Presence Of Non-aqueous Phase Liquids Research Articles

Mass flux from a non-aqueous phase liquid pool considering spontaneous expansion of a discontinuous gas phase

The partitioning of non-aqueous phase liquid (NAPL) compounds to a discontinuous gas phase results in the repeated spontaneous expansion, snap-off, and vertical mobilization of the gas phase. This mechanism has the potential to significantly affect the mass transfer processes that control the dissolution of NAPL pools by increasing the vertical transport of NAPL mass and increasing the total mass transfer rate from the surface of the pool. The extent to which this mechanism affects mass transfer from a NAPL pool depends on the rate of expansion and the mass of NAPL compound in the gas phase. This study used well-controlled bench-scale experiments under no-flow conditions to quantify for the first time the expansion of a discontinuous gas phase in the presence of NAPL. Air bubbles placed in glass vials containing NAPL increased significantly in volume, from a radius of 1.0 mm to 2.0 mm over 215 days in the presence of tetrachloroethene (PCE), and from a radius of 1.2 mm to 2.3 mm over 22 days in the presence of trans-1,2-dichloroethene (tDCE). A one-dimensional mass transfer model, fit to the experimental data, showed that this expansion could result in a mass flux from the NAPL pool that was similar in magnitude to the mass flux expected for the dissolution of a NAPL pool in a two-fluid (NAPL and water) system. Conditions favouring the significant effect of a discontinuous gas phase on mass transfer were identified as groundwater velocities less than ~ 0.01 m/day, and a gas phase that covers greater than ~ 10% of the pool surface area and is located within ~ 0.01 m of the pool surface. Under these conditions the mass transfer via a discontinuous gas phase is expected to affect, for example, efforts to locate NAPL source zones using aqueous concentration data, and predict the lifetime and risk associated with NAPL source zones in a way that is not currently included in the common conceptual models used to assess NAPL-contaminated sites.

Non-equilibrium partitioning tracer transport in porous media: 2-D physical modelling and imaging using a partitioning fluorescent dye

This paper describes an investigation into non-equilibrium partitioning tracer transport and interaction with non-aqueous-phase liquid (NAPL) contaminated water-saturated porous media using a two-dimensional (2-D) physical modelling methodology. A fluorescent partitioning tracer is employed within a transparent porous model which when imaged by a CCD digital camera can provide full spatial tracer concentrations and tracer breakthrough curves. Quasi one-dimensional (1-D) benchmarking tests in models packed with various combinations of clean quartz sand and NAPL are described. These modelled residual NAPL saturations, S n, of 0–15%. Results demonstrated that the fluorescent partitioning tracer was able to detect and quantify the presence of NAPL at low flow rates. At larger flow rates and/or higher NAPL saturations, the tracer increasingly underpredicted the NAPL volume as expected and this is attributed primarily to non-equilibrium partitioning. Despite little change in permeability, change in NAPL saturations from 4% to 8% resulted in significant NAPL saturation underestimates at the same flow rates implying coalescence of NAPL into wider separated but larger ganglia. A 2-D investigation of an idealised heterogeneous residual NAPL contaminated flow field indicated little permeability change in the NAPL contaminated zone and thus little flow bypassing, leading to reduced underpredictions of NAPL saturations than for equivalent quasi 1-D cases. This was attributed to increased ‘sampling’ of the NAPL by the tracer. The process is clearly visually identifiable from the experimental images. This rapid and relatively inexpensive experimental method is of value in laboratory studies of partitioning tracer behaviour in porous media; in particular, the ability to observe full field concentrations makes it valuable for the study of complex heterogeneous systems.

Effects of nonaqueous phase liquids on the washing of soil in the presence of nonionic surfactants

The removal of malathion from soil by surfactant washing was investigated under various physical-chemical states of the malathion. Three distinctive phases (without nonaqueous phase liquids (NAPL), with NAPL, and the transitional zone of NAPL) were found to be important for a better understanding of the washing process. When there is no NAPL in the system, the washing process is less dependent on the surfactant dose if the applied surfactant concentration is above the critical micelle concentration. The existence of a sorption site boundary, which for the determination of different washing mechanisms, was identified. In the presence of NAPL, the washing performance is generally independent of the organic content (f(oc)) of the soils but is dominated by the concentration of the surfactant used, due to the lesser resistance for mass transfer in NAPL. If the formation of NAPL is marginal, a two-stage washing pattern is observed, which has been quantified by the term 'unit extraction'. For this two-stage system, a mathematical model was derived based on the observed initial unit extraction and final extraction capacity, which eventually resulted in a practical design equation with the use of primary parameters such as f(oc) and surfactant dose.

TDR detection of nonaqueous phase liquids in sandy soils using the eigendecomposition method

In this study, the presence of nonaqueous phase liquids (NAPL) in sandy soils are detected using a TDR probe system and the eigendecomposition method of analysis. As a demonstration, five NAPLs with different physicochemical properties (acetone, benzene, heptane, trichloroethylene, and xylene; Table 1) were used. Samples were prepared in such a way that the soil pore fluid has different contents of deionized water and NAPLs. For each experiment, a pulse signal with known characteristics was used and reflected signals were captured by an oscilloscope and analyzed using the eigendecomposition method. Autoregressive modeling and singular value decomposition were used to calculate the eigenvalues. The most significant eigenvalues were identified based on their power spectrum. The relative eigenvalue of the first mode (Eow), which is a measure of the power carried by the signal, was calculated and correlated to NAPL type and content, and octanol water partition coefficient (log Kow). The results indicated that for the same NAPL content, as log Kow increases, Eow decreases due to increase of hydrophobicity. For the same log Kow, as the organic content in soil pore fluid increases, Eow increases due to decrease of dielectric properties of the pore fluids.

Estimation of anaerobic biodegradation rate constants at MGP sites.

Field data at six former manufactured gas plant sites in New Jersey were used to estimate the biodegradation rate constants for the anaerobic processes naturally occurring within the ground water contaminant plumes (primarily iron and sulfate reduction). Those rate constants turned out to be about an order of magnitude smaller than values reported for the same contaminants (primarily benzene and naphthalene) at fuel sites. At four of the sites, there appeared to be sufficient electron acceptor present to eventually degrade the contaminants in the plume. However, the presence of nonaqueous phase liquids tends to offset that capacity by continuing to act as a source of contaminants that can dissolve in the ground water.

Using amplitude variation with offset and normalized residual polarization analysis of ground penetrating radar data to differentiate an NAPL release from stratigraphic changes

Amplitude variation with offset analysis of ground penetrating radar data (AVO/GPR) may improve the differentiation of nonaqueous phase liquid (NAPL) from stratigraphic changes. Previous controlled experiments have shown that common offset (CO) GPR methods can detect the presence of NAPL in soil by examining amplitude and travel time (velocity) anomalies. Unfortunately, stratigraphic changes such as the presence of a silt or clay lens or perched water table may produce similar amplitude and velocity anomalies. Therefore, it is difficult to delineate NAPL in a terrain with unknown stratigraphy exclusively using CO data collection methods. Forward models based on the Fresnel equations predict that amplitude responses exist at various incidence angles that will allow for differentiating NAPL from hydrogeologic changes. Models generated as part of this study indicate that analyzing the difference in amplitude responses from linearly polarized electric field vertically oriented ( EV) to the horizontally oriented ( EH) signals at various incidence angles improves target discrimination. A case history is presented demonstrating that collecting common-midpoint (CMP) GPR data using EH and EV polarized signals at anomalous CO amplitude responses and analyzing the data using AVO and normalized residual polarization (NRP) methods may improve the detection and differentiation of NAPL from stratigraphic changes in the subsurface. These results are corroborated using a capacitively coupled resistivity instrument and subsequent intrusive sampling.

Influence of porous medium and NAPL distribution heterogeneities on partitioning inter-well tracer tests: a laboratory investigation

The inter-well partitioning tracer test seemingly provides an attractive way of investigating the presence of non-aqueous phase liquids (NAPLs) in aquifers. Although the feasibility of this rather new technique has been tested in the field, only few laboratory experiments have been performed to test its validity at scales greater than the column scale. In this study, a partitioning tracer test was conducted in a nominally two-dimensional, intermediate-scale flow cell containing a tetrachloroethene (PCE) spill. Tracer breakthrough curves were obtained at 14 sampling ports and an extraction well containing three outlets. Estimated PCE contents resulting from the tracer technique were compared to PCE saturations obtained at 800 locations by gamma radiation. An inverse procedure, based on the two-site, non-equilibrium convection–dispersion equation, was used in addition to the moment method for sampling port data analysis. Although the inverse procedure produced slightly better results than the moment method, the tracer technique generally underestimated the amount of DNAPL contained in our aquifer model. Notably, our results showed that the pooled part of the DNAPL spill was not detected. This was attributed to the low aqueous phase permeability within the DNAPL pool in combination with rate-limited partitioning of the tracers into the DNAPL. Our analysis indicate a general difficulty for the tracer technique to detect NAPL located in pools or large lenses.

Single-Well “Push−Pull” Partitioning Tracer Test for NAPL Detection in the Subsurface

Previous environmental applications of partitioning tracer tests to detect and quantify nonaqueous phase liquid (NAPL) contamination in the subsurface have been limited to well-to-well tests. However, theory and numerical modeling suggests that single-well injection-extraction ("push-pull") partitioning tracer tests can also potentially detect and quantify NAPL contamination. In this type of test, retardation factors for injected partitioning tracers are estimated from the increase in apparent dispersion observed in extraction-phase breakthrough curves in the presence of NAPL. A series of laboratory push-pull tests was conducted in physical aquifer models (PAMs) packed with natural aquifer sediment prepared with and without the presence of trichloroethene (TCE) NAPL. Field tests were conducted in an aquifer contaminated with petroleum hydrocarbon NAPL. Injected test solutions contained a suite of partitioning and conservative (nonpartitioning) alcohol tracers. Laboratory push-pull partitioning tracer tests were able to detect and quantify sorption of partitioning tracers to aquifer sediment (in the absence of NAPL) and to detect NAPL when it was present. NAPL saturations computed from estimated retardation factors bracketed those computed from known volumes of emplaced NAPL in the sediment pack. However, numerical modeling with assumed homogeneous NAPL distribution and linear equilibrium partitioning of tracers between aqueous and NAPL phases was unable to reproduce all features of observed breakthrough curves. Excavation of the sediment pack after all tests indicated that a portion of the emplaced NAPL had sunk to the bottom of the PAM invalidating the modeling assumption of homogeneous NAPL distribution. Moreover, the apparent dispersion in extraction-phase breakthrough curves decreased when the injection-extraction pumping rate was decreased, suggesting that mass transfer limitations existed during laboratory tests. Field push-pull partitioning tracer tests were able to detect NAPL in a portion of the aquifer known to contain NAPL; computed NAPL saturations were comparble to those obtained from sediment coring and the results of a partitioning interwell tracer test conducted in the same location. This study clearly demonstrates that the single-well partitioning tracer test can detect NAPL under both laboratory and field conditions. However, additional research is needed to verify the ability of the test to quantify NAPL saturations.

Enhanced mineralization of benzo[a]pyrene in the presence of nonaqueous phase liquids

Bacterial mineralization of [7-14C]benzo[a]pyrene (BaP) to 14CO2 was enhanced by the presence of nonaqueous phase liquids (NAPLs). Mineralization of BaP was affected differently by different NAPLs, and the mode of enhancement of mineralization by a NAPL most likely occurred by a combination of cometabolic and physical effects. Mineralization was enhanced to the greatest extent when BaP was dissolved in a high-boiling distillation product of diesel fuel.

Evaluation of Mass Flux to and from Ground Water Using a Vertical Flux Model (VFLUX): Application to the Soil Vacuum Extraction Closure Problem

AbstractSite closure for soil vacuum extraction (SVE) application typically requires attainment or specified soil concentration standards based on the premise that mass flux from the vadose zone to ground water not result in levels exceeding maximum contaminant levels (MCLs). Unfortunately, realization of MCLs in ground water may not be attainable at many sites. This results in soil remediation efforts that may be in excess of what is necessary for future protection of ground water and soil remediation goals which often cannot be achieved within a reasonable time period. Soil venting practitioners have attempted to circumvent these problems by basing closure on some predefined percent total mass removal, or an approach to a vapor concentration asymptote. These approaches, however, are subjective and influenced by venting design. We propose an alternative strategy based on evaluation of five components: (1) site characterization, (2) design. (3) performance monitoring, (4) rule‐limited vapor transport, and (5) mass flux to and from ground water. Demonstration of closure is dependent on satisfactory assessment of all five components. The focus of this paper is to support mass flux evaluation. We present a plan based on monitoring of three subsurface zones and develop an analytical one‐dimensional vertical flux model we term VFLUX. VFLUX is a significant improvement over the well‐known numerical one‐dimensional model. VLEACH, which is often used for estimation of mass flux to ground water, because it allows for the presence of nonaqueous phase liquids (NAPLs) in soil, degradation, and a lime‐dependent boundary condition at the water table inter‐face. The time‐dependent boundary condition is the center‐piece of our mass flux approach because it dynamically links performance of ground water remediation lo SVE closure. Progress or lack of progress in ground water remediation results in either increasingly or decreasingly stringent closure requirements, respectively.

Partitioning and interfacial tracers to characterize non-aqueous phase liquids (NAPLs) in natural aquifer material

Hydrophobic organic contaminants released from non-aqueous phase liquids (NAPL) impose a serious risk on groundwater quality. NAPL can exist as coherent “pools” or residual phase (“blobs” or “ganglia”) within the aquifer. Depending on the NAPL volume the time scale of contaminant emission may be centuries or longer. The concentration in groundwater depends on the NAPL volume and the interfacial area between NAPL and water. The knowledge of these parameters is essential for site characterization, risk assessment and planning of in-situ aquifer remediation techniques. This paper describes the characterization for NAPLs using partitioning (volume) and interfacial tracers. Laboratory studies (undisturbed soil cores, batch experiments) were conducted using coal tar contaminated material from a former manufactured gas plant site. Different alcohols (substituted pentanols) have been tested as partitioning tracers. An anionic surfactant (ethoxylated alkyl sulfate) was used as interfacial tracer. The alcohols have low but significant octanol/water partition coefficients Kow between 24 and 240 resulting in NAPL/water partition coefficients between 1 and 22. The surfactant accumulates significantly at the NAPL/water interface. While these reactive tracers were not retarded by the natural aquifer material, they were retarded in the presence of NAPL with respect to the transport of a conservative tracer. A surfactant flushing was applied to the contaminated column in order to enhance solubilization of tar oil constituents. The NAPL saturation calculated from the retardation factor and the equilibrium NAPL/water partition coefficients decreased from 30% to 17% in the pre- and post-surfactant flushing system, respectively.

Partitioning of hydrophobic contaminants in the vadose zone in the presence of a nonaqueous phase

Partitioning behavior of organic contaminants in the vadose zone may be influenced by the presence of a nonaqueous phase liquid (NAPL). The bulk NAPL phase not only serves as a sink for hydrophobic contaminants but may also cover a significant fraction of the soil surface, reducing the surface area available for vapor-solid adsorption. Experiments were carried out to examine the applicability of binary partitioning parameters when applied to a multiphase system. Dodecane was used as the model NAPL phase, while toluene and benzene were used as the model hydrophobic contaminants. Several soil types and water contents were examined. Results indicated that two-phase equilibrium parameters were sufficient to predict partitioning among the various phases if fractional soil surface coverage by each phase was considered. A previously developed model was extended to account for the presence of NAPL and was used to describe the multiphase contaminant partitioning presented in this work.

Laboratory Assessment of BTEX Soil Flushing

Soil cores from the unsaturated zone of a waste site contaminated with benzene, toluene, ethylbenzene, and xylenes (BTEX) were flushed with water under conditions representative of those proposed for site remediation. With a laboratory column dispersivity of 1 cm, kinetic modeling of the breakthrough curves gave desorption rate constants of 0.01−0.1 h-1. Desorption studies in batch tests gave similar rates after accounting for differences in the solid-to-water ratios. The presence of nonaqueous phase liquid (NAPL) in one soil column was deduced from the flushing behavior of the xylenes, which showed increasing effluent concentration with increasing time. The amount of NAPL was estimated to be 5 mgNAPL/gsoil from the flushing of the most water-soluble constituent, benzene. This quantity of NAPL would increase the site's estimated flushing time by more than a factor of 5 over the case with only kinetically limited desorption of BTEX from soil particles.

A Method for Assessing Residual NAPL Based on Organic Chemical Concentrations in Soil Samples

AbstractGround water contamination by non‐aqueous phase liquid (NAPL) chemicals is a serious concern at many industrial facilities and waste disposal sites. NAPL in the form of immobile residual contamination, or pools of mobile or potentially mobile NAPL, can represent continuing sources of ground water contamination. In order to develop rational and cost‐effective plans for remediation of soil and ground water contamination at such sites, it is essential to determine if non‐aqueous phase liquid (NAPL) chemicals are present in the subsurface and delineate the zones of NAPL contamination. The presence of NAPL pools may be evident as a floating or sinking phase in monitoring wells. The residual NAPL contamination may be identified in soil samples if residual contents are high and contaminated zones in the soil cores are thick. However, visual identification may not be effective if residual contents are low or if the NAPL residual is distributed heterogeneously in the samples. The chemical analysis of soil samples provides a measure of the total chemical concentration in the soil but cannot determine directly whether NAPL is present in the samples. Qualitatively, soil analyses that exhibit chemical concentrations in the percent range or >10,000 mg/kg would generally be considered to indicate the presence of NAPL. However, the results of soil analyses are seldom used in a quantitative manner to assess the possible presence of residual NAPL contamination when chemical concentrations are lower and the presence of NAPL is not obvious. The assessment of the presence of NAPL in soil samples is possible using the results of chemical and physical analyses of the soil, and the fundamental principles of chemical partitioning in unsaturated or saturated soil. The method requires information on the soil of the type typically considered in ground water contamination studies and provides a simple tool for the investigators of chemical spill and waste disposal sites to assess whether soil chemical analyses indicate the presence of residual NAPL in the subsurface.