No abstract

Limited knowledge about the spatial distribution of aquifer properties typically constrains our ability to predict subsurface flow and transport. Here we investigate the value of using high resolution full‐waveform inversion of cross‐borehole ground penetrating radar (GPR) data for aquifer characterization. By stitching together GPR tomograms from multiple adjacent crosshole planes, we are able to image, with a decimeter scale resolution, the dielectric permittivity and electrical conductivity of an alluvial aquifer along cross sections of 50 m length and 10 m depth. A logistic regression model is employed to predict the spatial distribution of lithological facies on the basis of the GPR results. Vertical profiles of porosity and hydraulic conductivity from direct‐push, flowmeter and grain size data suggest that the GPR predicted facies classification is meaningful with regard to porosity and hydraulic conductivity, even though the distributions of individual facies show some overlap and the absolute hydraulic conductivities from the different methods (direct‐push, flowmeter, grain size) differ up to approximately one order of magnitude. Comparison of the GPR predicted facies architecture with tracer test data suggests that the plume splitting observed in a tracer experiment was caused by a hydraulically low‐conductive sand layer with a thickness of only a few decimeters. Because this sand layer is identified by GPR full‐waveform inversion but not by conventional GPR ray‐based inversion we conclude that the improvement in spatial resolution due to full‐waveform inversion is crucial to detect small‐scale aquifer structures that are highly relevant for solute transport.

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Thermal use of the shallow subsurface for heat generation, cooling, and thermal energy storage is increasingly gaining importance in reconsideration of future energy supplies. Shallow geothermal energy use is often promoted as being of little or no costs during operation, while simultaneously being environmentally friendly. Hence, the number of installed systems has rapidly risen over the last few decades, especially among newly built houses. While the carbon dioxide reduction potential of this method remains undoubted, concerns about sustainability and potential negative effects on the soil and groundwater due to an intensified use have been raised-even as far back as 25 years ago. Nevertheless, consistent regulation and management schemes for the intensified thermal use of the shallow subsurface are still missing-mainly due to a lack of system understanding and process knowledge. In the meantime, large geothermal applications, for example, residential neighborhoods that are entirely dependent up on shallow geothermal energy use or low enthalpy aquifer heat storage, have been developed throughout Europe. Potential negative effects on the soil and groundwater due to an intensive thermal use of the shallow subsurface as well as the extent of potential system interaction still remain unknown.

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This study provides an example of fault structure delineation using both geophysical measurements and soilgas surveys. Seismic refraction and electrical resistivity tomography investigations were performed in combination with Direct Push (DP) soil gas concentration measurements, with the main objective being the characterization of an assumed permeable fault structure which is covered by sediments that are over 20 m thick. Geophysical methods were used to locate a potential fault zone and to provide an insight into the structural features of the covering sediments. Methods for quantifying the soil-gas concentration were applied to evaluate the permeability of the fault zone. The positioning of gas sampling points was based on results of a geophysical survey undertaken beforehand. Gas sampling was performed using DP-technology to obtain concentration profiles for the inert gas Radon-222 and its carrier gas CO 2 along the profile at different depths. Joint interpretation of the spatial distribution of geogenic gases and results from the geophysical survey allowed us to produce a representative model image of the fault structure consisting of two fault branches. Based on this image, it was possible to interpret the observed gas concentration patterns.

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Households accounted for 25.4% of the final energy consumption in the EU 28 in 2016 of which almost 80% of that energy was used for space heating and warm water provision (see Eurostat 2018). At the same time, only 16% of the final energy consumption of households was derived from renewables, including the renewable part of waste (see Eurostat 2018). Hence, strengthening the share of renewables for space heating and warm water provision is an important piece of the puzzle for the decarbonization of the building sector that plays an important role in reducing global greenhouse gas emissions (Lucon et al. 2014). The use of shallow geothermal energy has increasingly received attention as suitable alternative to fossil fuel-based space heating and cooling, warm water provision, as well as for seasonal heat storage over the last 20 years, e.g., see Sanner (2017) for the development of the German ground source heat pump market. This is because ground source heat pump technology can readily be used to provide sufficient space heating during winter periods even at locations that are not favored by an