Dense Nonaqueous Phase Liquid Flow Research Articles

DNAPL flow and complex electrical resistivity evolution in saturated porous media: A coupled numerical simulation

Induced Polarization (IP) is a non-intrusive geophysical method to monitor Dense Non-Aqueous Phase Liquid (DNAPL) contamination and remediation processes underground. In this study, an advanced numerical code simulating DNAPL flow and complex electrical resistivity is presented. The model was validated against existing IP results and image measurements that were carried out previously in a series of 2D tank experiment. Multiphase flow modeling in porous media is coupled with electrical current modeling to simulate the process of DNAPL migration and the associated IP response. This brings a broader view of the contamination in space and time compared to surface and borehole measurements, especially when the results are supported by field measurements or laboratory experiments. The simulations are developed in 3D and are performed in COMSOL Multiphysics®. The simulations using petrophysical relationships for in-phase and quadrature resistivity and the results of the experiments are in complete accordance with each other in the parts of the tank where the saturation of DNAPL is relatively low (i.e. especially in the cone of depression in the pumping scenario). However, the parts associated with high saturation of DNAPL show high errors between the in-phase resistivity simulations and the results from experiments. The present work can be regarded as a preliminary study toward further applications of coupled IP-multiphase flow for more accurate detection and monitoring of DNAPLs. It is suggested that the choice of tool/approach in this study be extended to larger-scale studies for further investigation.

DFN-M field characterization of sandstone for a process-based site conceptual model and numerical simulations of TCE transport with degradation

Plumes of trichloroethene (TCE) with degradation products occur at a large industrial site in California where TCE as a dense non-aqueous phase liquid (DNAPL) entered the fractured sandstone bedrock at many locations beginning in the late 1940s. Groundwater flows rapidly in closely spaced fractures but plume fronts are strongly retarded relative to groundwater flow velocities owing largely to matrix diffusion in early decades and degradation processes in later decades and going forward. Multiple data types show field evidence for both biotic and abiotic dechlorination of TCE and its degradation products, resulting in non-chlorinated compounds. Analyses were conducted on groundwater samples from hundreds of monitoring wells and on thousands of rock samples from continuous core over depths ranging from 6 to 426 metres below ground surface. Nearly all of the present-day mass of TCE and degradation products resides in the water-saturated, low-permeability rock matrix blocks. Although groundwater and DNAPL flow primarily occur in the fractures, DNAPL dissolution followed by diffusion and sorption readily transfers contaminant mass into the rock matrix. The presence of non-chlorinated degradation products (ethene, ethane, acetylene) and compound specific isotope analysis (CSIA) of TCE and cis-1,2-dichloroethene (cDCE) indicate at least some complete dechlorination by both biotic and abiotic pathways, consistent with the observed mineralogy and hydrogeochemistry and with published results from crushed rock microcosms. The rock matrix contains abundant iron-bearing minerals and solid-phase organic carbon with large surface areas and long contact times, suggesting degradation processes are occurring in the rock matrix. Multiple, high-resolution datasets provide strong evidence for spatially heterogeneous distributions of TCE and degradation products with varying degrees of degradation observed only when using new methods that achieve better detection of dissolved gases (i.e., Snap Sampler™) and contaminant mass stored in the low permeability rock matrix (i.e., CORE-DFN™). Simulations using a discrete fracture-matrix (DFN-M) numerical model capable of rigorously simulating flow and transport in both the fractures and matrix, including interactions, show that even slow, first-order degradation rates (i.e., 5- to 20-year half-lives) informed by site-derived parameters can contribute strongly to natural attenuation, resulting in TCE plumes that become stationary in space and might even retreat after 50 to 100 years, if the DNAPL sources become depleted due to the combination of diffusion and degradation processes.

Infiltration and Distribution of Elemental Mercury DNAPL in Water-Saturated Porous Media: Experimental and Numerical Investigation

Liquid elemental mercury occurrence in the subsurface as dense non-aqueous phase liquid (DNAPL) is reported worldwide in proximity of several industrial facilities, such as chlor-alkali plants. Insight into Hg0 DNAPL infiltration behavior is lacking and, to date, there are no experimental observations of its infiltration and distribution in water-saturated porous media, except for capillary pressure-saturation column experiments. To better understand the processes governing elemental mercury DNAPL flow behavior, a series of flow container experiments were performed using mercury DNAPL (in sands and glass beads) and tetrachloroethylene (PCE) (in sands). While liquid Hg0 was not able to infiltrate in the sand-filled container due to an overall lower permeability of the sample and a defect of the setup, in the glass beads experiment mercury DNAPL infiltration occurred. Dual gamma ray measurements showed that, in glass beads, liquid Hg0 preferentially migrated towards higher porosity zones. As for PCE, infiltration and distribution of Hg0 DNAPL are strongly affected by the heterogeneities within the porous formation. However, compared to other DNAPLs, liquid Hg0 shows a strong attenuation potential of gamma rays. Finally, numerical simulations of the glass beads experiment showed an overall good agreement with the experimental results, highlighting that, among the factors influencing the prediction of liquid Hg0 migration in water-saturated porous media, the most critical are (i) the knowledge of the inflow rate, (ii) the reliable estimation of the porous formation permeability, and (iii) the accurate representation of the correlation between retention properties and intrinsic permeability.

Pattern of Two-Phase (DNAPL-Water) Flow through Single Fracture under Temporal Evolution of Aperture

Coupling of two-phase flow with mechanical deformation has important applications in many fields, including dense nonaqueous phase liquid (DNAPL)–water transportation through the subsurface. To understand and couple the different processes, numerical model studies are inevitable at all temporal and spatial scales. This study presents the characterization of DNAPL and water flow in a fracture under confining and fluid pressures. A comprehensive and simplified mathematical model and the conditions under which DNAPL will enter an initially water-saturated fracture under deformation are discussed. A numerical model to predict the quantity of each phase of their saturations is developed. The effect of varying confining stresses on the traverse time of DNAPL across a fractured aquitard is studied. The sensitivity analysis for physical and hydraulic properties, such as apertures, fracture inclination, and fluid and confining pressures, are performed and discussed. The temporal evolution of aperture is necessary ...

Dense Nonaqueous Phase Liquid Architecture and Dissolution in Discretely Fractured Sandstone Blocks

Laboratory experiments were performed in discretely fractured sandstone blocks to evaluate residual dense nonaqueous phase liquid (DNAPL) architecture and dissolution. Tetrachloroethene (PCE) DNAPL residual saturations (DNAPL volume/ fracture volume) ranged between 0.18 and 0.52 for the rocks studied. DNAPL-water specific interfacial areas ranged between 19 and 57 cm2/cm3. No measurable correlation was observed between DNAPL-water interfacial area and aperture, aperture ratio, or residual saturation. DNAPL-water interfacial areas were comparable to those reported in sands with grain diameters similar to the rock apertures. However, the DNAPL residual saturation in the fractures were 2-4 times greater than in the sands, suggesting that PCE dissolution rates in rock fractures may be substantially less than in unconsolidated media, as the effective interfacial area per volume of DNAPL in rock fractures was less than in sands. Comparison of dissolution mass transfer coefficients in the bedrock fractures to corresponding mass transfer coefficients measured in sands indicated that dissolution rates in bedrock fractures were substantially less than dissolution rates measured in sands, even after normalization to DNAPL-water interfacial area. The presence of preferential water and DNAPL flow paths within the discrete fractures was shown to have a significant impact on observed DNAPL dissolution rates. DNAPL dissolution was reasonably described by a Reynolds number correlation that incorporated flow characteristics and the DNAPL-water interfacial area.

Effective properties of two‐phase flow in heterogeneous aquifers

The spectral perturbation approach is employed to predict the field‐scale relative permeabilities and the variance of capillary pressure and saturation under steady two‐phase flow conditions. The theoretical analysis treats stationary three‐dimensional variations in pressure and flow produced as a result of spatial variations of permeability and two‐phase flow characteristics. The analysis is developed in a general form that is applied to different commonly used two‐phase flow characterizations, and results are illustrated for two‐phase flow characteristics corresponding to those observed at the Borden site in Canada. For gravity‐driven flow in horizontally layered aquifers, both nonwetting phase flow (dense nonaqueous phase liquid (DNAPL) flow) and wetting phase flow (water, unsaturated flow) show effective vertical relative permeabilities that are substantially decreased in comparison with those corresponding to a homogeneous system. The resulting saturation‐dependent anisotropy of effective permeability is substantially larger for DNAPL flow than for the case of unsaturated flow. The Leverett scaling characterization frequently used in Monte Carlo simulations is shown to underestimate system anisotropy owing to omission of variability of the parameter governing the slope in the capillary pressure–saturation function. Application of the Brooks‐Corey characterization of the relative permeability resulted in significantly greater anisotropy than with the van Genuchten characterization. In contrast to the strong influence of heterogeneity predicted in the case of effective permeabilities, the effective capillary pressure characteristic (mean capillary pressure versus mean saturation curve) is only weakly influenced by heterogeneity.

Three‐phase flow analysis of dense nonaqueous phase liquid infiltration in horizontally layered porous media

We considered dense nonaqueous phase liquid (DNAPL) infiltration into a water‐unsaturated porous medium that consists of two horizontal layers, of which the top layer has a lower intrinsic permeability than the bottom layer. DNAPL is the intermediate‐wetting fluid with respect to the wetting water and the nonwetting air. The layer interface forms a barrier to DNAPL flow, which causes the DNAPL to spread out horizontally just above the interface. An analytical approximation has been developed to estimate the DNAPL pressure and saturation and the horizontal extension of the DNAPL above the layer interface at steady state for low water saturations. The analytical approximation shows that the DNAPL infiltration is determined by five dimensionless numbers: the heterogeneity factor γ, the capillary pressure parameter λ, the gravity number Ng, the ratio of the capillary and gravity numbers Nc/Ng, and the critical DNAPL pressure Poc. Its predictions were compared with the results of a numerical three‐phase flow simulator for a number of parameter combinations. For most of these combinations the analytical approximation predicts the DNAPL pressure and saturation profiles at the interface adequately. Using the analytical approximation, we carried out a sensitivity study with respect to the maximum horizontal extension of the plume. The extension of the plumes appears to be highly sensitive to variation of the dimensionless numbers Poc, λ and γ.

Noninvasive monitoring of DNAPL migration through a saturated porous medium using electrical impedance tomography

Electrical impedance tomography (EIT) was used to monitor the movement of a fluorinated hydrocarbon dense nonaqueous phase liquid (DNAPL) through a saturated porous medium within a laboratory column. Impedance measurements were made using a horizontal plane of 12 electrodes positioned at regular intervals around the centre of the column. A 2D inversion algorithm, which incorporated the cylindrical geometry of the column, was used to reconstruct resistivity and phase images from the measured data. Differential time-lapse images of DNAPL movement past the plane of electrodes were generated by the cell-by-cell subtraction of resistivity and phase baseline models from those associated with the DNAPL release stage of the experiment. The DNAPL pulse was clearly delineated as resistive anomalies in the differential time-lapse resistivity images. The spatial extent of the resistive anomalies indicated that in addition to vertical migration, some lateral spreading of the DNAPL had occurred. Residual contamination could be detected after quasi-static conditions were reestablished. Residual DNAPL saturation was estimated from the resistivity model data by applying Archie's second equation. Despite significant measured phase changes due to DNAPL contamination, the differential phase images revealed only weak anomalies associated with DNAPL flow; these anomalies could be seen only in the initial stages of the experiment during peak flow through the plane of electrodes.

Trichloroethene DNAPL flow and mass distribution in naturally fractured clay: Evidence of aperture variability

A cylindrical sample, 0.5 m in diameter and length, was obtained by excavation from 3.7–4.2 m depth below ground surface in a surficial deposit of fractured glaciolacustrine clay. The sample was enclosed in a triaxial cell in a cold room to impose conditions close to field temperature and stress. Hydraulic testing using a hydraulic gradient of 2 provided a saturated hydraulic conductivity of 7 × 10−10 m/s, which is only slightly larger than the matrix hydraulic conductivity. The cubic law provided a mean hydraulic aperture of 5–6 μm for the four continuous vertical fractures in the sample. A column of immiscible‐phase trichloroethene (TCE) imposed incrementally at the top of the sample provided an entry‐pressure‐derived aperture equal to 17 μm for a parallel‐plate fracture. TCE dense nonaqueous phase liquid (DNAPL) that flowed through the sample produced dissolved‐phase diffusion haloes in the matrix that indicated the preferential fracture pathways. These haloes indicated that DNAPL flowed through only 5–15% of the visible, oxidation‐stained fractures; hence natural fractures had variable apertures along their length, and the larger aperture regions provided channels for fluid flow. Using the hydraulic test results and applying the cubic law only to the aperture segments of confirmed DNAPL flow, an equivalent hydraulic aperture of 8–11 μm was obtained, which is greater than the conventional mean hydraulic aperture and smaller than the local aperture determined from the DNAPL entry pressure. These differences are large in the context of fluid flux, which is proportional to the aperture cubed.

DNAPL Transport Through Macroporous, Clayey Till Columns

AbstractThis paper provides the first experimental determination of the rates and distribution of transport of a dense, nonaqueous phase liquid (DNAPL) through a naturally bioporous and fractured clayey till deposit. Until now, assessment of DNAPL behavior in this type of deposit has relied on theoretical studies. Predictions of DNAPL transport have proven to be uncertain as a result of difficulties in measuring critical parameters such as DNAPL entry pressure and flow behavior in response to natural fracture/biopore apertures and the degree of interconnection of these structures. In the present investigation, the migration of free product trichloroethylene (TCE) was studied by means of two undisturbed clayey till columns (dimensions: 0.5 m diameter by 0.5 m long) under in situ effective soil stress conditions. The experiments revealed that transport of TCE was restricted to biopores in one column and fractures in another column, bypassing the low‐permeability clayey matrix. Effective porosities of the columns, i.e., bio‐pores and fractures, were two to three orders of magnitude lower than total porosities (25% and 32%, respectively, for the two columns), i.e., macropores and matrix. Single phase water flow rates through the columns at water‐saturated conditions followed a linear relationship with hydraulic gradient. TCE flow could not be predicted from the single‐phase calculations because of non‐linearity observed between applied TCE injection heads and resulting TCE flow. TCE flow rates were 24 and 10.3 m/day at TCE gradients of 1.18 and 0.91, respectively. The observed flow rates indicate that in cases where vertical biopores or fractures fully penetrate clayey till aquitards, a low‐viscosity DNAPL (e.g., TCE) may quickly enter underlying aquifers. The experiments further indicate that 100 liters of a low‐viscosity DNAPL are sufficient to contaminate approximately 25 to 100 m3 of till material because of the small effective porosity constituted by the biopores and fractures.

Modeling of air sparging in a layered soil: Numerical and analytical approximations

Air sparging in an aquifer below a less permeable horizontal layer is modeled using a two‐phase flow approach. Supported by numerical simulations, we show that a steady state situation is reached. For an analysis of the steady state, we distinguish three different flow regimes, which occur between the well screen and the unsaturated zone. Just below the interface that separates the high and the low permeable layers a regime with almost hydrostatic capillary pressures develops. We use this observation to derive an ordinary differential equation for the pressure at the interface, which leads to an approximation of the air flow pattern just below and within the low permeable layer. The approximation provides an estimate for the radius of influence as a function of the physical parameters. The agreement between the analytical approximation and the numerical steady state results is almost perfect when heterogeneity is increased. With a few modifications the analysis applies also to a dense non‐aqueous phase liquid (DNAPL) spill above a less permeable layer. Comparison with an illustrative numerical simulation shows that the analytical approximation provides a good estimate of the radial spreading of the DNAPL flow on top of and within the low permeable layer.

Mobile NAPL Recovery: Conceptual, Field, and Mathematical Considerations

AbstractRecovery of mobile Nonaqueous Phase Liquids (NAPLs), referred to as “oil recovery,” is one of the most common remedial technologies currently being implemented at sites where NAPLs have been released. The rationale for oil recovery typically includes resource recovery, mitigation of further NAPL migration, and compliance with regulatory mandates for source reduction. Efficient oil recovery can be achieved by optimizing conditions within the oil flow path. This concept is referred to as flow path management. Building on this concept, a waterflood oil recovery technique utilizing dual recovery and parallel delivery drain lines has been developed for recovery of creosote‐based wood‐treating oil, a Dense Nonaqueous Phase Liquid (DNAPL). Full‐scale application of this technique at a contaminated site has yielded 1.5 million gallons of DNAPL. Furthermore, an operational endpoint of 95 percent recovery of the mobile oil is being achieved. Building on the concept of flow path management and the observed performance of the waterflood oil recovery system, a first‐order analytical solution for DNAPL flow to a drain line has been derived and validated using field data. This solution leads to a set of useful design equations and further insight into the factors that control oil recovery.