Department Water Resources and Drinking Water

The sedimentary infill of glacially overdeepened valleys (i.e., structures eroded below the fluvial base level) is an excellent but yet underexplored archive with regard to the age, extent, and nature of past glaciations. The ICDP project DOVE (Drilling Overdeepened Alpine Valleys) Phase 1 investigates a series of drill cores from glacially overdeepened troughs at several locations along the northern front of the Alps. All sites will be investigated with regard to several aspects of environmental dynamics during the Quaternary, with focus on the glaciation, vegetation, and landscape history. Geophysical methods (e.g., seismic surveys), for example, will explore the geometry of overdeepened structures to better understand the process of overdeepening. Sedimentological analyses combined with downhole logging, analysis of biological remains, and state-of-the-art geochronological methods, will enable us to reconstruct the erosion and sedimentation history of the overdeepened troughs. This approach is expected to yield significant novel data quantifying the extent and timing of Middle and Late Pleistocene glaciations of the Alps. In a first phase, two sites were drilled in late 2021 into filled overdeepenings below the paleolobe of the Rhine Glacier, and both recovered a trough filling composed of multiphase glacial sequences. Fully cored Hole 5068_1_C reached a depth of 165 m and recovered 10 m molasse bedrock at the base. This hole will be used together with two flush holes (5068_1_A, 5068_1_B) for further geophysical cross-well experiments. Site 5068_2 reached a depth of 255 m and bottomed out near the soft rock–bedrock contact. These two sites are complemented by three legacy drill sites that previously recovered filled overdeepenings below the more eastern Alpine Isar-Loisach, Salzach, and Traun paleoglacier lobes (5068_3, 5068_4, 5068_5). All analysis and interpretations of this DOVE Phase 1 will eventually lay the ground for an upcoming Phase 2 that will complete the pan-Alpine approach. This follow-up phase will investigate overdeepenings formerly occupied by paleoglacier lobes from the western and southern Alpine margins through drilling sites in France, Italy, and Slovenia. Available geological information and infrastructure make the Alps an ideal area to study overdeepened structures; however, the expected results of this study will not be restricted to the Alps. Such features are also known from other formerly glaciated mountain ranges, which are less studied than the Alps and more problematic with regards to drilling logistics. The results of this study will serve as textbook concepts to understand a full range of geological processes relevant to formerly glaciated areas all over our planet.

Dietary deficiency of selenium is a global health threat related to low selenium concentrations in crops. Despite the chemical similarity of selenium to the two more abundantly studied elements sulfur and arsenic, the understanding of its accumulation in soils and availability for plants is limited. The lack of understanding of soil selenium cycling is largely due to the unavailability of methods to characterize selenium species in soils, especially the organic ones. Here we develop a size-resolved multi-elemental method using liquid chromatography and elemental mass spectrometry, which enables an advanced characterization of selenium, sulfur, and arsenic species in soil extracts. We apply the analytical approach to soils sampled along the Kohala rainfall gradient on Big Island (Hawaii), which cover a large range of organic carbon and (oxy)hydroxides contents. Similarly to sulfur but contrarily to arsenic, a large fraction of selenium is found associated with organic matter in these soils. However, while sulfur and arsenic are predominantly found as oxyanions in water extracts, selenium mainly exists as small hydrophilic organic compounds. Combining Kohala soil speciation data with concentrations in parent rock and plants further suggests that selenium association with organic matter limits its mobility in soils and availability for plants.

Biogeochemical gas production resulting in free gas phase formation can severely affect groundwater and solute transport in aquifers. Such gas–water interactions are important in aquifers affected by geogenic As, which are commonly associated with biogeochemical CH₄ production. Additionally, the influence of aquitards on As concentrations in contaminated aquifers has recently been challenged. These observations prompted the analysis through a heterogeneous aquitard overlying a high CH₄-gas-producing zone of an As-contaminated aquifer. A sediment core taken through the aquitard was analyzed for noble gases to assess how the aquitard physically contributes to the underlying gas production. Results reveal that the aquitard pore space is unsaturated in two separate layers resulting in hanging pore water constrained by an air-like gas phase. This interlayering of unsaturated and saturated zones identifies the aquitard's stratigraphy as key in determining hydrostatic pressure - a main control of free gas formation (i.e., CH₄) in the underlying aquifer. The partly unsaturated conditions reduce the hydrostatic pressure by 30% compared with fully saturated conditions. To our knowledge, this is the first study applying noble gases to examine the influence of an aquitards physical state on gas production in an underlying aquifer. Further, such partly unsaturated sediment layers of low conductivity might provide preferential pathways for periodic water flow, fostering aquitard–aquifer solute transport. Groundwater samples additionally collected throughout the study site confirm more widespread degassing than previously reported. Up to 90% of the expected atmospheric noble gas concentrations is lost from groundwater immediately below the investigated sediment core.