HS8.1.1

Bank filtration is a sustainable source for drinking water production in urbanized regions but is increasingly at risk by contamination with pathogenic bacteria and viruses from surface water receiving wastewater discharge. While recent advances have improved our process understanding for pathogen transport on laboratory scale, simulations and predictions on field scale under transient conditions, as in bank filtration, are still highly uncertain. To improve our understanding on field scale, we performed a sampling survey over 16 months at an observation well transect in a heterogeneous sand-gravel aquifer of an active bank filtration waterworks at the river Rhine in Germany. Water samples were collected from the river, the production well, and 4 multi-level observation wells. Samples were analysed for main anions/cations, and hygienic indicators (E. coli and coliform bacteria via plate counts, coliphages via plaque assay, and adenoviruses via ddPCR). A two-dimensional reactive transport model was set up using PFLOTRAN to simulate the transport of heat and dissolved species, aerobic respiration, denitrification, and colloid-based transport of bacteria and viruses. For the latter, adsorption to and desorption from the sediment, straining, blocking, and inactivation are considered. Model parameters were estimated from prior knowledge of the site or calibrated with the obtained data using particle swarm optimization.

Field observations show a strong seasonal variation of river hydraulics with up to 8 m difference in water level, a prolonged low in the summer/fall and short-termed river level increases in the winter. Aerobic respiration was strongly controlled by the temperature variation (6-24°C in groundwater), leading to an increase in oxygen consumption and limited denitrification during the warm summer/fall. Bacteria and virus concentrations in the groundwater were elevated following a flood in the first winter (up to 500 MPN/100mL coliforms, 2 PFU/100mL coliphages, 1000 copies/100mL adenovirus). Measurable concentrations were still observed during the summer (e.g., up to 10 MPN/100mL coliforms, 0.7 PFU/100mL coliphages, 500 copies/mL adenovirus), but concentrations were below the detection limit for most of the second winter, where no significant flood occurred. In the well closest to the river (40 m distance), the concentration reduction compared to the river varied over time between 1 to ≥4 log-units for coliforms, 1.5 to ≥3 log-units for coliphages, and 0.5 to ≥3 log-units for adenoviruses. The model results suggest the main driving processes for the variation in the bacteria and virus concentrations are (i) the changing groundwater velocity (driven by river level variations and pumping rate), (ii) occurrence of low dissolved oxygen concentrations which lower inactivation, and (iii) transient colmation layer properties (permeability and effective grain size). The colmation layer is affected by reworking of riverbed sediments during floods, bio-clogging during summer, and physical clogging due to constant forced infiltration caused by the bank filtration plant. This is supported by the observation of high bacteria concentrations in the aquifer for a short duration after pumps were reactivated following a 40-day maintenance period. Overall, bacteria and virus attenuation during bank filtration was high, only a strong flood resulted in significantly higher contaminant concentrations in the aquifer.

How to cite: Knabe, D., Wang, H., Griebler, C., and Engelhardt, I.: Impact of seasonal variations and transient colmation layer properties on bacteria and virus transport in bank filtration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4631, https://doi.org/10.5194/egusphere-egu21-4631, 2021.

The spatial distribution of a solute undergoing advection and diffusion is impacted by the velocity variability sampled by tracer particles. In spatially structured velocity fields, such as porous medium flows, Lagrangian velocities along streamlines are often characterized by a well-defined correlation length and can thus be described by spatial-Markov processes. Diffusion, on the other hand, is generally modeled as a temporal process, making it challenging to capture advective and diffusive dynamics in a single framework. In order to address this limitation, we have developed a description of transport based on a spatial-Markov velocity process along Lagrangian particle trajectories, incorporating the effect of diffusion as a local averaging process in velocity space. The impact of flow structure on this diffusive averaging is quantified through an effective shear rate. The latter is fully determined by the point statistics of velocity magnitudes together with characteristic longitudinal and transverse lengthscales associated with the flow field. For infinite longitudinal correlation length, our framework recovers Taylor dispersion, and in the absence of diffusion it reduces to a standard spatial-Markov velocity model. This novel framework allows us to derive dynamical equations governing the evolution of particle position and velocity, from which we obtain scaling laws for the dependence of longitudinal dispersion on Péclet number. Our results provide new insights into the role of shear and diffusion on dispersion processes in heterogeneous media.

In this presentation, I propose to discuss: (i) Spatial-Markov models and the modeling of diffusion as a spatial rather than temporal process; (ii) The concept of the effective shear rate and its role in the diffusive dynamics of tracer particle velocities; (iii) The role of transverse diffusion and its interplay with velocity heterogeneity on longitudinal solute dispersion.

How to cite: Aquino, T. and Le Borgne, T.: The diffusing-velocity random walk: Capturing the interplay of diffusion and heterogeneous advection within a spatial-Markov framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-875, https://doi.org/10.5194/egusphere-egu21-875, 2021.

We present new random walk methods to solve flow and transport problems in saturated/unsaturated porous media, including coupled flow and transport processes in soils, heterogeneous systems modeled through random hydraulic conductivity and recharge fields, processes at the field and regional scales. The numerical schemes are based on global random walk algorithms (GRW) which approximate the solution by moving large numbers of computational particles on regular lattices according to specific random walk rules. To cope with the nonlinearity and the degeneracy of the Richards equation and of the coupled system, we implemented the GRW algorithms by employing linearization techniques similar to the L-scheme developed in finite element/volume approaches. The resulting GRW L-schemes converge with the number of iterations and provide numerical solutions that are first-order accurate in time and second-order in space. A remarkable property of the flow and transport GRW solutions is that they are practically free of numerical diffusion. The GRW solvers are validated by comparisons with mixed finite element and finite volume solvers in one- and two-dimensional benchmark problems. They include Richards' equation fully coupled with the advection-diffusion-reaction equation and capture the transition from unsaturated to saturated flow regimes.  For completeness, we also consider decoupled flow and transport model problems for saturated aquifers.

How to cite: Suciu, N., Illiano, D., Prechtel, A., and Radu, F.: Global random walk solvers for fully coupled flow and transport in saturated/unsaturated porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1941, https://doi.org/10.5194/egusphere-egu21-1941, 2021.

In hydrothermal systems, the circulation of water through the porous matrix is strongly influenced by the joint effects of heat and salinity. Because of phase separation, layers of different salinities and temperature are thought to form, but their stability or their typical lifetime remains unclear. Moreover, the dynamics of heat transport across such a layered system is considerably enriched by double diffusive effects due to the slower diffusion of salinity relative to heat. Here, we study numerically the time evolution of an ideal two-layer configuration where a heavy layer of warm and salty water is overlain by a light layer of cold and fresh water. Thermal convection quickly develops in each layer and maintains a thin diffusive interface between the layers. There is long-standing controversy on the temporal evolution of such a system. Although Griffiths (1981) found experimentally that the sharp interface seemed to persist indefinitely, Schoofs & Hansen (2000) reported via numerical simulations systematic depletion and vanishing of the layers. We resolve this apparently inconsistency. In our simulations, we find systematic depletion of the two-layer initial condition in all cases. However, the timescale over which it occurs depends strongly on the ratio between salinity and temperature contributions to density. When salinity is weakly stabilising, thermal convection and layers are maintained over (very long) diffusive timescales. When salt is strongly stabilising, however, convection becomes quiescent over much shorter times and the sharp interface between layers is quickly diffused away. We determine scalings on the lifetime of the layers in both regimes as a function of the governing parameters.

How to cite: Le Reun, T. and Hewitt, D.: Double-diffusive depletion of layers in hydrothermal systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7289, https://doi.org/10.5194/egusphere-egu21-7289, 2021.

Groundwater is the largest source for freshwater which plays an important role in the hydrological cycle. The pollution of groundwater is on the rise due to various natural and anthropogenic sources such as landfills, agricultural lands, and underground waste storage facilities etc. These pollutants can be subjected to reactions depending on the contaminant type and the subsurface environment along with advection and dispersion processes.  As groundwater is used in various human activities such as drinking, agriculture and industrial activities, it is essential to track the contaminants in groundwater for assessing possible environmental impacts. The complex phenomena of flow and contaminant transport are represented by partial differential equations (PDEs) which are solved numerically throughout the problem domain. Although Finite Difference method (FDM) and Finite Element Method (FEM) based models are conventionally used for these simulations, these methods suffer from certain instabilities due to the presence of mesh/grid, for example, numerical dispersion and artificial oscillation for advection and reaction dominant problems. Moreover, these methods are not suitable for adaptive analysis which requires meshing and re-meshing in each simulation making the problem highly computationally expensive. Here, we present a strong form meshfree method named Radial Point Collocation Method (RPCM) for modelling flow and transport in groundwater. In contrast to mesh-based methods, the problem domain is discretised using only nodes in the proposed method. Moreover, unlike the mesh-based methods, it produces stable solutions for advection and reaction dominant problems without using special techniques such as up-winding, adaptive re-meshing or, operator splitting. The performance of the model is tested against analytical solutions, FDM and FEM based models for different reactive transport problems in groundwater involving adsorption, decay, multi-species decay network and biodegradation.

How to cite: Anshuman, A. and Eldho, T. I.: Meshfree models for simulation of reactive transport in groundwater systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3703, https://doi.org/10.5194/egusphere-egu21-3703, 2021.

The study of the flow in a single fracture is the starting point to understand the complex hydraulic behaviour of geological formations and fractured reservoirs, whose comprehension is of interest in many natural phenomena (e.g., magma intrusion) and the optimization of numerous industrial activities in fractured reservoirs (e.g., Enhanced Oil Recovery, drilling engineering, geothermal energy exploitation). Despite the considerable technical prospects of this topic, the associated mathematical complexity and computational burden have so far mostly discouraged investigations of the combined effects of fracture heterogeneity and of the complex rheology of relevant fluids. Indeed, magmas, foams, muds, and suspensions of natural colloids such as clay particles in water are complex fluids and often present in subsurface applications and natural processes. These fluids are characterized by a shear-thinning behavior, which can be well described by the Ellis model, a continuous three-parameter model that behaves as a power-law fluid at high shear rates and as a Newtonian fluid at low shear rates. The Ellis model parameters are: n the power law exponent, μ₀ the low shear rates viscosity, and τ_1/2 the shear rate such that μ_app(τ_1/2)=μ₀/2. We use this rheological description in combination with the lubrication theory, which is a depth-averaged formalism permitting us to reduce the full 3-D problem to a 2-D plane formulation. It has been applied to study Newtonian flow in a single fracture for decades and, as far as the aperture gradient remains small (∇d«1), the approximation error introduced by this model is limited. We present here a lubrication-based numerical code aiming at simulating the flow of an Ellis fluid in rough-walled fractures. The code is composed of two modules: a 2D FFT-based fracture aperture field generator and a lubrication-based non-Newtonian flow solver. The former module generates a random aperture field d(x,y) with isotropic spatial correlations, given a mean aperture ⟨d⟩, a coefficient of variation σ_d/⟨d⟩, a Hurst exponent (H) and a correlation length (l_c), reproducing realistic geometries of geological fractures. In the latter module, a 2-D finite volume scheme is adopted to solve the non-linear lubrication equation describing the flow of an Ellis fluid. The equation is discretized on a staggered grid, so that d(x,y) and the pressure field p(x,y) are defined at different locations. Computational efficiency is achieved by means of the inexact Newton algorithm, with the linearized symmetric system of equations solved via variable-fill-in Incomplete Cholesky Preconditioned Conjugate Gradient method (ICPCG), and a parameter-continuation strategy for the cases with strong nonlinearities. The code proves to be stable and robust when solving flow within strongly heterogeneous fractures (e.g., σ_d/⟨d⟩=1), even on very fine and coarse meshes (e.g., 2¹⁴×2¹⁴) and considering a wide range of power-law exponents (e.g., 0.1≤n≤1). The code is validated by comparing the results against analytical solutions (e.g., parallel plates model, sinusoidal profile) and full 3-D CFD simulations, considering different closures.

How to cite: Lenci, A., Méheust, Y., Putti, M., and Di Federico, V.: An efficient lubrication-based code for solving non-Newtonian flow in geological rough fractures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7820, https://doi.org/10.5194/egusphere-egu21-7820, 2021.

For most catchments, there is insufficient data to determine the location of the groundwater surface. For humid climates, it is, therefore, often assumed that the groundwater-surface follows the surface topography. This assumption allows using digital elevation models (DEMs) to estimate the flow directions and catchment boundaries. However, high-resolution elevation data also include many small-scale features that are unlikely to affect the direction of groundwater flow, or only affect it during specific conditions. Furthermore, flow directions may change during events or depending on the water level.

The optimal resolution of the DEM for determining groundwater flow directions is not known yet. Therefore, we studied how much DEM derived flow directions and catchment boundaries are affected by the resolution or smoothing of the elevation data for the Krycklan catchment in northern Sweden. We also measured the groundwater levels in two small sub-catchments to determine what DEM resolution best describes the actual groundwater-surface and flow directions.

For the topographic analyses, the LiDAR-based elevation data were first smoothed with various filters (e.g., Gaussian filters) and resampled to obtain lower resolution elevation data. We then determined the flow directions for these different DEMs. The aim was to determine where in the catchment the calculated flow directions are most sensitive to the resolution of the topographic data. The results of the topographic analyses show that for some areas, particularly flat areas, ridges, streambanks and locations where the local slope differs from the general slope, the calculated flow directions depend strongly on the resolution and smoothing of the elevation data.

To test how well the DEM based groundwater flow directions represent actual flow directions, we installed a dense (5-20 m spacing) network of shallow (1 to 6 m deep) groundwater wells (75 wells in total) in a 1 ha and a 2 ha gauged sub-catchment. The triangular nested design of the groundwater well network allowed us to determine the smaller (5 m) and larger scale (20 m) groundwater gradients. The recorded water levels were augmented and validated by manual measurements during the summers of 2018 and 2019. The high spatial and temporal resolution data allowed us to study the response of the groundwater level and the flow directions to different meteorological situations (e.g., large precipitation events after dry and wet conditions and during a very dry period in summer 2018). These observations indicate that the degree to which the groundwater-surface is a subdued copy of the surface topography varies throughout the year, and provides information on which DEM resolution most accurately represents the groundwater-surface and flow directions.

How to cite: Erdbrügger, J., van Meerveld, I., Seibert, J., and Bishop, K.: Flow directions of shallow groundwater in a boreal catchment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9384, https://doi.org/10.5194/egusphere-egu21-9384, 2021.

Methane is recognized as a potential energy source in the transition to carbon free energies. Appropriate modeling approaches to quantify methane migration in low permeability geomaterials can assist the appraisal of the feasibility of a methane recovery project. Wu et al. (2016) proposed a model enabling one to estimate the total mass flow rate of the gas as the sum of key processes, including (i) a surface diffusion and two weighted bulk diffusion components, (ii) slip flow, and (iii) Knudsen diffusion. In its isothermal form and taking pressure gradient as boundary condition, the model relies on 10 parameters. These are typically estimated through laboratory-scale experiments. Considering the mechanisms involved, such experiments are costly, time demanding, and their results are prone to uncertainty. The latter is also related to the intrinsic difficulties linked to replicating operational field conditions at the laboratory scale as well as to the desired transferability of results to heterogeneous field scale settings. Due to our still incomplete knowledge of the key mechanisms driving gas movement in low permeability geomaterials and the complexities associated with the estimation of model parameters, model outputs should be carefully analyzed considering all possible sources of uncertainty. In this sense, sensitivity analysis approaches may be used to enhance the quality of parameter estimation workflows, upon focusing efforts on parameters with the highest influence to target model outputs. We rely on two typical global sensitivity analysis approaches (i.e., Variance-based Sobol approach and Morris method) to analyze the behavior of the aforementioned gas migration model targeting low permeability media. Because of the complexity of the physical processes represented in the model and the typical frequency distributions of pore size in caprocks, the sensitivity analysis is performed in two differing settings, each corresponding to a given range of variability of characteristic pore sizes. When considering porous systems with pore size ranging between 2 and 100 nanometers, results based on Sobol indices identify (in decreasing order of importance) pore radius, porosity, pore pressure, and tortuosity as the parameters whose uncertainty significantly imprints model output uncertainty. Similar results are obtained through the analysis of the Morris indices, these identifying the pore radius parameter as the one with the highest contribution to non-linear (or interaction) effects on the model output. For tighter porous media (i.e., with pore size comprised between 2 and 10 nanometers), the Sobol indices analyses identify (in decreasing order of importance) pore pressure, porosity, blockage/migration ratio of adsorbed molecules, and pore radius as the most influential model parameters. The role of the blockage/migration ratio of adsorbed molecules suggests that surface diffusion is a dominant gas transport mechanism in these scenarios. The Morris approach identifies the same parameters as important, albeit in a different order of importance.

References.

Wu, K., Chen, Z., Li, X., Guo, C., Wei, M., 2016. A model for multiple transport mechanisms through nanopores of shale gas reservoirs with real gas effect-adsorption-mechanic coupling. International Journal of Heat and Mass Transfer 93, 408-426. doi: 10.1016/j.ijheatmasstransfer.2015.10.003

How to cite: Sandoval, L., Riva, M., Colombo, I., and Guadagnini, A.: Global sensitivity analysis of a low permeability media gas flow model with multiple transport mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9828, https://doi.org/10.5194/egusphere-egu21-9828, 2021.

Assessing the water balance including subsurface runoff in high Alpine catchments is still a major challenge due to environmental and meteorological complexity, and mostly data-lacking hydrology. The aim of this study is the determination of the water balance components and water budget with focus on approximation of interflow, subsurface runoff and groundwater interactions, depending on sediment and bedrock properties.

In this process we investigate a small, high data providing Alpine catchment in the Wipp Valley (Tyrol, AT) to evaluate the best modelling approach in order to apply it on catchments along the Austrian Brenner axis. Thus, a direct model comparison of the main study catchment, with its (moderate data providing) neighbouring valley is carried out. The main study catchment (Padaster Valley) covers 11.2 km² and is located east of Steinach am Brenner in the Wipp Valley. Due to its partially usage as a deposital site, respectively a landfill for the tunnel excavation material of the Brenner Base Tunnel, this valley represents a highly interesting site in a hydrological aspect. Thus, the Padaster Valley is highly monitored and hence predestined for hydrological investigations. Hydrological data such as discharge is measured high frequently on four gauges, meteorological data on two gauges. An additional study catchment (Navis Valley) covers 63 km² and is located northerly next the Padaster Valley. Seven gauges provide meteorological data, however, continuous discharge data is just measured at the valley mouth. Further meteorological data for both areas will be contributed by the ZAMG (Zentralanstalt für Meteorologie und Geodynamik), whose INCA model provide a high spatial resolution dataset of 1km. However, in order to gain a better overall understanding of subsurface runoff and hydrogeological processes, geological data will be considered and incorporated/integrated in the modelling process. This includes geological maps, - cross sections and geophysical analysis, which help to estimate the bedrock topography, and consequently the volume as well as deeper seated hydrogeological properties of the sediment cover. In this context, continuous data from 7 groundwater observation wells provide information regarding groundwater levels and hydraulic head. To increase the model accuracy regarding subsurface flow processes, subsurface-depending runoff types after Pirkl & Sausgruber (2015) are applied. Furthermore, several maps such as land use, surface runoff coefficient and soil map including grain size distribution of the layers have been compiled by in-situ fieldwork for this study. In order to model the water budget, subsurface runoff and overall hydrological slope properties, the distributed hydrological Model WaSIM (Richards version; Schulla, 1997) is applied. The model is based on a modular system which uses physically-based algorithms.

The present study is been carried out by the Austrian Research Centre for Forests (BFW) in collaboration with the Brenner Base Tunnel (BBT-SE).

How to cite: Bergmeister, D., Klebinder, K., Kohl, B., Burger, U., Orsi, G., and Lehner, F.: Water budget and subsurface runoff determination for small Alpine catchments using WaSIM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13436, https://doi.org/10.5194/egusphere-egu21-13436, 2021.