rn is highly uncertain.

Numerical simulations of a light nonaqueous phase liquid (LNAPL) movement in variably saturated soils with capillary hysteresis

The fate and transport of pollutants in the subsurface is usually predicted using numerical simulations. Comparisons of the simulation results with experimental data are required to validate the numerical codes. In this paper, the nonaqueous phase liquid (NAPL) simulator code was used to numerically simulate two centrifugal light nonaqueous phase liquid (LNAPL) transport experiments. The numerical predictions were compared with the centrifugal test results and the impact of capillary hysteresis was evaluated. It was concluded that the simulation with the nonhysteretic model slightly underestimated the LNAPL volume retained in the vadose zone, and the average error of the predicted LNAPL saturation in the vadose zone and along the plume centerline was approximately 5% to 10%. The application of capillary hysteresis for the numerical simulation of the second centrifuge test has eliminated this error. In addition, accounting for hysteresis in the numerical simulation of the second centrifuge test has considerably improved the predicted results with respect to the size of the lens-shaped plume within the capillary fringe. However, there were inconsistencies between the numerical and experimental results, especially with respect to the time for the LNAPL to reach the groundwater level. This was found to be due to overestimation of the simulated downward infiltration of the LNAPL in the capillary fringe. It was found that the pore-connectivity parameter with a value of 0.66 for the relative permeability–saturation model produced the best comparison with experimental results.

Effect of Soil Fabric on Transport of a LNAPL through Unsaturated Fine-Grained Soils: A Centrifugal Model Study

Centrifugal model tests were performed to study the impact of the fabric of a fine-grained soil on transport of a light non-aqueous phase liquid (LNAPL). An image processing technique was developed to extract contaminant transport and fate data from the centrifugal model. Two unconsolidated sites with different moisture contents and a saturated site consolidated due to self-weight were simulated using the centrifuge. The LNAPL migrated in the vertical direction as a narrow plume and formed a free product pool above the saturated zone in unsaturated and unconsolidated soils. However, the LNAPL migrated in the horizontal direction before moving in the vertical direction as a broad plume in the consolidated site. The test results showed that the final width of the plume in the unsaturated zone of the consolidated site was nearly two times as large as that for the unconsolidated sites. In addition, the rate of leak from the underground storage tanks (USTs) on consolidated soils was substantially higher when compared with those on the unconsolidated state. The comparison of LNAPL saturation profiles at the centerline of the centrifugal models during leakage showed that, depending on the soil fabric at a given time and depth, the LNAPL phase would be different; i.e., mobile or immobile (residual) in the same soil type. The test results provided additional insight into contribution of soil fabric on transport and fate of contaminants. The soil fabric controls the geological and hydro-geological properties of fine-grained soils and hence the contamination plume.

A Simplified Image Analysis Method to Study LNAPL Migration in Porous Media

A novel Simplified Image Analysis Method was developed and tested to assess the saturation distribution values for water and LNAPLs (Light Non-Aqueous Phase Liquids) in granular soils subjected to fluctuating groundwater conditions. This method, based on the Beer-Lambert Law of transmissivity, determines the saturation of water (Sw) and LNAPLs (So) by comparing the average optical densities (Di) for each matrix element of the tested domain to the corresponding average optical densities for three base pictures of the same domain taken with two digital cameras attached to two different band-pass filters (λ=450 nm and 640 nm). Two equations with two unknowns (Sw and So) are defined for each mesh element, which enables the saturation distribution to be calculated under dynamic conditions. The three base conditions for the domain are: (i) fully saturated with water (Di10), (ii) fully saturated with LNAPL (Di01), and (iii) completely dry (Di00). The Simplified Image Analysis Method was then applied to analyze the behavior of two fluctuating groundwater systems, namely, two-phase air-water and three-phase air-water-LNAPL, in a one-dimensional column, 3.5×3.5×50 cm, filled with Toyoura sand. The mass balance of the drainage-imbibition three-phase air-water-LNAPL system showed a difference of just 4.7% in LNAPL, demonstrating that this non-intrusive and non-destructive method is reliable for providing water and LNAPL saturation distributions throughout the domain when studying the effects of porous soil contamination by LNAPLs subjected to dynamic conditions.

Implementation of an image analysis technique to determine LNAPL infiltration and distribution in unsaturated porous media

Light non-aqueous phase liquids (LNAPLs) represent a great category of soil contaminants that can become a persistent, secondary and long-term source of contamination. In order to successfully confront with LNAPL pollution, their infiltration and distribution in the subsurface must be studied. The purpose of this article is to present the investigation results for the flow and distribution of two typical LNAPL (Soltrol 220 and Diesel Fuel) in two different types of unsaturated porous media (sand). Four LNAPL transient spills were simulated in an experimental sand matrix, using two different sand configurations. Photographs were frequently taken under stable conditions and the movement of the liquid was defined by an image analysis technique, producing saturation profiles. As it was proved, LNAPL move slower in the fine sand than in the coarse sand, due to its small coefficient of permeability. LNAPL saturation was estimated to be 80–100% mainly in the interface of the two sand layers, where vertical movement is limited and horizontal dominates, indicating that the interface of fine and coarse layer is able to act as a capillary barrier, which prevents LNAPL infiltration in the coarse layer.