Uwe Schneidewind

Determining groundwater‐surface water exchange from temperature‐time series: Combining a local polynomial method with a maximum likelihood estimator

The use of temperature-time series measured in streambed sediments as input to coupled water flow and heat transport models has become standard when quantifying vertical groundwater-surface water exchange fluxes. We develop a novel methodology, called LPML, to estimate the parameters for 1-D water flow and heat transport by combining a local polynomial (LP) signal processing technique with a maximum likelihood (ML) estimator. The LP method is used to estimate the frequency response functions (FRFs) and their uncertainties between the streambed top and several locations within the streambed from measured temperature-time series data. Additionally, we obtain the analytical expression of the FRFs assuming a pure sinusoidal input. The estimated and analytical FRFs are used in an ML estimator to deduce vertical groundwater-surface water exchange flux and its uncertainty as well as information regarding model quality. The LPML method is tested and verified with the heat transport models STRIVE and VFLUX. We demonstrate that the LPML method can correctly reproduce a priori known fluxes and thermal conductivities and also show that the LPML method can estimate averaged and time-variable fluxes from periodic and nonperiodic temperature records. The LPML method allows for a fast computation of exchange fluxes as well as model and parameter uncertainties from many temperature sensors. Moreover, it can utilize a broad frequency spectrum beyond the diel signal commonly used for flux calculations.

LPMLE3: A novel 1‐D approach to study water flow in streambeds using heat as a tracer

We introduce LPMLE3, a new 1-D approach to quantify vertical water flow components at streambeds using temperature data collected in different depths. LPMLE3 solves the partial differential equation for coupled water flow and heat transport in the frequency domain. Unlike other 1-D approaches it does not assume a semi-infinite halfspace with the location of the lower boundary condition approaching infinity. Instead, it uses local upper and lower boundary conditions. As such, the streambed can be divided into finite subdomains bound at the top and bottom by a temperature-time series. Information from a third temperature sensor within each subdomain is then used for parameter estimation. LPMLE3 applies a low order local polynomial to separate periodic and transient parts (including the noise contributions) of a temperature-time series and calculates the frequency response of each subdomain to a known temperature input at the streambed top. A maximum-likelihood estimator is used to estimate the vertical component of water flow, thermal diffusivity, and their uncertainties for each streambed subdomain and provides information regarding model quality. We tested the method on synthetic temperature data generated with the numerical model STRIVE and demonstrate how the vertical flow component can be quantified for field data collected in a Belgian stream. We show that by using the results in additional analyses, nonvertical flow components could be identified and by making certain assumptions they could be quantified for each subdomain. LPMLE3 performed well on both simulated and field data and can be considered a valuable addition to the existing 1-D methods.

Kinetics of dechlorination by Dehalococcoides mccartyi using different carbon sources

Stimulated anaerobic dechlorination is generally considered a valuable step for the remediation of aquifers polluted with chlorinated ethenes (CEs). Correct simulation and prediction of this process in situ, however, require good knowledge of the associated biological reactions. The aim of this study was to evaluate the dechlorination reaction in an aquifer contaminated with trichloroethene (TCE) and its daughter products, discharging into the Zenne River. Different carbon sources were used in batch cultures and these were related to the dechlorination reaction, together with the monitored biomarkers. Appropriate kinetic formulations were assessed. Reductive dechlorination of TCE took place only when external carbon sources were added to microcosms, and occurred concomitant with a pronounced increase in the Dehalococcoides mccartyi cell count as determined by 16S rRNA gene-targeted qPCR. This indicates that native dechlorinating bacteria are present in the aquifer of the Zenne site and that the oligotrophic nature of the aquifer prevents a complete degradation to ethene. The type of carbon source, the cell number of D. mccartyi or the reductive dehalogenase genes, however, did not unequivocally explain the observed differences in degradation rates or the extent of dechlorination. Neither first-order, Michaelis-Menten nor Monod kinetics could perfectly simulate the dechlorination reactions in TCE spiked microcosms. A sensitivity analysis indicated that the inclusion of donor limitation would not significantly enhance the simulations without a clear process understanding. Results point to the role of the supporting microbial community but it remains to be verified how the complexity of the microbial (inter)actions should be represented in a model framework.

Natural attenuation of chlorinated ethenes in hyporheic zones:: A review of key biogeochemical processes and in-situ transformation potential

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.

Transport and retention of surfactant- and polymer-stabilized engineered silver nanoparticles in silicate-dominated aquifer material

Packed column experiments were conducted to investigate the transport and blocking behavior of surfactant- and polymer-stabilized engineered silver nanoparticles (Ag-ENPs) in saturated natural aquifer media with varying content of material < 0.063 mm in diameter (silt and clay fraction), background solution chemistry, and flow velocity. Breakthrough curves for Ag-ENPs exhibited blocking behavior that frequently produced a delay in arrival time in comparison to a conservative tracer that was dependent on the physicochemical conditions, and then a rapid increase in the effluent concentration of Ag-ENPs. This breakthrough behavior was accurately described using one or two irreversible retention sites that accounted for Langmuirian blocking on one site. Simulated values for the total retention rate coefficient and the maximum solid phase concentration of Ag-ENPs increased with increasing solution ionic strength, cation valence, clay and silt content, decreasing flow velocity, and for polymer-instead of surfactant-stabilized Ag-ENPs. Increased Ag-ENP retention with ionic strength occurred because of compression of the double layer and lower magnitudes in the zeta potential, whereas lower velocities increased the residence time and decreased the hydrodynamics forces. Enhanced Ag-ENP interactions with cation valence and clay were attributed to the creation of cation bridging in the presence of Ca2+. The delay in breakthrough was always more pronounced for polymer-than surfactant-stabilized Ag-ENPs, because of differences in the properties of the stabilizing agents and the magnitude of their zeta-potential was lower. Our results clearly indicate that the long-term transport behavior of Ag-ENPs in natural, silicate dominated aquifer material will be strongly dependent on blocking behavior that changes with the physicochemical conditions and enhanced Ag-ENP transport may occur when retention sites are filled.

Transport and fate of manufactured silver nanoparticles in saturated heterogeneous natural porous media

Das Thema dieses Tagungsband lautet „Grundwasser, Mensch und Okosysteme“. Die vielfaltigen Aspekte dieses Themas werden in 13 thematischen Sessions und im Forum Junger Hydrogeologen intensiv diskutiert. Abgerundet wird das Programm durch drei spannende Keynote Lectures und eine popularwissenschaftliche Abendveranstaltung sowie Fortbildungskurse am Tag vor der Konferenz und Exkursionen im Anschluss an die Tagung.1 RWTH Aachen University, Department of Engineering Geology and Hydrogeology, Aachen, Germany 2 Vrije Universiteit Brussel (VUB), Department of Fundamental Electricity and Instrumentation, Brussels, Belgium 3 Vrije Universiteit Brussel (VUB), Department of Hydrology and Hydraulic Engineering, Brussels, Belgium 4 Helmholtz Centre for Environmental Research UFZ, Department of Hydrogeology, Leipzig, Germany

Quantifying Vertical Streambed Fluxes Using Heat as a Tracer: Applying Two Novel Frequency Domain Approaches

Das Thema dieses Tagungsband lautet „Grundwasser, Mensch und Okosysteme“. Die vielfaltigen Aspekte dieses Themas werden in 13 thematischen Sessions und im Forum Junger Hydrogeologen intensiv diskutiert. Abgerundet wird das Programm durch drei spannende Keynote Lectures und eine popularwissenschaftliche Abendveranstaltung sowie Fortbildungskurse am Tag vor der Konferenz und Exkursionen im Anschluss an die Tagung.1 RWTH Aachen University, Department of Engineering Geology and Hydrogeology, Aachen, Germany 2 Vrije Universiteit Brussel (VUB), Department of Fundamental Electricity and Instrumentation, Brussels, Belgium 3 Vrije Universiteit Brussel (VUB), Department of Hydrology and Hydraulic Engineering, Brussels, Belgium 4 Helmholtz Centre for Environmental Research UFZ, Department of Hydrogeology, Leipzig, Germany

Studying temporal and spatial variations of groundwater-surface water exchange flux for the Slootbeek (Belgium) using the LPML method

(1) Vrije Universiteit Brussel (VUB), Department of Hydrology and Hydraulic Engineering, Brussels, Belgium (canibas@vub.ac.be, mhuysman@vub.ac.be), (2) Flemish Institute for Technological Research (VITO), Environmental Modeling Unit, Mol, Belgium (uwe.schneidewind@vito.be), (3) Ghent University, Department of Soil Management, Ghent, Belgium, (4) Vrije Universiteit Brussel (VUB), Department of Fundamental Electricity and Instrumentation, Brussels, Belgium (Gerd.Vandersteen@vub.ac.be), (5) Flinders University, School of the Environment, Adelaide, Australia (okke.batelaan@flinders.edu.au)

LPMLE3 : A New Analytical Approach to Determine Vertical Groundwater-Surface Water Exchange Flux under Uncertainty and Heterogeneity

(1) VITO, Environmental Modeling Unit, Mol, Belgium (uwe.schneidewind@vito.be), (2) Ghent University, Department of Soil Management, Gent, Belgium, (3) Eindhoven University of Technology, Department of Mechanical Engineering, Eindhoven, The Netherlands, (4) FOM Institute DIFFER, Dutch Institute for Fundamental Energy Research, Nieuwegein, The Netherlands , (5) VUB Vrije Universiteit Brussel, Department of Hydrology and Hydraulic Engineering, Brussels, Belgium, (6) VUB Vrije Universiteit Brussel, Department of Fundamental Electricity and Instrumentation, Brussels, Belgium, (7) University of Antwerp, Department of Bioscience Engineering, Antwerp, Belgium, (8) Flinders University, School of the Environment, Adelaide, Australia

Determining Groundwater-Surface Water Exchange Fluxes and Their Spatial Variability Using the Local Polynomial Method LPML

(1) VITO, Environmental Modeling Unit, Mol, Belgium (uwe.schneidewind@vito.be), (2) Ghent University, Department of Soil Management, Ghent, Belgium, (3) VUB Vrije Universiteit Brussel, Department of Hydrology and Hydraulic Engineering, Brussels, Belgium, (4) VUB Vrije Universiteit Brussel, Department of Fundamental Electricity and Instrumentation, Brussels, Belgium, (5) University of Antwerp, Department of Bioscience Engineering, Antwerp, Belgium, (6) Flinders University, National Centre for Groundwater Research and Training, School of the Environment, Adelaide, SA, Australia