Residual Nonaqueous Phase Liquid Research Articles

Partitioning Tracers for Measuring Residual NAPL: Field-Scale Test Results

The difficult task of locating and quantifying nonaqueous phase liquids (NAPLs) present in the vadose and saturated zones has prompted the development of innovative, nondestructive characterization techniques. The use of the interwell partitioning tracer`s (IWPT) test, in which tracers that partition into the NAPL phase are displaced through the aquifer, is an attractive alternative to traditional coring and analysis. The first field test of IWPT was conducted in a hydraulically isolated test cell to quantify the total amount of a complex NAPL (a mixture of JP-4 jet fuel and chlorinated solvents) trapped within a 1.5-m smear zone in a shallow, unconfined sand and gravel aquifer at Hill Air Force Base (AFB), Utah. Tracer breakthrough curves (BTCs) were measured in three extraction wells (EWs) following a tracer pulse introduction through four injection wells (IWs). The measured retardation of the partitioning tracer (2,2-dimethyl-3-pentanol) relative to the nonreactive tracer (bromide) was used to quantify the NAPL present. The EW data were used to estimate an average NAPL saturation of 4.6--5.4% within the test cell. NAPL saturations estimated by using measured concentrations in soil cores of two significant compounds present in the NAPL were 3.0 and 4.6%.

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.

Influence of cation type, ionic strength, and pH on solubilization and mobilization of residual hydrocarbon by a biosurfactant

This study investigated the effect of cation type, ionic strength, and pH on the performance of an anionic monorhamnolipid biosurfactant for solubilization and removal of residual hexadecane from sand. Three common soil cations, Na +, Mg 2+, and Ca 2+, were used in these experiments and hexadecane was chosen to represent a nonaqueous phase liquid (NAPL) less dense than water. Results showed that hexadecane solubility in rhamnolipid solution was significantly increased by the addition of Na + and Mg 2+. Addition of up to 0.2 mM Ca 2+ also increased hexadecane solubility. For Ca 2+ concentrations greater than 0.2 mM there was little effect on hexadecane solubility due to competing effects of calcium-induced rhamnolipid precipitation and enhanced hexadecane solubilization. Efficiency of NAPL solubilization can be expressed in terms of molar solubilization ratios (MSR). The results showed that MSR values for hexadecane in rhamnolipid solutions increased 7.5-fold in the presence of 500 mM Na +, and 25-fold in the presence of 1 mM Mg 2+. The presence of cations also reduced the interfacial tension between rhamnolipid solutions and hexadecane. For example, an increase in Na + from 0 to 800 mM caused a decrease in interfacial tension from 2.2 to 0.89 dyn cm −1. Similarly, decreasing pH caused a reduction in interfacial tension. The lowest interfacial tension value observed in this study was 0.02 dyn cm −1 at pH 6 in the presence of 320 mM Na +. These conditions were also found to be optimal for removal of hexadecane residual from sand columns, with 58% of residual removed within three pore volumes. The removal of residual NAPL from the packed columns was primarily by mobilization, even though solubilization was significantly increased in the presence of Na +.

Dissolution of Trapped Nonaqueous Phase Liquids in Sand Columns

AbstractSoil column experiments were designed to evaluate the dissolution and leaching under continuous water flow of two immobilized nonaqueous phase liquids (NAPL): o‐xylene, which is lighter than water (LNAPL) and o‐dichlorobenzene, which is denser than water (DNAPL). Major differences were observed in the mobility and dissolution of the LNAPL and the DNAPL under continuous leaching. Under water‐saturated conditions and downward flow, the trapped LNAPL was removed from sand columns by dissolution and by mobilization of small droplets. In contrast, the DNAPL remained trapped in the soil matrix during the leaching phase and was removed by dissolution only. The model used to describe the data assumes that the NAPL is trapped in the soil volume as ideal spheres of initial diameter d0. The transport parameters of the model were determined by a separate tracer experiment, so that the only parameter fitted to the resident residual NAPL and outflow observations was the initial diameter of the spheres. The sphere model described the data fairly well, with optimum initial sphere diameter of 0.12 cm for o‐dichlorobenzene and varying between 0.17 to 0.21 cm for o‐xylene. The optimum diameter of the spheres was used to calculate the length of the mass transfer zone (Lm) over which solution passing through the NAPL region changes from zero NAPL concentration to NAPL water solubility. Lm varied between 7.9 to 11.1 cm for o‐xylene, and from 3.7 cm in the finer sand to 5.2 cm in the coarser material for o‐dichlorobenzene.

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.

Aqueous phase permeability in the region of discontinuous NAPL

In saturated soil, a nonaqueous phase liquid (NAPL) can become a discontinuous immobile phase when capillary forces are stronger than the opposing viscous and buoyancy forces. In regions where this occurs, the aqueous phase permeability is less than the water saturated permeability of the surrounding medium, which may cause treatment to bypass the targeted contaminated region. As an in situ treatment is applied, the saturation of residual NAPL (Srn) is reduced, while the aqueous phase saturation (Sa ) and the relative permeability of the aqueous phase (kra) increases. Hence, in situ treatment of NAPL must account for various levels of heterogeneity. In this study, column experiments measure Kra‐Sa during the application of a surfactant that enhances the dissolution of NAPL Into the aqueous phase. The results indicated that the surfactant initially decreases kra followed by a steady increase as Srn is reduced. In multiphase models, it is common practice to use empirical coefficients measured from pressure‐saturation (p‐S) data to predict kra‐Sa. This approach assumes that Kra‐Sa relationship in the region of discontinuous NAPL will follow the trend estimated in the continuous two‐phase flow region. The results document that this practice may not adequately predict Kra‐Sa at Sa> 1 ‐ Srn.

A composite numerical model for assessing subsurface transport of oily wastes and chemical constituents

Subsurface fate and transport models are utilized to predict concentrations of chemicals leaching from wastes into downgradient receptor wells. The contaminant concentrations in groundwater provide a measure of the risk to human health and the environment. The level of potential risk is currently used by the U.S. Environmental Protection Agency to determine whether management of the wastes should conform to hazardous waste management standards. It is important that the transport and fate of contaminants is simulated realistically. Most models in common use are inappropriate for simulating the migration of wastes containing significant fractions of nonaqueous-phase liquids (NAPLs). The migration of NAPL and its dissolved constituents may not be reliably predicted using conventional aqueous-phase transport simulations. To overcome this deficiency, an efficient and robust regulatory assessment model incorporating multiphase flow and transport in the unsaturated and saturated zones of the subsurface environment has been developed. The proposed composite model takes into account all of the major transport processes including infiltration and ambient flow of NAPL, entrapment of residual NAPL, adsorption, volatilization, degradation, dissolution of chemical constituents, and transport by advection and hydrodynamic dispersion. Conceptually, the subsurface is treated as a composite unsaturated zone-saturated zone system. The composite simulator consists of three major interconnected computational modules representing the following components of the migration pathway: (1) vertical multiphase flow and transport in the unsaturated zone; (2) areal movement of the free-product lens in the saturated zone with vertical equilibrium; and (3) three-dimensional aqueous-phase transport of dissolved chemicals in ambient groundwater. Such a composite model configuration promotes computational efficiency and robustness (desirable for regulatory assessment applications). Two examples are presented to demonstrate the model verification and a site application. Simulation results obtained using the composite modeling approach are compared with a rigorous numerical solution and field observations of crude oil saturations and plume concentrations of total dissolved organic carbon at a spill site in Minnesota, U.S.A. These comparisons demonstrate the ability of the present model to provide realistic depiction of field-scale situations.

Microemulsion‐mediated removal of residual gasoline from soil columns

In situ pumping of micellular solutions of surfactant (S) and cosurfactant (CoS) in water (W) through contaminated soils or aquifers offers potential for enhanced remediation of residual nonaqueous‐phase liquids (NAPLs). Extremely low interfacial tension generated between a W/S/CoS mixture and residual NAPL in soil pores may initially mobilize the NAPL, which is then transported temporarily as a separate phase by immiscible displacement. The NAPL is then solubilized by micro‐emulsification as the W/S/CoS mixture forms a stable W/S/CoS/NAPL micro‐emulsion that undergoes miscible displacement through the pore space. This remediation technique was tested under laboratory conditions by sequentially flushing a saline solution and a W/S/CoS mixture through columns of a sandy soil recently contaminated with residual leaded gasoline (LG). Prior to the flushings, the soil was initially contaminated by applying a W/S/CoS/LG microemulsion. A simple conceptual transport model with kinetic clogging of soil pores adequ...

Dissolution Fingering During the Solubilization of Nonaqueous Phase Liquids in Saturated Porous Media: 2. Experimental Observations

Nonaqueous phase liquids (NAPLs) are a common source of contamination at polluted groundwater sites, where they frequently remain trapped within the pore space at residual saturation and reduce the permeability of the medium to aqueous phase flow. The model presented in a companion paper [Imhoff and Miller, this issue] suggested that when fluid flow is imposed on such a system, the aqueous phase may interact with dissolution‐induced permeability changes, and lead to fingered patterns. In this investigation, a two‐dimensional flow cell was used to study the effects of porous medium structure, Darcy flux, initial residual NAPL saturation, median particle diameter, gravity, and NAPL composition on dissolution fingering. Fingering occurred when two conditions were met: (1) 11 to 80 e‐fold times had elapsed, where e‐fold time is the time required for the instability to grow by a factor e and was predicted from the linear stability analysis in the companion paper; and (2) the length of the dissolution front before finger development was smaller than the zone of NAPL residual. Where fingers formed, finger structure was similar and showed no systematic variation within the parameters investigated. Observed finger wavelengths compared well with model predictions. A single experiment in a three‐dimensional cell, 1 m long, demonstrated that fingers can grow to at least 30 cm in length. When experimental observations in this cell were compared with predictions of NAPL dissolution based on models that did not include fingering, the measurements of changing NAPL saturation differed significantly from model predictions.

Dissolution Fingering During the Solubilization of Nonaqueous Phase Liquids in Saturated Porous Media: 1. Model Predictions

The dissolution of nonaqueous phase liquids (NAPLs) trapped at residual saturation is an important problem at many contaminated groundwater sites. It is well known that NAPL ganglia trapped within the pore space reduce the permeability of the medium to aqueous phase flow. When fluid flow is imposed on such a system, the aqueous phase may interact with the dissolution‐induced permeability changes, leading to fingered patterns. This mechanism is very similar to that of mineral dissolution instabilities, which are a particular example of reactive infiltration instabilities. Extending that literature, we present a nonlinear model describing the dissolution of NAPL ganglia and perform a linear stability analysis of the resultant moving free boundary problem, demonstrating that instabilities may develop from a planar dissolution front. Predicted finger wavelengths are a function of both residual NAPL saturation and the imposed aqueous phase flow rate; they range from centimeters to meters. Experimental observations of dissolution fingering are presented in a companion paper [Imhoff et al., this issue] and are compared with predictions from this model. Dissolution fingering may affect the solubilization of NAPL ganglia in natural environments and in experimental studies of NAPL dissolution intended to quantify mass transfer rates.

The Long-Term Dissolution Characteristics of a Residually Trapped BTX Mixture in Soil

ABSTRACT A mass transfer limited model is presented to describe the long-term dissolution of organic compounds from a benzene, toluene, and xylenes (BTX) mixture residually trapped in a sandy soil. The model is an extension of a previously presented equilibrium dissolution model [1] which takes into consideration mass transfer limitations that develop later in the leaching process and is similar to that presented by Borden and Kao [2] for modeling BTX dissolution from residually trapped gasoline. The residual nonaqueous phase liquid (NAPL) is divided into multiple regions: one region which undergoes equilibrium dissolution and additional regions in which mass transfer is progressively limited. Application of the model to BTX column effluent data indicates that the initial dissolution (exponential decay region) of BTX can be effectively described by equilibrium dissolution. When applied to later dissolution times (asymptotic region) a multiple-region model is required to rationalize the data for all three ...

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.

Effect of a NAPL on migration of PCP in soil columns

Pentachlorophenol (PCP) is a widely used wood-treating chemical that is applied to wood under pressure in the form of concentrated solution in solvents that are nonaqueous phase liquids (NAPL). As a result, any spill at a wood-treating site using PCP results in soil contamination with PCP in the presence of a NAPL. Hence, a study of migration of PCP was conducted in presence of kerosene as a NAPL in unsaturated soils. Addition of kerosene to soil caused a change in the effective size of soil particles; the porosity, pore velocity, and dispersion coefficients, all changed (as measured by bromide break-through) in soil columns contaminated with kerosene. Moisture content of soil was found to be a key parameter determining the residual NAPL content in the soils. PCP/kerosene breakthrough experiments showed that the organic phase penetrated deeper and faster into soils with higher moisture content, and the residual kerosene acted as a sorptive phase that retarded the migration of PCP.

Direct Detection of Residual Nonaqueous Phase Liquid in the Saturated Zone Using SF6 as a Partitioning Tracer

Sulfur hexafluoride (SF 6 ) was investigated as a potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPL-water batch partitioning tests, SF 6 had a partitioning coefficient (K NW =concentration in NAPL÷ concentration in water) of 3211.33 to TCE, 2411.27 to DCM, and 45±1.89 to o-DCB. Tracer tests with SF 6 and bromide in packed sand columns containing residual DNAPL show that SF 6 breakthrough is retarded relative to bromide. The SF 6 retardation factor was 2.2 relative to bromide in a column containing 3.69% residual TCE and 2.6 in a column with o-DCB at 3.52% residual saturation. A relatively simple relationship based on equilibrium partitioning and retardation theory was developed that allows very accurate estimates of DNAPL residual content encountered by SF 6 during transport. Using observed retardation factors from the column tests and the experimentally determined K ow values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced

Partitioning Tracer Test for Detection, Estimation, and Remediation Performance Assessment of Subsurface Nonaqueous Phase Liquids

In this paper we present a partitioning interwell tracer test (PITT) technique for the detection, estimation, and remediation performance assessment of the subsurface contaminated by nonaqueous phase liquids (NAPLs). We demonstrate the effectiveness of this technique by examples of experimental and simulation results. The experimental results are from partitioning tracer experiments in columns packed with Ottawa sand. Both the method of moments and inverse modeling techniques for estimating NAPL saturation in the sand packs are demonstrated. In the simulation examples we use UTCHEM, a comprehensive three‐dimensional, chemical flood compositional simulator developed at the University of Texas, to simulate a hypothetical two‐dimensional aquifer with properties similar to the Borden site contaminated by tetrachloroethylene (PCE), and we show how partitioning interwell tracer tests can be used to estimate the amount of PCE contaminant before remedial action and as the remediation process proceeds. Tracer tests results from different stages of remediation are compared to determine the quantity of PCE removed and the amount remaining. Both the experimental (small‐scale) and simulation (large‐scale) results demonstrate that PITT can be used as an innovative and effective technique to detect and estimate the amount of residual NAPL and for remediation performance assessment in subsurface formations.

Vibratory Mobilization of Immiscible Liquid Ganglia in Sands

It is well known that residual NAPL (nonaqueous phase liquid) ganglia remain trapped in the saturated domain after pumpandtreat operations and create a longterm source of contamination. Mobilizatio...

Dissolution and biorestoration of nonaqueous phase hydrocarbons: Model development and laboratory evaluation

A one‐dimensional numerical model is presented for simulating the enhanced biorestoration of aquifer material contaminated with residual nonaqueous phase liquid (NAPL) hydrocarbon. The model simulates the simultaneous dissolution, transport, and biodegradation of individual hydrocarbons under oxygen‐limiting conditions. Partitioning between the NAPL and aqueous phase is modeled as a linear first‐order process where the NAPL is composed of two fractions, a fast oil fraction with a high mass transfer rate and a slow oil fraction with a much lower mass transfer rate. Microorganisms are assumed to be present attached to the soil grains and are assumed to be immobile. Biodegradation of the problem compounds is simulated as a multistep process where the parent compounds are first biotransformed to oxygenated intermediates. These intermediates may then be mineralized to CO2 and H2O. Comparison of model simulations with experimental results from soil columns containing residual hydrocarbon and an active microbial population indicate that the model is capable of simulating the extent of hydrocarbon biodegradation using reasonable model parameters. Model simulations indicate that if benzene, toluene, and xylene are assumed to be completely mineralized, the model will greatly underestimate the extent of biotransformation. The model overestimated the extent of hydrocarbon biotransformation at low substrate concentrations.

Transport in Porous Media Containing Residual Hydrocarbon. I: Model

When liquid hydrocarbons or nonaqueous phase liquids (NAPLs) become entrapped below the water table, flowing ground waters carry soluble NAPL components away from the spill zone. Transport of dissolved NAPL components is controlled by several processes including advection, dispersion, sorption onto aquifer materials, and liquid‐liquid partitioning. A model is developed to predict solute movement through heterogeneous porous media containing residual NAPL. Governing equations describe both instantaneous and time‐dependent processes including: (1) Exchange between mobile and immobile waters; (2) exchange between residual NAPL and water; and (3) exchange between aquifer solids and water. A one‐dimensional analytical Laplace‐space solution to governing equations is presented. The resulting transport model handles combinations of solute exchange activities that cannot be investigated with other analytical or numerical formulations. The model is tested during an investigation of several hypothetical systems. The investigation shows rate‐limited processes causing early solute arrival, a delay in total breakthrough, and asymmetric breakthrough curves.

Mass transfer from nonaqueous phase organic liquids in water‐saturated porous media

Results of dissolution experiments with trapped nonaqueous phase liquids (NAPLs) are modeled by a mass transfer analysis. The model represents the NAPL as isolated spheres that shrink with dissolution and uses a mass transfer coefficient correlation reported in the literature for dissolving spherical solids. The model accounts for the reduced permeability of a region of residual NAPL relative to the permeability of the surrounding clean media that causes the flowing water to partially bypass the residual NAPL. The dissolution experiments with toluene alone and a benzene-toluene mixture were conducted in a water-saturated column of homogeneous glass beads over a range of Darcy velocities from 0.5 to 10 m d(-1). The model could represent the observed effluent concentrations as the NAPL underwent complete dissolution. The changing pressure drop across the column was predicted following an initial period of NAPL reconfiguration. The fitted NAPL sphere diameters of 0.15 to 0.40 cm are consistent with the size of NAPL ganglia observed by others and are the smallest at the largest flow velocity.

A laboratory method for studying the aqueous phase transport of dissolved constituents from residually held NAPL in unsaturated soil columns

The potential for soils containing residual nonaqueous-phase liquid (NAPL) to contaminate infiltrating water was investigated using laboratory experiments. An experimental procedure was developed to leach soil-filled laboratory columns containing residual levels of gasoline. The water flow in the columns was maintained under steady state, unsaturated flow conditions. The 30 cm columns were constructed of glass, stainless steel and teflon components. A syringe collection technique was developed to collect effluent samples in a vacuum chamber in order to minimize losses of volatile compounds. Tensiometers were used to monitor unsaturated flow hydraulics. Four replicate column experiments were conducted using a sandy loam soil with gasoline as the residually held hydrocarbon source. Residual levels of gasoline were achieved using ceramic pressure plates. Maintaining unsaturated flow conditions, levels of benzene, toluene, ethylbenzene, m- and p-xylene and o-xylene (BTEX) were monitored over a period of one month. Initial effluent levels of BTEX agreed well with equilibrium values determined from batch solubility tests indicating that the system was operating under local equilibrium conditions. Minimum time for complete depletion of the five compounds from the soil was calculated based on the effluent concentration levels. Results from the four replicate columns were similar.