Non-aqueous Phase Liquid Contamination Research Articles

Predictions of bioenhancement of nonaqueous phase liquid ganglia dissolution using first- and zero-order biokinetic models

The bioenhanced dissolution of nonaqueous phase liquid (NAPL) contaminants that occurs as a result of an increased concentration gradient is influenced by several factors, including the biokinetics. This is important because available data suggest that at typical NAPL source zone concentrations, descriptions of dissolution bioenhancement may require kinetic expressions ranging from first- to zero-order. In this work, an analytical model for the bioenhancement factor, E, is developed for NAPL ganglia dissolution with zero-order kinetics, and compared to a model for E with first-order kinetics. The models are analyzed and an illustrative example is provided to demonstrate the importance of using the correct biokinetics when estimating the potential magnitude of the bioenhancement of NAPL ganglia dissolution.

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.

Reflection waveform inversion of ground-penetrating radar data for characterizing thin and ultrathin layers of nonaqueous phase liquid contaminants in stratified media

Accurately quantifying thin-layer parameters by applying a targeted reflection waveform inversion methodology to ground-penetrating radar (GPR) reflection data may provide a useful tool for near-surface investigation and especially for contaminated site investigation where nonaqueous phase liquid (NAPL) contaminants are present. We implemented a targeted reflection waveform inversion algorithm to quantify thin-layer permittivity, thickness, and conductivity for NAPL thin ([Formula: see text] dominant wavelength [Formula: see text]) and ultrathin ([Formula: see text]) layers using GPR reflection data. The inversion used a nonlinear grid search with a Monte Carlo scheme to initialize starting values to find the global minimum. By taking a targeted approach using a time window around the peak amplitude of the reflection event of interest, our algorithm reduced the complexity in the inverse problem. We tested the inversion on three different synthetic data sets and four field data sets. In all testing, the inversion solved for NAPL-layer properties within 15% of the measured values. This algorithm provides a tool for site managers to prioritize remediation efforts based on quantitative assessments of contaminant quantity and location using GPR.

Methodology for Optical Imaging of NAPL 3D Distribution in Transparent Porous Media

Three-dimensional mapping of non-aqueous phase liquid (NAPL) distribution within saturated porous media is an important issue in bench-scale geo-environmental studies. In this paper, a methodology for optical imaging of 3D multiphase liquid distribution is presented. The natural aquifer is simulated using a transparent soil surrogate that represents the macroscopic behavior of natural sand. To achieve transparency transparent fused quartz grains were saturated with a matched refractive index mineral oil solution that represents the natural aquifer and injected with a green-dyed sucrose solution to simulate dense NAPL contamination. The spatial volume was first estimated and then reconstructed using orthogonal images acquired at the model boundaries at various stages of contamination and remediation. Color space analysis was employed to segment the NAPL zone and transform pixel information into integrated concentration values using a previously published calibration model. The chromatic components CR and a of YCBCR and Lab, respectively, were combined to render the spatial concentration profile. A novel iterative reconstruction algorithm named 3D carving was used to resolve three 2D projections into a 3D model. The results show that the proposed methodology provides better efficacy for NAPL zone reconstruction in comparison with conventional image analysis routines. The technology presented in this paper is a sustainable, fast, accurate, non-intrusive, and inexpensive method for spatial mapping of contamination zones using bench-scale transparent soil models.

Numerical modeling analysis of hydrodynamic and microbial controls on DNAPL pool dissolution and detoxification: Dehalorespirers in co-culture

Dissolution of dense non-aqueous phase liquid (DNAPL) contaminants like tetrachloroethene (PCE) can be “bioenhanced” via biodegradation, which increases the concentration gradient at the DNAPL–water interface. Model simulations were used to evaluate the impact of ecological interactions between different dehalorespiring strains and hydrodynamics on the bioenhancement effect and the extent of PCE dechlorination. Simulations were performed using a two-dimensional coupled flow-transport model, with a DNAPL pool source and two microbial species, Dehalococcoides mccartyi 195 and Desulfuromonas michiganensis, which compete for electron acceptors (e.g., PCE), but not for their electron donors. Under biostimulation, low vx conditions, D. michiganensis alone significantly enhanced dissolution by rapidly utilizing aqueous-phase PCE. In co-culture under these conditions, D. mccartyi 195 increased this bioenhancement modestly and greatly increased the extent of PCE transformation. Although D. michiganensis was the dominant population under low velocity conditions, D. mccartyi 195 dominated under high velocity conditions due to bioclogging effects.

Tracing NAPLs contamination in the vadose zone using soil radon

Frequent spills of Non-Aqueous Phase Liquids (NAPLs), such as petroleum products or chlorinated solvents, occur all around the world contaminating the soil vadose zone or the aquifer. In order to trace the presence of these substances, radon concentration in soil gas can be employed because of its high solubility in NAPLs. Radon available in the soil pores accumulates in the NAPLs producing a local decrease of its concentration in the vicinity of contaminated areas. Thus radon minima in the vadose zone can be used to localize NAPL plumes indirectly.The suitability of this method is here checked by laboratory simulation of a gasoline spill within a known volume of soil. Radon activity concentration strongly drops down right after the injection of gasoline and progressively increases in the following days. This trend is compared with data from a simultaneous experiment carried out on a volume of the same soil, not contaminated by NAPL.Finally, an example of field measurements within a site affected by NAPL contamination is documented. Mapping of soil radon concentration at 80 cm depth shows an area where radon deficit is evident. This region is aligned along the directions of the groundwater flow induced by pumping for safety operation of the plant. Radon negative anomalies are not due to low gas permeability and are not correlated with soil CO2. The comparison of soil gas data with those from nearby background areas confirm the soundness of premises.

Numerical studies on kinetics of sorption and dissolution and their interactions for estimating mass removal of toluene from entrapped soil pores

When the non-aqueous phase liquid (NAPL) contaminant is entrapped in soil pores, their release to the subsurface environment is mainly limited by interphase mass transfer such as dissolution and sorption. Sorption onto the aquifer material plays an important role as a retardation mechanism in subsurface contaminant transport processes. Considering the heterogeneity associated with aquifer properties as well as contaminant source distribution, long-term contamination is an inevitable consequence of mass transfer limitation of residual mass from soil micropores. A comprehensive numerical model is presented in this study for understanding the kinetic nature of dissolution and sorption of hydrophobic hydrocarbons and their interactions in order to estimate the rate and extent of mass removal from different phases. The results showed that sorption by soils and sediments having different physicochemical properties generally follow non-linear behavior. It is observed from the study that sorption non-linearity can be effectively incorporated by the two-site kinetic model rather than combination of linear and non-linear sorption isotherms. Even though initial phase of mass transfer is dissolution-dominated, the extended tailing of concentration at higher pore volumes is controlled by non-equilibrium sorption, which is better explained by the two-site kinetic model.

Field study of using naturally occurring radon to assess the dense non-aqueous phase liquid distribution in saturated zone

The concept of radon deficiency such as the ratios of radon concentrations to the maximum measured value of a sample batch was employed as the survey methodology for this study to investigate contamination sources in an industrial zone that was suspected of causing subsurface dense non-aqueous phase liquid (DNAPL) contamination. The results showed that radon concentrations in certain wells were significantly lower than that in uncontaminated regions. Radon concentrations in groundwater are influenced by the in situ bioremediation of vegetable oil, which causes abnormal reductions of the radon in groundwater because radon partitions into vegetable oil and results in more variable for the radon deficit method to showing the impacts of remediation. Six contaminated regions were identified by integrating radon concentration ratios (divided into low (L), middle (M), and high (H) levels) and DNAPL concentrations (divided into low (L) and high (H) levels). Contaminated regions in the LH, MH, and HH categories are located in the vicinity of the contamination source, and those in the HL category are located far from the source zone. The ML and LL categories indicate the involvement of unknown factors, and that additional analyses are required to uncover the facts that affect radon and DNAPL concentrations.

Volumetric scale-up of smouldering remediation of contaminated materials

Smouldering remediation is a process that has been introduced recently to address non-aqueous phase liquid (NAPL) contamination in soils and other porous media. Previous work demonstrated this process to be highly effective across a wide range of contaminants and soil conditions at the bench scale. In this work, a suite of 12 experiments explored the effectiveness of the process as operating scale was increased 1000-fold from the bench (0.003m3) to intermediate (0.3m3) and pilot field-scale (3m3) with coal tar and petrochemical NAPLs. As scale increased, remediation efficiency of 97–99.95% was maintained. Smouldering propagation velocities of 0.6–14×10−5m/s at Darcy air fluxes of 1.54–9.15cm/s were consistent with observations in previous bench studies, as was the dependence on air flux. The pilot field-scale experiments demonstrated the robustness of the process despite heterogeneities, localised operation, controllability through airflow supply, and the importance of a minimum air flux for self-sustainability. Experiments at the intermediate scale established a minimum-observed, not minimum-possible, initial concentration of 12,000mg/kg in mixed oil waste, providing support for the expectation that lower thresholds for self-sustaining smouldering decreased with increasing scale. Once the threshold was exceeded, basic process characteristics of average peak temperature, destructive efficiency, and treatment velocity were relatively independent of scale.

Formulation design for target delivery of iron nanoparticles to TCE zones

Nanoparticles of zero-valent iron (NZVI) are effective reducing agents for some dense non-aqueous phase liquid (DNAPL) contaminants such as trichloroethylene (TCE). However, target delivery of iron nanoparticles to DNAPL zones in the aquifer remains an elusive feature for NZVI technologies. This work discusses three strategies to deliver iron nanoparticles to DNAPL zones. To this end, iron oxide nanoparticles coated with oleate (OL) ions were used as stable analogs for NZVI. The OL-coated iron oxide nanoparticles are rendered lipophilic via (a) the addition of CaCl2, (b) acidification, or (c) the addition of a cationic surfactant, benzethonium chloride (BC). Mixtures of OL and BC show promise as a target delivery strategy due to the high stability of the nanoparticles in water, and their preferential partition into TCE in batch experiments. Column tests show that while the OL-BC coated iron oxide nanoparticles remain largely mobile in TCE-free columns, a large fraction of these particles are retained in TCE-contaminated columns, confirming the effectiveness of this target delivery strategy.

Using 222Rn as a naturally occurring tracer to estimate NAPL contamination in an aquifer

The naturally occurring radioisotope 222Rn was used as a partitioning tracer to evaluate the presence and amount of a non-aqueous phase liquid (NAPL) in an aquifer. The batch experiment results of a diesel contaminated soil showed that the emanation rate of 222Rn decreased to 31%, relative to a background rate. Batch experiment results of water contaminated by gasoline, diesel, PCE and TCE were similar. A field study to examine TCE contamination was conducted using 54 groundwater samples in Wonju city, Republic of Korea. Estimates of TCE contamination ranged from <0.001mg/L to 14.3mg/L, and 222Rn concentrations ranged from 1.89Bq/L to 444. Results of 222Rn analysis showed that TCE contamination was mainly restricted to an asphalt laboratory area and that the 222Rn values of a NAPL-contaminated aquifer were correlated with TCE analytical results.

Integrating spatial and temporal oxygen data to improve the quantification of in situ petroleum biodegradation rates

Accurate estimation of biodegradation rates during remediation of petroleum impacted soil and groundwater is critical to avoid excessive costs and to ensure remedial effectiveness. Oxygen depth profiles or oxygen consumption over time are often used separately to estimate the magnitude and timeframe for biodegradation of petroleum hydrocarbons in soil and subsurface environments. Each method has limitations. Here we integrate spatial and temporal oxygen concentration data from a field experiment to develop better estimates and more reliably quantify biodegradation rates. During a nine-month bioremediation trial, 84 sets of respiration rate data (where aeration was halted and oxygen consumption was measured over time) were collected from in situ oxygen sensors at multiple locations and depths across a diesel non-aqueous phase liquid (NAPL) contaminated subsurface. Additionally, detailed vertical soil moisture (air-filled porosity) and NAPL content profiles were determined. The spatial and temporal oxygen concentration (respiration) data were modeled assuming one-dimensional diffusion of oxygen through the soil profile which was open to the atmosphere. Point and vertically averaged biodegradation rates were determined, and compared to modeled data from a previous field trial. Point estimates of biodegradation rates assuming no diffusion ranged up to 58 mg kg−1 day−1 while rates accounting for diffusion ranged up to 87 mg kg−1 day−1. Typically, accounting for diffusion increased point biodegradation rate estimates by 15–75% and vertically averaged rates by 60–80% depending on the averaging method adopted. Importantly, ignoring diffusion led to overestimation of biodegradation rates where the location of measurement was outside the zone of NAPL contamination. Over or underestimation of biodegradation rate estimates leads to cost implications for successful remediation of petroleum impacted sites.

Non-aqueous Phase Liquid Spills in Freezing and Thawing Soils: Critical Analysis of Pore-Scale Processes

The frequent use of non-aqueous phase liquids (NAPLs) in cold regions creates serious risks of soil and groundwater contamination. NAPL contaminants can stay in soil for long times due to their entrapment by strong interfacial forces, resulting in a source of pollution caused by their slow dissolution in groundwater over decades. The presence of these contaminants in ice-containing soils creates a four-phase problem, which must be fully understood to design/develop or improve remediation techniques in cold climate regions. In this review, the fate and transport of NAPL contaminants in periodically freezing-thawing and frozen soils is discussed, with emphasis on pore-scale processes. Three topics are identified for future research focus: (a) study of the dynamics of NAPLs during freezing and thawing of soils using non-destructive imaging techniques, and the effect of various factors, including wettability, pore size, consolidation of porous matrix, and fluid properties; (b) investigations of the fate and transport of NAPL contaminants in frozen soils with different wettabilities, and the effect of the spatial distribution of ice clusters on NAPL retention and movement; and (c) pore-scale modeling of the fate and transport of NAPL spills in freezing-thawing and frozen soils. This will lead us towards a complete pore-scale understanding of NAPL spills in cold climate soils, and their fate and transport over time.

Centrifuge simulation of LNAPL transportation in sand deposits with containment under flow and no-flow conditions

Light nonaqueous phase liquid (LNAPL) contamination of subsoil is a serious problem affecting human health and disturbing the environment. This paper describes centrifuge tests of LNAPL migration in a sandy soil layer that were focused on a soil-cement barrier as a containment to protect the groundwater from contamination. Two centrifuge tests were performed in this study for the following two scenarios: groundwater flow and no groundwater flow conditions. Liquid paraffin oil was used to represent LNAPL, and it was infiltrated into the partially saturated sand deposit. The centrifuge tests were performed at 30g, and pore pressure sensors and camera recordings were used to evaluate the model behaviour. The results show that groundwater flow accelerates LNAPL migration in sandy soil. In addition, LNAPL was found to migrate below the groundwater level, and the soil-cement barrier was found to serve effectively for containment.

Dielectric Response of a Variable Saturated Soil Contaminated by Non-Aqueous Phase Liquids (NAPLs)

In recent years, several studies have been conducted both in saturated and unsaturated soils to detect non-aqueous phase liquid (NAPL) hydrocarbon contamination in soils and groundwater by means of the time domain reflectometry (TDR) technique. This technique is widely used for measuring the dielectric permittivity and bulk electrical conductivity of multiphase systems. Only accurate knowledge of the dielectric response of soil matrix- water-NAPL (saturated condition) or soil matrix-air-water-NAPL (unsaturated condition) systems can allow the volumetric NAPL content (θNAPL) to be determined in the soil. This paper investigates the influence of NAPL contamination (corn oil, a non-volatile and non-toxic NAPL, was used) on TDR measurement in a volcanic soil, relating dielectric permittivity of the multiphase soil system to volumetric fluid content θf (i.e. water+NAPL). The soil samples were oven dried at 105°C and passed through a 2mm sieve. Known quantities of soil, water and oil were mixed and repacked into plastic cylinders (15cm high and 9.5cm in diameter); 40 different combinations of water and oil were tested, with θNAPL varying from 0.05 to 0.40 by 0.05cm3/cm3 increments. A volumetric mixing model with three (soil matrix-water-NAPL) or four (soil matrix-air-water-NAPL) phases permitted conversion from a dielectric permittivity domain into a θf domain. The results show that, the amount of contaminant in soil can be inferred if the total volume of pore fluid θf and the dielectric permittivity of the contaminated soil are known. Further work will be built on this initial study, concentrating on: i) enhancing the model linkage and validating it with new laboratory results; ii) validating the developed TDR interpretation tool with field results.

Remobilization of Residual Non-Aqueous Phase Liquid in Porous Media by Freeze−Thaw Cycles

The pore-scale behavior of a nonaqueous phase liquid (NAPL) trapped as residual contamination in a porous medium, subject to freeze-thaw cycles, was investigated by X-ray microcomputed tomography. It is shown that freeze-thaw cycles cause significant NAPL remobilization in the direction of the freezing front, due to the rupture and transport of a significant proportion of (supposedly entrapped) larger multipore NAPL ganglia. Significant NAPL remains in place, however, due to substantial ganglion fragmentation into single- and subpore ganglia. The contraction of branched ganglia into more rounded forms, especially near the top surface, is also observed. Three freezing-induced mechanisms are proposed to explain the results. The findings have important implications for NAPL contamination in cold regions, and for the behavior of water-hydrocarbon systems on the Earth and other planets.

Anionic Homopolymers Efficiently Target Zerovalent Iron Particles to Hydrophobic Contaminants in Sand Columns

The transport of microscale carbonyl iron powder suspensions modified with anionic homopolymers was studied in water-saturated sand columns containing well-dispersed hydrophobic sand grains. Sand grains functionalized with hexadecyltrimethoxysilane were coated with a eutectic mixture of dichlorobenzenes that was solid at -10 °C and was mixed by grinding with unmodified sand grains. The dichlorobenzene coating liquefied at the temperature of the transport experiments, and the coated grains were thus mimetic of uniform droplets of dense nonaqueous phase liquid (DNAPL) contaminants. By comparing iron particle transport in uncontaminated columns with those that contained a small fraction of DNAPL-coated sand grains, sticking coefficients for both types of grains could be estimated. The anionic polyelectrolytes tested (polyacrylate, carboxymethylcellulose, alginate, and metasilicate) all gave low particle sticking coefficients (0.004-0.05) to unmodified sand, as expected from earlier studies. However, iron particles modified with the two moderately hydrophobic polymers (carboxymethylcellulose and polyacrylate) had 30-fold higher sticking coefficients (0.40 and 0.13, respectively) to the model DNAPL surface than they did to the sand surface. In contrast, no significant difference between the two kinds of collector grains was found with the more polar polymers (metasilicate and alginate). The trend in sticking coefficients was correlated with the surface energy of the polymer-modified iron surface as measured by the static contact angle method. From these data one can conclude that the hydrophobicity of the polymer dispersant is a key factor in targeting zerovalent iron to DNAPL source zones in soil and groundwater.

Predicting Benzene Transport in Subsurface under Uncertainty through a Coupled Monte Carlo and Factorial Analysis Approach

Mathematical models have been widely used to simulate fate and transport of the nonaqueous phase liquids (NAPLs) contaminants for risk assessment and remediation design studies. However, many theoretical and field studies have recognized that the contaminant fate in subsurface is significantly influenced by uncertainties inherent in natural porous media and thus may affect model predictions. To tackle such a difficulty, a coupled Monte Carlo and factorial analysis modeling approach was developed in this study to systematically investigate impacts of uncertainties associated with hydrocarbon-contaminant transport in subsurface. The approach integrated a solute transport model, factorial analysis, and Monte Carlo technique into a general framework and effectively analyzed the individual and joint effects of input parameters' uncertainties that are associated with hydrogeological conditions. Through a hypothetical case study, the results demonstrated that the uncertainties in input parameters pose considerable influences on the predicted output. The results obtained from the systematic uncertainty analysis methods proposed in this study, such as mean, standard deviation, and percentile, can provide useful information for further decision making regarding the petroleum contamination problem.

Experimental investigation of the applications of natural freezing and curdlan biopolymer for permeability modification to remediate DNAPL contaminated aquifers in Alaska

Dense Non-aqueous Phase Liquids (DNAPL) contamination is one of the major environmental concerns today. DNAPL can remain in significant quantities as residual contaminants in the low permeability zones even after the bulk phase has been removed. As the drive fluid sweeps through the aquifer it follows the path of least resistance, which is the high permeability zone. Thus the contaminants trapped in the low permeability zones remain as residuals and serve as a source for prolonged contamination. Conventional remediation techniques are ill-equipped to deal with the heterogeneities of the aquifers. Various techniques to enhance the efficiency of the conventional methods have been tried without significant success. In this study we have tested a hypothesis, “A combination of natural freezing/thawing and microbial gel sealant in layered heterogeneous porous media along with a conventional remediation technique can be used effectively to reduce the volume of residual DNAPL in cold regions” to assess an alternative for remediation of residual DNAPL from soils and aquifers in cold regions such as Alaska. Reducing the temperature of soil formations can modify aquifer flow paths. The natural freezing of soils in winter may be used effectively to modify the flow paths. In summer, permeability modification can be accomplished by emplacement of microbial polymer gels. We have investigated a cost-effective and efficient technique to reduce the volume of residual DNAPL using a combination of natural freezing in winter and biopolymer application in summer through a series of one-dimensional laboratory scale column experiments. Specifically, we have investigated the amount of residual DNAPL volume reduction by utilizing the permeability modification approach and thus the modification of preferential flow path using freezing alone and by introducing curdlan biopolymer as a gelant to block the high conductive soil and sweep the DNAPL from residual zones. Our results suggest that a combination of natural freezing of soils in winter and a microbial biopolymer gelant in summer can be used effectively to reduce the volume of residual DNAPL by up to 90% of the total volume of residual DNAPL.

Diffusive partitioning tracer test for the quantification of nonaqueous phase liquid (NAPL) in the vadose zone: Performance evaluation for heterogeneous NAPL distribution

A partitioning tracer test based on gas-phase diffusion in the vadose zone yields estimates of the residual nonaqueous phase liquid (NAPL) saturation. The present paper investigates this technique further by studying diffusive tracer breakthrough curves in the vadose zone for a heterogeneous NAPL distribution. Tracer experiments were performed in a lysimeter with a horizontal layer of artificial kerosene embedded in unsaturated sand. Tracer disappearance curves at the injection point and tracer breakthrough curves at some distance from the injection point were measured inside and outside of the NAPL layer. A numerical code was used to generate independent model predictions based on the physicochemical sand, NAPL, and tracer properties. The measured and modeled tracer breakthrough curves were in good agreement confirming the validity of important modeling assumptions such as negligible sorption of chlorofluorocarbon (CFC) tracers to the uncontaminated sand and their fast reversible partitioning between the soil air and the NAPL phase. Subsequently, the model was used to investigate different configurations of NAPL contamination. The experimental and model results show that the tracer disappearance curves of a single-well diffusive partitioning tracer test (DPTT) are dominated by the near-field presence of NAPL around the tip of the soil gas probe. In contrast, breakthrough curves of inter-well tracer tests reflect the NAPL saturation in between the probes, although there is no unique interpretation of the tracer signals if the NAPL distribution is heterogeneous. Numerical modeling is useful for the planning of a DPTT application. Simulations suggest that several cubic meters of soil can be investigated with a single inter-well partitioning tracer test of 24-hour duration by placing the injection point in the center of the investigated soil volume and probes at up to 1 m distance for the monitoring of gaseous tracers.