Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elb... more Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elbe point at significant influences of groundwater on surface water, while the Brévilles Catchment shows a distinct problematic of pesticide loading to groundwater. Further modeling studies are currently being developed. In the Danube Basin no specific groundwater studies were carried out in the framework of AquaTerra. However on larger scales geochemical proxies such as strontium isotope ratios can give an insight into groundwater contributions to the river that reflects an integral signal of the environmental status of the Basin. Future local groundwater studies should be further correlated to the environmental status of rivers nearby.
Introduction: Recent field studies and laboratory experiments indicate that the emission of organ... more Introduction: Recent field studies and laboratory experiments indicate that the emission of organic contaminants appears to be controlled by the desorption of the contaminants from soil solids. The interactions of dissolved organic species with aquifer- or soil solids, e.g. sorption and desorption, depend on the physico-chemical parameters of the contaminant as well as on those of the aquifer material. The transfer of the contaminants from the mobile aquaeous phase to the sorption sites (sorptive uptake) or vice versa (desorption), is limited by slow molecular diffusion (1, 2). Therefore the time scale of remediation can reach years to decades and the transport of sorbing compounds in groundwater may occur under nonequilibrium conditions. In addition, hysteresis was observed in desorption of organic contaminants from soils (3).
Kurzfassung auf dem gelände eines ehemaligen gaswerks (testfeld Süd) wurden immissionspumpversuch... more Kurzfassung auf dem gelände eines ehemaligen gaswerks (testfeld Süd) wurden immissionspumpversuche (IPVs) zur Quantifizierung der Fracht und der mittleren Konzentration gaswerkstypischer Schadstoffe entlang von drei Kontrollquerschnitten durchgeführt. Die daraus resultierenden Konzentrationsganglinien wurden mithilfe des inversionsprogramms cStreaM und eines Strömungsund transportmodells des hydraulisch extrem heterogenen grundwasserleiters numerisch ausgewertet. Die den gesamten Abstrombereich der Verdachtsfläche erfassenden Kontrollquerschnitte ermöglichen aussagen über die Position und ausbreitung der Schadstofffahne auf dem gelände des testfeldes Süd. bisherige auswertungen von iPVs konnten die Verteilung der Schadstofffahne um den iPV-brunnen für genau drei Fälle berechnen: entweder befindet sich die Fahne links vom brunnen, rechts davon oder ist symmetrisch um ihn verteilt. Um eine realistischere Vorstellung von der Fahnenposition zu ermöglichen, wurden in dieser Studie zusätzlich Direct-Push-Messstellen entlang einer Kontroll ebene installiert. Die in diesen Messstellen gemessenen Konzentrationen wurden zur Konditionierung der numerischen inversionslösung herangezogen. Die ergebnisse ermöglichen eine genauere eingrenzung des Fahnenzentrums sowie des Fahnenrandes, was insbesondere die erarbeitung angepasster und optimierter Sanierungsstrategien unterstützt. Integral quantification of contaminant mass flux in groundwater-conditioning of the numerical inversion of concentration-time series of integral pumping tests Abstract a series of integral pumping tests (iPts) were conducted at a former gasworks site to quantify the contaminant mass flux and average concentration in groundwater along three control planes. the resulting concentration-time series were analysed numerically with the help of the inversion code CSTREAM and a flow and transport model representing the highly heterogeneous aquifer. Since the control planes cover the entire downstream width of the potentially contaminated area, they allow conclusions to be drawn about the current location and spread of the contaminant plume. Previous evaluations of integral pumping tests could calculate three scenarios concerning the spread of the plume around the iPt well: (i) the plume is located to the right of the well, (ii) to the left, or (iii) is distributed symmetrically around it. to create a more realistic picture of the plume position, a series of direct-push monitoring wells were installed along one control plane. the concentrations found in these wells were included in the numerical analysis to condition the inversion results, and resulted in a more pronounced plume centre and fringe, which supports the development of optimised remediation strategies. Keywords integral pumping tests • conditioning • plume delineation • remediation optimisation Einleitung Das herangehen an großräumige Schadensfälle unter berücksichtigung von Natural attenuation (Na) hat in den letzten Jahren mehr und mehr an bedeutung gewonnen, da im Vergleich zu Sanierungsmaßnahmen wie z. b. Pump-and
Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluati... more Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas‐phase diffusivities in the subsurface. We review methods to measure the tortuosity factor or the effective gas‐phase diffusion coefficient in situ. The strong dependency of these parameters on the structure and volume of the air‐filled pore space in the subsurface calls for an accurate and robust in situ measurement. A variety of approaches have been proposed during the last decades, each based on the observation and interpretation of gaseous tracer diffusion in near‐surface soils or the deeper vadose zone under various initial and boundary conditions. We briefly describe the conceptual basis and experimental setup of each method and give insight into error propagation. We then discuss 115 effective diffusion coefficients De compiled from the original method papers and applications. In situ methods and labora...
Coal-tar contaminated industrial sites are a wide-spread problem and represent a substantial risk... more Coal-tar contaminated industrial sites are a wide-spread problem and represent a substantial risk-potential because many of the tar constituents are toxic and/or cancerogenic. A coal-tar is a complex mixture of Phenols, Aromatics (BTX - Benzene, Toluene, Xylene), PAH and others [1]. The constituents vary with respect to their environmentally relevant chemical and physical properties, such as water-solubility and octanol-water partition coefficient over several orders of magnitude.
In recent years, soil gas measurements have successfully been used in investigations of subsurfac... more In recent years, soil gas measurements have successfully been used in investigations of subsurface contaminations of highly volatile pollutants in the unsaturated zone. Favorable partition coefficients (e.g. partition coefficient solid/water, Henry coefficient) of the compounds of interest (e.g. highly volatile chlorinated hydrocarbons) are necessary for their detection in soil gas.
Dispersive processes that diminish contaminant concentrations originating from an unsaturated sou... more Dispersive processes that diminish contaminant concentrations originating from an unsaturated source zone on the way to groundwater, were investigated. Simulations using the numerical model MIN3P were performed for a non-volatile, non-degrading contaminant from a persistent source after reaching a steady state. A 2-D vertical cross-section was used as geometry. Two different types of sandy sediment were simulated: a rather coarse sand with a capillary rise of 90% water saturation to 4 cm above the water table, and a silty sand showing a capillary fringe of 30 cm height (90% water saturation). Major dispersive fluxes were found to take place below the water table, thus dilution and concentration reduction at and above the water table is not very significant.
Aquifers at former gasworks sites are often contaminated with polycyclic aromatic hydrocarbons (P... more Aquifers at former gasworks sites are often contaminated with polycyclic aromatic hydrocarbons (PAH’s), which are toxic and in some cases carcinogenic. At such sites contamination of the soil and aquifer often occured several decades ago. It is likely that initially high contaminant concentrations in the groundwater have subsequently declined to the current low and almost constant values. Today the PAH’s are present in a residual phase of tar or taroil as well as sorbed onto the soil matrix. The concentrations in groundwater depend on the release characteristics of the PAH’s from residual and sorbed phases. In both cases, slow molecular diffusion in aqueous phase, either in the soil matrix or in stagnant water adjacent to residual coal tar, appears to limit the rates of the PAH emission. Thus, the release rates of PAH’s are not only functions of the contaminant concentration in the soil but also of soil characteristics such as porosity, pore size distribution, sorption capacity and organic carbon content. These parameters also affect the mobility of contaminants in groundwater, the accurate determination of which is essential for both risk assessment and soil remediation.
Relevance of local compound-specific transverse dispersion at the field scaleEffective parameters... more Relevance of local compound-specific transverse dispersion at the field scaleEffective parameters from flux-related measures of dilution show low uncertaintyReactive mixing: critical flux-related dilution index and scalar dissipation rate
The oxygen flux from the atmosphere into aquifers plays a key role for the biodegradation of orga... more The oxygen flux from the atmosphere into aquifers plays a key role for the biodegradation of organic solutes in groundwater and changing the water chemistry in general. In this paper, simple analytical solutions for the reactive transport of oxygen into groundwater are derived and compared to numerical models. The oxygen transfer is described analogous to diffusion into a semi‐infinite domain with instantaneous reaction. We assume the capillary fringe does not exist and the concentration of oxygen is constant at the water table. Steady state analytical solutions are derived for the oxygen flux and for the concentration profiles of the reactants and products that could be volatile and nonvolatile. The simple approach is validated with numerical simulations. We can show that the analytical solutions agree very well with the numerical model. On the basis of this finding, simple analytical solutions can be used to calculate the depth of oxidation fronts in groundwater and the reactive transport of oxygen into groundwater.
Transverse dispersion is the most relevant process in mass transfer of contaminants across the ca... more Transverse dispersion is the most relevant process in mass transfer of contaminants across the capillary fringe (both directions), dilution of contaminants, and mixing of electron acceptors and electron donors in biodegrading groundwater plumes. This paper gives an overview on literature values of transverse vertical dispersivities alpha(tv) measured at different flow velocities and compares them to results from well-controlled laboratory-tank experiments on mass transfer of trichloroethene (TCE) across the capillary fringe. The measured values of transverse vertical dispersion in the capillary fringe region were larger than in fully saturated media, which is credited to enhanced tortuosity of the flow paths due to entrapped air within the capillary fringe. In all cases, the values observed for alpha(tv) were < 1 mm. The new measurements and the literature values indicate that alpha(tv) apparently declines with increasing flow velocity. The latter is attributed to incomplete diffusive mixing at the pore scale (pore throats). A simple conceptual model, based on the mean square displacement and the pore size accounting for only partial diffusive mixing at increasing flow velocities, shows very good agreement with measured and published data.
In case of dissolved electron donors and acceptors, natural attenuation of organic contaminant pl... more In case of dissolved electron donors and acceptors, natural attenuation of organic contaminant plumes in aquifers is governed by hydrodynamic mixing and microbial activity. Main objectives of this work were (i) to determine whether aerobic and anaerobic biodegradation in porous sediments is controlled by transverse dispersion, (ii) to elucidate the effect of sediment heterogeneity on mixing and biodegradation, and (iii) to search for degradation-limiting factors. Comparative experiments were conducted in two-dimensional sediment microcosms. Aerobic toluene and later ethylbenzene degradation by Pseudomonas putida strain F1 was initially followed in a plume developing from oxic to anoxic conditions and later under steady-state mixingcontrolled conditions. Competitive anaerobic degradation was then initiated by introduction of the denitrifying strain Aromatoleum aromaticum EbN1. In homogeneous sand, aerobic toluene degradation was clearly controlled by dispersive mixing. Similarly, under denitrifying conditions, microbial activity was located at the plume's fringes. Sediment heterogeneity caused flow focusing and improved the mixing of reactants. Independent from the electron accepting process, net biodegradation was always higher in the heterogeneous setting with a calculated efficiency plus of 23-100% as compared to the homogeneous setup. Flow and reactive transport model simulations were performed in order to interpret and evaluate the experimental results.
In April 2004, the German Research Foundation (DFG) and the National Science Foundation (NSF) co-... more In April 2004, the German Research Foundation (DFG) and the National Science Foundation (NSF) co-organized a meeting for US and German junior geoscientists in Washington DC. As an outcome of this Research Conference titled 'Earth, Fire, Water, Air and Life', a follow-up workshop took place in May 2005 at the Eberhard-Karls University of Tuebingen in Germany. This workshop covered new aspects of research to improve the understanding of steep biogeochemical gradients covering pH changes, redox zones, as well as solute and particulate concentration variations in aqueous systems. Detailed understanding of biogeochemistry in this context delivers new fundamental aspects in interdisciplinary research. Such work is also urgently needed to control ever-increasing scarcity of water that is to large parts driven by decreasing water quality. Research ideas on gradients in a biogeochemical context that were discussed by a subgroup of biogeochemists during that workshop are summarized and presented here.
Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elb... more Several local groundwater studies within the EU project AquaTerra in the Basins of the Meuse, Elbe point at significant influences of groundwater on surface water, while the Brévilles Catchment shows a distinct problematic of pesticide loading to groundwater. Further modeling studies are currently being developed. In the Danube Basin no specific groundwater studies were carried out in the framework of AquaTerra. However on larger scales geochemical proxies such as strontium isotope ratios can give an insight into groundwater contributions to the river that reflects an integral signal of the environmental status of the Basin. Future local groundwater studies should be further correlated to the environmental status of rivers nearby.
Introduction: Recent field studies and laboratory experiments indicate that the emission of organ... more Introduction: Recent field studies and laboratory experiments indicate that the emission of organic contaminants appears to be controlled by the desorption of the contaminants from soil solids. The interactions of dissolved organic species with aquifer- or soil solids, e.g. sorption and desorption, depend on the physico-chemical parameters of the contaminant as well as on those of the aquifer material. The transfer of the contaminants from the mobile aquaeous phase to the sorption sites (sorptive uptake) or vice versa (desorption), is limited by slow molecular diffusion (1, 2). Therefore the time scale of remediation can reach years to decades and the transport of sorbing compounds in groundwater may occur under nonequilibrium conditions. In addition, hysteresis was observed in desorption of organic contaminants from soils (3).
Kurzfassung auf dem gelände eines ehemaligen gaswerks (testfeld Süd) wurden immissionspumpversuch... more Kurzfassung auf dem gelände eines ehemaligen gaswerks (testfeld Süd) wurden immissionspumpversuche (IPVs) zur Quantifizierung der Fracht und der mittleren Konzentration gaswerkstypischer Schadstoffe entlang von drei Kontrollquerschnitten durchgeführt. Die daraus resultierenden Konzentrationsganglinien wurden mithilfe des inversionsprogramms cStreaM und eines Strömungsund transportmodells des hydraulisch extrem heterogenen grundwasserleiters numerisch ausgewertet. Die den gesamten Abstrombereich der Verdachtsfläche erfassenden Kontrollquerschnitte ermöglichen aussagen über die Position und ausbreitung der Schadstofffahne auf dem gelände des testfeldes Süd. bisherige auswertungen von iPVs konnten die Verteilung der Schadstofffahne um den iPV-brunnen für genau drei Fälle berechnen: entweder befindet sich die Fahne links vom brunnen, rechts davon oder ist symmetrisch um ihn verteilt. Um eine realistischere Vorstellung von der Fahnenposition zu ermöglichen, wurden in dieser Studie zusätzlich Direct-Push-Messstellen entlang einer Kontroll ebene installiert. Die in diesen Messstellen gemessenen Konzentrationen wurden zur Konditionierung der numerischen inversionslösung herangezogen. Die ergebnisse ermöglichen eine genauere eingrenzung des Fahnenzentrums sowie des Fahnenrandes, was insbesondere die erarbeitung angepasster und optimierter Sanierungsstrategien unterstützt. Integral quantification of contaminant mass flux in groundwater-conditioning of the numerical inversion of concentration-time series of integral pumping tests Abstract a series of integral pumping tests (iPts) were conducted at a former gasworks site to quantify the contaminant mass flux and average concentration in groundwater along three control planes. the resulting concentration-time series were analysed numerically with the help of the inversion code CSTREAM and a flow and transport model representing the highly heterogeneous aquifer. Since the control planes cover the entire downstream width of the potentially contaminated area, they allow conclusions to be drawn about the current location and spread of the contaminant plume. Previous evaluations of integral pumping tests could calculate three scenarios concerning the spread of the plume around the iPt well: (i) the plume is located to the right of the well, (ii) to the left, or (iii) is distributed symmetrically around it. to create a more realistic picture of the plume position, a series of direct-push monitoring wells were installed along one control plane. the concentrations found in these wells were included in the numerical analysis to condition the inversion results, and resulted in a more pronounced plume centre and fringe, which supports the development of optimised remediation strategies. Keywords integral pumping tests • conditioning • plume delineation • remediation optimisation Einleitung Das herangehen an großräumige Schadensfälle unter berücksichtigung von Natural attenuation (Na) hat in den letzten Jahren mehr und mehr an bedeutung gewonnen, da im Vergleich zu Sanierungsmaßnahmen wie z. b. Pump-and
Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluati... more Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas‐phase diffusivities in the subsurface. We review methods to measure the tortuosity factor or the effective gas‐phase diffusion coefficient in situ. The strong dependency of these parameters on the structure and volume of the air‐filled pore space in the subsurface calls for an accurate and robust in situ measurement. A variety of approaches have been proposed during the last decades, each based on the observation and interpretation of gaseous tracer diffusion in near‐surface soils or the deeper vadose zone under various initial and boundary conditions. We briefly describe the conceptual basis and experimental setup of each method and give insight into error propagation. We then discuss 115 effective diffusion coefficients De compiled from the original method papers and applications. In situ methods and labora...
Coal-tar contaminated industrial sites are a wide-spread problem and represent a substantial risk... more Coal-tar contaminated industrial sites are a wide-spread problem and represent a substantial risk-potential because many of the tar constituents are toxic and/or cancerogenic. A coal-tar is a complex mixture of Phenols, Aromatics (BTX - Benzene, Toluene, Xylene), PAH and others [1]. The constituents vary with respect to their environmentally relevant chemical and physical properties, such as water-solubility and octanol-water partition coefficient over several orders of magnitude.
In recent years, soil gas measurements have successfully been used in investigations of subsurfac... more In recent years, soil gas measurements have successfully been used in investigations of subsurface contaminations of highly volatile pollutants in the unsaturated zone. Favorable partition coefficients (e.g. partition coefficient solid/water, Henry coefficient) of the compounds of interest (e.g. highly volatile chlorinated hydrocarbons) are necessary for their detection in soil gas.
Dispersive processes that diminish contaminant concentrations originating from an unsaturated sou... more Dispersive processes that diminish contaminant concentrations originating from an unsaturated source zone on the way to groundwater, were investigated. Simulations using the numerical model MIN3P were performed for a non-volatile, non-degrading contaminant from a persistent source after reaching a steady state. A 2-D vertical cross-section was used as geometry. Two different types of sandy sediment were simulated: a rather coarse sand with a capillary rise of 90% water saturation to 4 cm above the water table, and a silty sand showing a capillary fringe of 30 cm height (90% water saturation). Major dispersive fluxes were found to take place below the water table, thus dilution and concentration reduction at and above the water table is not very significant.
Aquifers at former gasworks sites are often contaminated with polycyclic aromatic hydrocarbons (P... more Aquifers at former gasworks sites are often contaminated with polycyclic aromatic hydrocarbons (PAH’s), which are toxic and in some cases carcinogenic. At such sites contamination of the soil and aquifer often occured several decades ago. It is likely that initially high contaminant concentrations in the groundwater have subsequently declined to the current low and almost constant values. Today the PAH’s are present in a residual phase of tar or taroil as well as sorbed onto the soil matrix. The concentrations in groundwater depend on the release characteristics of the PAH’s from residual and sorbed phases. In both cases, slow molecular diffusion in aqueous phase, either in the soil matrix or in stagnant water adjacent to residual coal tar, appears to limit the rates of the PAH emission. Thus, the release rates of PAH’s are not only functions of the contaminant concentration in the soil but also of soil characteristics such as porosity, pore size distribution, sorption capacity and organic carbon content. These parameters also affect the mobility of contaminants in groundwater, the accurate determination of which is essential for both risk assessment and soil remediation.
Relevance of local compound-specific transverse dispersion at the field scaleEffective parameters... more Relevance of local compound-specific transverse dispersion at the field scaleEffective parameters from flux-related measures of dilution show low uncertaintyReactive mixing: critical flux-related dilution index and scalar dissipation rate
The oxygen flux from the atmosphere into aquifers plays a key role for the biodegradation of orga... more The oxygen flux from the atmosphere into aquifers plays a key role for the biodegradation of organic solutes in groundwater and changing the water chemistry in general. In this paper, simple analytical solutions for the reactive transport of oxygen into groundwater are derived and compared to numerical models. The oxygen transfer is described analogous to diffusion into a semi‐infinite domain with instantaneous reaction. We assume the capillary fringe does not exist and the concentration of oxygen is constant at the water table. Steady state analytical solutions are derived for the oxygen flux and for the concentration profiles of the reactants and products that could be volatile and nonvolatile. The simple approach is validated with numerical simulations. We can show that the analytical solutions agree very well with the numerical model. On the basis of this finding, simple analytical solutions can be used to calculate the depth of oxidation fronts in groundwater and the reactive transport of oxygen into groundwater.
Transverse dispersion is the most relevant process in mass transfer of contaminants across the ca... more Transverse dispersion is the most relevant process in mass transfer of contaminants across the capillary fringe (both directions), dilution of contaminants, and mixing of electron acceptors and electron donors in biodegrading groundwater plumes. This paper gives an overview on literature values of transverse vertical dispersivities alpha(tv) measured at different flow velocities and compares them to results from well-controlled laboratory-tank experiments on mass transfer of trichloroethene (TCE) across the capillary fringe. The measured values of transverse vertical dispersion in the capillary fringe region were larger than in fully saturated media, which is credited to enhanced tortuosity of the flow paths due to entrapped air within the capillary fringe. In all cases, the values observed for alpha(tv) were < 1 mm. The new measurements and the literature values indicate that alpha(tv) apparently declines with increasing flow velocity. The latter is attributed to incomplete diffusive mixing at the pore scale (pore throats). A simple conceptual model, based on the mean square displacement and the pore size accounting for only partial diffusive mixing at increasing flow velocities, shows very good agreement with measured and published data.
In case of dissolved electron donors and acceptors, natural attenuation of organic contaminant pl... more In case of dissolved electron donors and acceptors, natural attenuation of organic contaminant plumes in aquifers is governed by hydrodynamic mixing and microbial activity. Main objectives of this work were (i) to determine whether aerobic and anaerobic biodegradation in porous sediments is controlled by transverse dispersion, (ii) to elucidate the effect of sediment heterogeneity on mixing and biodegradation, and (iii) to search for degradation-limiting factors. Comparative experiments were conducted in two-dimensional sediment microcosms. Aerobic toluene and later ethylbenzene degradation by Pseudomonas putida strain F1 was initially followed in a plume developing from oxic to anoxic conditions and later under steady-state mixingcontrolled conditions. Competitive anaerobic degradation was then initiated by introduction of the denitrifying strain Aromatoleum aromaticum EbN1. In homogeneous sand, aerobic toluene degradation was clearly controlled by dispersive mixing. Similarly, under denitrifying conditions, microbial activity was located at the plume's fringes. Sediment heterogeneity caused flow focusing and improved the mixing of reactants. Independent from the electron accepting process, net biodegradation was always higher in the heterogeneous setting with a calculated efficiency plus of 23-100% as compared to the homogeneous setup. Flow and reactive transport model simulations were performed in order to interpret and evaluate the experimental results.
In April 2004, the German Research Foundation (DFG) and the National Science Foundation (NSF) co-... more In April 2004, the German Research Foundation (DFG) and the National Science Foundation (NSF) co-organized a meeting for US and German junior geoscientists in Washington DC. As an outcome of this Research Conference titled 'Earth, Fire, Water, Air and Life', a follow-up workshop took place in May 2005 at the Eberhard-Karls University of Tuebingen in Germany. This workshop covered new aspects of research to improve the understanding of steep biogeochemical gradients covering pH changes, redox zones, as well as solute and particulate concentration variations in aqueous systems. Detailed understanding of biogeochemistry in this context delivers new fundamental aspects in interdisciplinary research. Such work is also urgently needed to control ever-increasing scarcity of water that is to large parts driven by decreasing water quality. Research ideas on gradients in a biogeochemical context that were discussed by a subgroup of biogeochemists during that workshop are summarized and presented here.
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Papers by P. Grathwohl