Knowing the centimeter- to meter-scale distribution of sand in clayey deposits is important for determining the dominating water flow pathways. Borehole information has a high vertical resolution, on the millimeter- to centimeter-scale, but provides poor lateral coverage. For highly heterogeneous deposits, such as glacial diamicts, this detailed borehole information may not be sufficient for creating reliable geological models. Crosshole ground-penetrating radar (GPR) can provide information on the decimeter- to meter-scale variation between boreholes, as the GPR response depends on the dielectric permittivity, electric conductivity, and the magnetic permeability of the subsurface. In this study, we investigate whether crosshole GPR can provide information on the material properties of diamicts, such as water content, bulk density, and clay content, as well as their structural relationships. To achieve ground truth, we compare the crosshole GPR data with geological information from both boreholes and excavation at the field site. The GPR data were analyzed comprehensively using several radar wave attributes in both time- and frequency domain, describing the signal velocity, strength, and shape. We found small variations in signal velocity (between 0.06-0.07 m/ns) but large variations in both amplitude and shape (either order of magnitude variation or doubling/tripling of attribute values). We see that the GPR response from wetter and more clayey diamicts have both lower amplitudes and lower centroid frequencies than the response from their drier and sandier counterparts. Furthermore, we find that the variation in amplitude and shape attributes are better correlated to the diamicts’ material properties than the signal velocity is.

Groundwater-dependent terrestrial ecosystems (GWDTE) have been increasingly under threat due to groundwater depletion globally. Within the past 200 years, there has been severe artificial drainage of low-lying areas in Denmark, leading to a gradual loss of GWDTE nature habitat areas. This study explores the spatial-temporal loss of Danish GWDTE using historically vectorized topographical maps. We carry out geographic information systems (GIS) overlap analysis between different historical topographic maps with signatures of GWDTE starting from the 19 th century up to a current river valley bottom map as a reference period. This is because farmworkers and monks have practiced drainage by ditching since the early middle ages (1100-1200). We then examine the changes in two protected GWDTE habitat types in different periods and different hydrologic spatial locations. Results reveal a decrease in the area of GWDTE over the last 200 years. We attribute this to different human interventions that through e.g., drainage, have impacted the low-lying landscape throughout history. We further conclude that downstream parts of the river network have been exposed to less GWDTE habitat loss than upstream ones. This indicates that upstream river valleys are more vulnerable to GWDTE decline. Therefore, as a management measure, caution should be exercised when designing these areas for agriculture activities using artificial drainage and groundwater abstraction since this may lead to further decline. In contrast, there is a higher potential for establishing constructed wetlands or rewetting peatlands to restore balance.