Carsten Leven

Delineation of subsurface hydrocarbon contamination at a former hydrogenation plant using spectral induced polarization imaging

Broadband spectral induced polarization (SIP) measurements were conducted at a former hydrogenation plant in Zeitz (NE Germany) to investigate the potential of SIP imaging to delineate areas with different BTEX (benzene, toluene, ethylbenzene, and xylene) concentrations. Conductivity images reveal a poor correlation with the distribution of contaminants; whereas phase images exhibit two main anomalies: low phase shift values (<5 mrad) for locations with high BTEX concentrations, including the occurrence of free-phase product (BTEX concentrations >1.7 g/l), and higher phase values for lower BTEX concentrations. Moreover, the spectral response of the areas with high BTEX concentration and free-phase products reveals a flattened spectrum in the low frequencies (<40 Hz), while areas with lower BTEX concentrations exhibit a response characterized by a frequency peak. The SIP response was modelled using a Debye decomposition to compute images of the median relaxation-time. Consistent with laboratory studies, we observed an increase in the relaxation-time associated with an increase in BTEX concentrations. Measurements were also collected in the time domain (TDIP), revealing imaging results consistent with those obtained for frequency domain (SIP) measurements. Results presented here demonstrate the potential of the SIP imaging method to discriminate source and plume of dissolved contaminants at BTEX contaminated sites.

Evaluation of Combined Direct-Push Methods Used for Aquifer Model Generation

Most established methods to characterize aquifer structure and hydraulic conductivities of hydrostratigraphical units are not capable of delivering sufficient information in the spatial resolution that is desired for sophisticated numerical contaminant transport modeling and adapted remediation design. With hydraulic investigation methods based on the direct-push (DP) technology such as DP slug tests, DP injection logging, and the hydraulic profiling tool, it is possible to rapidly delineate hydrogeological structures and estimate their hydraulic conductivity in shallow unconsolidated aquifers without the need for wells. A combined application of these tools was used for the investigation of a contaminated German refinery site and for the setup of hydraulic aquifer models. The quality of DP investigation and the models was evaluated by comparisons of tracer transport simulations using these models and measured breakthroughs of two natural gradient tracer tests. Model scenarios considering the information of all tools together showed good reproduction of the measured breakthroughs, indicating the suitability of the approach and a minor impact of potential technical limitations. Using the DP slug tests alone yielded significantly higher deviations for the determined hydraulic conductivities compared to considering two or three of the tools. Realistic aquifer models developed on basis of such combined DP investigation approaches can help optimize remediation concepts or identify flow regimes for aquifers with a complex structure.

Delineating subsurface heterogeneity at a loop of River Steinlach using geophysical and hydrogeological methods

Stratigraphic heterogeneity is a key controlling factor for flow and transport in groundwater systems. In this case study, we have combined two- and three-dimensional images of electric resistivity with hydrogeological data to delineate such stratigraphic heterogeneity at a loop of River Steinlach close to Tübingen, Germany, where we estimated spatially varying aquifer parameters using established evaluation techniques. We developed a three-layer stratigraphic model consisting of heterogeneous sandy gravel overlain by a fine alluvium including top soil, and underlain by sandstone weathered at its top. The sandy gravel makes up the aquifer unit while we interpret the sandstone as aquitard. We classified the area into two stratigraphic segments based on resistivity data. The Southern segment consists of a thicker alluvium where the aquifer material contains more clay than in the Northern segment. Two different stratigraphic structures were also delineated at the Southern and Northern segments, respectively. These structures are visible in inversion results as low-resistivity features trending in the SE–NW in the Southern segment and NE–NW in the Northern segment. Both structures may be previous flow paths of River Steinlach, and the low permeability material overlying them may impair flow exchange in the area. We performed slug and pumping tests to estimate the hydraulic conductivity (K) of the aquifer. The K distributions show slight variation with higher values obtained at wells within the Northern segment and close to the river bank. A qualitative comparison of measured hydraulic conductivities with the resistivity distribution shows good agreement of the spatial patterns. The stratigraphic and hydraulic heterogeneities delineated in this work are important for experimental and modeling studies of flow, transport, and hyporheic exchange at the site.

A laboratory study of tracer tomography

A tracer tomographic laboratory study was performed with consolidated fractured rock in three-dimensional space. The investigated fractured sandstone sample was characterized by significant matrix permeability. The laboratory transport experiments were conducted using gas-flow and gas-tracer transport techniques that enable the generation of various flow-field patterns via adjustable boundary conditions within a short experimental time period. In total, 72 gas-tracer (helium) tests were performed by systematically changing the injection and monitoring configuration after each test. For the inversion of the tracer breakthrough curves an inversion scheme was applied, based on the transformation of the governing transport equation into a form of the eikonal equation. The reliability of the inversion results was assessed with singular value decomposition of the trajectory density matrix. The applied inversion technique allowed for the three-dimensional reconstruction of the interstitial velocity with a high resolution. The three-dimensional interstitial velocity distribution shows clearly that the transport is dominated by the matrix while the fractures show no apparent influence on the transport responses.RésuméUne étude en laboratoire de tomographie par traceur a été réalisée sur une roche fracturée consolidée dans un espace à 3 dimensions. L’échantillon de grès fracturé étudié se caractérisait par une perméabilité de matrice élevée. Les expériences de traçage en laboratoire ont été conduites en utilisant des techniques de transfert d’un courant gazeux et d’un traceur gazeux permettant la création de modèles variés de champ d’écoulement, grâce à des conditions aux limites ajustables dans un laps de temps expérimental court. Au total, 72 tests avec traceur gazeux (hélium) ont été réalisés, en changeant systématiquement la configuration de l’injection et du système de mesure après chaque test. Pour l’inversion des courbes de marquage du traceur, on a appliqué un procédé d’inversion basé sur la transformation de l’équation de transfert principale en une forme d’équation iconale. La fiabilité des résultats de l’inversion a été démontrée par une décomposition en valeurs singulières de la matrice densité de trajectoire. La technique de l’inversion appliquée a tenu compte de la reconstruction à 3 dimensions de la vitesse interstitielle avec une haute résolution. La distribution de la vitesse interstitielle en 3 dimensions montre clairement que le transfert est contrôlé par la matrice tandis que les fractures ne montrent pas d’influence apparente sur les réponses du transfert.ResumenSe llevó a cabo un estudio tomográfico de laboratorio de trazadores en roca fracturada consolidada en el espacio tridimensional. La muestra investigada de arenisca fracturada se caracterizó por la significativa permeabilidad de la matriz. Los experimentos de transporte de laboratorio fueron realizados usando flujo de gas y técnicas de transporte de trazadores que permite la generación de varios esquemas de campos de flujo de campo a través de condiciones de límites ajustables dentro de corto periodo de tiempo experimental. En total, 72 pruebas con gas trazador (helio) fueron realizados cambiando sistemáticamente la configuración de inyección y monitoreo después de cada prueba. Para la inversión de las curvas de ruptura del trazador un esquema de inversión, basado en la transformación de una ecuación de transporte gobernante se aplica una forma de la ecuación eikonal. La confiabilidad de los resultados de inversión fue evaluada con una descomposición en valores singulares de la matriz de densidad de la trayectoria. La técnica de inversión aplicada permitió la reconstrucción tridimensional de la velocidad intersticial con una alta resolución. La distribución de la velocidad intersticial tridimensional muestra claramente que el transporte está dominado por la matriz mientras que las fracturas no muestran una influencia aparente en las respuestas de transporte.摘要对坚固裂隙岩体进行了三维空间示踪剂断层摄影术实验室研究。调查的断裂砂岩样品特点是具有显著的基质渗透率。采用气流和气体示踪运移技术进行了实验室运移实验,这些技术能够在短的试验时段内产生通过可调边界条件的各种流场模式。总共进行了72项气体示踪(氦)试验,每个试验后都系统地调整注入和监测配置。针对示踪剂突破曲线的反演,采用了基于调整运移公式转化成程函公式的反演方法。采用轨迹密度矩阵的奇值分解对反演结果的可靠性进行了评估。应用的反演技术可三维重建具有高分辨率的间隙速度。三维间隙流速分布清楚地表明,运移主要受控于基质,而断裂显示对运移反应无明显影响。ResumoFoi levado a cabo um estudo laboratorial de tomografia de um traçador, no espaço tridimensional, numa rocha consolidada fraturada. A amostra estudada, arenito fraturado, foi caraterizada por permeabilidade matricial significativa. Os testes laboratoriais de transporte foram realizados utilizando técnicas de transporte de fluxo de gás e de gás traçador que permitiram a geração de vários padrões de campo de fluxo através de condições de fronteira ajustáveis dentro de um período de tempo experimental curto. No total foram realizados 72 testes de gás traçador (hélio), alterando sistematicamente o esquema de injeção e de monitorização após cada teste. Para inversão das curvas de chegada do traçador foi aplicado um esquema de inversão, a partir da transformação da equação de transporte numa forma da equação iconal. A confiança dos resultados da inversão foi avaliada pela decomposição em valores singulares da matriz densidade da trajetória. A técnica de inversão aplicada permitiu a reconstrução tridimensional da velocidade intersticial com uma alta resolução. A distribuição da velocidade intersticial tridimensional mostra claramente que o transporte é dominado pela matriz, enquanto as fraturas não mostram, aparentemente, ter influência nas respostas de transporte.

Tracer Tomography: Design Concepts and Field Experiments Using Heat as a Tracer

Numerical and laboratory studies have provided evidence that combining hydraulic tomography with tomographic tracer tests could improve the estimation of hydraulic conductivity compared with using hydraulic data alone. Field demonstrations, however, have been lacking so far, which we attribute to experimental difficulties. In this study, we present a conceptual design and experimental applications of tracer tomography at the field scale using heat as a tracer. In our experimental design, we improve active heat tracer testing by minimizing possible effects of heat losses, buoyancy, viscosity, and changing boundary conditions. We also utilize a cost-effective approach of measuring temperature changes in situ at high resolution. We apply the presented method to the 8 m thick heterogeneous, sandy gravel, alluvial aquifer at the Lauswiesen Hydrogeological Research Site in Tübingen, Germany. Results of our tomographic heat-tracer experiments are in line with earlier work on characterizing the aquifer at the test site. We demonstrate from the experimental perspective that tracer tomography is applicable and suitable at the field scale using heat as a tracer. The experimental results also demonstrate the potential of heat-tracer tomography as a cost-effective means for characterizing aquifer heterogeneity.

Combining 3D Hydraulic Tomography with Tracer Tests for Improved Transport Characterization

Hydraulic tomography (HT) is a method for resolving the spatial distribution of hydraulic parameters to some extent, but many details important for solute transport usually remain unresolved. We present a methodology to improve solute transport predictions by combining data from HT with the breakthrough curve (BTC) of a single forced-gradient tracer test. We estimated the three dimensional (3D) hydraulic-conductivity field in an alluvial aquifer by inverting tomographic pumping tests performed at the Hydrogeological Research Site Lauswiesen close to Tübingen, Germany, using a regularized pilot-point method. We compared the estimated parameter field to available profiles of hydraulic-conductivity variations from direct-push injection logging (DPIL), and validated the hydraulic-conductivity field with hydraulic-head measurements of tests not used in the inversion. After validation, spatially uniform parameters for dual-domain transport were estimated by fitting tracer data collected during a forced-gradient tracer test. The dual-domain assumption was used to parameterize effects of the unresolved heterogeneity of the aquifer and deemed necessary to fit the shape of the BTC using reasonable parameter values. The estimated hydraulic-conductivity field and transport parameters were subsequently used to successfully predict a second independent tracer test. Our work provides an efficient and practical approach to predict solute transport in heterogeneous aquifers without performing elaborate field tracer tests with a tomographic layout.

Effect of hydrocarbon-contaminated fluctuating groundwater on magnetic properties of shallow sediments

We investigate magnetic phase (trans)formation in the presence of petroleum hydrocarbons and its relation to bacterial activity, in particular in the zone of remediation driven fluctuating water levels at a former military air base in the Czech Republic. In a previous study an increase of magnetite concentration from the groundwater table towards the top of the groundwater fluctuation zone (GWFZ) was reported, however with limited reliability as there was no control on small-scale effects. To recognize statistically significant magnetic signatures versus depth, we obtained multiple sediment cores from three locations in January 2011 and April 2012, penetrating the unsaturated zone, the GWFZ and the uppermost one meter below the groundwater level (∼2.3 m depth at the time of sampling). Magnetic concentration variation versus depth was determined by measuring magnetic susceptibility (MS) and remanence parameters. Small-scale features were identified and eliminated by statistical processing of multiple cores. A trend of increasing MS values from the lowermost position of the groundwater table upward was verified and highest magnetic concentration was found at the top of the GWFZ. Magnetic mineralogy indicates that newly formed fine-grained magnetite in the single domain to small pseudo-single domain range is responsible for the MS enhancement confirming previous results. There is no correlation with the depth variation of hydrocarbon (HC) concentrations; however, total organic carbon is linked to MS and may represent a degradation product of HC that is bioavailable for microorganisms. Bacterial activity is likely responsible for magnetite formation as indicated by most probable number (MPN) results of iron-metabolizing bacteria. The comparison of our results with an earlier study conducted at the same site revealed that magnetic concentration clearly decreased since remediation was terminated in 2008, possibly due to dissolution of magnetite.

MONACO—Monitoring Approach for Geological CO2 Storage Sites Using a Hierarchical Observation Concept

The reliable detection and assessment of potential CO2 leakages from storage formations require the application of assurance monitoring tools at different spatial scales. Such tools also play an important role in helping to establish a risk assessment strategy at carbon dioxide capture and storage (CCS) facilities. Within the framework of the MONACO project (“Monitoring approach for geological CO2 storage sites using a hierarchical observation concept”), an integrative hierarchical assurance monitoring concept was developed and validated with the aim of establishing a modular observation strategy including investigations in the shallow subsurface, at ground surface level, and in the atmosphere. Numerous methods and technologies from different disciplines (such as chemistry, hydrogeology, meteorology, and geophysics) were either combined or used complementarily to one another, with results subsequently being jointly interpreted. Patterns of atmospheric CO2 distributions in terms of leakage detection can be observed on large scales with the help of infrared spectroscopy or micrometeorological methods, which aim to identify zones with unexpected or anomalous atmospheric CO2 concentrations. On the meso-scale, exchange processes between ground surface level and subsurface structures need to be localized using geophysical methods and soil gas surveys. Subsequently, the resulting images and maps can be used for selecting profiles for detailed in situ soil gas and geophysical monitoring, which helps to constrain the extent of leakages and allows us to understand controlling features of the observable fluid flow patterns. The tools utilized were tested at several natural and industrial analogues with various CO2 sources. A comprehensive validation of the opportunities and limitations of all applied method combinations is given and it shows that large spatial areas need to be consistently covered in sufficient spatial and temporal resolutions.

Tomographic Methods in Hydrogeology

The extraction of groundwater for drinking water purposes is one of the most important uses of the natural subsurface. Sustainable management of groundwater resources requires detailed knowledge of the hydraulic properties within the subsurface. Typically, these properties are not directly accessible. The evaluation of hydraulic properties therefore requires hydraulic stimuli of the subsurface (e.g., injection and extraction of groundwater, tracer tests, etc.) with subsequent data analysis. In this context, tomographic techniques and inversion strategies originally derived for geophysical surveying can be transferred to hydraulic applications. In addition, geophysical techniques may be used to monitor hydraulic tests. The latter requires fully coupled hydrogeophysical inversion strategies, accounting for the entire process chain from hydraulic properties via flow and transport to the application of the geophysical surveying techniques. The project “Tomographic methods in hydrogeology” focuses on the development of a geostatistical inversion method for transient tomographic data of multiple hydraulic investigation techniques, the model-based optimal design of tomographic surveys, and the development of experimental techniques and equipment for an efficient execution of tomographic surveys in a hydrogeological context using the model-based design and providing data for the inversion. In this chapter we will show selected examples of the project’s outcome. The examples include developments related to the joint geostatistical inversion of tomographic data sets, its efficient parallelization, and its application to a 3D-inversion of tomographic thermal tracer tests. Furthermore we present a method for solving the inversion of transient tomographic data sets which usually suffer from high computational efforts. Related to the acquisition of tomographic data sets, we also discuss the development of tracer-tomographic methods using heat as tracer.

Where can cone penetrometer technology be applied? Development of a map of Europe regarding the soil penetrability.

Over the past decades, significant efforts have been invested in the development of push-in technology for site characterization and monitoring for geotechnical and environmental purposes and have especially been undertaken in the Netherlands and Germany. These technologies provide the opportunity for faster, cheaper, and collection of more reliable subsurface data. However, to maximize the technology both from a development and implementation point of view, it is necessary to have an overview of the areas suitable for the application of this type of technology. Such an overview is missing and cannot simply be read from existing maps and material. This paper describes the development of a map showing the feasibility or applicability of Direct Push/Cone Penetrometer Technology (DPT/CPT) in Europe which depends on the subsurface and its extremely varying properties throughout Europe. Subsurface penetrability is dependent on a range of factors that have not been mapped directly or can easily be inferred from existing databases, especially the maximum depth reachable would be of interest. Among others, it mainly depends on the geology, the soil mechanical properties, the type of equipment used as well as soil-forming processes. This study starts by looking at different geological databases available at the European scale. Next, a scheme has been developed linking geological properties mapped to geotechnical properties to determine basic penetrability categories. From this, a map of soil penetrability is developed and presented. Validating the output by performing field tests was beyond the scope of this study, but for the country of the Netherlands, this map has been compared against a database containing actual cone penetrometer depth data to look for possible contradictory results that would negate the approach. The map for the largest part of Europe clearly shows that there is a much wider potential for the application of Direct Push Technology than is currently seen. The study also shows that there is a lack of large-scale databases that contain depth-resolved data as well as soil mechanical and physical properties that can be used for engineering purposes in relation to the subsurface.