In many situations, fluids other than groundwater such as non‑aqueous phase liquids (NAPLs) are present in the subsurface, many of which are hazardous industrial chemicals. NAPLs include liquids lighter than water (LNAPLs)—for example, gasoline and diesel fuel—and liquids denser than water (DNAPLs)—including chlorinated solvents and coal tar. This book is an introduction to the fundamentals of NAPL flow in porous media, and presents properties and concepts that dictate how NAPLs are distributed in porous media, including the importance of interfacial tension, wettability, and capillary pressure. Further, the authors describe the relationships among capillary pressure, relative permeability, and fluid saturation, and include more detailed discussions of LNAPL and DNAPL distribution. This book provides the reader with a foundation for understanding conceptual and numerical models of groundwater contamination as well as strategies for the investigation and remediation of NAPL‑affected sites.

The 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.

We investigate the movement of LNAPL (light non-aqueous phase liquid) into and out of monitoring wells in an immediate-scale experimental cell. Aquifer material grain size and LNAPL viscosity are two factors that are varied in three experiments involving lowering and rising water levels. There are six monitoring wells at varying distances from a LNAPL injection point and a water pumping well. We established steady water flow through the aquifer materials prior to LNAPL injection. Water pumping lowered the water levels in the aquifer materials. Terminating water pumping raised the water levels in the aquifer materials. Our focus was to record the LNAPL thickness in the monitoring wells under transient conditions. Throughout the experiments, we measured the elevations of the air-LNAPL and LNAPL-water interfaces in the monitoring wells to obtain the LNAPL thicknesses in the wells. We analyze the results and give plausible explanations. The data presented can be employed to test multiphase flow numerical models.

Leakage of hydrocarbon fuel (light nonaqueous-phase liquid, LNAPL) from petroleum processing facilities and storage tanks may result in significant subsurface contamination. Remediating the contaminated areas represent considerable challenges, especially when remediation resources are limited and site data are incomplete. A reasonable management strategy under this scenario may be to identify sites where LNAPL recovery operations should be located that would provide the largest LNAPL recovery initially while minimizing the LNAPL remaining in the subsurface (entrapped and residual LNAPL), which may serve as future sources for groundwater contamination. To accomplish this objective, we use estimates of subsurface recoverable and total LNAPL specific volumes and LNAPL transmissivities to generate GIS maps that can be combined to highlight locations where to develop LNAPL recovery operations. When the approach is applied to a LNAPL-contaminated area in Iran, we were able to narrow the locations for potential LNAPL recovery operations. Specifically, we combine maps of the LNAPL specific yield, an introduced term, and the LNAPL transmissivity where the LNAPL specific yield is the ratio of the recoverable to total LNAPL specific volumes. The LNAPL specific yield is a relative measure of the amount of LNAPL that potentially can be recovered while minimizing residual LNAPL in soils. The approach can be applied to sites where the recoverable and total LNAPL specific volumes and LNAPL transmissivities can be estimated using data from boreholes in the contaminated area.

In this paper we present modifications to previously published models for determining the specific volume of non-aqueous phase liquids (LNAPLs) in the subsurface at and near the groundwater table following a spill or leak from the soil surface. The modifications account for porous media having multimodal pore-size distributions as is often the case with tropical soils. Data from the literature are used to show that the use of multimodal pore-size distributions can lead to significantly different subsurface LNAPL specific volume predictions and possible LNAPL recovery rates, compared to when only unimodal pore-size distributions are considered. Differences of up to 200% are possible when the dual-porosity nature of the pore system is ignored, which can yield erroneous estimates of the time needed to remediate LNAPLs from contaminated areas when conventional systems are employed.

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.

This chapter discusses techniques for characterizing properties that are needed to predict multiphase fluid flow involving nonaqueous phase liquids, which is a difficult process. It also discusses fluid content measurement techniques for multiphase fluid systems. Capillary pressure–saturation relations, defined at a physical point, are important for the prediction of multiphase fluid flow. The chapter presents a discussion concerning the measurement of relations between fluid contents and pressures in multiphase fluid systems. A brief discussion of the measurement of relative permeability relations is provided. The chapter describes modeling approaches that have been used to predict relations among fluid contents, pressures, and relative permeabilities, including effects of hysteresis. Finally, also presents a discussion of the latest developments in measuring the interfacial areas of immiscible fluids, as they are important for understanding of multiphase fluid flow and retention and the transfer of mass, heat, and momentum between the fluids.

A recent analytical model predicts free, entrapped, and residual LNAPL saturations and the LNAPL transmissivity in the subsurface from current and historic fluid levels in groundwater wells. As such, the model accounts for effects of fluid level fluctuations in a well. The model was developed to predict LNAPL specific volumes and transmissivities from current fluid level measurements in wells and either recorded historic fluid level fluctuations in wells or estimates. An assumption is made in the model that the predictions are not dependent on whether the historic highest or lowest fluid level elevations in a well occur first. To test the assumption, we conduct two simulations with a modified multiphase flow numerical code TMVOC that incorporates relative permeability-saturation-capillary head relations employed in the model. In one simulation, the initial condition is for fluid levels in a well at the historic highest elevations. In the other simulation, the initial condition is for fluid levels in a well at the historic lowest elevations. We change the boundary conditions so both historical conditions occur followed by generating the current condition. Results from the numerical simulations are compared to model predictions and show the assumption in the analytical model is reasonable. The analytical model can be used to develop/refine conceptual site models and for assessing potential LNAPL recovery endpoints, especially on sites with fluctuating fluid levels in wells.

A model is presented to account for elevation-dependent residual and entrapped LNAPL above and below, respectively, the water-saturated zone when predicting subsurface LNAPL specific volume (fluid volume per unit area) and transmissivity from current and historic fluid levels in wells. Physically-based free, residual, and entrapped LNAPL saturation distributions and LNAPL relative permeabilities are integrated over a vertical slice of the subsurface to yield the LNAPL specific volumes and transmissivity. The model accounts for effects of fluctuating water tables. Hypothetical predictions are given for different porous media (loamy sand and clay loam), fluid levels in wells, and historic water-table fluctuations. It is shown the elevation range from the LNAPL-water interface in a well to the upper elevation where the free LNAPL saturation approaches zero is the same for a given LNAPL thickness in a well regardless of porous media type. Further, the LNAPL transmissivity is largely dependent on current fluid levels in wells and not historic levels. Results from the model can aid developing successful LNAPL remediation strategies and improving the design and operation of remedial activities. Results of the model also can aid in accessing the LNAPL recovery technology endpoint, based on the predicted transmissivity.

A meter‐scale experimental system (2‐m high by 2‐m deep by 3‐m wide) was used to investigate the behavior of water in a model system consisting of two unconsolidated sediment layers separated by a layer containing discrete flow channels. Stainless steel tubes were inserted vertically through a clayey matrix to represent the discrete flow‐channel layer. The experimental system was well characterized, and results from water infiltration experiments were analyzed. A time series of water arrival at a network of 86 probes located in the unconsolidated sediment layers is presented, as well as water pressure histories at specific locations. Some probes were located at opposite ends of the flow channels to assess water migration through the discrete flow‐channel layer. Analyses of the experimental results focused on capillary break phenomenon at the interface between the overlying unconsolidated layer and the underlying discrete flow channels. Dissimilar water pressure histories were measured at probes near the upper boundary of the discrete flow‐channel layer, suggesting varied and complex water flow behavior. At some locations, a steady or periodic “leaking” of water through the discrete flow channels appeared to occur, contrary to capillary break theory. The authors advocate larger‐scale experiments to advance our understanding and ability to model fluid flow across a wide range of spatial and temporal scales.

A major shortcoming of multifluid flow simulators is the inability to predict the retention of nonaqueous phase liquid (NAPL) in the vadose zone after long drainage periods. Recently, three theoretical models—the Wipfler and van der Zee (WVDZ) model; the Van Geel and Roy (VGR) model; and the Lenhard, Oostrom, and Dane (LOD) model—have been proposed for describing residual NAPL formation. The WVDZ model assumes a critical total liquid saturation below which all NAPL becomes residual. The VGR and LOD models are extensions of an existing hysteretic relative permeability–saturation–capillary pressure model and assume formation of residual NAPL during NAPL drainage and imbibition, respectively. In this study, we compared model predictions against results of a series of static pressure cell experiments. We found no experimental evidence supporting the WVDZ concept of a critical total liquid saturation. The other two models yielded reasonable predictions. The VGR and LOD models were then incorporated into a multifluid flow simulator, and simulations of two transient column experiments were conducted. Both models performed considerably better than simulations without considering the formation of residual NAPL, underwriting the importance of incorporating this process in simulators. Although the VGR and LOD models are based on different conceptual models, no clear performance differences could be observed when simulation results were compared against the transient experimental data.

Mesoscale tank experiments were performed to simulate bioremediation of saturated zone carbon tetrachloride (CCl4) originating from a vadose zone carbon tetrachloride source. The mesoscale tank is 2-m wide by 2-m high by 3-m long and was constructed of stainless steel, yielding a total volume of 12 m3. Simulated geology within the tank consisted of two unconsolidated sand layers separated by a clay layer containing variable-sized stainless steel tubes that represented fractures within a consolidated porous medium. The thickness of the upper sand layer was approximately 55 cm, the thickness of the virtual fracture layer was 25 cm, and the thickness of the lower sand layer was approximately 98 cm. The water table was located at an elevation of approximately 54 cm from the bottom of the tank. CCl4 was added to the sealed tank by pouring 500 ml of neat CCl4 into a beaker buried approximately 10 cm below the upper sand surface through a stainless steel tube. The CCl4 was then allowed to partition through the reactor over time, eventually coming to equilibrium. Once CCl4 equilibrium had occurred in the saturated zone (~500 ppb); the reactor was bioaugmented with a CCl4 degrading culture enriched from the Subsurface Disposal Area at the INEEL. The culture was grown to a cell density of ~ 1.0 x 108 cells/ml and injected into the simulated aquifer through a center sampling port. Following injection of the culture, an initial aliquot of lactate (1,000 g/L), nitrogen, and phosphorus were added to the reactor. Lactate was injected every 3 – 5 days for one month. After 1 month of operation, a continuous supply of lactate (1,000 g/L) was pumped into the reactor at an average rate of 50 mL/min. CCl4 concentrations in the unsaturated zone were measured using hollow fiber membrane samplers, while liquid samples were analyzed to monitor levels in the simulated aquifer zone. Samples were also taken for analysis of volatile organic acids and cell density. As would be expected, increases in cell density over the length of the cell correlated with the flow of the water through the cell. One week following injection microbes and lactate, cell numbers were in the range of 5.0 x 106 cells/mL, by the end of the experiment cell numbers had increased to approximately 1.94 x 107 cells/mL. Five days after lactate injection was initiated, chloroform appeared in liquid samples taken for chlorinated VOC analysis. CCl4 concentrations in the liquid phase dropped to approximately 180 ppb. At the conclusion of the batch lactate injection phase of the bioaugmentation, CCl4 levels averaged 40 ppb and chloroform levels averaged 44 ppb. Interestingly, once continuous lactate addition was initiated, CCl4 concentrations in the saturated zone increased with spikes as high as 3,000 ppb. Chloroform concentrations also increased following continuous addition of lactate. Since the CCl4 source in the breaker had been depleted, vadose zone concentrations of CCl4 dropped steadily during addition of lactate to the saturated zone. CCl4 levels of ~ 800 ppmv were noted at the beginning of the experiment, levels decreased to below 200 ppmv by the end of the bioaugmentation phase. No chloroform was noted in the vadose zone during testing.

Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueous-gas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.

In a remote site in eastern Idaho, now known as the Idaho National Engineering and Environmental Laboratory (INEEL) Site, the U.S. Government established a facility to test‐fire naval gunnery during World War II. The mission after the war switched to development and demonstration of nuclear technology. For more than 50 yr, the site has been devoted to nuclear energy research. Because of the remote location of the site west of Idaho Falls, ID, wastes containing radioactive and hazardous materials were disposed to the subsurface. It was felt that any disposed materials would not travel downward through the vadose zone, which is 100 to 300 m thick, to the underlying Snake River Aquifer. However, some materials have traveled through the thick vadose zone and contaminated the aquifer. Other wastes were injected directly into the aquifer. To provide a general background for papers in this special issue of Vadose Zone Journal on research at the INEEL, we give a brief historical perspective of work conducted at the INEEL Site west of Idaho Falls and associated subsurface contamination issues. We furthermore give an overview of the research papers presented in this special issue.

where Sn(j) is the actual NAPL saturation and Slr is the residual aqueous phase saturation. The NAPL saturation is the sum of mobile and immobile NAPL (Equation 1a). Immobile NAPL is the sum of residual and entrapped NAPL (Equation 1b). Free NAPL, as opposed to entrapped NAPL, comprises mobile and residual NAPL, and occurs within pore space radii larger than the ones occupied by the apparent aqueous saturation, which is defined as the sum of the effective aqueous and entrapped NAPL saturations (Equation 1c). Residual Sn(j) 5 Sn(j) 1 2 Slr Sn 5 Snf 1 Sne Sn 5 Snm 1 Snr 1 Sne Abstract Flow of nonvolatile nonaqueous phase liquid (NAPL) and aqueous phases that account for mobile, entrapped, and residual NAPL in variably saturated water-wet porous media is modeled and compared against results from detailed laboratory experiments. Residual saturation formation in the vadose zone is a process that is often ignored in multifluid flow simulators, which might cause an overestimation of the volume of NAPL that reaches the ground water. Mobile NAPL is defined as being continuous in the pore space and flows under a pressure gradient or gravitational body force. Entrapped NAPL is defined as being occluded by the aqueous phase, occurring as immobile ganglia surrounded by aqueous phase in the pore space and formed when NAPL is replaced by the aqueous phase. Residual NAPL is defined as immobile, nonwater entrapped NAPL that does not drain from the pore spaces and is conceptualized as being either continuous or discontinuous. Free NAPL comprises mobile and residual NAPL. The numerical model is formulated on mass conservation equations for oil and water, transported via NAPL and aqueous phases through variably saturated porous media. To account for phase transitions, a primary variable switching scheme is implemented for the oil-mass conservation equation over three phase conditions: (1) aqueous or aqueousgas with dissolved oil, (2) aqueous or aqueous-gas with entrapped NAPL, and (3) aqueous or aqueous gas with free NAPL. Two laboratory-scale column experiments are modeled to verify the numerical model. Comparisons between the numerical simulations and experiments demonstrate the necessity to include the residual NAPL formation process in multifluid flow simulators.

A hysteretic constitutive model describing relations among relative permeabilities, saturations, and pressures in fluid systems consisting of air, nonaqueous-phase liquid (NAPL), and water is modified to account for NAPL that is postulated to be immobile in small pores and pore wedges and as films or lenses on water surfaces. A direct outcome of the model is prediction of the NAPL saturation that remains in the vadose zone after long drainage periods (residual NAPL). Using the modified model, water and NAPL (free, entrapped by water, and residual) saturations can be predicted from the capillary pressures and the water and total-liquid saturation-path histories. Relations between relative permeabilities and saturations are modified to account for the residual NAPL by adjusting the limits of integration in the integral expression used for predicting the NAPL relative permeability. When all of the NAPL is either residual or entrapped (i.e., no free NAPL), then the NAPL relative permeability will be zero. We model residual NAPL using concepts similar to those used to model residual water. As an initial test of the constitutive model, we compare predictions to published measurements of residual NAPL. Furthermore, we present results using the modified constitutive theory for a scenario involving NAPL imbibition and drainage.

In a remote site in eastern Idaho, now known as the Idaho National Engineering and Environmental Laboratory (INEEL) Site, the U.S. Government established a facility to test-fire naval gunnery during World War II. The mission after the war switched to development and demonstration of nuclear technology. For more than 50 yr, the site has been devoted to nuclear energy research. Because of the remote location of the site west of Idaho Falls, ID, wastes containing radioactive and hazardous materials were disposed to the subsurface. It was felt that any disposed materials would not travel downward through the vadose zone, which is 100 to 300 m thick, to the underlying Snake River Aquifer. However, some materials have traveled through the thick vadose zone and contaminated the aquifer. Other wastes were injected directly into the aquifer. To provide a general background for papers in this special issue of Vadose Zone Journal on research at the INEEL, we give a brief historical perspective of work conducted at the INEEL Site west of Idaho Falls and associated subsurface contamination issues. We furthermore give an overview of the research papers presented in this special issue.

The formation of residual, discontinuous nonaqueous phase liquids (NAPLs) in the vadose zone is a process that is not well understood. To obtain data that can be used to study the development of a residual NAPL saturation in the vadose zone and to test current corresponding models, detailed transient experiments were conducted in intermediate-scale columns and flow cell. The column experiments were conducted to determine residual carbon tetrachloride (CCl(4)) saturations of two sands and to evaluate the effect of CCl(4) vapors on the water distribution. In the intermediate-scale flow cell experiment, a rectangular zone of the fine-grained sand was packed in an otherwise medium-grained matrix. A limited amount of CCl(4) was injected from a small source and allowed to redistribute until a pseudo steady state situation had developed. A dual-energy gamma radiation system was used to determine fluid saturations at numerous locations. The experiments clearly demonstrated the formation of residual CCl(4) saturations in both sands. Simulations with an established multifluid flow simulator show the shortcomings of current relative permeability-saturation-capillary pressure (k-S-P) models. The results indicate that nonspreading behavior of NAPLs should be implemented in simulators to account for the formation of residual saturations.

At many contaminated sites, nonaqueous phase liquids (NAPLs) persist in the vadose zone for long periods of time. This occurs because the permeability of the NAPL becomes negligible at some saturation and downward movement ceases, resulting in residual NAPL. To obtain data that can be used to study the development of a residual NAPL saturation and to test corresponding models, a detailed transient experiment was conducted in a 170 cm long by 90 cm high by 5.5 cm wide flow cell. Fluid saturation measurements were obtained with a dual‐energy γ radiation system. The experimental conditions reflected those at the Hanford Site in Washington State, where an estimated 363 to 580 m3 of carbon tetrachloride (CCl4) was disposed to the subsurface. A key subsurface feature at the Hanford Site is a sloped Plio‐Pleistocene caliche layer, which was reproduced in the experiment as a sloped lens in a medium‐grained, uniform, sand matrix. The caliche contains considerable amounts of CaCO3 and may have fluid wettability properties other than strongly water wet. A total of 800 mL of CCl4 was injected into the experimental domain at a rate of 0.5 mL min−1 from a small source area located at the surface. After apparent static conditions were obtained with respect to CCl4 redistribution, saturation measurements indicated that all of the dense nonaqueous phase liquids (DNAPL) that had initially moved into the caliche remained in this layer. Water was subsequently applied to the surface at a constant rate over the full length of the caliche layer to study CCl4 displacement as a result of changing water saturations. Water saturation in the caliche layer rose to as high as 0.91 during water infiltration. Results show that 25% of the DNAPL present in the caliche migrated from this layer as a consequence of water infiltration, while 75% remained in the caliche layer. The experimental results could not be reproduced with numerical multifluid flow simulations based on common constitutive theory. This indicates that improvements in constitutive theory may be needed to accurately model air−DNAPL−water flow behavior.