Residual Nonaqueous Phase Liquid 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.

Enhanced mass transfer of residual NAPL by convection in stagnant zone

Cosolvent flushing is a promising method to improve NAPLs (non-aqueous phase liquids) removal. Nevertheless, the removal performance is undermined by the tailing phenomenon, i.e., the contaminant concentration tends to remain unchanged in later period as NAPL is residually trapped in the stagnant zone. High proportion of stagnant zone leads to more severe tailing phenomenon and how to accelerate dissolution of NAPL in the stagnant zone is important for NAPL remediation. However, few work has studied the remediation process at pore-scale due to limited observation techniques. As a result, the fundamental process of convection enhancement remains unclear. In this work, dynamics of NAPL removal in the stagnant zone is quantitatively investigated in transparent micromodels via the use of Raman spectroscopy. Impacts of injection condition and properties of the porous media are systematically investigated. Results indicate that convection dominants the mass transfer in aqueous phase and enhances the NAPL dissolution. Increasing the flushing velocity can enhance convection, but when the flushing velocity exceeds 3 cm/min, convection in the region away from the dominant channel is little affected by the flushing velocity. Differently, increasing the pore throat size will enhance convection in the entire stagnant zone and accelerate the whole dissolution process.

Modeling of Complex, Multi‐Component NAPL Remediation for Decision Support

AbstractIn situ remediation of nonaqueous phase liquid (NAPL)‐impacted sites is difficult and costly. Even with enhancements (e.g., chemical and thermal) and partial NAPL recovery, mass transfer constraints associated with partitioning from residual NAPL typically control the depletion rate of sources and attainment of cleanup goals. Practical methodologies are needed to support strategic evaluation, planning, and implementation of cost‐effective remedial approaches that are considered protective. A modeling system averaging over the NAPL‐impacted saturated soil volume was developed and demonstrated at the former Williams Air Force Base. The system combines upscaled, physically based mass transfer coefficients for multi‐component NAPL dissolution with theoretical enhancements specific to multiple remediation processes. Dissolution increases in the presence of aqueous phase reactions, according to first‐ or second‐order kinetics, by increasing concentration gradients. Slow biological processes are considered first order in modeling natural attenuation and enhanced biological degradation. Fast reactions associated with chemical oxidation are considered second order. These enhancement models are equally applicable to numerical simulations of NAPL remediation. Pump and treat enhances dissolution in proportion to increases in the characteristic velocity associated with dissolution. The demonstration yielded realistic predictions, with greater certainty, for outcomes from of multiple technologies intended to reduce remedial timeframes and life cycle costs. The enhanced dissolution modeling provides a site‐specific, quantitative assessment of changes in NAPL source discharge concentration and mass discharge over time for various remedial options that is equivalent to assessments from complex numerical transport models, given typical input data limitations.

Residual NAPL Morphology Effects on Electrical Resistivity: Insights From Micromodel Displacement Experiments and Pore Network Simulations

AbstractCharacterizing residual non‐aqueous phase liquids (NAPLs) in porous media is essential for designing contaminated site remediation strategies. The direct current resistivity method is increasingly being used at sites contaminated by NAPLs, which uses the Archie's second law to connect NAPL saturation with the resistivity of porous media. A power‐law relationship connecting the phase morphology measured by Euler characteristic with resistivity has also been observed at the pore‐scale. Because these relationships are limited for porous media with specific wettability, previous research works have demonstrated deviations from Archie's second law in both the laboratory and field experiments. To evaluate the effects of residual NAPL morphology with different wettability on bulk resistivity, we developed two 2D‐micromodels and measured their electrical resistivity and phase morphology during phase displacement under laboratory‐controlled conditions. Furthermore, pore network simulations of a variety of conditions were performed in absence of surface conduction. Results show that NAPL distributed in dead ends of the pore space has limited influence on bulk resistivity since they do not control the current path and voltage distribution. The correlations between resistivity index and Euler characteristic are determined by NAPL ganglia size and pore throat width. A power‐law relationship is found between the Archie's saturation exponent and the average size of the residual NAPL ganglia, indicating that this exponent can be interpreted as the rate of change of the pore water connectivity with saturation. These insights provide a physical framework connecting the morphology and wettability of the NAPL ganglia with the Archie's saturation exponent.

Application of a Developed Numerical Model for Surfactant Flushing Combined with Intermittent Air Injection at Field Scale

Surfactant flushing with intermittent air injection, referred to as enhanced flushing, has been proposed at a site in Korea contaminated by military activity to overcome the difficulty of treatment caused by a layered geological structure. In this study, we developed a simple numerical model for exploring the effects of various physical and chemical processes associated with enhanced flushing on pollutant removal efficiency and applied it in a field-scale test. This simple numerical model considers only enhanced hydraulic conductivity rather than all of the interacting parameters associated with the complex chemical and physical processes related to air and surfactant behavior during enhanced flushing treatment. In the numerical experiment, the removal efficiency of residual non-aqueous phase liquid (NAPL) was approximately 12% greater with enhanced, rather than conventional, flushing because the hydraulic conductivity of the low-permeability layer was enhanced 5-fold, thus accelerating surfactant transport in the low-permeability layer and facilitating enhanced dissolution of residual NAPL. To test whether the enhanced flushing method is superior to conventional flushing, as observed in the field-scale test, successive soil flushing operations were simulated using the newly developed model, and the results were compared to field data. Overall, the simulation results aligned well with the field data.

Dissolution and remobilization of NAPL in surfactant-enhanced aquifer remediation from microscopic scale simulations

In this paper, the dissolution and mobilization of non-aqueous phase liquid (NAPL) blobs in the Surfactant-Enhanced Aquifer Remediation (SEAR) process were upscaled using dynamic pore network modeling (PNM) of three-dimensional and unstructured networks. We considered corner flow and micro-flow mechanisms including snap-off and piston-like movement for two-phase flow. Moreover, NAPL entrapment and remobilization were evaluated using force analysis to develop the capillary desaturation curve (CDC) and predict the onset of remobilization. The corner diffusion mechanism was also applied in the modeling of interphase mass transfer to represent NAPL dissolution as the dominant mass transfer process. In addition, the effect of pore-scale heterogeneity on mass transfer rate coefficient and recovered residual NAPL was considered in the simulations. Sodium dodecyl sulfate (SDS) and Triton X-100 were used as the surfactant for the SEAR process. The results indicate that although surfactants enhance NAPL recovery during two-phase flow, surfactant-enhanced remediation of residual NAPL through dissolution is highly dependent on surfactant type. When SDS ─as a surfactant with high critical micelle concentration (CMC) and low micelle partition coefficient (Km)─ was injected into a NAPL contaminated site, the mass transfer rate coefficient decreased (due to considerable changes in interface chemical potentials) which leads to a significant reduction in NAPL recovery after the end of two-phase flow. In contrast, Triton X-100 (with low CMC and high Km) improved NAPL recovery, by enhancing solubility at surfactant concentrations greater than CMC which overcompensates the interphase mass transfer reduction.

Modeling of soil gas radon as an in situ partitioning tracer for quantifying LNAPL contamination

In the last decades radon (Rn) has been widely proposed as a naturally occurring tracer for non-aqueous phase liquids (NAPL) in the soil. This work examines the feasibility of using soil gas data collected at some distance from the source zone for the application of the Rn deficit technique for the identification and quantification of NAPL contamination. To this end, we used a steady-state 1-D analytical solution that is based on a 3-layer model that allows to simulate the transport and distribution of Rn in the source zone, capillary fringe and overlying unsaturated soil. The analytical solution was first validated against a more detailed numerical model available in the literature. Then, a series of simulations were carried out to evaluate the vertical concentration profiles of Rn in soil gas above the source zone and in background location not impacted by NAPL. Simulation results showed that the parameters that most influence the migration and distribution of Rn in the subsurface are the distance of the soil gas probe from the source zone and, to a lower extent, the type of contamination (e.g. diesel or gasoline) and the soil type. On the basis of these results, we developed some easy-to-use nomographs to estimate the residual NAPL phase based on the observed radon deficit in soil gas and on the probe to source distance and soil and NAPL characteristics. According to the obtained results, the radon deficit technique results a feasible method for a qualitative identification of residual NAPL when radon in soil gas is measured at distances lower than 2 m from the contaminated zone. However, for an accurate quantitative estimation of the NAPL phase content, soil gas probes should be preferably located at distances lower than 1 m from the source zone.

Pore-scale investigation of wettability impact on residual nonaqueous phase liquid dissolution in natural porous media

Understanding the dissolution behavior of residual nonaqueous phase liquids (NAPLs) in soil and aquifer is essential for remediation activities and related prediction models. In this study, the impact of wettability of pore surfaces on the residual NAPL dissolution was investigated in natural porous media using X-ray microtomography. Three-dimensional pore-scale characteristics of NAPL blobs were determined in water-wet and neutral-wet media. Time-resolved images enable to estimate the temporal changes in the residual saturation and interfacial area, then to calculate the mass transfer coefficient. The results showed that the wettability has a significant impact on the morphology of residual NAPL; thus, affecting the dissolution behavior in natural porous media. In the neutral-wet system, residual NAPL blobs are trapped in small and large pores, resulting in a uniform distribution and a higher specific total interfacial area. The NAPL dissolution rate in the neutral-wet media is faster due to the higher specific interfacial area and more uniform spatial distribution. Considering the total interfacial area as the active mass transfer place, the wettability has a slight effect on the mass transfer coefficient. The mass transfer coefficient is independent of the residual saturation, whereas the lumped mass transfer coefficient decreases significantly during the dissolution process. Under different flow rates, the experimental data of Sherwood number Sh and modified Sherwood number Sh' could be well fitted with Sh ~ Ren and Sh′ ~ Renθn. These results are essential because the effect of wettability on dissolution was first studied from the perspective of pore scale, and the prediction models will help optimize the remediation strategies.

Influence of aqueous film forming foams on the solubility and mobilization of non-aqueous phase liquid contaminants in quartz sands

At sites where aqueous film forming foams (AFFFs) are used for fire suppression or training activities, interactions between dissolved foam constituents and organic liquids could alter contaminant migration in the subsurface. In this study, batch reactor and column experiments were conducted to investigate the potential for AFFF solutions to enhance the solubility and mobility of three representative non-aqueous phase liquid (NAPLs), JP-4 jet fuel, trichloroethene (TCE), and tetrachloroethene (PCE). For AFFF concentrations up to 5% wt. (50,000 mg/L), aqueous solubilities of TCE and PCE increased by less than 50%, indicating the absence of micellar solubilization. However, NAPL-water interfacial tensions were reduced to less than 1.5 mN/m and resulted in accumulation of up to 2.25 mg/m2 of AFFF at the NAPL-water interface. To assess the potential for AFFF to mobilize residual (entrapped) NAPL at a field application rate of 3% wt. (30,000 mg/L), columns were packed with two size fractions of Ottawa sands (20–30 mesh and 60–80 mesh) that yielded residual NAPL saturations ranging from 11.7 to 17.6%. Following injection of 3 pore volumes of the 3% wt. AFFF solution, partial mobilization of residual NAPL was observed for PCE, TCE, and JP-4, with saturation reductions of 0.7 to 2% in 20–30 mesh and 0.3% to 1.3% in 60–80 mesh Ottawa sand. The columns were then flushed with an ultralow-IFT surfactant solution consisting of 4% wt. 1:1 Aerosol AY/OT, which resulted in nearly complete mobilization of the remaining residual NAPL. When NAPL desaturation curves were expressed in terms of the total trapping number (NT), the threshold value of NT required for NAPL mobilization by 3% wt. AFFF was approximately 2 × 10−5, consistent with previous studies. These findings demonstrate that AFFF solutions can lead to partial mobilization of residual NAPL, and that the total trapping number concept can be used to predict such behavior a priori. In addition, the observed IFT reductions are sufficient to alter NAPL accumulation and redistribution behavior in the subsurface through enhanced spreading above the water table (e.g., JP-4) or penetration into lower-permeability media (e.g., TCE, PCE).

The mechanism of NAPL layer formation in a microfluidic device with dual-permeability: experiments and numerical simulation

In-situ remediation is an important technique for non-aqueous phase liquid (NAPL) contamination remediation in soils. Understanding the formation and distribution of NAPL contaminated layers in heterogeneous soils is essential to propose cost-effective remediation methods. Therefore, a two-dimensional microfluidic device with dual permeability zones was designed to simulate the soil-groundwater system and experimentally investigated the formation process of the NAPL contamination layer. Numerical modeling of phase field coupled with laminar flow was used to simulate the distribution of NAPL in soil-groundwater system, and the formation of NAPL contamination layer under typical groundwater flow rate and wetting angle was evaluated. The NAPL in the low permeable zone formed a stable contamination layer, while the NAPL in the high permeable zone was washed out to varying degrees and mainly resided in the junction of high-low-permeability regions of the chip, with residues of 42.2% and 23.3% in the chip high permeable region at groundwater velocities of 6.17 m/d and 10.16 m/d, respectively, the numerical simulation results were consistent with the experimental observations. This retention was diminished in the high permeable zone as the NAPL-to-wall wetting angle increased. In addition, when the contact angle was increased from 33° (lipophilic) to 108° (hydrophilic), the residual NAPL content in the high permeable zone decreased from 48.86% to 28.22%. This paper provides a reliable visualizable experimental platform for the study of NAPL pollution formation at micro-scale in heterogeneous groundwater system, and lays a foundation for the subsequent optimization of the remediation strategy.

Radon as a Natural Tracer for Monitoring NAPL Groundwater Contamination

In this research, the radioactive noble gas radon was used as a tracer for Non-Aqueous Phase Liquids (NAPLs) contamination, since it is much more soluble in these substances than in air or water. Soil radon remains trapped within the NAPLs, resulting in a local reduction in the radon concentration within close proximity to the contaminated area. This technique was applied to a contaminated site in Roma (Italy). The main residual NAPLs are total hydrocarbons and methyl-tertiary-butyl ether (MTBE), a water-soluble additive. The monitoring activities included two sampling campaigns of groundwater from 18 wells in February and May 2020. Concentration maps were produced using radon data. The results show that the radon deficit traces the location of NAPLs in the fuelling station very well, with a residual source zone extending in a NNW-SSE direction. A good correspondence between a low amount of radon and a higher concentration of NAPLs was found. A reduction in the average amount of radon in the May 2020 survey indicated a stronger remobilization of NAPLs compared to that of the February 2020 monitoring campaign. The peaks of Volatile Organic Compounds (VOCs) detected between 8–9 and 11–12 m depths indicate the presence of residual blobs of NAPLs in the vadose zone of the aquifer.

Aqueous Film-Forming Foams Exhibit Greater Interfacial Activity than PFOA, PFOS, or FOSA.

Perfluoroalkyl acids spontaneously concentrate at air-water and non-aqueous phase liquid (NAPL)-water interfaces, which can influence their retention during subsurface transport. This work presents measurements of air- and NAPL-water interfacial tension for synthetic groundwater containing perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorooctanesulfonamide (FOSA), or aqueous film-forming foam (AFFF) formulations at concentrations ranging from 0.1 to greater than 1000 mg/L. The NAPLs tested included dodecane, tetrachloroethylene, and jet fuel. AFFF formulations were less efficient at lowering interfacial tension than PFOA, FPOS, or FOSA substances below 100 mg/L, while above 100 mg/L, these formulations were more effective, achieving tensions of less than 3 mN/m. Infiltration of solutions with such low tension could lead to mobilization of residual NAPL. Equations based on interfacial tension measurements show that concentrations of PFOA, PFOS, and FOSA at the air-water interface were from 2 to 16 times greater than at the NAPL-water interface below 100 mg/L and were 10-50 times greater for AFFF below 20 mg/L. Calculations for unsaturated soil estimate that up to 87% of PFOS mass was at the air-water interface and less than 4% at the dodecane-water interface for bulk-water concentrations below 1 mg/L.

Dynamics of microbial populations and diversity in NAPL contaminated peat soil under varying water table conditions

Despite the risks that hydrocarbon contamination from pipeline leaks or train derailments impose on the health of peatlands in hydrocarbon production areas and transportation corridors, assessing the effect of such contaminations on the health and sustainability of peatlands has received little attention. This study investigates the impacts of hydrocarbons on peat microbial communities. Column experiments were conducted on non-aqueous phase liquid (NAPL) contaminated undisturbed peat core (0–35 cm) under static and fluctuating water table conditions. Water table fluctuations reduced residual NAPL saturation from 8.1-11.3% to 7.7–9.5%. Biodegradation of n-C8 and n-C12 along with oxidation of CH4 together produced high CO2 concentrations in the headspace. Clear patterns in dynamics in the microbial community structure were observed, with a more pronounced population growth. However, a significant loss of microbial richness was observed in contaminated columns. The result indicates that the phylum Proteobacteria benefited most from NAPL; however, their families differed between static and fluctuating water table conditions. This study established strong evidence that peat microbes and water table fluctuation can be an excellent tool for hydrocarbon removal and its control in peatlands.

Impact of mineralogy and wettability on pore-scale displacement of NAPLs in heterogeneous porous media

Subsurface formations often contain multiple minerals with different wettability characteristics upon contact with nonaqueous-phase liquids (NAPLs). Constitutive relationships between microstructure heterogeneity and NAPL fate and transport in these formations are difficult to predict. Several studies have used pore-scale network models with faithful representations of rock pore space topology to predict macroscopic descriptors of two-phase flow, however wettability is usually considered as a spatially random variable. This study attempts to overcome this limitation by considering more realistic representations of rock mineralogy and wettability in these models. This is especially important for heterogeneous rocks where properties vary at the pore-scale. The work was carried out in two phases. First, pore-fluid occupancy maps during waterflooding were obtained by X-ray microtomography to elucidate the impact of pore wall mineralogy and wettability on water preferential flow paths and NAPL trapping within a heterogeneous aquifer sandstone (Arkose). Then, microtomography images of the rock were used to generate a hybrid pore network model (PNM) that incorporated both pore space topology and pore wall mineralogy. In-situ contact angles (CA) measured on the surface of different minerals were assigned to the network on a pore-by-pore basis to describe the exact wettability distribution of the rock (Pore-by-pore model). The equivalent network was used as input in a quasi-static flow model to simulate waterflooding, and the predictions of residual NAPL saturation and relative permeabilities were compared against their experimental counterparts. To examine the sensitivity of the model to the underlying fluid-solid interactions, we also used traditional methods of wettability characterization in the input data and assigned them randomly to the PNM. Wettability in this case was assessed from macroscale CA distribution of oil droplets on the surface of unpolished Arkose substrates released by spontaneous imbibition of water (Arkose model) and from pendant drop measurements on polished quartz (Quartz model). Our results revealed that the Pore-by-pore model predicted waterflooding with the highest accuracy among all three cases. The Arkose model slightly overestimated NAPL removal whereas the Quartz model failed to predict the experiments. More in-depth analysis of the Pore-by-pore and Arkose models showed that macroscopic transport quantities are less dependent to microstructure heterogeneity if minerals are distributed uniformly across the rock. The predictions herein indicate the importance of incorporating mineralogy and wettability maps to improve the prediction capabilities of PNMs especially in systems with high mineral heterogeneity, where minerals are nonuniformly distributed, or selective fluid-mineral interactions are targeted.

Remediation of trapped DNAPL enhanced by SDS surfactant and silica nanoparticles in heterogeneous porous media: experimental data and empirical models.

The remediation of nonaqueous phase liquids (NAPLs) enhanced by surfactant and nanoparticles (NP) has been investigated in numerous studies. However, the role of NP-assisted surfactants in the dissolution process is still not well discussed. Besides, there is a lack of empirical dissolution models considering the effects of initial residual saturation Strap, NAPL distribution, and surfactant concentration in NAPL-aqueous phase systems. In this work, micromodel experiments are conducted to quantify mass transfer coefficients for different injected aqueous phases including deionized water, SDS surfactant solutions, and NP-assisted solutions with different levels of concentrations and flow rates. Observations reveal that silica nanoparticles (SNP) can significantly enhance interphase mass transfer, while SDS surfactant reduces the mass transfer coefficient. In addition, Strap and intrinsic interfacial area ai, as an indicator of dense nonaqueous phase liquids (DNAPL) distribution, influence the interphase mass transfer. The ai is also independent of DNAPL saturation SNAPL except for SNAPL < 7% when ganglia breakup occurs. Based on these observations, new empirical dissolution models are proposed in the presence and the absence of SDS surfactant and SNP in which ai, Strap, and surfactant concentrations are introduced as new parameters. The evaluated mass transfer rate coefficients using the proposed models show a significant improvement compared to available empirical models. The finding of this study might be attractive for application in field-scale simulations of surfactant-enhanced aquifer remediation (SEAR) and NP-assisted methods.

Experimentally based pore network modeling of NAPL dissolution process in heterogeneous porous media

Practical designs of non-aqueous phase liquids (NAPLs) remediation strategies require reliable modeling of interphase mass transfer to predict the retraction of NAPL during processes such as dissolution. In this work, the dissolution process of NAPL during two-phase flow in heterogeneous porous media is studied using pore-network modeling and micromodel experiments. A new physical-experimental approach is proposed to enhance the prediction of the dissolution process during modeling of interphase mass transfer. In this regard, the normalized average resident solute concentration is evaluated for describing the dissolution process at pore-level. To incorporate the effect of medium heterogeneities, a new experimental factor is considered for enhancing corner diffusion modeling. In addition, capillary desaturation curves (CDCs) are predicted during hydraulic flow modeling to estimate initial residual NAPL saturation. The developed network model can predict residual NAPL saturations and mass transfer rate coefficient for a NAPL-water system at different injection rates and fluid saturations. The evaluated mass transfer rate coefficients using the proposed physical-experimental approach show a significant improvement compared to either mechanistic or empirical methods. The proposed approach in this study can be attractive for possible applications in commercial simulators of contaminant transport in porous media.

An analytical model of bubble-facilitated vapor intrusion

Mass transfer from nonaqueous phase liquid (NAPL) to entrapped air induced by a fluctuating water table commonly occurs in residual NAPL zones in aquifers. Gas bubble expansion and vertical migration due to interphase mass transfer could facilitate the upward transport of volatile organic compounds (VOCs) in the aquifer and result in higher mass fluxes into a building relative to those of diffusion-limited (D-L) VOC transport. However, the current vapor intrusion models have not considered bubble migration. In this study, an analytical solution of bubble-facilitated (B–F) VOC transport in the unsaturated-saturated zone was developed. The analytical solution was tested by a numerical solution using the finite-difference method. Sensitivity analyses of model parameters were implemented to understand the VOC transport behaviors. The effects of bubble migration on vapor intrusion pathway completion time (tc) and the attenuation factor (AF) were investigated by comparison with the D-L VOC transport model. The results indicate that the D-L model significantly overestimates the tc and underestimates the AF because the model neglects the impacts of bubble migration. Therefore, one may make an inappropriate decision and set up an inappropriate response action schedule if using the D-L model to assess the risk of bubble-facilitated vapor intrusion. The analytical solution was applied to a laboratory experiment. The analytical model managed to interpret the laboratory experiment data, showing that the mass flux of B–F VOC transport is two orders of magnitude higher than that of D-L VOC transport.

Effect of Residual NAPLs on the Transport of Bisphenol A and Bisphenol S in Saturated Porous Media

Endocrine disrupter, like bisphenol A (BPA) and bisphenol S (BPS), is frequently detected in the subsurface environment, imposing threats to the groundwater quality and public health. However, current understanding of environmental fate and transport of BPA/BPS is still not clear, especially with respect to those in the subsurface media with trapped non-aqueous phase liquids (NAPLs). In this study, the effect of residual NAPLs on the retention and transport of BPA/BPS in the saturated sand and soil media was investigated using column experiments. The results showed that residual NAPLs (i.e., xylene and perchloroethylene (PCE)) inhibited the transport of BPA in the sand columns, and the inhibit effect of xylene was greater than that of PCE. While the presence of NAPLs showed little influence on the transport of BPS in the sand columns, in soil A (sandy soil), the residual NAPLs had similar effect on the retention and transport of BPA and BPS. Both BPA and BPS showed higher retention in the soil B columns than in the sand and soil A columns due to soil B’s higher organic matter contents, which can strongly sorb BPA/BPS. The presence of residual NAPLs reduced the retention of BPA/BPS in the soil B columns because the NAPLs blocked the sorption sites of the soil organic matter for BPA/BPS. This study demonstrates the importance of residual NAPLs, types of media, properties of contaminants, and their interactions to the fate and transport of BPA/BPS in saturated porous media.

Deviation from capillary number scaling of nonlinear viscous fingering formed by the injection of Newtonian surfactant solution

An experimental study of immiscible viscous fingering (VF), formed during the injection of surfactant solutions into viscous oil in a radial Hele-Shaw cell, in which both fluids are Newtonian, is described. VF in such a system is related to the enhanced oil recovery method known as surfactant flooding and to the recovery of residual nonaqueous phase liquids (NAPLs) in aquifers. Previous studies show that, for a given viscosity contrast, immiscible VF formed by Newtonian fluids is dominated by the capillary number, which is defined as the ratio between the viscous force and the interfacial tension, and that the finger width decreases with increasing capillary number. However, in the present study, phenomena contrary to these rules were observed: wider fingers occurred in the surfactant solution system compared to those in the water system, in the nonlinear stage of VF evolution, despite the fact that the capillary number had the same value for both systems. In addition, even though the surfactant system had a higher capillary number than the water system, wider surfactant fingers were observed. A possible mechanism explaining this is discussed by comparing with previous studies regarding VF with surfactants. The present study indicates that the capillary number does not control the nonlinear VF width in the surfactant system. Our results and discussion can be used to contribute to the establishment of well-controlled processes for surfactant flooding and the recovery of residual NAPL in aquifers.

The Frequent Effects of Hydroxyl Radicals on the Oxidation of Crude Oil in Soil

The characteristics of hydroxyl radicals (•OH) frequently produced in the Fenton process and the effects of these radicals on the oxidation of hydrophobic total petroleum hydrocarbons (TPH) in soil are investigated. Twelve groups of Fenton oxidation experiments are conducted using two kinds of crude‐oil‐contaminated soil samples (S1 and S2) with different soil organic matter (SOM) contents. The results show that the decrease of TPH increases by 6774 mg kg−1 (S1) and 5875 mg kg−1 (S2) by using the frequent •OH (•OHf). These results are two times the results obtained by using the single •OH (•OHs) (S1: 3410 mg kg−1, S2: 2918 mg kg−1). The decrease of hydrophobic long‐chain alkanes C21–C30 is greatly improved by using the •OHf, that is, oxidation of 1557–3011 mg kg−1 in soil S1 and 1371–2532 mg kg−1 in soil S2. Moreover, the C21–C30 alkanes in soil S1 with low SOM (3.89%) are more degraded after oxidation. The residual non‐aqueous phase liquid (NAPL) TPH content by using the •OHf (266–1200 mg kg−1) is much lower than that by using the •OHs (2519–4070 mg kg−1). The residual of the NAPL TPH by using the •OHf is 11–29% of the residual by using the •OHs. This result indicates that residual NAPL TPH is reduced by using the •OHf, and the TPH contents in the two soil samples are efficiently oxidized.