Fractured-Aquifer Hydrogeology from Geophysical Logs; The Passaic Formation, New Jersey

The Passaic Formation consists of gradational sequences of mudstone, siltstone, and sandstone, and is a principal aquifer in central New Jersey. Ground-water flow is primarily controlled by fractures interspersed throughout these sedimentary rocks and characterizing these fractures in terms of type, orientation, spatial distribution, frequency, and transmissivity is fundamental towards understanding local fluid-transport processes. To obtain this information, a comprehensive suite of geophysical logs was collected in 10 wells roughly 46 m in depth and located within a .05 km2 area in Hopewell Township, New Jersey. A seemingly complex, heterogeneous network of fractures identified with an acoustic televiewer was statistically reduced to two principal subsets corresponding to two distinct fracture types: (1) bedding-plane partings and (2) high-angle fractures. Bedding-plane partings are the most numerous and have an average strike of N84° Wand dip of 20° N. The high-angle fractures are oriented subparallel to these features, with an average strike of N79° E and dip of 71 0 S, making the two fracture types roughly orthogonal. Their intersections form linear features that also retain this approximately east-west strike. Inspection of fluid temperature and conductance logs in conjunction with flowmeter measurements obtained during pumping allows the transmissive fractures to be distinguished from the general fracture population. These results show that, within the resolution capabilities ofthe logging tools, approximately 51 (or 18 percent) of the 280 total fractures are water producing. The bedding-plane partings exhibit transmissivities that average roughly 5 m2/day and that generally diminish in magnitude and frequency with depth. The high-angle fractures have average transmissivities that are about half those of the bedding-plane partings and show no apparent dependence upon depth. The geophysical logging results allow us to infer a distinct hydrogeologic structure within this aquifer that is defined by fracture type and orientation. Fluid flow near the surface is controlled primarily by the highly transmissive, subhorizontal bedding-plane partings. As depth increases, the high-angle fractures apparently become more dominant hydrologically

Sensitivity Analysis for the Appraisal of Hydrofractures in Horizontal Wells with Borehole Resistivity Measurements

This paper numerically evaluates the possibility of using borehole electromagnetic (EM) measurements to diagnose and quantify hydraulic fractures that have been arti ficially generated in a horizontal well. Hydrofractures are modeled as thin disks perpendicular to the well and filled with either sand-based or electrically-conductive proppant. The study focuses on the e ect of both thickness and length (radius) of hydrofractures to assess their eff ects on speci fic con figurations of borehole resistivity instruments. Numerical results indicate that several measurements (e.g. those obtained with low- and high-frequency solenoids) could be used to asses the thickness of a fracture. However, only low-frequency measurements performed with electrodes and large-spacing between transmitter and receivers (18 m) exhibit the necessary sensitivity to reliably and accurately estimate the length of long hydrofractures (up to 150 m) in open-hole wells. In the case of steel-cased wells, the casing acts as a long electrode, whereby conventional low-frequency short-spaced, through-casing measurements are suitable for the accurate diagnosis of long hydrofractures (up to 150 m in length).MTM2010-16511 P711RT027

Topographical, structural and geophysical characterization of fracture zones : implications for groundwater flow and vulnerability

The main objective of this study is to evaluate selected geophysical, structural and topographic methods on regional, local, and tunnel and borehole scales, as indicators of the properties of fracture zones or fractures relevant to groundwater flow. Such information serves, for example, groundwater exploration and prediction of the risk of groundwater inflow in underground construction. This study aims to address how the features detected by these methods link to groundwater flow in qualitative and semi-quantitative terms and how well the methods reveal properties of fracturing affecting groundwater flow in the studied sites. The investigated areas are: (1) the Päijänne Tunnel for water-conveyance whose study serves as a verification of structures identified on regional and local scales; (2) the Oitti fuel spill site, to telescope across scales and compare geometries of structural assessment; and (3) Leppävirta, where fracturing and hydrogeological environment have been studied on the scale of a drilled well.The methods applied in this study include: the interpretation of lineaments from topographic data and their comparison with aeromagnetic data; the analysis of geological structures mapped in the Päijänne Tunnel; borehole video surveying; groundwater inflow measurements; groundwater level observations; and information on the tunnel's deterioration as demonstrated by block falls. The study combined geological and geotechnical information on relevant factors governing groundwater inflow into a tunnel and indicators of fracturing, as well as environmental datasets as overlays for spatial analysis using GIS. Geophysical borehole logging and fluid logging were used in Leppävirta to compare the responses of different methods to fracturing and other geological features on the scale of a drilled well.Results from some of the geophysical measurements of boreholes were affected by the large diameter (gamma radiation) or uneven surface (caliper) of these structures. However, different anomalies indicating more fractured upper part of the bedrock traversed by well HN4 in Leppävirta suggest that several methods can be used for detecting fracturing.Fracture trends appear to align similarly on different scales in the zone of the Päijänne Tunnel. For example, similarities of patterns were found between the regional magnetic trends, correlating with orientations of topographic lineaments interpreted as expressions of fracture zones. The same structural orientations as those of the larger structures on local or regional scales were observed in the tunnel, even though a match could not be made in every case. The size and orientation of the observation space (patch of terrain at the surface, tunnel section, or borehole), the characterization method, with its typical sensitivity, and the characteristics of the location, influence the identification of the fracture pattern. Through due consideration of the influence of the sampling geometry and by utilizing complementary fracture characterization methods in tandem, some of the complexities of the relationship between fracturing and groundwater flow can be addressed.The flow connections demonstrated by the response of the groundwater level in monitoring wells to pressure decrease in the tunnel and the transport of MTBE through fractures in bedrock in Oitti, high­light the importance of protecting the tunnel water from a risk of contamination. In general, the largest values of drawdown occurred in monitoring wells closest to the tunnel and/or close to the topographically interpreted fracture zones. It seems that, to some degree, the rate of inflow shows a positive correlation with the level of reinforcement, as both are connected with the fracturing in the bedrock.The following geological features increased the vulnerability of tunnel sections to pollution, especially when several factors affected the same locations: (1) fractured bedrock, particularly with associated groundwater inflow; (2) thin or permeable overburden above fractured rock; (3) a hydraulically conductive layer underneath the surface soil; and (4) a relatively thin bedrock roof above the tunnel. The observed anisotropy of the geological media should ideally be taken into account in the assessment of vulnerability of tunnel sections and eventually for directing protective measures

Geophysical and geological characterization of fault-controlled geothermal systems: The Vallès Basin case of study

[eng] Geothermal energy is a renewable source of energy that harnesses heat from the Earth's interior. Temperature increases with depth, defining the geothermal gradient, which can be variable depending on the geological context. The geological setting of western Europe favors a relatively high geothermal gradient that could be exploited to generate electricity or for its direct use, for example, for its application in industry, greenhouses, or heating systems. In each of these cases, geothermal could favor the community's energy independence and reduce the use of polluting energy sources. To appropriately exploit areas with a significant geothermal gradient, it is essential to know the origin of the temperature anomaly and the system's functioning. In this context, developing appropriate exploration methodologies and techniques is essential for its adequate and efficient use. This thesis develops a methodology focused on a geothermal system type characterized by being located in highly fractured zones. These fractures connect the surface with great depths, allowing the rapid ascent of deep fluids at high temperatures without giving them time to cool down. Specifically, this thesis applies this methodology to a study case located in the Vallès Basin, close to Barcelona city (NE Iberian Peninsula), where some localities, such as La Garriga and Caldes de Montbui towns, have thermal hot springs (60ºC and 70ºC, respectively). In particular, the methodology applied to study the Vallès Basin geothermal fractured system, is focused on two main cores, geophysical and geological techniques. Geophysical methods allow the characterization of the subsurface physical properties, reaching great depths without having to drill. For example, if the physical characteristics of the subsurface have enough contrast, they could allow distinguishing between different types of rocks, fractured zones, or if there is any fluid circulation. However, the geophysical results have to be complemented with other geoscientific studies in order to make a proper interpretation. In this case, this thesis includes a characterization of the area's geology, fracturing, and hydrology. Finally, the integration of the applied techniques has allowed the understanding of the origin and system's functioning, which is presented in the form of a 3D conceptual model, geological model, and temperature model. This innovative methodology, which integrates different geoscientific techniques at different scales, combining traditional techniques with novel digital tools, has facilitated the characterization of a geothermal system controlled by geological structures. Therefore, it is established as a methodical option to characterize systems of similar origin.[cat] La Geotèrmia és una font renovable d'energia que aprofita la temperatura de l'interior de la Terra. El grau en què aquesta temperatura augmenta en profunditat, ve definint pel gradient geotèrmic, el qual pot ser variable segons el context geològic. La geologia de la regió oest del continent europeu afavoreix un gradient geotèrmic relativament alt que podria ser aprofitat per generar electricitat o per a ús directe, com és el cas d'aplicacions en indústria, hivernacles o sistemes de calefacció. En qualsevol cas, la geotèrmia podria afavorir la independència energètica i una disminució en l’ús de fonts d’energia contaminants. Per a un aprofitament d'aquestes zones amb un gradient geotèrmic significatiu, és essencial conèixer-ne l'origen i el funcionament. En aquest context, és basic desenvolupar metodologies d'exploració que siguin adequades i eficients. Aquesta tesis desenvolupa una metodologia aplicada a un exemple de sistema geotèrmic caracteritzat per estar ubicat en una zona molt fracturada. Aquestes fractures connecten la superfície amb grans profunditats, permetent l'ascens ràpid de fluids profunds que es troben a temperatures altes, sense que els doni temps a refredar-se. Concretament, aquesta zona d'estudi es situa a la Conca del Vallès (NE Península Ibèrica), on algunes localitats com La Garriga i Caldes de Montbui, tenen surgències d'aigua termal (60ºC i 70ºC, respectivament). Concretament, la metodologia aplicada es basa en dues parts principals: l'exploració geofísica i la geològica. Els mètodes geofísics ens permeten conèixer les propietats físiques del subsol arribant a grans profunditats sense haver de fer perforacions. Si les característiques físiques del terreny presenten un contrast suficient, poden permetre, per exemple, distingir entre tipus de roques, zones fracturades, o si hi ha circulació d'algun fluid. Tot i així, els resultats geofísics s'han de complementar amb altres estudis geocientífics per una correcta interpretació dels resultats. En aquest cas, aquesta tesis inclou una caracterització de la geologia, la fracturació i la hidrologia de la zona. La integració final de totes les dades ha permès entendre l'origen i el funcionament d'aquest sistema, resultat del qual es presenta en forma d'un model 3D conceptual, geològic i de temperatures. Aquesta metodologia innovadora, que integra diferents tècniques geocientífiques a escala diferent, ha combinat tècniques tradicionals amb eines digitals noves, facilitant la caracterització d'un sistema geotèrmic controlat per estructures geològiques. Per tant, s’estableix com una opció metòdica a seguir per a la caracterització de sistemes d’origen similar.[spa] La Geotermia es una fuente renovable de energía que aprovecha el calor del interior de la Tierra. La temperatura del interior de la Tierra aumenta con la profundidad, y este aumento, definido como gradiente geotérmico, puede ser variable según el contexto geológico. El contexto geológico del oeste del continente europeo favorece un gradiente geotérmico relativamente alto que podría ser aprovechado para generar electricidad o para su uso directo, como es el caso de aplicaciones en industria, invernaderos o sistemas de calefacción. En cualquier caso, la geotermia podría favorecer la independencia energética y una disminución del uso de fuentes de energía contaminantes. Para un apropiado aprovechamiento de estas zonas con un gradiente geotérmico significativo, es esencial conocer su origen y funcionamiento. En este contexto, es necesario un avance en el desarrollo de metodologías de exploración que sean adecuadas y eficientes. Esta tesis desarrolla una metodología aplicada a un tipo de sistema geotérmico caracterizado por estar ubicado en zonas muy fracturadas. Estas fracturas conectan la superficie con grandes profundidades, permitiendo el ascenso rápido de fluidos profundos que se encuentran a altas temperaturas sin que les dé tiempo a enfriarse. Geográficamente, esta zona de estudio se encuentra en la Cuenca del Vallès, cerca de Barcelona (NE Península Ibérica), donde algunas localidades como La Garriga y Caldes de Montbui, tienen surgencias de agua termal (60ºC y 70ºC, respectivamente). Concretamente, esta metodología se puede separar en dos partes principales, la exploración geofísica y la geológica. Los métodos geofísicos nos permiten conocer las propiedades físicas del subsuelo, llegando a grandes profundidades, sin tener que hacer perforaciones. Si las características físicas del terreno presentan un contraste suficiente, nos pueden permitir, por ejemplo, distinguir entre tipos de rocas, zonas fracturadas, o si hay circulación de algún fluido. Aun así, los resultados geofísicos tienen que complementarse con otros estudios geocientíficos para poder hacer una apropiada interpretación. Esta tesis incluye una caracterización de la geología, la fracturación y la hidrología de la zona, cuya integración final ha permitido entender el origen y funcionamiento de este sistema. Los resultados finales se presentan en forma de un modelo 3D conceptual, geológico y de temperaturas. Esta metodología innovadora integra distintas técnicas geocientíficas a distinta escala, combinando técnicas tradicionales con herramientas digitales novedosas, facilitando la caracterización de un sistema geotérmico controlado por estructuras geológicas. Por lo tanto, se establece como una opción metódica a seguir para la caracterización de sistemas de origen similar

Geophysical and geological characterization of fault-controlled geothermal systems: The Vallès Basin case of study

[eng] Geothermal energy is a renewable source of energy that harnesses heat from the Earth's interior. Temperature increases with depth, defining the geothermal gradient, which can be variable depending on the geological context. The geological setting of western Europe favors a relatively high geothermal gradient that could be exploited to generate electricity or for its direct use, for example, for its application in industry, greenhouses, or heating systems. In each of these cases, geothermal could favor the community's energy independence and reduce the use of polluting energy sources. To appropriately exploit areas with a significant geothermal gradient, it is essential to know the origin of the temperature anomaly and the system's functioning. In this context, developing appropriate exploration methodologies and techniques is essential for its adequate and efficient use. This thesis develops a methodology focused on a geothermal system type characterized by being located in highly fractured zones. These fractures connect the surface with great depths, allowing the rapid ascent of deep fluids at high temperatures without giving them time to cool down. Specifically, this thesis applies this methodology to a study case located in the Vallès Basin, close to Barcelona city (NE Iberian Peninsula), where some localities, such as La Garriga and Caldes de Montbui towns, have thermal hot springs (60ºC and 70ºC, respectively). In particular, the methodology applied to study the Vallès Basin geothermal fractured system, is focused on two main cores, geophysical and geological techniques. Geophysical methods allow the characterization of the subsurface physical properties, reaching great depths without having to drill. For example, if the physical characteristics of the subsurface have enough contrast, they could allow distinguishing between different types of rocks, fractured zones, or if there is any fluid circulation. However, the geophysical results have to be complemented with other geoscientific studies in order to make a proper interpretation. In this case, this thesis includes a characterization of the area's geology, fracturing, and hydrology. Finally, the integration of the applied techniques has allowed the understanding of the origin and system's functioning, which is presented in the form of a 3D conceptual model, geological model, and temperature model. This innovative methodology, which integrates different geoscientific techniques at different scales, combining traditional techniques with novel digital tools, has facilitated the characterization of a geothermal system controlled by geological structures. Therefore, it is established as a methodical option to characterize systems of similar origin.[cat] La Geotèrmia és una font renovable d'energia que aprofita la temperatura de l'interior de la Terra. El grau en què aquesta temperatura augmenta en profunditat, ve definint pel gradient geotèrmic, el qual pot ser variable segons el context geològic. La geologia de la regió oest del continent europeu afavoreix un gradient geotèrmic relativament alt que podria ser aprofitat per generar electricitat o per a ús directe, com és el cas d'aplicacions en indústria, hivernacles o sistemes de calefacció. En qualsevol cas, la geotèrmia podria afavorir la independència energètica i una disminució en l’ús de fonts d’energia contaminants. Per a un aprofitament d'aquestes zones amb un gradient geotèrmic significatiu, és essencial conèixer-ne l'origen i el funcionament. En aquest context, és basic desenvolupar metodologies d'exploració que siguin adequades i eficients. Aquesta tesis desenvolupa una metodologia aplicada a un exemple de sistema geotèrmic caracteritzat per estar ubicat en una zona molt fracturada. Aquestes fractures connecten la superfície amb grans profunditats, permetent l'ascens ràpid de fluids profunds que es troben a temperatures altes, sense que els doni temps a refredar-se. Concretament, aquesta zona d'estudi es situa a la Conca del Vallès (NE Península Ibèrica), on algunes localitats com La Garriga i Caldes de Montbui, tenen surgències d'aigua termal (60ºC i 70ºC, respectivament). Concretament, la metodologia aplicada es basa en dues parts principals: l'exploració geofísica i la geològica. Els mètodes geofísics ens permeten conèixer les propietats físiques del subsol arribant a grans profunditats sense haver de fer perforacions. Si les característiques físiques del terreny presenten un contrast suficient, poden permetre, per exemple, distingir entre tipus de roques, zones fracturades, o si hi ha circulació d'algun fluid. Tot i així, els resultats geofísics s'han de complementar amb altres estudis geocientífics per una correcta interpretació dels resultats. En aquest cas, aquesta tesis inclou una caracterització de la geologia, la fracturació i la hidrologia de la zona. La integració final de totes les dades ha permès entendre l'origen i el funcionament d'aquest sistema, resultat del qual es presenta en forma d'un model 3D conceptual, geològic i de temperatures. Aquesta metodologia innovadora, que integra diferents tècniques geocientífiques a escala diferent, ha combinat tècniques tradicionals amb eines digitals noves, facilitant la caracterització d'un sistema geotèrmic controlat per estructures geològiques. Per tant, s’estableix com una opció metòdica a seguir per a la caracterització de sistemes d’origen similar

Enhancing the information content of geophysical data for nuclear site characterisation

Our knowledge and understanding to the heterogeneous structure and processes occurring in the Earth’s subsurface is limited and uncertain. The above is true even for the upper 100m of the subsurface, yet many processes occur within it (e.g. migration of solutes, landslides, crop water uptake, etc.) are important to human activities. Geophysical methods such as electrical resistivity tomography (ERT) greatly improve our ability to observe the subsurface due to their higher sampling frequency (especially with autonomous time-lapse systems), larger spatial coverage and less invasive operation, in addition to being more cost-effective than traditional point-based sampling. However, the process of using geophysical data for inference is prone to uncertainty. There is a need to better understand the uncertainties embedded in geophysical data and how they translate themselves when they are subsequently used, for example, for hydrological or site management interpretations and decisions. This understanding is critical to maximize the extraction of information in geophysical data. To this end, in this thesis, I examine various aspects of uncertainty in ERT and develop new methods to better use geophysical data quantitatively. The core of the thesis is based on two literature reviews and three papers. In the first review, I provide a comprehensive overview of the use of geophysical data for nuclear site characterization, especially in the context of site clean-up and leak detection. In the second review, I survey the various sources of uncertainties in ERT studies and the existing work to better quantify or reduce them. I propose that the various steps in the general workflow of an ERT study can be viewed as a pipeline for information and uncertainty propagation and suggested some areas have been understudied. One of these areas is measurement errors. In paper 1, I compare various methods to estimate and model ERT measurement errors using two long-term ERT monitoring datasets. I also develop a new error model that considers the fact that each electrode is used to make multiple measurements. In paper 2, I discuss the development and implementation of a new method for geoelectrical leak detection. While existing methods rely on obtaining resistivity images through inversion of ERT data first, the approach described here estimates leak parameters directly from raw ERT data. This is achieved by constructing hydrological models from prior site information and couple it with an ERT forward model, and then update the leak (and other hydrological) parameters through data assimilation. The approach shows promising results and is applied to data from a controlled injection experiment in Yorkshire, UK. The approach complements ERT imaging and provides a new way to utilize ERT data to inform site characterisation. In addition to leak detection, ERT is also commonly used for monitoring soil moisture in the vadose zone, and increasingly so in a quantitative manner. Though both the petrophysical relationships (i.e., choices of appropriate model and parameterization) and the derived moisture content are known to be subject to uncertainty, they are commonly treated as exact and error‐free. In paper 3, I examine the impact of uncertain petrophysical relationships on the moisture content estimates derived from electrical geophysics. Data from a collection of core samples show that the variability in such relationships can be large, and they in turn can lead to high uncertainty in moisture content estimates, and they appear to be the dominating source of uncertainty in many cases. In the closing chapters, I discuss and synthesize the findings in the thesis within the larger context of enhancing the information content of geophysical data, and provide an outlook on further research in this topic

Detection and quantification of permafrost change in alpine rock walls and implications for rock instability

The perennial presence of ice in permafrost rock walls alters thermal, hydraulic and mechanic properties of the rock mass. Temperature-related changes in both, rock mechanical properties (compressive and tensile strength of water-saturated rock) and ice mechanical properties (creep, fracture and cohesive properties) account for the internal mechanical destabilisation of permafrost rocks. Two hypothetical ice-/rock mechanical models were developed based on the principle of superposition. Failure along existing sliding planes is explained by the impact of temperature on shear stress uptake by creep deformation of ice, the propensity of failure along rock-ice fractures and reduced total friction along rough rock-rock contacts. This model may account for the rapid response of rockslides to warming (reaction time). In the long term, brittle fracture propagation is initialised. Warming reduces the shear stress uptake by total friction and decreases the critical fracture toughness along rock bridges. The latter model accounts for slow subcritical destabilisation of whole rock slopes over decades to millennia, subsequent to the warming impulse (relaxation time). To gain further understanding of thermal, hydraulic and mechanic properties of permafrost rocks, multidimensional and multi-temporal insights into the system are required. This Ph.D. adopted, modified and calibrated existing ERT (electrical resistivity tomography) techniques for the use in permafrost rocks. Laboratory analysis of electrical properties of eight rock samples from permafrost summits brought upon evidence that the general exponential temperature-resistivity relation, proposed by McGinnis (1973), is not applicable for frozen rocks, due to the effects of freezing in confined space. We found, that separate linear temperature-resistivity (T- ρ) approximation of unfrozen, supercooled and frozen behaviour offers a better explanation of the involved physics. Frozen T-ρ gradients approach 29.8 ±10.6 %/°C while unfrozen gradients were confirmed at 2.9 ±0.3 %/°C. Both increase with porosity. Path-dependent supercooling T-ρ behavior (3.3 ±2.3 %/°C) until the spontaneous freezing temperature -1.2 (±0.2) °C resembles unfrozen behavior. Spontaneous freezing subsequent to supercooling coincides with sudden self-induced temperature increases of 0.8 (±0.1) °C and resistivity increases of 2.9 (±1.4) kΩm. As temperature-resistivity gradients of frozen rocks are steep, temperature-referenced ERT with accuracies in the range of 1 °C is technically feasible in frozen rock. Technical design for field measurements in permafrost-affected bedrock was developed from 2005 to 2008 in consecutive measurements at a rock crest in the Swiss Alps (Steintaelli, 3150 m a.s.l., Matter Valley) and in a gallery along a north face in the German/ Austrian Alps (Zugspitze, 2800 m a.s.l.). 2D measurements in the Steintaelli along S-, NE-, NW- and Wfacing rock walls showed that ERT provides information on temporal and spatial patterns of thawing, refreezing, cleftwater flow and permafrost distribution in a decameter scale. Monthly, annual and multiannual data were compared using a time-lapse inversion technique and showed consistent results. Seasonal thaw at the Zugspitze was observed in February and monthly from May to October 2007 with high-resolution ERT (140 electrodes). An error model based on the measured offset of normal-reciprocal measurements was operated to empirically fit inherent error. A smoothness-constrained, error-controlled inversion routine (CRTomo) was applied to gain quantitatively reliable ERT data. Application of temperature-referenced laboratory data is consistent with temperature data observed in the adjacent borehole and with temperature logger data. Calculated temperature changes are in accordance with slow thermal conduction away from the rock surface and subsequent refreezing from the rock face in September/October. Smoothness-constrained, error-controlled inversion was transferred to pseudo-3D measurements collated from five 2D-transects with an offset of 4 m across a NE-SW facing ridge in the Steintaelli. In spite of the enormous topography, ERT transects were capable of resolving permafrost and thaw dynamics at the NE facing slope and along ice-filled crevices as well as disclosing unfrozen rock on the SW-facing rock slope. Consecutive measurements of 2006, 2007 and 2008 provide coherent results in line with temperature logger data. ERT measurements confirm that aspect is the most important control of permafrost distribution in rock walls, for a given altitude. At 3150 m a.sl., rock permafrost was found in NE-, NW- and E-facing rock walls in the Steintaelli but not in S-facing transects. Multiannual 3D data show that all NE-facing rock slopes still comprise decameter large permafrost bodies, but the 104.5 Ωm (31.6 kΩm) line which represents a definite transition to the –2 °C range is not reached in any of the transects apart from the surrounding of ice-filled clefts or at the surface. Semiconductive effects of centimetre to decimetre wide frozen fractures significantly cool ambient bedrock and have a dominant influence on the spatial distribution of permafrost under the crestline. Multiannual 2D data reveal that cleftwater inundation in two fracture systems can effectively prevent a decametre large rockwall from cooling below –1 °C (20 kΩm) during two years with permafrost aggradation (August 2005 to August 2007) in sheltered positions. An adjacent rockwall with similar surface characteristics but no hydraulic interconnectivity cooled significantly below –3 °C (> 60 kΩm) in the same time. Steep, highly dissected rock masses can create local permafrost occurrences of meter size even on SW-facing rock slopes. Seasonal thaw of rock permafrost occurs much faster than expected. Monthly measurements at the Zugspitze showed that maximum thaw depth in 2007 was already reached in July/August. In May, rapid warming of permafrost rocks with a resistivity increase equivalent to 1.5 °C warming and more was observed along a fracture zone with active cleftwater flows up to 30 m away from the rock face. Eighteen extensometer transects along the 3D-ERT array in the Steintaelli indicate that rock deformation on the permafrost-affected crest line and in the NE-facing slope is 3-4 times higher than in the non-perennially-frozen SW-facing slope. The velocity of rock displacements in late summer is 20 times higher than in all-season measurements. Velocities along a directly ERT-approved permafrost rock slope respond exponentially to mean air temperature during observation period with an R2; of 0.86. These findings support the hypothesised rapid sliding response to temperature change due to enhanced ice-creep and failure of ice in fractures

Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities

Sandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavy fractured when rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. In this paper, well test and outcrop-scale studies reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤ 180 m), similar to limestone and crystalline aquifers. However, sedimentary heterogeneities can primarily control fluid flow where fracture apertures are reduced by overburden pressures or mineral infills at greater depths. The Triassic St Bees Sandstone Formation (UK) of the East Irish Sea Basin represents an optimum example for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This fluvial sedimentary succession accumulated in rapidly subsiding basins, which typically favours preservation of complete depositional cycles including fine grained layers (mudstone and silty sandstone) interbedded in sandstone fluvial channels. Additionally, vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding discontinuities. Additionally, normal faults are present through the succession showing particular development of open-fractures. Here, the shallow aquifer (depth ≤ 180 m) was characterized using hydro-geophysics. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicates that zones of fault-related open fractures characterize ~ 50% of water flow. The remaining flow component is dominated by bed-parallel fractures. However, such sub-horizontal fissures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas. The findings of this study have been compared with those of an earlier investigation of the deeper St Bees Sandstone aquifer (180 to 400 m subsurface depth) undertaken as part of an investigation for a proposed nuclear waste repository. The deeper aquifer is characterized by significantly lower transmissivities. High overburden pressure and the presence of mineral infillings, have reduced the relative impact of tectonic heterogeneities on transmissivity here, thereby allowing matrix flow in the deeper part of the aquifer. The St Bees Sandstone aquifer contrasts the hydraulic behaviour of low-mechanically resistant sandstone rock-types. In fact, the UK Triassic Sandstone of the Cheshire Basin is low-mechanically resistant and flow is supported both by matrix and fracture. Additionally, faults in such weak-rocks are dominated by granulation seams representing flow-barriers which strongly compartmentalize the UK Triassic Sandstone in the Cheshire Basin

Geochemical Attributes of Hydraulically Active Fractures and Their Influence on Groundwater Quality

This study utilized discrete interval diffusion sampling of a fractured bedrock well completed in schist to investigate if a natural weathering signal can be used to identify hydraulically active fractures. The open borehole well MFS-1, is the focus of the study, which is in close proximity to the recharge zone making it ideal for testing the hypotheses of this study. The hydraulically active fractures of Well MFS-1 were identified, and the dominant mixing force of the water column of the well was determined to be thermal convection in the upper portion of the well and upward gradients. The isotopic data collected from the well concludes that these forces are sufficient to fully mix the water in Well MFS-1 over the one month diffusion sampler deployment period. Iron in groundwater is in the reduced form while flowing through the fractured system, and becomes oxygenated when it enters the well, resulting in a significant amount of iron hydroxide precipitate. The best signal of natural weathering and the systems response to changes in recharge is shown by the changes in silicon concentration in solution. The highest silicon concentrations measured in the groundwater samples were related to waters recharged during the highest precipitation period of the year. The greatest response to system perturbations were observed at the hydraulically active fractures, as shown in the changes in measured concentrations in this portion of Well MFS-1. Even though in-well mixing causes the concentrations of natural weathering products to be measureable throughout the water column, the signal and response to changes in system chemistry are greatest at the hydraulically active fractures. Studying the rock core collected from the fractured bedrock well showed that the hydraulically active fractures are all associated with red staining on the fracture surface and in the matrix of the rock adjacent to the fracture, but the data does not support the conclusion that iron oxide is currently being precipitated. Other observations about the rock, such as the presence and nature of garnet crystals show that the current zone of advective flow in Well MFS‑1 was previously active during a period of hydrothermal activity