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Partial source zone removal
Journal of Contaminant Hydrology 102 (2008) 1–2
Contents lists available at ScienceDirect
Journal of Contaminant Hydrology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j c o n h y d
Editorial
Partial source zone removal ARTICLE INFO Keywords: Monitored natural attenuation (MNA) Mass flux Non-aqueous phase liquids (NAPL) Source depletion Source zone remediation
Extensive worldwide releases of non-aqueous phase liquids (NAPLs) at private, public and industrial facilities still pose significant challenges in site remediation and long-term site management. Active remediation techniques such as pump-and-treat or excavation have not proven economical for a large number of soil and groundwater contamination scenarios. As a result monitored natural attenuation (MNA), as a passive remediation strategy, is increasingly considered a viable option for management of contaminant plumes. Short contaminant plumes that have reached steady state are considered acceptable with MNA typically implemented when no receptor is at risk (NRC, 1999; Rügner et al., 2006). Following the management of large contaminant plumes, NAPL source zones often contribute the highest environmental risk and liability for many sites. Thus, the potential benefit of NAPL source depletion and partial source zone removal has been the subject of significant and still on-going technical and policy debates (EPA, 2003). The decision to address the NAPL source area and/or the dissolved plume is burdened with a high degree of uncertainty on the benefits of various remedial strategies. This is often due to the complexity of the NAPL source architecture (particularly DNAPLs) and the lack of natural attenuation processes taking place in the plume. Research efforts in recent years have sought to better define the nature of the relationship between the NAPL source zone, the dissolved plume and the benefits derived from remedial activities. In the current special issue on “Partial Source Zone Removal”, a number of research efforts addressing components of this complex source–plume relationship are presented including studies at pore, laboratory and field scales. Some research focuses on understanding complex distributions of NAPL in heterogeneous media in laboratory scale 0169-7722/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2008.09.003
systems while others interpret field scale data collected to better understand the problem and changes that result following remedial action. Several papers present results based on experiments conducted using laboratory aquifer models. Experiments that incorporate complex NAPL distributions in both the vertical and groundwater flow directions are critical to advancing our ability to understand and predict field behaviour. Heiderscheidt et al. investigate the effectiveness of potassium permanganate for remediation of a DNAPL source zone using a laboratory aquifer model. Permeability changes caused by manganese dioxide deposition during remediation were monitored in a complex DNAPL distribution that included pools and residual. Oxidation of tetrachloroethylene (PCE) was quantified and found to be rate limited by mass transfer from the DNAPL to the aqueous phase. In a paper by Kaye et al., fluid density contrasts typically encountered in cosolvent foods were evaluated with respect to effectiveness for reducing contaminant flux from DNAPL source zones. Experiments using four different flushing agents with varying density contrasts were employed in continuous and pulsed remedial applications. Mass removal– flux reduction relationships were observed and modelled to assess the dissolution of the complex DNAPL distributions. Research by Wang et al. investigates five light transmission visualization techniques including a novel multiple wavelength method. The tests were conducted in systems with dye included in the aqueous phase but not the DNAPL. This approach allowed assessment of surfactant enhanced dissolution avoiding the complications of dye partitioning. In a set of experiments evaluating fundamental relationships between immiscible fluids in porous media, Ryder and Demond focus on wettability hysteresis and how this can
2
impact DNAPL source zone distribution. The relationship between fluids and natural aquifer materials is critical to understanding the behaviour of NAPL in porous media and developing management solutions. Zhang et al. investigate simplified mass transfer models to simulate NAPL dissolution. In this work spatial and temporal distributions of NAPL are measured using magnetic resonance imaging. The experiments are used to test six different dissolution models. The results indicate that effective interfacial area is important to consider for modelling dissolution behaviour. The theory behind NAPL dissolution from heterogeneous source zones continues to develop and this issue includes papers that contribute to this advancement. Processes leading to plume attenuation are explored by Parker, Park and Tang. In this research, a vertically-integrated analytical model for dissolved phase transport is developed. The model includes an error analysis module. The application to a hypothetical case study allowed assessment of propagation of error uncertainty and ability to predict true performance. The effect of flux reduction from a DNAPL source area on downgradient water quality is investigated by Sale et al. An idealized two layer scenario is simulated using a unique analytical solution for source loading and back diffusion. The results are compared to laboratory experiments and a sensitivity analysis is conducted. The outcome of this study provides a basis for testing more complex modelling approaches. The final group of papers focuses on field site assessment including a study by Parker, Chapman and Guilbeault on plume persistence caused by back diffusion from thin clay layers in a sand aquifer following trichloroethylene (TCE) source zone hydraulic isolation. A Florida site is investigated, where the source zone was hydraulically isolated and included a clean-water displacement zone downgradient of the source. The investigation concludes that back diffusion from thin clay beds (less than 0.2 m thick) was persistent for several years. Maji and Sudicky investigate the influence of mass transfer characteristics for DNAPL source depletion and contaminant flux in a highly characterized glaciofluvial aquifer using various alternative DNAPL-aqueous-phase mass transfer models. D'Affonseca et al. simulate source zone depletion at a field site with a complex multicomponent coal tar source zone for a period of 1000 years. The problem was simplified using 3 composite and 2 individual constituents. The results indicate that mass flux is governed by the geometry of the residual coal tar and that some constituents are extremely persistent. In a comprehensive field study of mass flux at two DNAPL sites before and after remedial actions, Brooks et al. provide results indicating that sub-
Editorial
stantial reductions in mass flux (N90%) were achieved based on field measurements taken along a downgradient transect. The methods included integral pump tests and passive flux meters while remediation consisted of a surfactant flood at Hill Air Force Base and a thermal treatment system at Ft. Lewis Army Base. Finally, Thomson et al. investigate rebound at a coal tar contaminated site following treatment with potassium permanganate. A pilot scale test was conducted over a 35 day period oxidizing b10% of the initial coal tar. Mass discharge in the source and plume was monitored over a 4 year period. Rebound was observed in both mass discharge and plume mass during the course of the monitoring period. The papers presented in this special issue not only represent our current state of understanding in this area but also provide information on areas in need of future research efforts to improve simulation and field assessment methods. Research needs include linking source architecture with mass discharge characteristics and how those change with different remedial solutions. Furthermore, the issue of back diffusion has been documented in short term responses to source remediation but requires further quantification under a range of site conditions and for long term site management implications. We hope that the papers presented in this special issue will be of general interest, provide new insights, and guide future research in the area of partial source zone removal and the long-term management of contaminated sites. References Environmental Protection Agency, 2003. The DNAPL Remediation Challenge: Is There a Case for Source Depletion?. EPA/600/R-03/143, Cincinnati, Ohio. National Research Council, 1999. Natural Attenuation for Ground Water Remediation. National Academy Press, Washington, DC. 274 pp. Rügner, H., Finkel, M., Kaschl, A., Bittens, M., 2006. Application of monitored natural attenuation in contaminated land management. A review and recommended approach for Europe. Environ. Sci. Pol. 9, 568–576.
Philipp Blum University of Tübingen, Center for Applied Geoscience (ZAG), Sigwartstr. 10, 72076 Tübingen, Germany E-mail address: [email protected]. Michael D. Annable University of Florida, Department of Environmental Engineering Sciences, 220 Black Hall, Gainesville, FL 32611, USA E-mail address: annable@ufl.edu. Corresponding author. Tel.: +1 352 392 3294; fax: +1 352 392 3076
Contents lists available at ScienceDirect
Journal of Contaminant Hydrology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j c o n h y d
Editorial
Partial source zone removal ARTICLE INFO Keywords: Monitored natural attenuation (MNA) Mass flux Non-aqueous phase liquids (NAPL) Source depletion Source zone remediation
Extensive worldwide releases of non-aqueous phase liquids (NAPLs) at private, public and industrial facilities still pose significant challenges in site remediation and long-term site management. Active remediation techniques such as pump-and-treat or excavation have not proven economical for a large number of soil and groundwater contamination scenarios. As a result monitored natural attenuation (MNA), as a passive remediation strategy, is increasingly considered a viable option for management of contaminant plumes. Short contaminant plumes that have reached steady state are considered acceptable with MNA typically implemented when no receptor is at risk (NRC, 1999; Rügner et al., 2006). Following the management of large contaminant plumes, NAPL source zones often contribute the highest environmental risk and liability for many sites. Thus, the potential benefit of NAPL source depletion and partial source zone removal has been the subject of significant and still on-going technical and policy debates (EPA, 2003). The decision to address the NAPL source area and/or the dissolved plume is burdened with a high degree of uncertainty on the benefits of various remedial strategies. This is often due to the complexity of the NAPL source architecture (particularly DNAPLs) and the lack of natural attenuation processes taking place in the plume. Research efforts in recent years have sought to better define the nature of the relationship between the NAPL source zone, the dissolved plume and the benefits derived from remedial activities. In the current special issue on “Partial Source Zone Removal”, a number of research efforts addressing components of this complex source–plume relationship are presented including studies at pore, laboratory and field scales. Some research focuses on understanding complex distributions of NAPL in heterogeneous media in laboratory scale 0169-7722/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2008.09.003
systems while others interpret field scale data collected to better understand the problem and changes that result following remedial action. Several papers present results based on experiments conducted using laboratory aquifer models. Experiments that incorporate complex NAPL distributions in both the vertical and groundwater flow directions are critical to advancing our ability to understand and predict field behaviour. Heiderscheidt et al. investigate the effectiveness of potassium permanganate for remediation of a DNAPL source zone using a laboratory aquifer model. Permeability changes caused by manganese dioxide deposition during remediation were monitored in a complex DNAPL distribution that included pools and residual. Oxidation of tetrachloroethylene (PCE) was quantified and found to be rate limited by mass transfer from the DNAPL to the aqueous phase. In a paper by Kaye et al., fluid density contrasts typically encountered in cosolvent foods were evaluated with respect to effectiveness for reducing contaminant flux from DNAPL source zones. Experiments using four different flushing agents with varying density contrasts were employed in continuous and pulsed remedial applications. Mass removal– flux reduction relationships were observed and modelled to assess the dissolution of the complex DNAPL distributions. Research by Wang et al. investigates five light transmission visualization techniques including a novel multiple wavelength method. The tests were conducted in systems with dye included in the aqueous phase but not the DNAPL. This approach allowed assessment of surfactant enhanced dissolution avoiding the complications of dye partitioning. In a set of experiments evaluating fundamental relationships between immiscible fluids in porous media, Ryder and Demond focus on wettability hysteresis and how this can
2
impact DNAPL source zone distribution. The relationship between fluids and natural aquifer materials is critical to understanding the behaviour of NAPL in porous media and developing management solutions. Zhang et al. investigate simplified mass transfer models to simulate NAPL dissolution. In this work spatial and temporal distributions of NAPL are measured using magnetic resonance imaging. The experiments are used to test six different dissolution models. The results indicate that effective interfacial area is important to consider for modelling dissolution behaviour. The theory behind NAPL dissolution from heterogeneous source zones continues to develop and this issue includes papers that contribute to this advancement. Processes leading to plume attenuation are explored by Parker, Park and Tang. In this research, a vertically-integrated analytical model for dissolved phase transport is developed. The model includes an error analysis module. The application to a hypothetical case study allowed assessment of propagation of error uncertainty and ability to predict true performance. The effect of flux reduction from a DNAPL source area on downgradient water quality is investigated by Sale et al. An idealized two layer scenario is simulated using a unique analytical solution for source loading and back diffusion. The results are compared to laboratory experiments and a sensitivity analysis is conducted. The outcome of this study provides a basis for testing more complex modelling approaches. The final group of papers focuses on field site assessment including a study by Parker, Chapman and Guilbeault on plume persistence caused by back diffusion from thin clay layers in a sand aquifer following trichloroethylene (TCE) source zone hydraulic isolation. A Florida site is investigated, where the source zone was hydraulically isolated and included a clean-water displacement zone downgradient of the source. The investigation concludes that back diffusion from thin clay beds (less than 0.2 m thick) was persistent for several years. Maji and Sudicky investigate the influence of mass transfer characteristics for DNAPL source depletion and contaminant flux in a highly characterized glaciofluvial aquifer using various alternative DNAPL-aqueous-phase mass transfer models. D'Affonseca et al. simulate source zone depletion at a field site with a complex multicomponent coal tar source zone for a period of 1000 years. The problem was simplified using 3 composite and 2 individual constituents. The results indicate that mass flux is governed by the geometry of the residual coal tar and that some constituents are extremely persistent. In a comprehensive field study of mass flux at two DNAPL sites before and after remedial actions, Brooks et al. provide results indicating that sub-
Editorial
stantial reductions in mass flux (N90%) were achieved based on field measurements taken along a downgradient transect. The methods included integral pump tests and passive flux meters while remediation consisted of a surfactant flood at Hill Air Force Base and a thermal treatment system at Ft. Lewis Army Base. Finally, Thomson et al. investigate rebound at a coal tar contaminated site following treatment with potassium permanganate. A pilot scale test was conducted over a 35 day period oxidizing b10% of the initial coal tar. Mass discharge in the source and plume was monitored over a 4 year period. Rebound was observed in both mass discharge and plume mass during the course of the monitoring period. The papers presented in this special issue not only represent our current state of understanding in this area but also provide information on areas in need of future research efforts to improve simulation and field assessment methods. Research needs include linking source architecture with mass discharge characteristics and how those change with different remedial solutions. Furthermore, the issue of back diffusion has been documented in short term responses to source remediation but requires further quantification under a range of site conditions and for long term site management implications. We hope that the papers presented in this special issue will be of general interest, provide new insights, and guide future research in the area of partial source zone removal and the long-term management of contaminated sites. References Environmental Protection Agency, 2003. The DNAPL Remediation Challenge: Is There a Case for Source Depletion?. EPA/600/R-03/143, Cincinnati, Ohio. National Research Council, 1999. Natural Attenuation for Ground Water Remediation. National Academy Press, Washington, DC. 274 pp. Rügner, H., Finkel, M., Kaschl, A., Bittens, M., 2006. Application of monitored natural attenuation in contaminated land management. A review and recommended approach for Europe. Environ. Sci. Pol. 9, 568–576.
Philipp Blum University of Tübingen, Center for Applied Geoscience (ZAG), Sigwartstr. 10, 72076 Tübingen, Germany E-mail address: [email protected]. Michael D. Annable University of Florida, Department of Environmental Engineering Sciences, 220 Black Hall, Gainesville, FL 32611, USA E-mail address: annable@ufl.edu. Corresponding author. Tel.: +1 352 392 3294; fax: +1 352 392 3076