Nonaqueous Phase Liquid Composition Research Articles

Combining Radon Deficit, NAPL Concentration, and Groundwater Table Dynamics to Assess Soil and Groundwater Contamination by NAPLs and Related Attenuation Processes

Soil and groundwater contamination by NAPLs (Non-Aqueous Phase Liquids) is certainly a big issue for protecting the environment. In situ clean-up actions are routinely applied to mitigate the risk and are supplemented by monitoring surveys to assess the degree, extension, and evolution of the contamination. Radon gas is here used as a tracer of contamination because of its high solubility in non-polar solvents that produce a reduced concentration of the gas in polluted soil and groundwater with reference to radon levels in adjacent “clean” areas. This approach was employed in two sites where gasoline and diesel spillage occurred, causing soil and groundwater contamination. The two case studies were chosen because of their difference in terms of the hydrogeological features, age of the spillage, composition of residual NAPLs, and clean-up measures to test the advantages and limits of this approach in a variety of settings. Radon data, NAPL concentration in the groundwater (mainly total hydrocarbons, Methyl Tertiary-Butyl Ether and Ethyl Tertiary-Butyl Ether) and the depth of the groundwater table were periodically collected in surveys that spanned a period of two years. This dataset was statistically processed using principal component analysis to unravel which factors and attenuation processes are working in the sites and the response of the radon deficit approach to this complex series of phenomena concurrently occurring there.

Evidence of precipitate formation and byproduct transfer to nonaqueous phase liquids as a result of persulfate exposure

AbstractFor optimal treatment of a source zone using in situ chemical oxidation (ISCO), the oxidant needs to be delivered as close to the nonaqueous phase liquid (NAPL) as possible to enhance the rate of dissolution. However, the interaction between the oxidant and NAPL may result in physical and chemical changes that affect both treatment effectiveness and post‐treatment dissolution behavior from remnant NAPL. Due to a host of favorable attributes, persulfate is frequently selected as the preferred oxidant but little is known about changes that can occur when persulfate interacts with a NAPL. In this study, two series of static batch experiments were conducted. In the first series, NAPL collected from a former manufactured gas plant (FMGP) site was exposed to a persulfate solution for ~60 days, while in the second series, individual and a mixture of pure organic liquids were exposed to a persulfate solution for ~50 days. It was observed that dark‐brown precipitates were formed in almost all the experimental systems. Notably, a layer of precipitates accumulated along the NAPL/water interface in the FMGP NAPL system and inhibited dissolution. In some of the pure organic liquid systems, the aqueous phase appeared yellow and precipitates were formed. Precipitate formation was attributed to the polymerization of aromatic compounds initiated by sulfate and hydroxyl radicals. Although precipitate formation is detrimental to mass transfer, surfactant‐like byproducts which may also be produced could lead to an emulsified aqueous phase, thereby enhancing dissolution and altering NAPL/water surface tension. Several of the organic byproducts that were detected in the aqueous phase were also detected in the NAPL, suggesting that these byproducts which were generated in aqueous phase migrated into the NAPL. Depending on the concentration of these byproducts in the NAPL, they could impact important physical properties (e.g., viscosity and density) and affect NAPL composition, which can affect intra‐NAPL diffusion and post‐treatment dissolution behavior. Although additional investigation is required, the results from this study indicate that the impacts of byproducts generated when using persulfate in ISCO applications need to be considered.

Nontargeted Analysis of a Non-Aqueous-Phase Liquid From a Chemical Manufacturing Site Using Nuclear Magnetic Resonance Spectroscopy.

Non-aqueous-phase liquids (NAPLs), composed primarily of organic solvents and other immiscible liquids, can be found in the subsurface at many industrial sites. The chemical composition of NAPLs is often complex and, in many instances, difficult to fully characterize using conventional analytical techniques based on targeted compound analysis. Incomplete characterization of NAPLs leaves gaps in the understanding of the chemical profile at an impacted site. Previous work has shown that nuclear magnetic resonance (NMR) spectroscopy may be able to assist in the improved characterization of complex NAPL samples. In general, NMR spectroscopy provides an unbiased approach for the analysis of organic compounds because different classes of compounds are all treated and analyzed using the same methods. In addition, NMR spectroscopy provides unique structural information that can be used to elucidate unknowns. The present study describes the use of NMR spectroscopy as a nontargeted tool to characterize the composition of NAPLs collected from an impacted site. It is shown that NMR spectroscopy can be a complementary tool to be used in site assessments to help provide improved understanding of NAPL chemistry, leading to the development of improved conceptual site models and improved strategies for remedial and managerial activities at impacted sites. Environ Toxicol Chem 2019;00:1-9. © 2019 SETAC.

A mobile, modular and rapidly-acting treatment system for optimizing and improving the removal of non-aqueous phase liquids (NAPLs) in groundwater.

Non-aqueous phase liquids (NAPLs) in pumped groundwater are highly variable, challenging the selection of above-ground treatment strategies in pump-and-treat system. Adjustable systems with multiple treatment units are urgently required. In the present study, a mobile, modular and rapidly-acting treatment system was developed to treat groundwater contaminated by NAPLs at a chemical industrial site. The system integrated four units of coagulation sedimentation, air flotation, air stripping and chemical oxidation. During a 3-month onsite operation, the composition of groundwater NAPLs had huge fluctuations and different treatment units had unique advantages in eliminating some components. For instance, air stripping exhibited satisfactory removal efficiencies (>80%) for short-chain petroleum hydrocarbons and chloroalkanes, but poor performance for others comparing to other units. A decision-making tool and a central control system were further developed to combine and adjust the four units in proper orders, achieving satisfied removal efficiency (70-90%) for multi-component NAPLs, regardless of composition fluctuation. These findings raise the state-of-the-art modular and rapidly-acting groundwater treatment system to clean up NAPLs contaminated groundwater through pump-and-treat strategy, help in better understanding on the decision and management to improve the treatment performance, and provide guidelines for its implication at other sites contaminated with multi-component NAPLs or undergoing accidental contamination.

In situ stabilization of NAPL contaminant source-zones as a remediation technique to reduce mass discharge and flux to groundwater

Widely used flushing and in-situ destruction based remediation techniques (i.e. pump-and treat, enhanced-solubilization, and chemical oxidation/reduction) for sites contaminated by nonaqueous phase liquid (NAPL) contaminant sources have been shown to be ineffective at complete mass removal and reducing aqueous-phase contaminant of concern (COC) concentrations to levels suitable for site closure. A remediation method was developed to reduce the aqueous solubility and mass-flux of COCs within NAPL through the in-situ creation of a NAPL mixture source-zone. In contrast to remediation techniques that rely on the rapid removal of contaminant mass, this technique relies on the stabilization of difficult-to-access NAPL sources to reduce COC mass flux to groundwater. A specific amount (volume) of relatively insoluble n-hexadecane (HEXDEC) or vegetable oil (VO) was injected into a trichloroethene (TCE) contaminant source-zone through a bench-scale flow cell port (i.e. well) to form a NAPL mixture of targeted mole fraction (TCE:HEXDEC or TCE:VO). NAPL-aqueous phase batch tests were conducted prior to the flow-cell experiments to evaluate the effects of various NAPL mixture ratios on equilibrium aqueous-phase concentrations of TCE to design optimal NAPL (HEXDEC or VO) injection volumes for the flow-cell experiments. The NAPL-stabilization flow-cell experiments initiated and sustained significant reductions in COC concentration and mass flux due to a combination of both reduced relative permeability (increased NAPL-saturation) and via modification of NAPL composition (decreased TCE mole fraction). Variations in remediation performance (i.e. impacts on TCE concentration and mass flux reduction) between the different HEXDEC injection volumes were relatively minor, and therefore inconsistent with Raoult's Law predictions. This phenomenon likely resulted from non-uniform mixing of the injected HEXDEC with TCE in the source-zone. VO injection caused TCE concentrations and mass-flux to decrease more rapidly than with HEXDEC injections. This phenomenon occurred because the injected VO was observed to mix more uniformly with TCE in the source-zone due to a lower mobilization potential. The relative lower density differences (buoyancy effects) between VO and the flushing solution (water) was the primary factor contributing to the lower mobilization potential for VO. Overall, this study indicated that the delivery of HEXDEC or VO into the toxic TCE source-zone was effective in significantly reducing contaminant aqueous-phase concentration and mass-flux. However, the effectiveness of this in-situ NAPL stabilization technique depends on source delivery, uniform mixing of amendment, and that the amendment remains immobilized within and around the NAPL contaminant source.

Predicting Chemical Fingerprints of Vadose Zone Soil Gas and Indoor Air from Non-Aqueous Phase Liquid Composition

Complex mixtures of volatile organic chemical (VOC) vapors can exist over subsurface accumulations of non-aqueous phase liquids (NAPLs) and contaminated soils. The ability to predict the relative soil gas chemical composition arising from such NAPLs is relevant to studies of the sources and fate of soil gas, and in assessing the possible intrusion of soil gas chemical constituents to indoor air. However, it is difficult to directly compare the chemical ‘fingerprints’ of NAPL and vapor because the differences are significant in liquid-gas partitioning coefficients among the many volatile hydrocarbons that compose the liquid NAPL. In this article, detailed chemical characterization data from analysis of liquid NAPLs are used to calculate equilibrium vapor phase distributions of 66 diagnostic paraffin, isoparaffin, aromatic, naphthene, and olefin (PIANO) compounds. Measured NAPL and predicted vapor phase chemical composition are presented for a diverse group of 10 hydrocarbon products that include crude oil, solvents, gasoline, distillate fuels, and coal tar. The laboratory investigation and field study described in this article demonstrate that chemical fingerprints of such predicted vapor phase hydrocarbons can be used as compositional benchmarks for reconciling source(s) of soil gas and indoor air-borne hydrocarbons.

Simultaneous estimation of diffusive Volatile Organic Compound (VOC) fluxes and Non‐Aqueous Phase Liquid (NAPL) saturation in the vadose zone

AbstractSoil‐gas monitoring is a widely used tool to observe the migration of volatile organic compounds (VOCs) at contaminated sites. By combining this technique with natural gradient tracer methods, diffusive contaminant fluxes can be measured in situ, and non–aqueous phase liquid (NAPL) can be detected and roughly quantified. This work describes the new approach and its application at a field site in Denmark with an emplaced NAPL contamination. Soil‐gas probes with a low dead volume were installed at 1‐m depths in the sandy vadose zone, and a small volume of gas containing conservative and partitioning tracers was injected. Soil‐gas samples were withdrawn subsequently during 1 to 4 h and analyzed simultaneously for VOCs and tracers. Tracers detected the NAPL reliably, and the combined data allowed for a close delineation of the source zone. The calculated NAPL saturation deviated by up to a factor of 3 from the analyses of soil cores. Better agreement was found by taking the NAPL composition into consideration, which is, however, generally unknown at the actual field sites. In addition, the tracers were also used to estimate effective diffusion coefficients in situ, which varied by a factor of 2 between various locations. From these data, diffusive contaminant vapor fluxes were quantified without additional laboratory experiments or the use of empirical relationships. The new approach yields a better site investigation with a few additional measurements.

Flux and product distribution during biological treatment of tetrachloroethene dense non-aqueous-phase liquid.

Flux in non-aqueous-phase liquid (NAPL)-contaminated systems containing active microbial populations (including Dehalococcoides sp.) was investigated using a quantitative mass balance and phase distribution approach. Batch systems containing mixed NAPL with an initial tetrachloroethene (PCE) mole fraction ranging from 0.1 to 0.4 provided a means for comparing systems where mass transfer and aqueous concentration were controlled by the initial NAPL composition. Although the use of mixed NAPL with increasing PCE mole fractions introduced a mass-transfer variable on the abiotic dissolution rate, it was determined that biological systems produced flux rates that were similar to each other regardless of the initial PCE mole fraction. Thus, organisms appeared to be dechlorinating near their maximum conversion rates, and the result was the accumulation of cis-1,2-dichloroethene (cDCE) followed by slow conversion to vinyl chloride (VC). Increases in the initial PCE mole fractions in the NAPL had a negative impact on product distribution due to the presence of a larger concentration of a more favorable electron acceptor. Because the mass converted to cDCE was present largely in the dissolved phase in all systems, the production of this metabolite was a favorable outcome in terms of NAPL dissolution. The pH dropped as low as 4.9 in active systems, indicating that the amount of HCl released during the reductive dechlorination process was large enough to overwhelm the buffering capacity. This pH effect was more pronounced in systems that exhibited extensive dechlorination to VC, further suggesting that rapid dechlorination of PCE NAPL can alter chemical characteristics in source zone regions.

Complex NAPL Site Characterization Using Fluorescence Part 1: Selection of Excitation Wavelength Based on NAPL Composition

Fluorescence has been demonstrated to be a viable method for detecting non-aqueous phase liquid (NAPL) contaminants comprised of polycyclic aromatic hydrocarbons (PAHs). Commercially available cone penetrometer (CPT)induced fluorescence based sensor platforms can be used to detect NAPLs such as petroleum oils and lubricants in-situ. In addition, these approaches can be used to detect dense non-aqueous phase liquid (DNAPL) source zones by detecting commingled oils, fuels, and naturally oc curring organic materials entrained by or in solution with DNAPLs and carried to depths below the water table. The currently available CPT-based fluorescence systems are typically restricted to a single wavelength excitation source, each demonstrating specific advantages and disadvantages with respect to detection capabilities for partic ular fluorophores. Several neat NAPLs and mixtures were analyzedfor specificfluores-cence characteristics to determine the optimal excitation source for site characterization efforts. Commercially available cone penetrometer based fluorescence detection systems were ranked according to the potential for likelihood of detection. Our work demon strates that an optimal range of excitation wavelength can be determined for specific fluorophores within NAPL mixtures, and that available systems can be ranked based on the specific contaminant and site characteristics. We have identified optimal excitation sourcesfor a number of common NAPL mixtures, including petroleum-basedfuels and a lubricant mixed with a chlorinated solvent.

Identifying the effect of polar constituents in coal-derived NAPLs on interfacial tension.

Interfacial tension, which is a critical variable affecting multiphase flow of nonaqueous phase liquids (NAPLs) in the subsurface, varies greatly with the composition of complex NAPLs recovered from field sites. Much of this variability stems from the presence of acid and base molecules in the NAPL mixture. The interfacial tension and acid and base concentration in six coal-derived NAPLs were measured. Creosotes generally have lower interfacial tensions due to their higher concentrations of organic acid and base macromolecules as compared with coal tar samples. Interfacial tension is a function of pH, with lower values measured at pH greater than approximately 9. At a neutral pH, the interfacial tensions are inversely proportional to the total acid concentration. Asphaltenes in these coal-derived NAPLs account for most of the acid and base constituents. It is found in this study that acid and base numbers are valuable indicators of interfacial tension and, therefore, the capillary phenomena associated with multiphase flow behavior of NAPLs in the subsurface.

Multicomponent NAPL Solidification Thermodynamics

Nonaqueous phase liquid (NAPL) contaminants that are chemical mixtures often contain compounds that are solids in their pure states. In the environment, weathering processes cause shifts in multicomponent NAPL composition, thereby enriching the NAPL in the less soluble compounds which may result in their eventual solidification. In this paper, we review the thermodynamic theory governing solid-liquid phase equilibria for the multicomponent NAPLs, and we present experimental observations of such phase equilibria for binary, ternary, and quaternary mixtures of polycyclic aromatic hydrocarbons (PAHs). If the NAPL phase behaves as an ideal solution and if the solid precipitate is pure, then a compound's mole fraction solubility limit in the NAPL phase equals its solid-liquid reference fugacity ratio. This value is a constant at the temperature of the system. If the NAPL phase is a non-ideal solvent or if the solid is a solid solution, prediction of NAPL solidification in the environment is considerably more difficult. Experimental results indicate that for compounds such as naphthalene and acenaphthene, the solid-liquid reference fugacity ratio serves as a good indicator of the solubility limits in the NAPL phase. For phenanthrene, the solids that form when this compound exceeds its solubility limit are solid solutions that consistently include large portions of 2-methylnaphthalene. These results suggest that the independent behavior implied by ideal solubility theory may not be an accurate descriptor of NAPL solidification phenomena for all PAH-containing NAPLs.

The variability and intrinsic remediation of a BTEX plume in anaerobic sulphate-rich groundwater

Data from long-term groundwater sampling, limited coring, and associated studies are synthesised to assess the variability and intrinsic remediation/natural attenuation of a dissolved hydrocarbon plume in sulphate-rich anaerobic groundwater. Fine vertical scale (0.25- and 0.5-m depth intervals) and horizontal plume-scale (>400 m) characteristics of the plume were mapped over a 5-year period from 1991 to 1996. The plume of dissolved BTEX (benzene, toluene, ethylbenzene, xylene) and other organic compounds originated from leakage of gasoline from a subsurface fuel storage tank. The plume was up to 420 m long, less than 50 m wide and 3 m thick. In the first few years of monitoring, BTEX concentrations near the point of leakage were in approximate equilibrium with non-aqueous phase liquid (NAPL) gasoline. NAPL composition of core material and long-term trends in ratios of BTEX concentrations in groundwater indicated significant depletion (water washing, volatilisation and possibly biodegradation) of benzene from residual NAPL after 1992. Large fluctuations in BTEX concentrations in individual boreholes were shown to be largely attributable to seasonal groundwater flow variations. A combination of temporal and spatial groundwater quality data was required to adequately assess the stationarity of plumes, so as to allow inference of intrinsic remediation. Contoured concentration data for the period 1991 to 1996 indicated that plumes of toluene and o-xylene were, at best, only partially steady state (pseudo-steady state) due to seasonal groundwater flow changes. From this analysis, it was inferred that significant remediation by natural biodegradation was occurring for BTEX component plumes such as toluene and o-xylene, but provided no conclusive evidence of benzene biodegradation. Issues associated with field quantification of intrinsic remediation from groundwater sampling are highlighted. Preferential intrinsic biodegradation of selected organic compounds within the BTEX plume was shown to be occurring, in parallel with sulphate reduction and bicarbonate production. Ratios of average hydrocarbon concentrations to benzene for the period 1991 to 1992 were used to estimate degradation rates (half-lives) at various distances along the plume. The estimates varied with distance, the narrowest range being, for toluene, 110 to 260 days. These estimates were comparable to rates determined previously from an in situ tracer test and from plume-scale modelling.

Phase Stability of Multicomponent NAPLs Containing PAHs

Multicomponent nonaqueous phase liquid (NAPL) contaminants that contain compounds that are solids in their pure states can undergo phase transformations. This paper examines the relationship between NAPL composition and liquid phase stability for mixtures of polycyclic aromatic hydrocarbons (PAHs), most of which are solids in pure form at ambient temperatures. Because any natural or engineered process that acts to selectively extract compounds will alter NAPL composition, knowledge of phase stability is an important determinant in risk assessment, remediation effectiveness, and research. Ideal solubility theory dictates that a NAPL will be a homogeneous liquid if each constituent's mole fraction is less than the solid−liquid reference fugacity ratio at the system temperature. Through experimental observation of binary and ternary systems, it is shown that ideal solubility theory is a reliable guide for predicting PAH−NAPL phase stability. Uncertainty in melting temperature and entropy of fusion is a signi...