Nonaqueous Phase Liquid Distribution Research Articles

Effects of flow bypassing and nonuniform NAPL distribution on the mass transfer characteristics of NAPL dissolution

Ground water concentrations of nonaqueous phase liquid (NAPL) are usually less than their aqueous solubility; this can be attributed to irregular NAPL distribution, nonuniform flow patterns, dilution effects, and rate‐limited mass transfer between the NAPL and aqueous phases. This paper uses two‐domain and parallel column models to demonstrate the effects of nonuniform NAPL distribution and flow bypassing on apparent mass transfer kinetics for NAPL dissolution. The hypothesis of this study is that much of the apparent nonequilibrium dissolution can be explained by the effects of heterogeneity in NAPL distribution and in porous media properties. Models incorporating two‐domain concepts to represent heterogeneity are able to reproduce the timescale dependency of mass transfer coefficients which has often been observed but is inconsistent with mass transfer theory. Parallel column models, with equilibrium partitioning assumed in each column, are able to reproduce the concentration drop‐off and tailing observed in published column studies of NAPL dissolution. The parallel column model is also able to reproduce the experimentally observed phenomenon of mass transfer zones which lengthen with time and distance traveled. The results of this study support the hypothesis that much of the apparent nonequilibrium mass transfer kinetics of NAPL dissolution can be described by heterogeneity in NAPL distribution and in porous media properties.

Optimal estimation of residual non–aqueous phase liquid saturations using partitioning tracer concentration data

Stochastic methods are applied to the analysis of partitioning and nonpartitioning tracer breakthrough data to obtain optimal estimates of the spatial distribution of subsurface residual non–aqueous phase liquid (NAPL). Uncertainty in the transport of the partitioning tracer is assumed to result from small‐scale spatial variations in a steady state velocity field as well as spatial variations in NAPL saturation. In contrast, uncertainty in the transport of the nonpartitioning tracer is assumed to be due solely to the velocity variations. Partial differential equations for the covariances and cross cpvariances between the partitioning tracer temporal moments, nonpartitioning tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields are derived assuming steady flow in an infinite domain [Gelhar, 1993] and the advection‐dispersion equation for temporal moment transport [Harvey and Gorelick, 1995]. These equations are solved using a finite difference technique. The resulting covariance matrices are incorporated into a conditioning algorithm which provides optimal estimates of the tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields given available measurements of any of these random fields. The algorithm was tested on a synthetically generated data set, patterned after the partitioning tracer test conducted at Hill AFB by Annable et al. [1997]. Results show that the algorithm successfully estimates major features of the random NAPL distribution. The performance of the algorithm, as indicated by analysis of the “true” estimation errors, is consistent with the theoretical estimation errors predicted by the conditioning algorithm.

Effect of soil layering on NAPL removal behavior in soil-heated vapor extraction

Soil heating has been proposed as a method to enhance the vapor extraction of NAPLs from contaminated soils. Three-dimensional fluid flow and heat transfer simulations have been performed for soil-heated vapor extraction to determine the transient system performance for a hypothetical configuration. Soil layering has been considered in evaluation of the initial non-aqueous phase liquid (NAPL) distribution and in evaporation and transport to the vapor extraction location. Results from this layered model are compared with results for a homogeneous system with an initially uniform NAPL, indicating the influence of layering, the initial NAPL distribution, the type of NAPL, and the possibility of enhanced vapor diffusion. Not only is the NAPL removal time reduced significantly with the addition of heat, but the uncertainty in the removal time owing to a number of difficult to characterize in situ factors, such as layering and the initial NAPL distribution, is much less than for standard soil vapor extraction without heating, owing to the rise in temperature and increase in NAPL vapor pressure with time.

Multivariate Analyses to Improve Understanding of NAPL Pollutant Sources

AbstractCurrent site assessment techniques do not always generate adequate information regarding the presence, type, or distribution of nonaqueous phase liquids (NAPLs) at sites with ground water contamination. Without this information, however, the design of remediation methods is uncertain, often resulting in costly and/or failed attempts to minimize risks associated with the contamination. In this work, it is proposed that a thorough multivariate analysis of data from ground water sampling efforts could improve our overall understanding of these sites. Multivariate analyses can provide considerable insight into the contaminant source characteristics by elucidating correlations in ground water concentrations that identify recurring chemical patterns or “signatures.” These correlations are related to the type of NAPL and proximity to the contaminant source.Multivariate plots and a principal components analysis (PCA) are used to interpret ground water data from a manufactured gas plant site in Iowa contaminated with both gasoline and coal tar. Conclusions from these analyses regarding the distribution of NAPL contaminants were generally consistent with those derived from direct physical evidence of the NAPL sources. The multivariate analyses, however, provide an additional level of interpretation regarding the distribution of coal tar in the subsurface that was not possible with the standard evaluation techniques used during the remedial investigation (RI). Comparison of chemical signatures of ground water samples among wells identified two distinct regions of coal tar contamination, and suggests which wells are impacted by each source providing greater confidence in the location of the DNAPL sources.

The influence of mass transfer characteristics and porous media heterogeneity on nonaqueous phase dissolution

A two‐dimensional multiphase flow and species transport model was developed and applied to the case of nonaqueous phase liquid (NAPL) emplacement and dissolution in both homogeneous and heterogeneous porous media systems. Simulations were performed to observe dissolution rate variations and the degree of NAPL‐aqueous phase nonequilibrium as a function of two aqueous phase velocities and five forms of the NAPL‐aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL‐aqueous phase nonequilibrium. Mass removal rates for homogeneous media were insensitive to the form of the NAPL‐aqueous phase mass transfer formulation, yielding results similar to a local equilibrium approach for all but one mass transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, which was manifested as a decrease in NAPL dissolution rates as the NAPL volume fraction decreased. Variations in mass elution rates between homogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for horizontal correlation lengths. In heterogeneous media, decreases in dissolution rates were attributed to the existence of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so‐called NAPL pools. These results illustrate the importance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolution processes.

An Experimental Investigation of Rate‐Limited Nonaqueous Phase Liquid Volatilization in Unsaturated Porous Media: Steady State Mass Transfer

Results of one‐dimensional soil column experiments are presented to evaluate the factors influencing volatilization of entrapped nonaqueous phase liquids (NAPLs) in unsaturated sandy porous media. Three‐phase fluid saturations measured in Ottawa and Wagner sands were found to depend upon porous media grain size and distribution, with residual water and NAPL saturations ranging from 8 to 16% and 4 to 10%, respectively. In general, residual NAPL saturations were 2–3 times less than NAPL entrapped in similar two‐phase (organic‐water) systems. During volatilization of three single‐component NAPLs (styrene, toluene, and tetrachloroethylene), contaminant vapor phase effluent concentrations deviated from local equilibrium values by 10–40% for pore velocities ranging from 0.25 to 1.5 cm/s. In contrast to NAPL dissolution, mass transfer rates were found to decrease with decreasing soil mean grain size. An empirical correlation based on the modified Sherwood number and Peclet number was developed which incorporates the soil mean grain size as a surrogate measure of NAPL distribution. The utility of this model is demonstrated for the prediction of steady state volatilization rates in independent NAPL‐porous media systems.

Ultrasonic imaging of organic liquid contaminants in unconsolidated porous media

Laboratory experiments were conducted to measure the change in the ultrasonic wave signature as a function of the fraction of nonaqueous-phase liquid (NAPL) contaminants in initially water-saturated sand samples. This represents a fundamental step in the application of high-frequency seismic measurements to detect and delineate NAPL contamination in groundwater. The NAPL's used are n-dodecane, iso-octane and Freon 113®. P-wave velocity changes almost linearly as a function of the fraction of pore space occupied by NAPL. At a given NAPL fraction, the velocities rank with the bulk moduli of the NAPL's. The percent change in velocity at residual water saturation relative to the water-saturated medium ranges from 15% for n-dodecane to 31% for Freon 113®. Velocity changes are significant relative to the ability of seismic methods to detect changes on the order of 5%. These data are described by a model previously derived by G.T. Kuster and M.N. Toksöz of P-wave transmission through a fluid matrix with spherical inclusions that is modified to account for a two-fluid-phase matrix. Amplitude data as a function of NAPL fraction are described by assuming the distribution of NAPL in the column and using attenuation coefficients for the water-saturated and residual-water medium. Amplitudes are shown to be sensitive to both the fraction of NAPL and its distribution, whereas velocity is only a function of NAPL fraction.

An experimental investigation of pore-scale distributions of nonaqueous phase liquids at residual saturation

A set of experiments was conducted to analyze the size distribution of nonaqueous phase liquid (NAPL) blobs as a function of aqueous phase velocities, NAPL densities, and interfacial tension between the aqueous phase and the NAPL. Blob size distributions were analyzed in a model cell after a state of residual saturation was achieved, using image analysis methods. Blob lengths parallel to the direction of flow and parallel to the direction of gravity ranged over about two orders of magnitude, while blob areas varied over about three orders of magnitude. Form factors, which are a measure of blob shapes, varied over a single order of magnitude, with a median form factor of about 0.6. Blobs with smaller areas occurred much more frequently than larger blobs. However, the few larger blobs contributed to the majority of the total NAPL saturation in the model cell. The geometric standard deviations of blob length parallel to the direction of gravity and square root of area exhibited inverse relationships to Bond number (Bo), for 1.3×10−2<Bo<7.1×10−2. Blob length parallel to the direction of flow and area exhibited a weak inverse correlation with capillary number (Ca), for 3×10−8<Ca<3.1×10−5.

An experimental investigation of nonaqueous phase liquid dissolution in saturated subsurface systems: Steady state mass transfer rates

Results of an experimental investigation into steady state dissolution of nonaqueous phase liquids (NAPLs) entrapped within water saturated porous media are presented. The influence of porous media characteristics, NAPL type, and aqueous phase velocity on NAPL dissolution rates is explored through evaluation of a series of laboratory column studies. For many of the conditions tested, measured organic solute concentrations in the column effluents were below solubility limits, indicating nonequilibrium conditions. Entrapped NAPL distributions are examined and shown to depend upon porous media grading and mean grain size. Experimental results reveal a dependence of dissolution rate on the distribution pattern of entrapped NAPL, as well as upon aqueous phase velocity. A phenomenological model for the mass transfer process is developed which incorporates grain size parameters as surrogate measures of NAPL distribution. An additional set of column experiments, employing another porous medium and NAPL type, confirm the usefulness of the model as a predictor of steady state mass transfer rates in similar systems.