Light Non-aqueous Phase Liquids Distribution Research Articles

Imaging LNAPL distribution at a former chemical plant with time-domain induced polarization

Light non-aqueous phase liquids (LNAPLs) that infiltrate into the subsurface are commonly described in two distinct zones: the source zone and the plume zone. A precise differentiation between these zones is essential for constraining further migration and selecting an effective remediation method. In this study, we employ the induced polarization (IP) method to characterize the contaminants. Six time domain IP survey lines were conducted at a former chemical plant contaminated with LNAPLs. Even though the contaminated areas corresponding to BTEX concentration above 180 mg/kg are less than 5 mS/m, the source and plume zones cannot be distinguished by conductivity alone. However, a noticeable difference in phase (φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varphi$$\\end{document}) between the two zones is observed, and the threshold phase value corresponding to a critical concentration of 450 mg/kg is 20 mrad. Moreover, the normalized chargeability (Mn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_{n}$$\\end{document}) threshold for the source zone is 80 mS/m, and the corresponding Mn\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_{n}$$\\end{document} differences between the source and plume zones are more significant than those in φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varphi$$\\end{document}. These results illustrate that changes in polarization characteristics associated with BTEX concentrations can aid in further distinguishing between the source and plume zones. Ultimately, it is concluded that IP imaging is a well-suited method for LNAPL investigations that permits an improved characterization of different contaminated zones, which can facilitate the optimization of drillings for further site assessment and remediation.

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.

The impact of water table fluctuation and salinity on LNAPL distribution and geochemical properties in the smear zone under completely anaerobic conditions

Climate and groundwater are always in a state of dynamic equilibrium. Subsurface systems contaminated by light non-aqueous phase liquid (LNAPL) present a challenge to understand the overall impact of water table dynamics, due to various interacting mechanisms, including volatilization, and LNAPL mobilization/dissolution along the groundwater flow direction and oscillating redox conditions. We investigated the impact of water table fluctuations on LNAPL natural attenuation and soil geochemical characteristics in semi-arid coastal areas under saline conditions. Four soil columns operated for 151 days under anoxic conditions where a layer of benzene and toluene were subjected to a stable and fluctuating water table associated with low and high salinity conditions. The bottom of stable and fluctuating columns reached an anaerobic state after 40 days, while the middle of stable column took 60 days. pH values of the fluctuating columns covered a wide range, and at the end shifted towards alkaline conditions, unlike the stable columns. In fluctuating columns, pore water sulfate decreased in the middle, but in stable columns, it decreased in the first 40 days, which suggested that sulfate was the primary electron donor and sulfate-reducing bacteria were present. At the source zone, benzene and toluene reached their maximum concentration after 30 and 10 days for the stable and the fluctuating columns, respectively. Significant decrease in benzene and toluene concentrations occurred under the fluctuating water table. Salinity did not affect benzene and toluene concentrations in the aqueous phase, although water table fluctuations have the most effect. Soil solid-phase analysis shows fluctuating columns have less toluene than stable columns. Solid-phase analysis showed the fluctuating columns have less benzene and toluene concentrations as compared to the stable columns.

Spatial-temporal trends and correlations from a large natural source zone depletion (NSZD) research project at a site with LNAPL

Results are reported from the third phase of an extensive Natural Source Zone Depletion (NSZD) research project with NSZD measurements at 12 locations with up to four years of monitoring using two measurement technologies: Carbon Traps and the temperature based, Thermal NSZD technology. Additionally, location-specific site characterization data was compiled to evaluate factors impacting NSZD rates. The two methods both resulted in site wide average NSZD rates in the low hundreds of gallons of light non-aqueous phase liquid (LNAPL) biodegraded per acre per year (gal/acre/year): 280 gal/acre/year for Thermal NSZD measurements and 360 gal/acre/year for Carbon Trap measurements. Based on the temporal and spatial variability in the NSZD data, these values were considered to be relatively consistent. In addition, these data support the case that NSZD is actively removing LNAPL at this site. Annual temporal rates generally varied by a factor of about two during the two- to four-year monitoring period, and the spatial distribution of NSZD rates across the site for the two methods ranged from 80 to 890 gal/acre/year. While the site-wide NSZD rates were relatively consistent, the location-specific Carbon Trap and Thermal NSZD data only had a weak correlation with a linear regression r2 of 0.4. The correlation between the spatial distribution of the NSZD rates and nine specific lithological, concentration, LNAPL distribution, and LNAPL composition factors showed strong correlations with only two of the factors: (1) a positive correlation to the fraction of LNAPL in the saturated zone as indicated by a rapid optical screening tool (ROST); and (2) a negative correlation to locations with significant clay in the geologic boring log. Overall, these results confirmed the observations from other NSZD field studies that showed significant temporal, spatial, and methodological variation in the NSZD rates at LNAPL sites. More importantly, these results support the conclusion that NSZD is active even for submerged LNAPL.

Predicting Vertical LNAPL Distribution in the Subsurface under the Fluctuating Water Table Effect

AbstractThe present study proposes a methodology for predicting the vertical light nonaqueous‐phase liquids (LNAPLs) distribution within an aquifer by considering the influence of water table fluctuations. The LNAPL distribution is predicted by combining (1) information on air/LNAPL and LNAPL/water interface elevations with (2) the initial elevation of the water table without LNAPL effect. Data used in the present study were collected during groundwater monitoring undertaken over a period of 4 months at a LNAPL‐impacted observation well. In this study, the water table fluctuations raised the free LNAPL in the subsurface to an elevation of 206.63 m, while the lowest elevation was 205.70 m, forming a thickness of 0.93 m of LNAPL‐impacted soil. Results show that the apparent LNAPL thickness in the observation well is found to be three times greater than the actual free LNAPL thickness in soil; a finding that agrees with previous studies reporting that apparent LNAPL thickness in observation wells typically exceeds the free LNAPL thickness within soil by a factor estimated to range between 2 and 10. The present study provides insights concerning the transient variation of LNAPL distribution within the subsurface and highlights the capability of the proposed methodology to mathematically predict the actual LNAPL thickness in the subsurface, without the need to conduct laborious field tests. Practitioners can use the proposed methodology to determine by how much the water table should be lowered, through pumping, to isolate the LNAPL‐impacted soil within the unsaturated zone, which can then be subjected to in situ vadose zone remedial treatment.

Remobilization of LNAPL in unsaturated porous media subject to freeze-thaw cycles using modified light transmission visualization technique

The frequent use of light non-aqueous phase liquids (LNAPLs) in cold regions creates serious risks of soil and groundwater contamination. However, the subsurface remobilization behaviors of LNAPL in freezing and thawing unsaturated porous media are not well understood. The light transmission visualization (LTV) technique, which be previously used in two-phase system (water–air or water-NAPL), is modified to quantify LNAPL in three-(water–air-LNAPL) or four-phase (water–ice-air-LNAPL) systems during freezing and thawing. The pattern of LNAPL remobilization was observed in a two-dimensional flow chamber filled with unsaturated porous media subject to freeze–thaw cycles (FTCs) using the modified LTV technique. It is shown that LNAPL remobilization mainly occurs in the direction of the freezing front and horizontal with the number of FTCs. Meanwhile, LNAPL in the area below the freezing front is promoted to remigrate downward during freezing and the opposite direction during thawing. The analysis suggests that LNAPL remobilization is caused by a combination of freezing-induced pressure and changes in capillary pressure controlled by water migration, rather than through microcracks in the frozen porous media. The proposed modified LTV technique shows great promise for investigating LNAPL transport and distribution in freezing (unfreezing) unsaturated porous media that is non-destructive. The study offers a scientific basis for the prediction of the subsurface behavior of LNAPL at contaminated sites in widely distributed cold regions.

Delineation of LNAPL contaminant plumes at a former perfumery plant using electrical resistivity tomography

Light non-aqueous phase liquids (LNAPLs) are commonly used in industrial processes, and they are well known for their potential to contaminate groundwater and their toxic effects on ecosystems. The adequate delineation of contaminant plume distribution is critical for the effective remediation of contaminated sites and aquifers. Electrical resistivity tomography (ERT) surveys on LNAPL contaminated soils have shown great potential in this regard. In this study in China, six ERT profiles were conducted at a former perfumery plant with a benzene and ethylbenzene spill history to evaluate whether ERT could be used to delineate the distribution of the LNAPL plume beneath the plant. Based on the survey results, the electrical resistivity was consistent with borehole sampling results, where high resistivity corresponded to increased LNAPL concentration. A linear relationship was built between resistivity and contaminant concentration, with a threshold value of 18 Ω·m used to identify contaminated areas. It was possible to construct a detailed three-dimensional characterization of the LNAPL distribution. In addition, four local sites were excavated to verify the results of the ERT profiles. The contamination sources were further categorized into four types that were considered useful for the selection of remediation strategies. In conclusion, ERT was an effective non-invasive technique for delineating LNAPL plume distribution at a high resolution.

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.

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.

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.

An analytical model for predicting LNAPL distribution and recovery from multi-layered soils

An analytical model was developed for estimating the distribution and recovery of light nonaqueous phase liquids (LNAPL) in heterogeneous aquifers. Various scenarios of LNAPL recovery may be simulated using LDRM for LNAPL recovery systems such as skimmer wells, water-enhanced wells, air-enhanced wells, and trenches from heterogeneous aquifers. LDRM uses multiple horizontal soil layers to model a heterogeneous aquifer. Up to three soil layers may be configured with unique soil properties for each layer. Simulation results suggest that LNAPL distribution and its recovery volume are highly affected by soil properties. In sandy soils LNAPL can be highly mobile and the recovery efficiency can be high. In contrast, even at high LNAPL saturations, LNAPL mobility is typically low in fine-grained soils. This characteristic of LNAPL with respect to soil texture has to be carefully accounted for in the model to better predict the recovery of LNAPL from heterogeneous soils. The impact of vertical hydraulic gradient in fine grain zone was assessed. A sensitivity analysis suggests that the formation LNAPL volume can be significantly affected by a downward vertical hydraulic gradient if the magnitude is near a critical amount (=ρr-1). Sensitivity of input parameters with respect to LNAPL formation in soils and LNAPL recovery volume were identified through a sensitivity analysis. The performance of LDRM on predicting the distribution and recovery of LNAP was reasonably accurate for a short-term analysis as demonstrated in a case study. However, further validation is needed to ascertain the model's performance in long-term simulations.

Identification and Assessment of Confined and Perched LNAPL Conditions

Although confined and perched light nonaqueous phase liquids (LNAPLs) have previously been recognized, the majority of technical LNAPL literature focuses on unconfined LNAPL. Little information exists regarding the appropriate use of LNAPL distribution and transmissivity data to distinguish between confined, perched, and unconfined LNAPL hydrogeological scenarios. This paper describes three case histories that illustrate how the observed behavior of LNAPL can be used to identify the hydrogeologic condition of LNAPL at a given site and improved methods for calculating LNAPL drawdown based on these hydrogeologic conditions. The assessment methodology uses routinely available data such as fluid gauging, boring lo, laser-induced fluorescence, visual observations of soil cores, and LNAPL baildown testing. Identification of the correct LNAPL hydrogeologic condition results in more accurate LNAPL conceptual site models, improved estimates of LNAPL recovery rates and volumes, more appropriate technology applications, and improved accuracy of LNAPL remediation metrics such as LNAPL transmissivity.

Free-product plume distribution and recovery modeling prediction in a diesel-contaminated volcanic aquifer

Light non-aqueous phase liquids (LNAPL) represent one of the most serious problems in aquifers contaminated with petroleum hydrocarbons liquids. To design an appropriate remediation strategy it is essential to understand the behavior of the plume. The aim of this paper is threefold: (1) to characterize the fluid distribution of an LNAPL plume detected in a volcanic low-conductivity aquifer (∼0.4m/day from slug tests interpretation), (2) to simulate the recovery processes of the free-product contamination and (3) to evaluate the primary recovery efficiency of the following alternatives: skimming, dual-phase extraction, Bioslurping and multi-phase extraction wells. The API/Charbeneau analytical model was used to investigate the recovery feasibility based on the geological properties and hydrogeological conditions with a multi-phase (water, air, LNAPL) transport approach in the vadose zone. The modeling performed in this research, in terms of LNAPL distribution in the subsurface, show that oil saturation is 7% in the air–oil interface, with a maximum value of 70% in the capillary fringe. Equilibrium between water and LNAPL phases is reached at a depth of 1.80m from the air–oil interface. On the other hand, the LNAPL recovery model results suggest a remarkable enhancement of the free-product recovery when simultaneous extra-phase extraction was simulated from wells, in addition to the LNAPL lens. Recovery efficiencies were 27%, 65%, 66% and 67% for skimming, dual-phase extraction, Bioslurping and multi-phase extraction, respectively. During a 3-year simulation, skimmer wells and multi-phase extraction showed the lowest and highest LNAPL recovery rates, with expected values from 207 to 163 and 2305 to 707l-LNAPL/day, respectively. At a field level we are proposing a well distribution arrangement that alternates pairs of dual-phase well-Bioslurping well. This not only improves the recovery of the free-product plume, but also pumps the dissolve plume and enhances in situ biodegradation in the vadose zone. Thus, aquifer and soil remediation can be achieved at a shorter time. Rough calculations suggest that LNAPL can be recovered at an approximate cost of $6–$10/l.

Probabilistic data integration for characterization of spatial distribution of residual LNAPL

The spatial distribution of residual light non-aqueous phase liquid (LNAPL) is an important factor in reactive solute transport modeling studies. There is great uncertainty associated with both the areal limits of LNAPL source zones and smaller scale variability within the areal limits. A statistical approach is proposed to construct a probabilistic model for the spatial distribution of residual NAPL and it is applied to a site characterized by ultra-violet-induced-cone-penetration testing (CPT–UVIF). The uncertainty in areal limits is explicitly addressed by a novel distance function (DF) approach. In modeling the small-scale variability within the areal limits, the CPT–UVIF data are used as primary source of information, while soil texture and distance to water table are treated as secondary data. Two widely used geostatistical techniques are applied for the data integration, namely sequential indicator simulation with locally varying means (SIS–LVM) and Bayesian updating (BU). A close match between the calibrated uncertainty band (UB) and the target probabilities shows the performance of the proposed DF technique in characterization of uncertainty in the areal limits. A cross-validation study also shows that the integration of the secondary data sources substantially improves the prediction of contaminated and uncontaminated locations and that the SIS–LVM algorithm gives a more accurate prediction of residual NAPL contamination. The proposed DF approach is useful in modeling the areal limits of the non-stationary continuous or categorical random variables, and in providing a prior probability map for source zone sizes to be used in Monte Carlo simulations of contaminant transport or Monte Carlo type inverse modeling studies.

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.

A pragmatic approach for estimation of source-zone emissions at LNAPL contaminated sites

When considering natural attenuation as a remediation strategy at a site contaminated by a light non-aqueous phase liquid (LNAPL), it is important to consider the emission of contaminants from the source zone. A quantification of source-zone emissions is essential both for comparison with down-gradient mass fluxes to provide an estimate of fractional mass flux reduction, as well as for estimating the source lifetime. Because the spatial distribution of LNAPL at a field site is strongly dependent on both the spill circumstances and the heterogeneity of the geologic materials, which can be problematic for in-situ determination, alternative methods for estimating source-zone emissions are needed. In this work, a three-dimensional multiphase flow and transport modelling approach is used to investigate the relationship between the lateral extent of an LNAPL body and the emission of contaminants to groundwater at a contaminated site. For simulations involving an LNAPL release in an aquifer comprised of heterogeneous porosity and permeability distributions that were generated geostatistically, it is shown that a simple linear relationship exists between the lateral extent of the LNAPL body in the capillary fringe and the emission to the aqueous phase. The parameters describing the relationship are found to be linear functions of the groundwater flow velocity and the vertical infiltration rate. This site-specific relationship provides a simple method to estimate contaminant emissions to groundwater at LNAPL contaminated sites.

LNAPL in fine‐grained soils: Conceptualization of saturation, distribution, recovery, and their modeling

AbstractA simple conceptual model is presented that leads to a quantitative description of the behavior of light non–aqueous phase liquid (LNAPL) in fine‐grained soil (FGS). The occurrence of large (15 feet) (4.6 m) LNAPL accumulations in observation wells in FGS and of LNAPL located below the water table is explained by macropore theory and capillarity of the FGS. Using soil capillary data, fluid property data, and a simple spreadsheet model, the LNAPL saturation in a soil profile and LNAPL recovery were predicted for a field study site. The predicted LNAPL distribution, saturation, and recovery matched the field observations and actual LNAPL recovery. Measured LNAPL saturations were <2%, while model‐predicted values were <3%. The model predicted recovery of ∼530 gallons (2009 L). After 1.5 years of continuous operation, a three‐phase, high‐vacuum extraction system recovered 150 gallons (568 L) of LNAPL. Application of a model that assumes homogeneity of the soil that is heterogeneous at a small scale may seem to be a misapplication; however, conceptualizing the model domain at a sufficiently large scale (3 to 6 feet; 0.9 to 1.8 m) allows for the FGS to be viewed as a homogeneous medium with small effective porosity.

Two-dimensional laboratory simulation of LNAPL infiltration and redistribution in the vadose zone

A quantitative two-dimensional laboratory experiment was conducted to investigate the immiscible flow of a light non-aqueous phase liquid (LNAPL) in the vadose zone. An image analysis technique was used to determine the two-dimensional saturation distribution of LNAPL, water and air during LNAPL infiltration and redistribution. Vertical water saturation variations were also continuously monitored with miniature resistivity probes. LNAPL and water pressures were measured using hydrophobic and hydrophilic tensiometers. This study is limited to homogeneous geological conditions, but the unique experimental methods developed will be used to examine more complex systems. The pressure measurements and the quantification of the saturation distribution of all the fluids in the entire flow domain under transient conditions provide quantitative data essential for testing the predictive capability of numerical models. The data are used to examine the adequacy of the constitutive pressure-saturation relations that are used in multiphase flow models. The results indicate that refinement of these commonly used hydraulic relations is needed for accurate model prediction. It is noted in particular that, in three-fluid phase systems, models should account for the existence of a residual NAPL saturation occurring after NAPL drainage. This is of notable importance because residual NAPL can act as a non negligible persistent source of contamination.

Infiltration and redistribution of LNAPL into unsaturated layered porous media

Enhanced understanding of light non-aqueous phase liquid (LNAPL) infiltration into heterogeneous porous media is important for the effective design of remediation strategies. We used a 2-D experimental facility that allows for visual observation of LNAPL contours in order to study LNAPL redistribution in a layered porous medium. The layers are situated in the unsaturated zone near the watertable and they are inclined to be able to observe the effect of discontinuities in capillary forces and relative permeabilities. Two experiments were performed. The first experiment consisted of LNAPL infiltration into a fine sand matrix with a coarse sand layer, and the second experiment consisted of a coarse sand matrix and a fine sand layer. The numerical multi-phase flow model STOMP was validated with regard to the experimental results. This model is able to adequately reproduce the experimental LNAPL contours. Numerical sensitivity analysis was also performed. The capillarity contrast between sands was found to be the main controlling factor determining the final LNAPL distribution.

Characterization of LNAPL in fractured rock

Behaviour of light non-aqueous phase liquids (LNAPLs) within a fractured rock mass is a function of the properties of the immiscible fluid, the fracture network, rock matrix properties, and the groundwater regime. LNAPL behaves differently in rock with open fractures than it does in porous media. Relatively small volumes of LNAPL within vertical or subvertical fractures can produce significant LNAPLpressure heads, resulting in LNAPL penetration into the saturated zone. Penetration can be significantly deeper than predicted by porous medium models. Groundwater surface fluctuations can cause lateral LNAPL migration, even up-gradient to natural gradients. Characterization of LNAPL-contaminated fractured rock masses must take into account these fundamental differences in behaviour. Site investigation should focus on determination of fracture network and rock matrix properties, understanding of groundwater surface fluctuation dynamics, and consideration of spatial LNAPL distribution. A combination of techniques, many not used in porous medium investigations, but can used to develop a detailed conceptual model. These include coring, angled holes, digital borehole imaging, and fracture casting for aperturedetermination. The data provide information on LNAPL occurrence and behaviour, allow LNAPL spill volume to be estimated, indicate future movement, and ultimately allow for more effective and economic remedial decision making.