Non-aqueous Phase Liquid Contamination Research Articles

Microbial activity in soils following steam treatment.

Steam enhanced extraction (SEE) is an aquifer remediation technique that can be effective at removing the bulk of non-aqueous phase liquid (NAPL) contamination from the subsurface, particularly highly volatile contaminants. However, low volatility compounds such as polynuclear aromatic hydrocarbons (PAHs) are less efficiently removed by this process. This research evaluated the effects of steam injection on soil microbial activity, community structure, and the potential for biodegradation of contaminants following steam treatment. Three different soils were evaluated: a laboratory-prepared microbially-enriched soil, soil from a creosote contaminated field site, and soil from a chlorinated solvent and waste oil contaminated field site. Results from field-scale steaming are also presented. Microbial activity before and after steam treatment was evaluated using direct epifluorescent microscopy (DEM) using the respiratory activity dye 5-cyano-2,3, ditolyl tetrazolium chloride (CTC) in conjunction with the fluorochrome 5-(4,6-dichlorotriazinyl) aminofluorescein (DTAF) to yield a quantitative assessment of active and total microbial numbers. DEM results indicate that steamed soils that were analyzed while still hot exhibited microbial activity levels that were below detection. However, soil samples that were slowly cooled, more closely reflecting the conditions of applied SEE, exhibited microbial activity levels that were comparable to presteamed soils. Samples from a field-site where steam was applied continuously for 6 weeks also showed high levels of microbial activity following cooling. The metabolic capabilities of the steamed communities were investigated by measuring cell growth in enrichment cultures on various substrates. These studies provided evidence that organisms capable of biodegradation were among the mesophilic populations that survived steam treatment. Fluorescent in situ hybridization (FISH) analysis of the soils with domain-level rRNA probes suggest that both Archaea and Bacteria survived steam exposure.

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.

Numerical simulation of two-phase flow in conceptualized fractures

Two-phase flow in fractured rock is an important phenomenon related to a range of practical problems, including non-aqueous phase liquid contamination of groundwater. Although fractured rocks consist of fracture networks, the study of two-phase flow in a single fracture is a pre-requisite. This paper presents a conceptual and numerical model of two-phase flow in a variable fracture. The void space of the fracture is conceptualized as a system of independent channels with position-dependent apertures. Fundamental equations, governing two-phase displacement in each channel, are derived to represent the interface positions and fractional flows in the fracture. For lognormal aperture distributions, simple approximations to fractional flows are obtained in analytical form by assuming void occupancy based on a local capillary allowability criterion. The model is verified by analytical solutions including two-phase flow in a parallel-plate fracture, and used to study the impacts of aperture variation, mobility ratio and fracture orientation on properties of two-phase flow. Illustrative examples indicate that aperture variation may control the distribution of wetting and non-wetting fluids within the fracture plane and hence the ability of the fracture to transmit these fluids. The presence of wetting fluid does little to hinder non-wetting fluid flow in fractures with large aperture variations, whereas a small volume of non-wetting fluid present in the fracture can significantly reduce wetting fluid flow. Large mobility ratios and high fracture slope angles facilitates the migration of non-wetting fluid through fractures.

Biosurfactant-enhanced solubilization of NAPL mixtures

Remediation of nonaqueous phase liquids (NAPLs) by conventional pump-and-treat methods (i.e., water flushing) is generally considered to be ineffective due to low water solubilities of NAPLs and to mass-transfer constraints. Chemical flushing techniques, such as surfactant flushing, can greatly improve NAPL remediation primarily by increasing the apparent solubility of NAPL contaminants. NAPLs at hazardous waste sites are often complex mixtures. However, the equilibrium and nonequilibrium mass-transfer characteristics between NAPL mixtures and aqueous surfactant solutions are not well understood. This research investigates the equilibrium solubilization behavior of two- and three-component NAPL mixtures (containing akylbenzenes) in biosurfactant solutions. NAPL solubilization is found to be ideal in water (i.e., obeys Raoult's Law), while solubilization in biosurfactant solutions was observed to be nonideal. Specifically, the relatively hydrophobic compounds in the mixture experienced solubility enhancements that were greater than those predicted by ideal enhanced solubilization theory, while the solubility enhancements for the relatively hydrophilic compounds were less than predicted. The degree of nonideality is shown to be a nonlinear function of the NAPL-phase mole fraction. Empirical relationships based on the NAPL-phase mole fraction and/or micelle-aqueous partition coefficients measured in single-component NAPL systems are developed to estimate values for the multicomponent partition coefficients. Empirical relationships that incorporate both the NAPL-phase mole fraction and single-component partition coefficients yield much improved estimates for the multicomponent partition coefficient.

Experimental Study of Horizontal Barrier Formation by Colloidal Silica

Migration of non-aqueous-phase liquids (NAPLs) such as trichloroethylene or gasoline can be controlled by barrier systems. The research presented in this paper aims to form a horizontal layer by injecting gelling liquids and to test the ability of horizontal barriers to isolate the downward migration of NAPLs in subsurface environments. We developed a methodology in the laboratory to form a contiguous horizontal barrier by injecting gelling liquids through horizontal and vertical wells. A series of batch, column, and 2D sandbox experiments were conducted to investigate the gel development and horizontal barrier formation. Prior to 2D barrier formation experiments, two different methods were employed to measure pre- and postinjection hydraulic conductivity in 1D columns to quantify hydraulic conductivity reductions. After an impervious grout formed, the durability of the grouted porous media in the presence of two NAPL contaminants was investigated. The hydraulic conductivity of filter sand treated with co...

Characterization of Contaminated Soils Using Confocal Laser Scanning Microscopy and Cryogenic-Scanning Electron Microscopy

The distribution of nonaqueous phase liquid (NAPL) contaminants in the microstructure of soil was examined by microscopy. The fluorescence of contaminant polynuclear aromatic hydrocarbons allowed imaging of soil aggregates using fluorescent confocal laser scanning microscopy (CLSM). Cryogenic scanning electron microscopy was then used to examine samples at higher magnification. The results indicate that the NAPL covers the surface of aggregates and fills the accessible interaggregate pores of diameter 0.05−5 μm, creating a discrete phase inside the pores. The NAPL phase was highly associated with clay minerals. These physical observations suggest that diffusion of contaminants out of NAPL-clay structures and NAPL-filled pores is important in release of contaminants from soil.

Effect of groundwater flow dimensionality on mass transfer from entrapped nonaqueous phase liquid contaminants

Mass transfer from entrapped nonaqueous phase liquids (NAPLs) at subsurface locations of environmental contamination typically takes place in three‐dimensional groundwater flow fields. Yet most laboratory studies of NAPLs dissolution have been one‐dimensional, eliminating more realistic conditions such as the heterogeneous distribution of entrapped NAPL ganglia and the potential for flow bypassing due to reduced water permeability in contaminated zones. In this study, experiments in two‐dimensional flow fields were used to evaluate the effects of flow dimensionality on NAPL dissolution. Modifications of the transport code MT3D provided the capability to simulate NAPL dissolution. Regression analysis, matching experimental observations to simulated predictions, provided parameter values for a proposed phenomenological model of dissolution in two‐dimensional flow fields. The proposed model predicted lower NAPL dissolution rates relative to models developed on the basis of published one‐dimensional experimental measurements. The results indicate potential for significant errors using the one‐dimensionally based models for NAPL dissolution in field applications.

Partitioning of Ethoxylated Nonionic Surfactants in Water/NAPL Systems: Effects of Surfactant and NAPL Properties

Surfactant-based remediation has received considerable attention as a potential technology for enhancing conventional pump-and-treat remediation processes for aquifers contaminated with organic nonaqueous-phase liquids (NAPLs). In surfactant remediation, NAPL contaminants can be removed by two processes: solubilization and mobilization. Although these processes have been extensively studied, limited information is available regarding the partitioning of nonionic surfactants between aqueous and organic phases. Because surfactants are amphiphilic molecules that have a finite solubility in aqueous phases and NAPL, partitioning between the two phases is expected. The study presented here examines the effect of surfactant and NAPL properties on the partitioning of nonionic surfactants between different aqueous and nonaqueous phases using ethoxylated nonionic surfactants with different chemical structures and mixture distributions and NAPLs covering a wide range of NAPL/water interfacial tensions. In general, ...

A Physically Based Model of Dissolution of Nonaqueous Phase Liquids in the Saturated Zone

The design of remediation strategies for nonaqueous phase liquid (NAPL) contaminants involves predicting the rate of NAPL dissolution. A physically based model of an idealized pore geometry was developed to predict nonaqueous phase liquid dissolution rate coefficients. A bundle of parallel pores in series model is used to represent NAPL dissolution as a function of three processes: pore diffusion, corner diffusion, and mixing and multiple contact. The dissolution rate coefficient is expressed in terms of the modified Sherwood number (Sh′) and is a function of Peclet (Pe) number. The model captures the complex behavior of Sh′ versus Pe data for both water-wet (Powers, 1992) and NAPL-wet (Parker et al., 1991) media. For water-wet media, the observed behavior can be broken down into four distinct regions. Each region represents a different physical process controlling NAPL dissolution: the low-Pe region is controlled by pore diffusion; the low- to moderate-Pe region is a transition zone; the moderate-Pe region is controlled by mixing and multiple contact; and the high-Pe region is controlled by corner diffusion. For the high-Pe conditions typical of most column experiments, the model involves only one fitting parameter. For NAPL-wet media, NAPL dissolution is governed exclusively by corner diffusion, and the model again involves only one fitting parameter.

Multicomponent NAPL Solidification Thermodynamics

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

Numerical simulation of multiphase flow and phase partitioning in discretely fractured geologic media

The three-dimensional compositional model CompFlow has been extended to allow the simulation of the multiphase advective, dispersive and diffusive flux of non-aqueous phase liquid (NAPL) contaminants in a discrete-fracture network, allowing for phase partitioning and dynamic interactions between the fracture network and the surrounding low-permeability rock matrix. The approach used to couple fluxes between the fractures and matrix allows representation of capillary pressure differences within the fractures and matrix and makes no assumption of equilibrium hydraulic conditions between the two. The model is verified for the case of aqueous-phase solute transport by comparison with an analytical solution. An example problem is presented involving the migration of a dense non-aqueous phase liquid (DNAPL) consisting of trichlorethylene (TCE) in a single vertical fracture within a low-permeability material with significant matrix porosity. The simulation results demonstrate that matrix diffusion acts to transfer significant amounts of contaminant to the matrix in the aqueous phase. After the DNAPL source is removed, the NAPL ultimately disappears from the fracture due to partitioning of contaminant into the aqueous phase with concomitant matrix diffusion. It is shown that as the porosity of the matrix increases, the rate of migration of the TCE DNAPL front within fractures is retarded, due to dissolution and matrix diffusion. The sensitivity of DNAPL migration within the fracture to the form of the relative permeability relationship is also discussed. The model is then used to highlight the potential for deep DNAPL penetration through a vertical cross-section consisting of a shallow unconfined sand aquifer and a deeper sand aquifer separated by a layer of fractured clay. Vertical fractures through the clay that hydraulically connects the shallow and deep aquifers are shown to be capable of transmitting both dissolved contaminant and DNAPL to the underlying aquifer, with DNAPL travel times through the 5-m thick clay unit being on the order of days. For the scenarios examined, the DNAPL entering the lower aquifer via the fractured clay unit may completely dissolve at the aquifer–aquitard interface due to lateral groundwater flow in the lower aquifer, provided the aperture of the vertical fractures in the clay layer are less than about 30 μm; however, some DNAPLs may penetrate into the lower aquifer and exist as a non-aqueous phase if the fracture apertures in the clay layer are 50 μm in size or larger. In this latter case, the presence of the DNAPL in the lower aquifer acts as a persistent source of groundwater contamination and can produce an extensive plume in the direction of groundwater flow.

Laboratory Studies of Aqueous Partitioning Tracer Tests for Measuring Nonaqueous Phase Liquid Volumes

The partitioning tracer test (PTT) is an innovative method for measuring nonaqueous phase liquid (NAPL) volume distributions in contaminated aquifers. During a PTT, a tracer pulse containing nonpartitioning and partitioning tracers is injected to an aquifer and extracted from downgradient wells. The appearance of the tracers at downgradient locations is analyzed to determine tracer mean residence times, for example, using the methods of moments and retardation factors based on residence times. The NAPL volume present in the tracer flow path is then determined based on retardation factors and independently measured tracer partition coefficients. Column experiments were performed to assess the validity of PTTs conducted at a former chemical disposal pit at Hill Air Force Base, Utah. The NAPL contamination was a complex mixture of volatile and semivolatile organic chemicals. Up to four alcohol tracers were used in PTTs: methanol; n‐hexanol; 2,2‐dimethyl‐3‐pentanol; and 6‐methyl‐2‐heptanol and batch test, conducted at 10 °C yielded partition coefficients of 0, 2.07, 7.42, and 18.8, respectively. Partition coefficients increased between 53 and 60% with a 12 °C increase in temperature. Column experiments were used to evaluate the validity of using a PTT for field test conditions of temperature and average flow velocity. Measured NAPL volumes based on comparing three replicate column tests using two to three partitioning tracers and different flow rates were within 84.3 to 101% of the NAPL volume used to contaminate the column (overall, 8.91% of the pore volume contained NAPL). The average NAPL saturations from all the tracers yielded estimates between 89 to 99% of the known value. Afterwards, the column was sparged with 5800 pore volumes of air, and the subsequent PTT yielded an estimate of the reduction in NAPL volume of 16%.

Laboratory and Field Investigation of Surfactant Sorption Using Single‐Well, “Push‐Pull” Tests

AbstractSorption to aquifer sediments can limit the effectiveness of surfactants injected to solubilize residual nonaqueous phase liquid contaminants in the subsurface. The objective of this study was to evaluate the ability of the single‐well, push‐pull test to characterize sorption of linear alkylbenzene sulfonate (LAS) and hexadecyl diphenyl oxide disulfonate (DOWFAX) surfactants to natural aquifer sediment in situ. Batch sorption isotherms for both surfactants exhibited Langmuir‐type sorption behavior with larger apparent sorption maxima for LAS than for DOWFAX. However, numerical transport simulations based on batch sorption isotherms were unable to predict the retardation and chromatographic separation of LAS homologs and conservative transport of DOWFAX observed during laboratory and field push‐pull tests, indicating that the single‐well, push‐pull test method can more accurately describe in situ surfactant sorption and transport behavior than batch sorption isotherms.

Unifac modeling of multicomponent nonaqueous phase liquids containing polycyclic aromatic hydrocarbons

AbstractMulticomponent nonaqueous phase liquid (NAPL) contaminants that are mixtures of polycyclic aromatic hydrocarbons (PAHs) are examined using UNIFAC. The NAPL phase activity coefficients for constituent compounds of four different coal tar materials are estimated. The group contribution approach is adapted to describe representative molecular structures of the uncharacterized fractions of the tar materials, determined from average molecular weights and nonparametric regressions of the functional group parameters against molecular weight. The molecular structure characterization of the tars elucidates the similar chemistry of the materials from different sites, despite their different chemical compositions. Nearly ideal behavior is predicted for most tar constituents, with the majority of activity coefficients in the range of 0.9 to 1.1. These results provide a firm theoretical basis for an assumption of NAPL‐phase ideality for many tar constituents. The robustness of this conclusion is indicated through comparable results across different tar materials and through a sensitivity analysis of the effect of the estimated characteristics of the uncharacterized fractions.

Literature Review of 2-D Laboratory Experiments in NAPL Flow, Transport, and Remediation

The migration of nonaqueous phase liquid (NAPL) contaminants in the subsurface results in a complex multiphase environment due in part to heterogeneity in both the soil and fluid saturations. To accurately predict the flow, transport, and remediation of NAPL contaminants, research has focused on laboratory and numerical modeling of the subsurface environment. Within this research, 2-D laboratory model are advantageous due to the fact that the capillary, viscous, and buoyancy forces found in the subsurface environment can be reproduced. Thus, they can be used to study flow and transport, test and develop remediation technologies, and verify numerical models. However, to date a comprehensive review of the 2-D experimental work has not been compiled. The review presented in this article should be of interest to geohydrologists, engineers and scientists involved in both applied and research aspects of NAPL-contaminated aquifers.

Student awards given at 18th Annual Hydrology Days

Four students received awards for best student presentations at the 18th annual Hydrology Days (HD) held earlier this year at Colorado State University. The event is sponsored by AGU's Hydrology Section.Mohamed Bennani Lakhim, Colorado State University, received the award in the Ph.D. oral presentation category for developing a stochastic risk‐assessment methodology, based on numerical modeling of contaminant fate and transport in a heterogeneous aquifer, to better estimate the potential magnitude of exposure of an individual. His study analyzed the effects of the uncertainty of different groundwater parameters on the uncertainty of contaminant concentration in the aquifer. The Ph.D. poster category award went to Tarek A. Saba from the University of Colorado, Boulder, for his studies of the effect of groundwater flow dimensionality on mass transfer from entrapped nonaqueous phase liquid contaminants.

Site Investigations of Fuel Spill Migration into Permafrost

In the summers of 1995 and 1996 subsurface investigations were conducted to examine the vertical migration of petroleum hydrocarbons into the permafrost at two different sites in the Canadian Arctic where fuel spills had previously occurred: Canadian Forces Station Alert on the Northern tip of Ellesmere Island, and Isachsen High Arctic Weather Station on Ellef Ringnes Island. Petroleum hydrocarbon contamination was found below the permafrost table in excess of 1.5 m at Alert, and in excess of 0.6 m at Isachsen, both depths being the maximum depths of the investigations. This paper provides the detailed results of these investigations with an explanation of the expected transport mechanisms. The results show that the permafrost may not be an impermeable barrier to nonaqueous phase liquid (NAPL) contamination under some circumstances.

Micellar solubilization of nonaqueous phase liquid contaminants by nonionic surfactant mixtures: Effects of sorption, partitioning and mixing

Surfactant micellar solubilization is under investigation as a promising method for significantly increasing the efficiency of remediation of aquifers contaminated with nonaqueous phase liquids (NAPLs). In this study, the influence of three parameters on the micellar solubilization of NAPLs by a mixture of ethoxylated dodecyl alcohol surfactants was investigated: Presence of model aquifer materials, extent of equilibrium partitioning of surfactant into the nonaqueous phase, and sample mixing regime. The micellar solubilization of two NAPLs, hexane, representative of nonbranched alkanes, and 1,2-dichlorobenzene, representative of polar and aromatic compounds, was studied. Batch experiments indicated that surfactant losses to sorption on organic-coated silica were relatively small for these experimental systems. Surfactant partitioning into hexane was not significant, but there was significant surfactant partitioning into the 1,2-DCB phase, resulting in a total surfactant dose required to achieve 1,2-DCB micellar solubilization greater than that predicted from the surfactant critical micelle concentration. Partitioning of surfactant into 1,2-DCB was selective, with the more hydrophobic surfactant molecules favoring the 1,2-DCB phase, resulting in an aqueous phase surfactant mixture with different solubilization properties than the original. Mixing conditions had a significant effect on the extent of solubilization of 1,2-DCB, attributed to the greater tendency of 1,2-DCB versus hexane to form emulsions. Results indicate that partitioning may be significant for certain nonionic surfactant/NAPL systems and should be investigated prior to remedial activities. Total surfactant concentration, aqueous to nonaqueous phase volume ratio, and surfactant formulation should be chosen to minimize losses to nonaqueous phases.

Laboratory and controlled field experiments using potassium permanganate to remediate trichloroethylene and perchloroethylene DNAPLs in porous media

Few proven technologies exist that may be used to treat dense non-aqueous phase liquid (DNAPL) contaminants. In-situ chemical flushing is a proposed technology which consists of flushing DNAPL source zones with a reactive solution to degrade the contaminant mass below ground. A laboratory and controlled field experimental program was conducted to assess the potential of potassium permanganate (KMnO 4) as a reagent for in-situ DNAPL remediation. The results of laboratory experiments indicated that two common DNAPL contaminants, perchloroethylene (PCE) and trichloroethylene (TCE), were rapidly degraded to chloride and carbon dioxide. Column experiments, using residual PCE flushed with oxidant concentrations as high as 10 g L −1, indicated that chloride could be used as a reaction tracer. From the chloride data, it appeared that the rate of PCE removal from the columns was a complex process dependent upon the kinetics of both dissolution and oxidation. Two experimental applications of in-situ oxidation were conducted in the Borden aquifer isolated within a 7.5 m 3 double sheet-pile cell. The cell was fitted with injection and recovery wells through which aqueous solutions of KMnO 4 were flushed to oxidize solvent source zones in situ. In the initial experiment, flushing of a 1 L PCE residual source with 10 g L −1 KMnO 4 at total flow rates of up to 100 L per day, completely removed the source within 120 days. A second experiment, using an 8 L mixture of PCE and TCE slowly allowed to infiltrate into the cell, was conducted using a system to recycle the oxidant. The oxidant was added at 10 g L −1 with a flow of approximately 50 L per day. After 290 days of flushing, it was concluded from the monitoring data that 62% of the initial source (as equivalent chloride mass) has been oxidized and it was evident that oxidation was continuing in the upper third of the cell. These experiments have suggested that the effectiveness of in-situ chemical oxidation will depend primarily upon the distribution of the DNAPL in the subsurface and its effects upon dissoluttion. In both experiments, spatial variability of chloride measurements appeared to reflect both the DNAPL location and distribution.

Applying AVO analysis to GPR data

For reflected‐wave phenomena in both seismology and ground‐penetrating radar (GPR), the amplitudes of the reflected waves depend on the incidence angles and the physical properties of the media above and below an interface. Amplitude variation with offset (AVO) analysis employs knowledge of reflected‐wave phenomena to constrain the physical properties of two media at an interface. Ostrander (1984) demonstrated that AVO analysis can be used on seismic reflection data in certain instances to obtain information about the subsurface not revealed by traveltime‐offset relationships (i. e., traditional seismic cross sections). Quantifiable information about the physical properties of the subsurface may then be extracted. AVO analysis, proven for a decade to work well in reflection seismology, may be useful in near‐surface hydrogeological situations when applied carefully to GPR data to better constrain the presence or absence of nonaqueous phase liquid contaminants (NAPLs) in the subsurface.