Non-aqueous Phase Liquid Water Research Articles

Enhanced mobility of non aqueous phase liquid (NAPL) during drying of wet sand

Enhanced upward mobility of a non aqueous phase liquid (NAPL) present in wet sand during natural drying, and in the absence of any external pressure gradients, is reported for the first time. This mobility was significantly higher than that expected from capillary rise. Experiments were performed in a glass column with a small layer of NAPL-saturated sand trapped between two layers of water-saturated sand. Drying of the wet sand was induced by flow of air across the top surface of the wet sand. The upward movement of the NAPL, in the direction of water transport, commenced when the drying effect reached the location of the NAPL and continued as long as there was significant water evaporation in the vicinity of NAPL, indicating a clear correlation between the NAPL rise and water evaporation. The magnitude and the rate of NAPL rise was measured at different water evaporation rates, different initial locations of the NAPL, different grain size of the sand and the type of NAPL (on the basis of different NAPL-glass contact angle, viscosity and density). A positive correlation was observed between average rate of NAPL rise and the water evaporation while a negative correlation was obtained between the average NAPL rise rate and the NAPL properties of contact angle, viscosity and density. There was no significant correlation of average NAPL rise rate with variation of sand grain size between 0.1 to 0.5mm. Based on these observations and on previous studies reported in the literature, two possible mechanisms are hypothesized –a) the effect of the spreading coefficient resulting in the wetting of NAPL on the water films created and b) a moving water film due to evaporation that “drags” the NAPL upwards. The NAPL rise reported in this paper has implications in fate and transport of chemicals in NAPL contaminated porous media such as soils and exposed dredged sediment material, which are subjected to varying water saturation levels due to drying and rewetting.

Electrical Resistance Heating of Clay Layers in Water‐Saturated Sand

AbstractElectrical resistance heating (ERH) experiments were performed in a two‐dimensional water‐saturated porous medium comprising an electrically conductive, low‐permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non‐aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand–clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL–water co‐boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co‐boiling plateaus as an indicator of NAPL–water co‐boiling could be misleading during applications of ERH at sites containing electrically conductive, low‐permeability clay lenses embedded within less electrically conductive, higher‐permeability sands.

Laboratory study of non-aqueous phase liquid and water co-boiling during thermal treatment

In situ thermal treatment technologies, such as electrical resistance heating and thermal conductive heating, use subsurface temperature measurements in addition to the analysis of soil and groundwater samples to monitor remediation performance. One potential indication of non-aqueous phase liquid (NAPL) removal is an increase in temperature following observations of a co-boiling plateau, during which subsurface temperatures remain constant as NAPL and water co-boil. However, observed co-boiling temperatures can be affected by the composition of the NAPL and the proximity of the NAPL to the temperature measurement location. Results of laboratory heating experiments using single-component and multi-component NAPLs showed that local-scale temperature measurements can be mistakenly interpreted as an indication of the end of NAPL–water co-boiling, and that significant NAPL saturations (1% to 9%) remain despite observed increases in temperature. Furthermore, co-boiling of multi-component NAPL results in gradually increasing temperature, rather than a co-boiling plateau. Measurements of gas production can serve as a complementary metric for assessing NAPL removal by providing a larger-scale measurement integrated over multiple smaller-scale NAPL locations. Measurements of the composition of the NAPL condensate can provide ISTT operators with information regarding the progress of NAPL removal for multi-component sources.

Evaluation of simplified mass transfer models to simulate the impacts of source zone architecture on nonaqueous phase liquid dissolution in heterogeneous porous media

Nonaqueous phase liquid (NAPL) dissolution was studied in three-dimensional (3D) heterogeneous experimental aquifers (25.5 cm × 9 cm × 8.5 cm) with two different longitudinal correlation lengths (2.1 cm and 1.1 cm) and initial spill volumes (22.5 ml and 10.5 ml). Spatial and temporal distributions of NAPL during dissolution were measured using magnetic resonance imaging (MRI). At high NAPL spill volume, average effluent concentrations initially increased during dissolution, as NAPL pools transitioned to NAPL ganglia, and then decreased as the total NAPL–water interfacial area decreased over time. Experimental results were used to test six dissolution models: (i and ii) a one-dimensional (1D) model using either specific NAPL–water interfacial area values estimated from MR images at each time step (i.e., 1D quasi-steady state model), or an empirical mass transfer ( Sh′) correlation (i.e., 1D transient model), (iii and iv) a multiple analytical source superposition technique (MASST) using either the NAPL distribution determined from MR images at each time step (i.e., MASST steady state model), or the NAPL distribution determined from mass balance calculations (i.e., MASST transient model), (v) an equilibrium streamtube model, and (vi) a 3D grid-scale pool dissolution model (PDM) with a dispersive mass flux term. The 1D quasi-steady state model and 3D PDM captured effluent concentration values most closely, including some concentration fluctuations due to changes in the extent of flow reduction. The 1D transient, MASST steady state and transient, and streamtube models all showed a monotonic decrease in effluent concentration values over time, and the streamtube model was the most computationally efficient. Changes during dissolution of the effective NAPL–water interfacial area estimated from imaging data are similar to changes in effluent concentration values. The 1D steady state model incorporates estimates of the effective NAPL–water interfacial area directly at each time point; the 3D PDM does so indirectly through mass balance and a relative permeability function, which causes reduced water flow through high saturation NAPL regions. Hence, when model accuracy is required, the results indicate that a surrogate of this effective interfacial area is required. Approaches to include this surrogate in the MASST and streamtube models are recommended.

Saturation‐Dependent Hydraulic Conductivity Anisotropy for Multifluid Systems in Porous Media

The hydraulic conductivity of unsaturated anisotropic soils has recently been described with a tensorial connectivity–tortuosity (TCT) concept. We extend this concept to unsaturated porous media with two or three immiscible fluids. Mathematical expressions to describe the conductivity of each fluid in anisotropic porous media under unsaturated condition are derived in the form of symmetric second order tensors. The theory is applicable to the generalized hydraulic conductivity model and compatible types of saturation–pressure formulation. The extended model shows that the anisotropic coefficient of any one of the fluids is independent of the saturation of other fluids. Synthetic Miller‐similar soils generated to be anisotropic were defined by allowing the saturated hydraulic conductivity to have different correlation ranges for different directions of flow. The extended TCT concept was tested using synthetic soils with four levels of heterogeneity and four levels of anisotropy. Numerical experiments of infiltration of two liquid phases, water and the nonaqueous phase liquid (NAPL) carbon tetrachloride, were performed to test the extended model. The results show that, similar to water in a two‐fluid (air–water) system, NAPL retention curves in a three‐fluid (air–NAPL–water) system were independent of flow direction but dependent on soil heterogeneity, whereas the connectivity–tortuosity coefficients are functions of both soil heterogeneity and anisotropy. The extended TCT model accurately describes unsaturated hydraulic functions of anisotropic soils and can be combined into commonly used relative permeability functions for use in multifluid flow and transport numerical simulations.

Effects of fractional wettability on capillary pressure–saturation–relative permeability relations of two-fluid systems

Capillary pressure ( P c)–saturation ( S)–relative permeability ( k r) relationships must be quantified to accurately predict non-aqueous phase liquid (NAPL) distribution in the subsurface. Several experimental techniques are presented here for two-fluid P c– S– k r relationships for various saturation paths to better define the effect of fractional wettability on these relationships. During the primary drainage path of the P c– S curves, the air–water system showed no distinct trend as a function of the fraction of sand treated by organosilane (S) to render it non-water wetting. In a NAPL–water system, however, a consistent decrease of capillary pressure with increase of the fraction of non-water wetting sands was observed. The much lower contact angle for air–water (a–w) system may result in the observed insensitivity of the a–w P c– S curves to fractional wettability, at least for the PD pathway. For the main imbibition path of NAPL–water system, capillary pressure decreased as the fraction of the S component increased, requiring forced imbibition (negative capillary pressures) for a certain range of saturations. Systems with an increasing percentage of the S component also exhibited a higher water k r and lower NAPL or air k r at a given saturation for the primary drainage and main imbibition paths in both air–water and NAPL–water systems. The increase of water k r with increase of the fraction of the S component can be explained by the ability of water to occupy larger and highly conductive pores in such a system. Experimental k r– S data for the primary drainage path of NAPL–water system presented here were used to test the Bradford et al. [Bradford SA, Abriola LM, Leij FJ. Wettability effects on two- and three-fluid relative permeabilities. J Contam Hydrol 1997;28:171–91] model and the modified Mualem model for estimating the k r– S curves from measured P c– S data as a function of fractional wettability. Both models predicted significantly less variation in the k r– S curves than measured indicating that they did not adequately represent the system under investigation.

Partitioning of mono- and polycyclic aromatic hydrocarbons in a river sediment adjacent to a former manufactured gas plant site

The equilibrium distributions, between water and coal-tar contaminated sediment, of 16 monocyclic and polycyclic aromatic hydrocarbons were measured and evaluated for consistency with a Raoult’s Law-based quantitative relationship. The quantitative relationship calculates the pore water concentration as the product of the aqueous solubility (or for compounds that are solid at room temperature, the aqueous super-cooled liquid solubility) and the mole fraction concentration of the compound within the liquid coal tar. Sediment was collected at five locations at two depths within a 120 m stretch of a river adjacent to a former manufactured gas plant, and all samples contained non-aqueous phase liquid (NAPL) coal tar. Although the amount of coal tar varied between samples by over an order of magnitude, the Raoult’s Law-based NAPL–water partition coefficients for each monocyclic or 2- or 3-ring polycyclic aromatic hydrocarbon measured in this study generally varied within a factor of 2 over all sediments.

A constitutive model for air–NAPL–water flow in the vadose zone accounting for immobile, non-occluded (residual) NAPL in strongly water-wet porous media

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.

Helium and Neon Groundwater Tracers to Measure Residual DNAPL: Laboratory Investigation

A laboratory investigation was conducted to evaluate the applicability of dissolved He and Ne as partitioning tracers for detecting and quantifying nonaqueous phase liquid (NAPL) in the saturated zone. Based on the results of batch experiments, the equilibrium NAPL–water partition coefficients ( K N,W ) of these tracers for two common dense NAPLs (DNAPLs), tetrachloroethene (PCE) and trichloroethene (TCE), are: K PCE,W = 1.28 and K TCE,W = 2.42 for He, and K PCE,W = 1.84 and K TCE,W = 3.24 for Ne. Tracer partitioning is linear across the range of concentrations tested, and appears to be linear even near aqueous solubility limits of the gases. Multiple partitioning tracer tests (PTTs) were conducted in columns, and residual TCE saturations ( S TCE ) ranging from 4.7 to 10.5% were successfully measured by the tracers. Sensitivity analysis for the column experiments indicates that random tracer-measurement error of up to ±20% had little effect on results; however, accurate characterization of the tail region of the tracer curves is particularly important. Therefore, the low analytical detection limits possible with dissolved He and Ne (4 to 5 orders of magnitude below aqueous solubility) may permit better tracer curve characterization than commonly used alcohol partitioning tracers, and is a notable advantage for these tracers. Due to their high Henry9s Law constants, these gases will also partition into trapped air present in the tracer sweep zone. Equations are presented for estimating both trapped air and NAPL saturation for PTTs where three phases are present (water, trapped air, and residual NAPL). The results of this investigation provide a basis for field-scale application of dissolved He and Ne as groundwater partitioning tracers.

Helium and Neon Groundwater Tracers to Measure Residual DNAPL: Laboratory Investigation

A laboratory investigation was conducted to evaluate the applicability of dissolved He and Ne as partitioning tracers for detecting and quantifying nonaqueous phase liquid (NAPL) in the saturated zone. Based on the results of batch experiments, the equilibrium NAPL–water partition coefficients (KN,W) of these tracers for two common dense NAPLs (DNAPLs), tetrachloroethene (PCE) and trichloroethene (TCE), are: KPCE,W = 1.28 and KTCE,W = 2.42 for He, and KPCE,W = 1.84 and KTCE,W = 3.24 for Ne. Tracer partitioning is linear across the range of concentrations tested, and appears to be linear even near aqueous solubility limits of the gases. Multiple partitioning tracer tests (PTTs) were conducted in columns, and residual TCE saturations (STCE) ranging from 4.7 to 10.5% were successfully measured by the tracers. Sensitivity analysis for the column experiments indicates that random tracer‐measurement error of up to ±20% had little effect on results; however, accurate characterization of the tail region of the tracer curves is particularly important. Therefore, the low analytical detection limits possible with dissolved He and Ne (4 to 5 orders of magnitude below aqueous solubility) may permit better tracer curve characterization than commonly used alcohol partitioning tracers, and is a notable advantage for these tracers. Due to their high Henry's Law constants, these gases will also partition into trapped air present in the tracer sweep zone. Equations are presented for estimating both trapped air and NAPL saturation for PTTs where three phases are present (water, trapped air, and residual NAPL). The results of this investigation provide a basis for field‐scale application of dissolved He and Ne as groundwater partitioning tracers.

NAPL source zone characterization and remediation technology performance assessment: recent developments and applications of tracer techniques

Several innovative tracer techniques have been introduced during the past 5 years for enhanced characterization of the “source zones” at sites contaminated with non-aqueous phase liquid (NAPL) wastes. These tracer techniques allow for an in situ estimation of domain-averaged values and spatial patterns for NAPL saturation ( S n), NAPL–water interfacial area ( a nw), and bio-geochemical reactivity ( k s) within the target test zone. The tracer tests can be used to evaluate the spatial patterns in these parameters, both before and after implementing some in situ technique for site cleanup, in order to evaluate the effectiveness of remediation achieved and the possible impacts of the cleanup technology on hydrodynamic and bio-geochemical processes. Here, we review the theoretical and experimental basis for these tracer methods, present selected examples of recent field-scale applications, and examine their reliability.

Determination of specific NAPL–water interfacial areas of residual NAPLs in porous media using the interfacial tracers technique

We present here an application of a new experimental method called the `interfacial tracers technique' (IFTT), using tracer retardation by adsorption as the basis, to determine the specific NAPL–water interfacial areas ( a nw, cm 2/cm 3) in packed columns of sand and glass beads containing residual saturation of non-aqueous phase liquid (NAPLs). Four NAPLs, two types of sand and uniform glass beads, and two interfacial tracers were used in the tracer experiments. The a nw values determined for residual NAPLs in sand or glassbead columns were in the range of 60 to 300 (with one exception of 1200) cm 2/cm 3, and are in agreement with the current understanding of immiscible fluid behavior in porous media. For a given NAPL, the a nw value was found to be about twice as large when the porous medium was NAPL wet first, compared to when the medium was first water wet. The interfacial tracer technique will be useful in assessing NAPL morphology, dissolution and mass transfer, and is useful in the characterization and remediation assessment at NAPL-contaminated waste sites.

Factors affecting the biodegradation of phenanthrene initially dissolved in different nonaqueous‐phase liquids

AbstractA study was conducted of the importance of measured partitioning rate, the nonaqueous‐phase liquid (NAPL)–water interfacial area, and the toxicity of NAPLs to the biodegradation of constituents of NAPLs. Bacterial mineralization of phenanthrene was slower if the compound was initially dissolved in phthalate esters than in aliphatic hydrocarbons with several NAPL–water interfacial areas. The differences were not the result of toxicity of the test NAPLs. The rates of partitioning of phenanthrene from NAPLs to water were faster with larger interfacial areas, but a consistent influence of interfacial area on the rate of mineralization was not evident. The measured rates of partitioning from NAPLs to water under sterile conditions varied among the NAPLs, the mass transfer rates being slower with phthalate esters than with alkanes. The rates of mineralization of phenanthrene initially in NAPLs were correlated with measured partitioning rates, but the rates of biodegradation were sometimes faster than the partitioning rates measured under sterile conditions. Although the rates of biodegradation of a constituent of nontoxic NAPLs are generally related to rates of mass transfer determined under sterile conditions, we suggest that the partitioning rate determined in the absence of microorganisms is not an adequate predictor of the maximum rate of biodegradation of such constituents.