Christian Anibas

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.

Improving surface–subsurface water budgeting using high resolution satellite imagery applied on a brownfield

The estimation of surface-subsurface water interactions is complex and highly variable in space and time. It is even more complex when it has to be estimated in urban areas, because of the complex patterns of the land-cover in these areas. In this research a modeling approach with integrated remote sensing analysis has been developed for estimating water fluxes in urban environments. The methodology was developed with the aim to simulate fluxes of contaminants from polluted sites. Groundwater pollution in urban environments is linked to patterns of land use and hence it is essential to characterize the land cover in a detail. An object-oriented classification approach applied on high-resolution satellite data has been adopted. To assign the image objects to one of the land-cover classes a multiple layer perceptron approach was adopted (Kappa of 0.86). Groundwater recharge has been simulated using the spatially distributed WetSpass model and the subsurface water flow using MODFLOW in order to identify and budget water fluxes. The developed methodology is applied to a brownfield case site in Vilvoorde, Brussels (Belgium). The obtained land use map has a strong impact on the groundwater recharge, resulting in a high spatial variability. Simulated groundwater fluxes from brownfield to the receiving River Zenne were independently verified by measurements and simulation of groundwater-surface water interaction based on thermal gradients in the river bed. It is concluded that in order to better quantify total fluxes of contaminants from brownfields in the groundwater, remote sensing imagery can be operationally integrated in a modeling procedure.

Determining discharges from the Table Mountain Group (TMG) aquifer to wetlands in the Southern Cape, South Africa

The focus of this study was to determine whether coastal wetlands in lowland settings could be dependent on groundwater from the deep circulating confined Table Mountain Group (TMG) aquifer. Groundwater interactions with wetlands are normally perceived to be limited to primary aquifers. A comparative study was done between two endorheic coastal wetlands in the southern Cape. Earlier reports stated that these groundwater dependent wetlands were fed by discharges from the fixed dunes surrounding them. On the basis of a three-dimensional electrical conductivity (EC) interpolation for Groenvlei, a hydrological link between the TMG aquifer and Groenvlei and Van Kervelsvlei was investigated by measuring water level and quality of groundwater and surface water. Water quality parameters used were EC, pH, Na+, Fe2+ and Cl−. The results from this, and an accompanying study, on the basis of water quality and plant nutrient cycling assessments, indicated direct groundwater discharges from the TMG to at least Van Kervelsvlei, with Groenvlei receiving secondary discharges from the TMG via Van Kervelslvlei. These findings significantly affect the current knowledge on which water balance models are based for the determination of groundwater availability for the area.

Reactive transport modeling of redox processes to assess Fe(OH)3 precipitation around aquifer thermal energy storage wells in phreatic aquifers

Well clogging due to iron (hydr)oxide precipitation can negatively influence the performance, or even cause failure, of aquifer thermal energy storage (ATES) wells in aquifers with varying redox conditions. The interactions between physical and chemical processes during ATES operation are, however, not well understood. The reactive transport modeling code PHT3D is used to assess the effects of alternating pumping by ATES systems near the redox boundary on the precipitation of iron hydroxides for two cases, Leuven and Antwerp in the north of Belgium. Results show in both investigated cases that initial mixing plays an important role in the development of Fe(OH)3 precipitation around the wells, with the highest concentration of Fe(OH)3 around the cold well. The initial injection into the warm well causes both the initial mixing and temperature effects to counteract each other, so that the Fe(OH)3 concentration at the cold well is lower and closer to those of the warm well. Since the temperature dependence of the reaction rate of Fe(OH)+, Fe(OH)2 and other reactive species is not taken into account, the impact of the temperature effect on iron (hydr)oxide precipitation should not be viewed quantitatively. However, avoiding the mixing of oxygen/nitrate rich water with iron rich water remains the best strategy to prevent well clogging. Feasibility studies for ATES should therefore assess water quality variations with depth, and use this information to optimize filter screen settings. In this way, the well screen setting can be optimized and the risk of well clogging due to iron (hydr)oxide precipitation is reduced.

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