[1] Exchange of water and solutes across the stream-sediment interface is an important control for biogeochemical transformations in the hyporheic zone (HZ). In this paper, we investigate the interplay between turbulent stream flow and HZ flow in pool-riffle streams under various ambient groundwater flow conditions. Streambed pressures, derived from a computational fluid dynamics (CFD) model, are assigned at the top of the groundwater model, and fluxes at the bottom of the groundwater model domain represent losing and gaining conditions. Simulations for different Reynolds numbers (Re) and pool-riffle morphologies are performed. Results show increasing hyporheic exchange flows (m 3 /d) for larger Re and a concurrent decrease in residence time (RT). Losing and gaining conditions were found to significantly affect the hyporheic flow field and diminish its spatial extent as well as rates of hyporheic exchange flow. The fraction of stream water circulating through the hyporheic zone is in the range of 1 Â 10 À5 to 1 Â 10 À6 per meter stream length, decreasing with increasing discharge. Complex distributions of pressure across the streambed cause significant lateral hyporheic flow components with a mean lateral travel distance of 20% of the longitudinal flow paths length. We found that the relationship between pool-riffle height and hyporheic exchange flow is characterized by a threshold in pool-riffle amplitude, beyond which hyporheic exchange flow becomes independent of riffle height. Hyporheic residence time distributions (RTD) are log-normally distributed with medians ranging between 0.7 and 19 h.

At the interface between stream water, groundwater, and the hyporheic zone (HZ), important biogeochemical processes that play a crucial role in fluvial ecology occur. Solutes that infiltrate into the HZ can react with each other and possibly also with upwelling solutes from the groundwater. In this study, we systematically evaluate how variations of gaining and losing conditions, stream discharge, and pool-riffle morphology affect aerobic respiration (AR) and denitrification (DN) in the HZ. For this purpose, a computational fluid dynamics model of stream water flow is coupled to a reactive transport model. Scenarios of variations of the solute concentration in the upwelling groundwater were conducted. Our results show that solute influx, residence time, and the size of reactive zones strongly depend on presence, magnitude, and direction of ambient groundwater flow. High magnitudes of ambient groundwater flow lower AR efficiency by up to 4 times and DN by up to 3 orders of magnitude, compared to neutral conditions. The influence of stream discharge and morphology on the efficiency of AR and DN are minor, in comparison to that of ambient groundwater flow. Different scenarios of O 2 and NO 3 concentrations in the upwelling groundwater reveal that DN efficiency of the HZ is highest under low upwelling magnitudes accompanied with low concentrations of O 2 and NO 3 . Our results demonstrate how ambient groundwater flow influences solute transport, AR, and DN in the HZ. Neglecting groundwater flow in stream-groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems.

Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three-dimensional numerical model of a natural in-stream gravel bar (20 m 3 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater-borne nitrate when mixed with stream-sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log-space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures.

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Superconducting power cables represent a recent innovative development for highcapacity underground transmission. Their promise lies principally in their high efficiency associated with a small size and with potential advantages in terms of environmental impact. Within the BEST PATHS European project, the DEMO 5 demonstrator aims to illustrate the technological maturity of superconducting HVDC links for operation in the grid. At the same time, this demonstrator is also a first attempt to employ MgB 2 as a superconductor for HVDC cables. More concretely, DEMO 5 aims to develop a monopole superconducting cable designed to operate in helium gas at 10 kA and 320 kV, corresponding to a transferred power of up to 3.2 GW. The project is coordinated by leading cable manufacturer Nexans and encompasses expertise from transmission system operators, industry, and research organizations. Thus, in addition to the design, development, optimization, manufacturing and testing activities, special attention will be devoted to studying the integration of a superconducting link into the future transmission grid and to assessing the availability and economic viability of the system. An overview of the project will be presented at the meeting, including the main tasks and challenges ahead as well as preliminary results after one year of activity.

Aerobic respiration is an important component of in‐stream metabolism. The larger part occurs in the streambed, where it is difficult to directly determine actual respiration rates. Existing methods for determining respiration are based on indirect estimates from whole‐stream metabolism or provide time invariant results estimated from oxygen consumption measurements in enclosed chambers that do not account for the influence of hydrological changes. In this study we demonstrate a simple method for determining time‐variable hyporheic respiration. We use a windowed cross‐correlation approach for deriving time‐variable travel times from the naturally changing electrical conductivity signal that is transferred into the sediment. By combining the results with continuous in situ dissolved oxygen measurements, variable oxygen consumption rate coefficients in the streambed are obtained. An empirical temperature relationship is derived and used for standardizing the respiration rate coefficients to isothermal conditions. For demonstrating the method, we compare two independent measurement spots in the streambed, which were located upstream and downstream of an in‐stream gravel bar and thus exposed strongly diverse travel times. The derived respiration rate results are in accordance with findings of other stream studies. By comparing the travel time and respiration rate coefficient (i.e., Damköhler number) we estimate the contribution of each to the oxygen consumption in the streambed.

Abstract. Spatial patterns of water flux in the stream bed are controlled by the distribution of hydraulic conductivity, bedform-induced head gradients and the connectivity to the adjoining groundwater system. The water fluxes vary over time driven by short-term flood events or seasonal variations in stream flow and groundwater level. Variations of electrical conductivity (EC) are used as a natural tracer to detect transient travel times and flow velocities in an in-stream gravel bar. We present a method to estimate travel times between the stream and measuring locations in the gravel bar by nonlinearly matching the EC signals in the time domain. The amount of temporal distortion required to obtain the optimal matching is related to the travel time of the signal. Our analysis revealed that the travel times increase at higher stream flows because lateral head gradients across the gravel bar become significantly smaller at the time.

One of the key environmental conditions controlling biogeochemical reactions in aquatic sediments like streambeds is the distribution of dissolved oxygen. We present a novel approach for the in situ measurement of vertical oxygen profiles using a planar luminescence-based optical sensor. The instrument consists of a transparent acrylic tube with the oxygen-sensitive layer mounted on the outside. The luminescence is excited and detected by a moveable piston inside the acrylic tube. Since no moving parts are in contact with the streambed, the disturbance of the subsurface flow field is minimized. The precision of the distributed oxygen sensor (DOS) was assessed by a comparison with spot optodes. Although the precision of the DOS, expressed as standard deviation of calculated oxygen air saturation, is lower (0.2-6.2%) compared to spot optodes (<0.1-0.6%), variations of the oxygen content along the profile can be resolved. The uncertainty of the calculated oxygen is assessed with a Monte Carlo uncertainty assessment. The obtained vertical oxygen profiles of 40 cm in length reveal variations of the oxygen content reaching from 90% to 0% air saturation and are characterized by patches of low oxygen rather than a continuous decrease with depth.