Nonaqueous Phase Liquids In Subsurface Research Articles

Experiments and sensitivity coefficients analysis for multiphase flow model calibration of enhanced DNAPL dissolution

Multiphase flow modeling is often used for the comparison and optimization of subsurface nonaqueous phase liquid (NAPL) remediation schemes. The calibration of such models is a challenging task due to the lack of detailed data describing the initial NAPL spatial distribution and the processes governing the fate and transport of NAPLs in porous media. In this study laboratory scale experiments were conducted to evaluate reagent-enhanced dense nonaqueous phase liquid (DNAPL) solubilization in saturated heterogeneous media. The DNAPL consisted of both pooled and residual saturation forms. To gain insight into the influence of various input parameters on effluent concentrations, the multiphase flow program was used to compute the sensitivity coefficients of key parameters, relating to the flow, flushing solution properties, soil parameters, NAPL distribution and mass transfer coefficient. The sensitivity coefficients were, in turn, used to aid in the model calibration and to underline the difficulties associated with the calibration of multiphase flow models, most notably the non-uniqueness of the calibration process when complete information is lacking. To alleviate this uncertainty and provide additional constraints, the conducted flushing experiments were jointly used to calibrate the multiphase flow model. The results of the model calibration suggest that the interphase mass transfer coefficient is dependent on the properties of the reagent aqueous solution used for DNAPL remediation, most notably the viscosity and interfacial tension.

A design of experiment aided stochastic parameterization method for modeling aquifer NAPL contamination

Numerical models have been widely applied in simulating subsurface Non-aqueous Phase Liquid (NAPL) contamination processes. In order to examine modeling uncertainties and improve simulation performance, a new hybrid stochastic - design of experiment (DOE) aided parameterization method was developed by using a coupled experimental and modeling approach. In a case study, an existing commercial groundwater modeling tool BioF&T 3D was applied to conduct numerical simulations of subsurface contamination processes based on flow cell experiments. Parameterization results indicated that porosity, distribution coefficient, and Henry's constant were the most significant parameters. The result also revealed their interactions. The DOE predicted responses were found reasonably close to the actual ones from the models' simulations. Monte Carlo simulation was applied to conduct uncertainty analysis within the narrowed parameters ranges, which were generated by centralizing the DOE optimized values, and the combinations of parameters were further updated when better responses were found. After parameterization, R2 valued 0.80, 0.91, 0.89, and 0.90 for benzene, toluene, ethylbenzene, and xylene (BTEX), respectively. A good consistency (R2 = 0.76 to 0.90 for BTEX) was also achieved during the model verification, which confirmed that after the parameterization processes, the simulation model can potentially be used for predictions under similar conditions.

Optimizing Ethanol Enhanced NAPL Remediation Using Evolutionary Algorithms

In the past decades, subsurface non-aqueous phase liquid (NAPL) contamination has been recognized as one of the most widespread and challenging environmental problems. Thus, researchers have focused their efforts on developing and testing the efficiency of remediation methodologies, able to address the unique nature of these contaminants. Recently, in-situ flooding techniques for the accelerated removal of NAPLs trapped in the subsurface have been proposed, where additives are injected together with water upgradient of the NAPL-contaminated area in order to alter the physio-chemical properties of the contaminants, such as interfacial tension, and enhance their solubilities. In this work, the efficiency of ethanol enhanced NAPL remediation is addressed. To this end, a non-linear, multi-objective optimization strategy is developed by combining a multiphase flow simulation model with evolutionary algorithms. Two conflicting optimization objectives are considered: minimizing operation cost and maximizing remediation efficiency, while preventing uncontrolled NAPL mobilization. More specifically, the first objective involves the operation cost of the procedure, which is directly proportional to the pumping rate, duration and ethanol volume used. The second represents the environmental considerations of the problem that, in this work, are described by the maximization of free product removal and the prevention of DNAPL vertical spreading.

Using radon as environmental tracer for the assessment of subsurface Non-Aqueous Phase Liquid (NAPL) contamination – A review

The radioactive noble gas radon has an ambivalent nature: on the one hand is it of main concern with regard to radiation protection, on the other hand can it be applied as powerful tracer tool in various fields of applied geosciences. Due to its omnipresence in nature, its chemical and physical properties, and its uncomplicated detectability radon fulfils all requirements for being used as environmental tracer. This application is discussed in the paper with focus on the use of radon as tracer for subsurface contamination with Non-Aqueous Phase Liquids (NAPL). After a short introduction in the ambivalence and ubiquitous presence of radon in nature, the theoretical background of its suitability as NAPL tracer is summarized. Finally three potential applications are discussed. Background information and practical examples are given for (i) the investigation of residual NAPL contamination in soils, (ii) the investigation of residual NAPL contamination in aquifers and (iii) the monitoring of the remediation of dissolved NAPL contamination in groundwater. The presented information reveals that radon is an ideal tracer for the assessment of a wide range of subsurface NAPL contamination. Still, its application is not without restrictions. Problems may occur due to mineralogical heterogeneity of the soil or aquifer matrix. Furthermore, local changes in the permeability of the subsurface may be associated with preferential groundwater or soil gas flow paths bypassing isolated sub-domains of an investigated NAPL source zone. Moreover, NAPL aging may result in alterations in the composition of a complex NAPL mixture thus giving rise to significant changes of the radon partition coefficient between NAPL and water or soil gas. However, since radon shows a strong affinity to NAPLs in general, semi-quantitative results will always be possible.

Testing of NAPL simulator to predict migration of a light nonaqueous phase liquid (LNAPL) under water table fluctuation in a sandy medium

Nanoqueous phase liquid (NAPL) simulator is a powerful and popular mathematical model for modeling the flow and transport of non-aqueous phase liquids in subsurface, but the testing of its feasibility under water table fluctuation has received insufficient attention. The feature in a column test was tested through two cycles of water table fluctuation. The sandy medium in the column was initially saturated, and each cycle of water table fluctuation consisted of one water table falling and one rising, resulting in a drainage and an imbibition of the medium, respectively. It was found that the difference between the simulated and measured results in the first drainage of the column test was minor. However, with the propagation of the water table fluctuations, the simulation errors increased, and the simulation accuracy was not acceptable except for the first drainage in the two fluctuation cycles. The main reason was proved to be the estimation method of residual saturation used in this simulator. Also, based on the column tests, it was assumed that the resulting residual saturation from an incomplete imbibition process was a constant, with a value equal to that of the residual value resulting from the main imbibition process. The results obtained after modifying NAPL simulator with this assumption were found to be more accurate in the first cycle of water table fluctuation, but this accuracy decreased rapidly in the second one. It is concluded that NAPL simulator is not adequate in the case of LNAPL migration under water table fluctuation in sandy medium, unless a feasible assumption to estimate residual saturation is put forward.

Simulation‐based optimization of dual‐phase vacuum extraction to remove nonaqueous phase liquids in subsurface

An integrated simulation‐optimization system was developed for supporting decisions of the dual phase vacuum extraction (DPVE) processes. The system coupled a DPVE process simulator, a multivariate regression tool and a nonlinear optimization model into a general framework. A stepwise‐cluster‐analysis technique was provided for establishing a DPVE process forecasting system for describing the relationships between remediation actions and system responses (i.e., total extracted volume of oil/water, elevation distribution of water table, and specific volume of oil). The forecasting system was then embedded into a multiobjective optimization framework, where the objectives were to minimize the operation cost and maximize the remediation efficiency. The constraints include environmental, hydraulic and technical restrictions to the DPVE processes. A case study was conducted for a petroleum‐contaminated site in western Canada. The results from the stepwise cluster analysis indicated that the generated cluster trees could be used for predicting system responses of the DPVE process, given inputs of the operating conditions. The prediction accuracies of the generated cluster trees were verified using randomly generated data sets. The optimum operating conditions could vary significantly under different cost‐efficiency targets. When a stricter environmental target (i.e., the amount of pollutants in subsurface) was concerned, a higher system cost had to be paid; when the cost became a critical factor, the performance of contaminant removal would have to be compromised. The developed system could be used to analyze tradeoffs between system cost and process efficiency in the DPVE operations; it could also support the formulation of an on‐site process‐control system with vacuum levels and extraction rates being the main control variables.

Estimating Partition Coefficients of Tracers

AbstractProper characterization of subsurface nonaqueous phase liquids is essential for effective remediation efforts. Since tracers are increasingly being used for characterization efforts, it would be helpful if a correlation concerning the distribution of a tracer between water and organic solvents existed such that researchers could estimate partition coefficients. Dwarakanath and Pope (1998) reported a bilinear relationship between tracer partition coefficients and the equivalent alkane carbon numbers of both the contaminant and the alcohol. Since further work by other researchers has not been found in the literature, a similar study was conducted to contribute additional insight. The resulting bilinear relationship was consistent for hydrocarbons and chlorocarbons, linear and branched alcohol tracers, neat compounds, and mixtures. The ability to estimate partition coefficients, when coupled with modeling efforts, enables the researcher to focus on tracers possessing partitioning characteristics most suitable for a given site during the design of tracer tests.

Determination of radon partition coefficients between water and organic liquids and their utilization for the assessment of subsurface NAPL contamination

The inhomogeneous distribution of radon between water and non-aqueous phase liquid (NAPL) allows for utilizing naturally occurring radon as aqueous tracer for the assessment of residual NAPL contamination of aquifers ("NAPL source zones"). For the qualitative assessment (i.e. the localization) of NAPL source zones depleted radon concentrations that occur locally in the groundwater can be used as indicator. However, quantitative estimation of the NAPL saturation of the aquifer pore space requires the knowledge of the radon partition coefficient specific for the NAPL present. While radon partition coefficients are known for a wide range of pure substances, few reliable data is available on radon partitioning into complex NAPL mixtures. At the same time, widely used NAPL mixtures, such as diesel fuel, gasoline, and kerosene, have to be named as main contaminants at many NAPL contaminated sites. The paper presents radon partition coefficients for the three NAPL mixtures mentioned, achieved by application of an analytical method based on radon partitioning between air, water, and NAPL in a closed system, which is described in detail. Based on the experimental results a respective potential theoretical approach employing the regular-solution theory of Hildebrand and Scatchard is discussed and evaluated critically. Finally, the general practical applicability of naturally occurring radon as an indicator for the quantitative evaluation of NAPL source zones is assessed through laboratory experiments carried out in NAPL-contaminated sand columns. The distinct negative correlation between radon concentration and NAPL saturation of the pore space suggests the general applicability of radon for quantitative estimation of NAPL contamination of aquifers.

Determination of radon distribution patterns in the upper soil as a tool for the localization of subsurface NAPL contamination

A radon survey was carried out at an abandoned military airfield, heavily contaminated with non-aqueous phase-liquids (NAPLs). Geo-statistical analysis of the data was used to confirm the validity of the chosen soil gas sampling pattern. The survey revealed a non-uniform distribution of the soil gas radon concentration in the upper soil in spite of a virtually homogenous geological situation. The radon distribution pattern showed minimum zones with radon concentrations decreased by up to 90% with regard to the local background level. The determined radon minimum anomalies could be explicitly associated with the NAPL subsurface contamination. The observed effect is due to the strong partitioning of radon into NAPLs from soil gas or groundwater. Corresponding partitioning coefficients were determined in the laboratory for some NAPL. As result of the study, it was shown that naturally occurring soil gas radon has the potential to be used as an indicator for the localization of subsurface NAPL contamination. As possible options for survey equipment, the AlphaGUARD radon monitor and passive solid-state nuclear track detectors were successfully evaluated.

Hindered gas-phase partitioning of trichloroethylene from aqueous cyclodextrin systems: implications for treatment and analysis.

Chemically enhanced flushing has shown great promise for attenuating subsurface nonaqueous phase liquid (NAPL) contamination. One particular chemically enhanced remediation technology is cyclodextrin enhanced flushing (CDEF). CDEF has been demonstrated as a viable alternative to conventional and innovative remediation methods. However, the presence of cyclodextrin (CD) in solution complicates the treatment and analysis of volatile organic compounds, such as trichloroethylene (TCE). The principal reason for the complications is the presence of TCE in three compartments instead of two, i.e., the aqueous solution, the vapor phase, and complexed inside the soluble CD molecule. Aqueous TCE-CD systems were examined at various concentration and temperature conditions and their respective Henry's law constants were measured. The presence of CD significantly decreased Henry's law constant of TCE. On the basis of these results, a quantitative model was developed to predict the additional effort that becomes necessary when air-stripping TCE from CDEF flushing solution. The modeling results demonstrate that the presence of CD requires significantly higher gas flow rates or longer residence times of the flushing solution inside an air stripper. Similarly, current gas chromatographic purge-and-trap methods for TCE analysis in CD solution appear to underestimate the aqueous phase TCE concentration if the CD concentration of the sample is not accounted for. Although this model was developed specifically for CD-TCE systems, it is likely that these results have implications for other VOCs and other solubilization enhancing agents, such as surfactants or cosolvents.

Investigation of Fluorescent Dyes as Partitioning Tracers for Subsurface Nonaqueous Phase Liquid (NAPL) Characterization

Five fluorescent dyes were evaluated as partitioning tracers for detection and quantification of tetrachloroethylene (PCE): fluorescein, rhodamine WT (RWT), sulforhodamine B (SRB), eosine, and pyranine. Results obtained from batch tests were (a) sorption partitioning coefficients (KP) 0.088±0.025 and 0.091 cm3/g, for RWT and SRB, respectively (eosine and pyranine increased fluorescence after soil/dye batch tests; thus, KP could not be obtained for these tracers) and (b) nonaqueous phase liquid/water partitioning coefficients (KNW) for RWT, SRB, and eosine were 0.1703, 0.0603, and 0.049, respectively (pyranine also increased fluorescence in PCE/dye tests). Retardation factors (Rf) measured in column tests for fluorescein, RWT, SRB, eosine, and pyranine were (a) 1.08±0.07, 1.64±0.38, 1.23, 1.45, and 2.97, in the absence of PCE; and (b) 1.31, 2.3, 1.33, 1.79, and 3.51, in the presence of PCE, respectively. The results of this study suggested that the partitioning behavior of the tracers tested is different f...

Influence of a solubility‐enhancing agent (cyclodextrin) on NAPL‐water partition coefficients, with implications for partitioning tracer tests

The partitioning tracer test (PTT) is used to quantify subsurface nonaqueous phase liquid (NAPL) saturations (SN). The laboratory‐measured NAPL‐water partitioning coefficient (KNW) is required to calculate the SN. The complexing agent hydroxypropyl−β−cyclodextrin (HPCD) has been used to successfully remediate NAPL‐contaminated sites. Residual concentrations of HPCD may remain in the groundwater during a postremediation PTT. This investigation examines the influence of HPCD on the trichlorethene NAPL‐water partitioning coefficients of several alcohol tracers. Laboratory results indicate that aqueous HPCD concentrations of 2000 mg/L result in the reduction of KNW values by up to 33%. This effect can result in up to 33% error in the PTT‐estimated SN. The effect decreases as the original KNW value of a tracer decreases. The relationship between KNW values and HPCD concentrations is nonlinear and yields curves characteristic of each tracer. Empirical relationships are offered to reproduce the characteristic curves based on tracer KNW values to allow estimates of corrected KNW values for an arbitrary tracer.

Numerical Simulation of Dual-Phase Vacuum Extraction to Remove Nonaqueous Phase Liquids in Subsurface

Dual-phase vacuum extraction (DPVE) is an emerging cost-effective technology used to enhance remediation by recovering petroleum hydrocarbon from the subsurface. To improve its efficiency, insight into the complex processes and phase/component interactions in the remediation system needs to be gained before the system can be properly designed and operated. In this paper, a numerical simulation model is proposed to analyze the behavior of the complex DPVE remediation system. The simulator developed is then applied to a petroleum-contaminated site in western Canada that is undergoing the DPVE process. The influence of vacuum pressure employed at extraction wells on system performance is investigated. Moreover, the effects of parameter uncertainties in modeling results are also examined. The proposed approach offers an effective tool for evaluating remediation system performance and supporting decisions on site remediation and management practices.

Using the soil gas radon as an indicator for ground contamination by non-aqueous phase-liquids

One of the major problems facing risk assessment at polluted industrial sites and military bases is subsurface contamination by non-aqueous phase-liquids (NAPLs), since tracing the extent of a NAPL plume using conventional methods (drive point profiling) is usually associated with difficulties. In an effort to trace subsurface contamination as precisely as possible, monitoring points are placed in the area that might be affected by contaminants, and groundwater and soil samples are taken to the laboratory for analysis. However, the final number of monitoring points is hardly ever sufficient for distinctive contamination mapping, and this may ultimately result in an unsuitable remediation action being taken. To obtain a more detailed image of a subsurface NAPL plume and, hence, to facilitate remediation measures that are best suited for the site in question, a denser network of monitoring points is desirable. The aim of the investigation described in this paper was therefore to develop a new detection method for subsurface NAPL contamination, which is based on an easily accessibleindicator for NAPLs rather than on the analysis of soil and groundwater samples taken at the site. Based on the good solubility of radon in NAPLs, the idea was put forward that subsurface NAPL contamination should have an influence on the natural radon concentration of the soil gas. Provided this effect is significant, it would be possible to carry out a straightforward radon survey on an appropriate sampling grid covering the suspected site and thus enabling the NAPL contamination to be detected by the localization of anomalous low radon concentrations in the soil. The overall aim of the investigation was to assess the general suitability of the soil-gas radon concentration as an indirect tracer for NAPL contamination in the ground. The partitioning coefficient KNAPL/air is one of the most influential parameters governing the decrease of the radon concentration in the soil gas in the presence of a subsurface NAPL contamination. Since NAPL mixtures such as gasoline, diesel fuel and paraffin are among the most important NAPLs regarding remediation activities, laboratory experiments were performed to determine the radon-partitioning coefficient for these three NAPL mixtures. Field experiments were carried out as well. The aim of the field experiments was to test the use of the soil-gas radon concentration as a tracer for NAPL contamination on-site. For the field experiments, each site was covered with a suitable grid of soil gas sampling points. Finally, the lateral radon distribution pattern achieved on each of the sites was compared to the respective findings of the earlier research performed by conventional means. The results of the laboratory experiments clearly show a very strong affinity of radon to the NAPL mixtures examined. The partitioning coefficients achieved correspond to those published for pure NAPLs (Clever 1979) and are thus in the expected range. The results of the field experiments showed that the minimum radon concentrations detected match the respective NAPL plumes traced previously. Both the results of the lab experiments and the on-site findings demonstrate that the soil-gas radon concentration can be used as an indicator for subsurface NAPL contamination. The investigation showed that NAPL-contaminated soil volumes give rise to anomalous low soil-gas radon concentrations in the close vicinity of the contamination. The reason for this decrease in the soil-gas radon concentration is the good solubility of radon in NAPLs, which enables the NAPLs to accumulate and ‘trap’ part of the radon available in the soil pores. Further research is required into contamination with rather volatile NAPLs such as BTEX. Further research is also needed to examine whether it is possible to not only localize a NAPL plume, but also to obtain some quantitative information about the subsurface NAPL contamination. The authors also believe that additional investigations should be carried out to study the ability of the method to not just localize a NAPL contamination, but also to monitor on-site, clean-up measures.

Laboratory characterization of non-aqueous phase liquid/tracer interaction in support of a vadose zone partitioning interwell tracer test

Contaminant characterization is important for successful remediation of non-aqueous phase liquids (NAPLs) in the unsaturated zone. A partitioning interwell tracer test (PITT) can provide a good estimate of average subsurface NAPL saturations. Screening experiments were completed in the laboratory to evaluate several gas tracers for a PITT study to be completed in the vadose zone at Kirtland Air Force Base in Albuquerque, NM. Four perfluorocarbon tracers were found to be suitable for this PITT. Further laboratory column studies were completed using contaminated field soil to measure the partition coefficients between the tracers and the NAPL. The results from the column studies showed that the air/NAPL tracer partition coefficients ranged from 8.8±0.6 to 71±3. This range of partition coefficients is suitable for detection of NAPL saturations in the field of 0.002 to 0.14.

Field test of high molecular weight alcohol flushing for subsurface nonaqueous phase liquid remediation

A pilot scale field test of non‐aqueous phase liquid (NAPL) removal using high molecular weight alcohols was conducted at Operable Unit 1, Hill Air Force Base, Utah. Petroleum hydrocarbons and spent solvents were disposed of in chemical disposal pits at this site, and these materials are now present in the subsurface in the form of a light non‐aqueous phase liquid (LNAPL). This LNAPL is a complex mixture of aromatic and aliphatic hydrocarbons, chlorinated solvents, and other compounds. The field experiment was performed in a 5 m by 3 m confined test cell, formed by driving interlocking sheet pile walls through the contaminated zone into an underlying clay. The test involved the injection and extraction of about four pore volumes (1 pore volume=7000 L) of a mixture of 80% tert‐butanol and 15% n‐hexanol. The contaminants were removed by a combination of NAPL mobilization and enhanced dissolution, and the results of postflood soil coring indicate better than 90% removal of the more soluble contaminants (trichloroethane, toluene, ethylbenzene, xylenes, trimethylbenzene, naphthalene) and 70–80% removal of less soluble compounds (decane and undecane). The results of preflood and postflood NAPL partitioning tracer tests show nearly 80% removal of the total NAPL content from the test cell. The field data suggest that a somewhat higher level of removal could be achieved with a longer alcohol injection.

Difluoromethane as Partitioning Tracer to Estimate Vadose Water Saturations

Water saturation in the vadose zone is an important parameter for many nonaqueous phase liquid (NAPL) remediation technologies. Conventional soil boring analyses may not provide accurate average water saturation data. Previous studies have shown that a partitioning interwell tracer test (PITT) can provide an accurate estimate of average subsurface NAPL saturations. The PITT is proposed as a suitable technology for estimating average subsurface water saturations. In this research, difluoromethane was evaluated as a suitable tracer compound to partition among air, water, and NAPL phases. Method of moments analyses were used to develop equations necessary to determine water saturations from difluoromethane PITT results. One-dimensional column experiments provided air-water and air-NAPL partition coefficient data for difluoromethane. Difluoromethane is shown to be a suitable tracer for use in field PITTs to predict water saturations in the vadose zone.

SIMILITUDE REQUIREMENTS FOR MODELLING NAPL MOVEMENT WITH A GEOTECHNICAL CENTRIFUGE.

Keywords MASSACHUSETTS TECHNOLOGY INSTITUTE, MIT, EDINBURGH UNIVERSITY CENTRIFUGES, GEOTECHNICAL, MOVEMENT, MODELLING, NON AQUEOUS, PHASE, LIQUIDS, NAPL, TRANSPORT, SIMILITUDE, MODELS, GEOTECHNICS, MEDIA, POROUS, SCALING, STUDIES, SUBSURFACE, UNDERGROUND, WETTING, MIGRATION, INSTABILITY, BEHAVIOUR, GROUNDWATER, POLLUTION, REQUIREMENTS, RELATIONSHIPS... Show All

Considerations for robust compositional simulations of subsurface nonaqueous phase liquid contamination and remediation

Considerations for robust compositional simulations of subsurface nonaqueous phase liquid contamination and remediation

Considerations for robust compositional simulations of subsurface nonaqueous phase liquid contamination and remediation : S. Panday, P. A. Forsyth, R. W. Falta, Yu-Shu Wu & P. S. Huyakorn, Water Resources Research, 31(5), 1995, pp 1273–1289

Considerations for robust compositional simulations of subsurface nonaqueous phase liquid contamination and remediation : S. Panday, P. A. Forsyth, R. W. Falta, Yu-Shu Wu & P. S. Huyakorn, Water Resources Research, 31(5), 1995, pp 1273–1289