Combination of geophysical and hydrogeological methods for the targeted assessment of fine-grained valley fills

Abstract

Sedimentary features in large fine-grained valley fills potentially influence regional groundwater flow and solute turnover processes, yet could be missed with classical site characterization approaches. Hydraulically relevant features, such as gravel-filled channel structures, particularly control the transport pathways and residence times of solutes. In turn, biogeochemically relevant features, such as zones of high organic carbon content, are responsible for the turnover capacity in aquifers. Such features should therefore be reliably identified and characterized in terms of their geometry, internal heterogeneity, and hydraulic and biogeochemical properties. Along these lines, this thesis aims to characterize sedimentary features in fine-grained valley fills via a combination of complementary geophysical and hydrogeological methods. At first, typical sedimentary features are identified in a representative fine-grained valley fill in a hydrogeological site characterization. The influence of individual features on the hydrogeology and hydrochemistry of the valley is discussed based on the depositional history of the valley fill and simplistic water balance calculations. Two exemplary features, a gravel channel, potentially influencing the water balance and a peat layer, functioning as a biogeochemical hotspot, are subsequently characterized on their spatial extent and hydraulic and biogeochemical properties, respectively. In a second step, target-specific methods are developed to reliably delineate the spatial extent of the sedimentary features based on their contrasting properties to the surrounding sediments. The gravel channel is identified as a higher resistive anomaly in electrical resistivity tomography (ERT) measurements. Therefore, I developed a method to identify suitable measurement configurations to map the geoelectric signature of the channel across the entire valley [Klingler et al., 2020a]. The dark peat layer in turn show strong color contrasts to the surrounding sediments. Thus, I recorded vertical logs of the in-situ sediment colors via an optical direct-push probe to map the peat layer within the floodplain [Klingler et al., 2020b]. The color logs resolve the layer contacts of individual colorfacies and indicate the in-situ distribution of organic carbon in the sedimentary sequence. Finally, both previously developed methods are integrated into a combination of different geophysical and hydrogeological investigation techniques to characterize the hydrogeological and hydrochemical influence of the exemplary features [Klingler et al., 2021]. The thickness and internal heterogeneity of the gravel channel is characterized by different direct-push probings within the geoelectrically mapped extent. The geometry and hydraulic properties of the peat layer are determined by the color logs and additional direct-push methods. These steps of a targeted characterization of sedimentary features via complementary investigation techniques limit labor-intensive measurements to representative locations in the delineated subareas of the study area. This way, relevant hydraulic and geochemical properties are measured only at representative locations via pumping tests and sediment samples. The approach is therefore applicable to efficiently characterize large areas, enables the integration of newly developed and target-specific methods, and allows a targeted characterization of features of interdisciplinary relevance.