Displays

Dissolution by reactive fluid flow is a fundamental process in geological systems. It controls diagenesis and karst evolution and has broad implications for groundwater hydrology. Specifically, reactive flow controls the evolution of the void-space structure via the feedback between the reaction and transport. In some instances, advective transport rate is high compared to that of geochemical reactions (low Damkӧhler number, Da), such that the reactive fluid penetrates the system before its reactivity is exhausted, resulting in a relatively spatially-uniform dissolution. Despite the importance of low Da conditions, the emerging transformations in the medium structure, flow field, and its bulk properties are not well understood. Likewise, our ability to decipher diagenetic history and preexisting structure is lacking.

Here, using a network model, we investigate the evolution of heterogeneous and anisotropic medium during dissolution at low Da conditions. The numerical simulations show that the medium progressively becomes more homogeneous as well as isotropic, which consequently makes the flow field more uniform. Homogenization is particularly notable for anisotropic media, in which the transverse channels are wide relative to the channels parallel to the main flow direction. In this case, flow is initially focused within a few highly tortuous pathways, hence emphasizing the effect of dissolution on flow heterogeneity and tortuosity. The homogenization process is further enhanced when the surface reaction is transport-controlled—that is, when diffusion of dissolved ions away from the mineral surface to the bulk fluid is slow, reducing the reactivity adjacent to the surface: At first, since diffusive transport is more effective in narrow channels, they undergo faster dissolution, which selectively enlarges them leading to an initial steep rise in permeability. Later, however, as dissolution proceeds and the channels broaden, the overall dissolution rate drops, diminishing the growth rate of permeability. Our findings provide fundamental insights into reactive transport and hydrogeological processes in fractured and porous media.

How to cite: Aharonov, E., Roded, R., Holtzman, R., and Szymczak, P.: Reactive flow and homogenization in anisotropic media , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2756, https://doi.org/10.5194/egusphere-egu2020-2756, 2020.

Some porous media such as clay have charged surfaces. The presence of these charged surfaces results in a complex system where water flow, salt transport, and the electric field are coupled. This system is important in many fields, such as geotechnical engineering, storage of radioactive waste in clay barriers, enhanced oil recovery, and irrigation with marginal water. The charged surfaces alter the transport properties of ions. For example, clay minerals are often negatively charged due to isomorphous substitution. Cations are therefore attracted to the mineral surface, while anions are repelled, creating a diffuse double layer around the clay particle. Cations are therefore transported preferably over anions through such charged pores. To conserve electroneutrality, a streaming potential develops to counteract diffusion by electromigration. This results in smaller effective diffusion coefficients compared to uncharged porous media. We developed a pore-network model to quantify the effect of the double layer processes on the effective diffusion coefficient. Pore-network models are a suitable tool to include the heterogeneity of pore sizes and surface charge densities seen in nature. In pore-network modeling, the geometry of the pore space is simplified, but the network properties are based on realistic statistics such as pore size distribution and connectivity. The larger scale behavior can be identified by averaging over a large number of pores. The results were strongly dependent on the salinity, as this controls the thickness of the double layers. At high salt concentrations, the diffuse double layer is thin and the differences between charged and uncharged porous media are negligible. However, at low salinity, the double layers are thick and the effective diffusion coefficient of salt was reduced by 25% in charged porous media compared to uncharged porous media, due to salt transport being slowed down to conserve electroneutrality. Hence, the presence of charged mineral surfaces can significantly alter transport rates under low salinity conditions.

How to cite: Cornelissen, P., Leijnse, A., Joekar-Niasar, V., and van der Zee, S.: Salt diffusion in charged porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17553, https://doi.org/10.5194/egusphere-egu2020-17553, 2020.

Traditional concepts for flow in porous media assume that the heterogenous distribution of hydraulic conductivity is the source for the contaminant temporal and spatial heavy tail, a process known as anomalous or non-Fickian transport; this anomalous transport behavior can be captured by the β parameter in the continues time random walk (CTRW) framework. In previous studies we showed that there is a functional form relating the β parameter to the permeability variance¹ and fracture alignment in fracture fields². Moreover, we showed that this variance is strongly influencing the reaction pattern during transport³.  This study shows that as the spatial correlation length, between these heterogenous distribution of hydraulic conductivities, increase, the anomaly of the flow reduces, yet the β value is unchanged suggesting that there is a topological component to the flow field, captured by the β⁴. This finding is verified by an analysis on the flow field, showing that the changes in the conductivity values have little effect on the flow field morphology, which points to the topological component in the flow.

How to cite: Edery, Y.: The topological origin of anomalous transport: Persistence of β in the face of varying correlation length., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7871, https://doi.org/10.5194/egusphere-egu2020-7871, 2020.

Advective trapping occurs when solutes enter a low velocity zone in the porous medium. Current multirate mass transfer (MRMT) models consider slow advection and diffusion but do not separate these processes, which makes parameterization difficult. Here we investigate the impact of advective trapping on transport in media consisting of isolated low permeability inclusions. Breakthrough curves show that effective transport changes from a streamtube model to genuine MRMT as the degree of disorder of the inclusion arrangement increases. We discuss the mathematical formulation in the MRMT and CTRW frameworks and the impact of the spatial geometry on the ergodicity and stationarity of large scale transport. These finding give new insight into transport into transport in highly heterogeneous media.

How to cite: Hidalgo, J. J., Neuweiler, I., and Dentz, M.: Transport under advective trapping, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8686, https://doi.org/10.5194/egusphere-egu2020-8686, 2020.

Regional scale transport models are needed to support the long-term evaluation of groundwater quality and to develop management strategies aiming to prevent serious groundwater degradation. The transport dominant process, advection or diffusion, was identified for flow fields with different primary flow directions. The capacities of Multi-Rate Mass Transfer (MRMT) and adaptive Multi-rate Mass Transfer (aMMT), modified from MRMT by updating mass transfer rates with changing velocities, to adequately describe the main solute transport processes, including the capture of late-time tails under changing boundary conditions were evaluated. Advective-dispersive contaminant transport simulated in a 3D heterogeneous medium was used as a reference solution. Equivalent transport under homogeneous flow conditions was then evaluated by applying the MRMT or aMMT models for upscaling. Results indicated that for advection-dominated transport, both the MRMT and aMMT methods can upscale the anomalous transport dynamics affected by sub-grid heterogeneity under transient flow conditions. Whereas, for diffusion-dominated systems, the MRMT model failed to capture the tails of tracer breakthrough curves (BTCs) after the boundary condition changed, but the results from the aMMT model were significantly improved. However, if the overall flow direction changed, both MRMT and aMMT failed to represent the BTC tail generated by the heterogeneous system. In this study, an indicator that describe the primary flow direction in anisotropic heterogeneous domain was developed, and the relationship between the flow direction and the dominant transport process was investigated. The ranges of the indicator, within which the advection or diffusion is dominant, are determined. Therefore, this study not only show the capability of upscaling methods on describing the transport that dominated by different processes, but provides a guide on choosing upscaling methods in field site, which supports long-term management of groundwater.

How to cite: Guo, Z., Pauloo, R., Fogg, G. E., Henri, C., and Zheng, C.: Study on transport upscaling of Advection or Diffusion dominated process, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21773, https://doi.org/10.5194/egusphere-egu2020-21773, 2020.

Enhanced spreading of contaminants by groundwater (macrodispersion) is governed by advection by the velocity field, whose spatial variability is caused by the heterogeneity of the hydraulic conductivity K. Characterization of K distribution in space is a major topic of research. While considerable knowledge has been accumulated for natural gradient flows, hydraulic tomography methods have been forwarded only recently. A typical setup consists of short segments of a well through which water is pumped (injected) and the head H response is measured by pressure transducers along observation piezometers at different distances and elevations. Attempts in the past were done mainly to derive K from measured H  by numerical inversion of the flow equation accordingly to a global optimality condition. The present study considers stochastic hydraulic tomography by which measured H are employed in order to identify the statistical parameters of the log-conductivity Y= ln K field (mean, variance, integral scales). As a first step we investigate and present the solution of the steady flow equations relating  H statistical moments to those of the K field for the strongly nonuniform source flow, which approximates the main constitutive element of the tomographic setup. This is achieved by numerical simulations for values of the Y variance up to 4 and the derivation of type curves which helps in the identification of K statistics. Application to identification of logconductivity moments for a hydraulic tomography setup is illustrated by a synthetic example.

How to cite: Bellin, A., Fiori, A., and Dagan, G.: Source Flow in Heterogeneous Aquifers with Application to Hydraulic Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11294, https://doi.org/10.5194/egusphere-egu2020-11294, 2020.

The underground fracture pattern, which results from tectonic, climatic and biological stresses, drives water storage dynamic and nutrient cycling in the deep critical zone. Despite a gradual decrease of fracture density with depth, the fracture network is strongly heterogeneous and anisotropic, resulting in a complex pathway distribution with variable hydraulic conductivities. High celerities occurring in preferential flowpaths govern the dynamic response of discharge flows to extreme recharge events. However, the role of preferential flowpaths in transporting fresh meteoritic water and biota remains poorly studied, while the delivery of meteoritic reactants is crucial to initiate underground chemical reactions.

Here, we study a fractured aquifer in a crystalline catchment located in Brittany (Guidel, France) to investigate the link between depth, water transit time and subsurface reactivity in fractures. Oxygen is used as a tracer of fresh water inputs because its availability has a tremendous impact on oxidation-driven reactions such as weathering processes and microbial activity. We performed vertically sampling of fracture fluid with an inflatable packer capable of isolating fractures in an artesian well located in the discharge chemically-reduced zone of the aquifer. Major ions, dissolved reactive gases, dissolved anthropogenic gases, stable isotopes (O, Sr and Si) and microbial diversity were analysed on five fracture waters sampled at depth between 20 and 55 m. Significant differences have been observed between fractures and younger and more oxygenated waters were found intermittently in fractures at 47 and 54m, with dissolved oxygen concentrations ranging between 0.1 and 0.5 mg/L. The penetration of oxygen in deep fractures reveals either a rapid transport of oxygen or a low consumption of oxygen in preferential flowpaths. These hypotheses are tested with a Discrete Fracture Network model, where first-order reaction rates have been implemented, and the temporal dynamic of oxygen is assessed and linked to water transit time in fractures. We investigate the concept of transit time and water-rock contact time and discuss the relevance of mean transit time to evaluate subsurface reactivity.

Preferential flowpaths thus not only make fractured aquifers more dynamic but can also, under extreme recharge conditions, efficiently transport fresh water at high depth. The advective-dominant transport of oxygen through artery-like fractures could have a significant impact on short term microbial activity and the associated nutrient cycling but also on long term weathering front propagation.

How to cite: Bouchez, C., Lavenant, N., Farasin, J., Labasque, T., Osorio, I., Bouchez, J., Gomez, F., Longuevergne, L., and Le Borgne, T.: Which role do preferential flowpaths and fractures play in the subsurface reactivity in heterogeneous aquifers?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17564, https://doi.org/10.5194/egusphere-egu2020-17564, 2020.

Transient boundary conditions are important in the transport of dissolved solutes and contaminants in the subsurface, as they can generate complex groundwater flow fields that influence mixing and mixing-controlled reactions. An important example of highly transient boundaries are rivers in which the river stage changes fast and sharply due to the operation of hydropower plants. In order to better understand how these rivers influence the groundwater flow field, we performed a laboratory experimental study. A quasi two-dimensional flow-through chamber was filled with glass beads and sand and the surface of the saturated porous medium was connected to two water reservoirs representing two rivers in hydraulic contact with the aquifer. The river stage could be changed independently by a separate lifting system for each reservoir. This experimental set-up was inspired by shallow unconfined aquifers in hydraulic contact with rivers affected by hydropeaking. In order to observe the influence of the transient boundary conditions, we injected a color tracer and observed the evolution of the solute plume across the tank. We monitored the spatial distribution of the tracer and the impact of the dynamic river boundaries on the groundwater plume with a non-invasive image analysis technique, consisting in a background light foil and a CCD camera. Additionally, to analyze the breakthrough curves we also took samples at multiple outlet ports and measured the tracer concentration using a spectrophotometer. We characterized the plume behavior using different metrics, including the dilution index, the flux-related dilution index and the moments of the tracer distribution. We conducted experiments both under steady-state flow conditions (with and without rivers) and under different transient conditions, obtained by variations in the order and the intensity of the river fluctuations, and groundwater flow velocities.

How to cite: Basilio Hazas, M., Ziliotto, F., Rolle, M., and Chiogna, G.: Plume evolution, transport and mixing processes under highly transient boundary conditions: A laboratory-scale study , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5629, https://doi.org/10.5194/egusphere-egu2020-5629, 2020.

Managed aquifer recharge through riverbank filtration is an important method to produce drinking water in densely populated regions. Due to the discharge of wastewater into streams, this type of drinking water production can be affected by organic contaminants originating from surface water inflow. Transport and degradation of anthropogenic contaminants depend on several factors, such as pH, temperature, oxygen content, and redox conditions. One of the key factors that drive the degradation of organic contaminants like x-ray contrast media is the prevailing redox system as many pharmaceuticals and pesticides are transformed under aerobic conditions but are more persistent under anaerobic conditions.

We conducted a 1-year monitoring campaign at an active riverbank filtration plant at the Rhine river in Düsseldorf, Germany. Samples were taken every two weeks from the Rhine, a production well, and five observation wells with three different depths along a transect perpendicular to the river and parallel to the main flow direction. Samples were analyzed for main cations and anions, redox-species, and microbiological parameters. Water samples were also screened for 100 organic contaminants, pharmaceuticals, and pesticides.

A 2D reactive transport model was set-up using PFLOTRAN to simulate the redox zonation during a hydrological year. It includes aerobic respiration and denitrification with dissolved organic carbon using Monod kinetics and also accounts for temperature-dependency. Our results show that hot spots for biogeochemical processes develop close to the river, and thus most of the inflowing oxygen is already consumed within the first few decimeters. We also found a substantial seasonal variability of reaction rates due to seasonal temperature variations leading to oxygen depletion and limited denitrification in the warmest period (late summer/early fall).

Reactive transport is affected by the hydrogeological properties of the aquifer, which are influenced by its geological development. Thus, model results will depend on the reliability and accuracy of the employed conceptual geological model. Based on structural information obtained from grain size sieve analysis, and geophysical investigations such as geoelectric and natural gamma-ray measurements, we created a set of plausible conceptual models with increasing complexity. These models range from a simple homogeneous aquifer, to a multi-layer aquifer system or a cross-bedded aquifer structure. The conceptual models include different representations of the colmation layer at the interface between river and aquifer.

Numerical analysis of the different conceptual models indicates that a homogeneous aquifer can represent a single flow path over a hydrological year. However, only more complex aquifer structures were able to reproduce the spatial and seasonal temporal variability of temperature and redox species observations (O₂, NO₃^-). Additionally, proper integration of the colmation layer is the key factor to simulate heat transport as well as the spatial distribution of redox-species and thus redox-zonation during the entire hydrological year, including droughts and flooding periods. Therefore, an accurate and detailed integration of the geological system into the reactive transport model, especially characteristics (e.g., size, type of material) of the colmation layer, are of highest relevance for enhanced predictions of redox zones in highly transient hydrogeological systems and at hydrodynamic interfaces.

How to cite: Knabe, D., Dwivedi, D., Werban, U., and Engelhardt, I.: Impact of the accuracy of the conceptual geological model on predicting hot spots of redox-zones at the surface water – groundwater interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5887, https://doi.org/10.5194/egusphere-egu2020-5887, 2020.

Subsurface flow in small first-order catchments is dominated by both, precipitation patterns and subsurface structure. We report on a series of repeated tracer experiments under transient conditions in a small forested first-order catchment (F4, 2.3 ha) at Gårdsjön in SW Sweden. Podsols are the dominant soil types, soil thickness varies strongly (0-50 cm) and bedrock outcrops are partly visible at the surface. A small wetland is situated directly upstream of the runoff weir. A hillslope of the catchment is equipped with a sprinkler system and can be irrigated at around 38-45 m³ day^-1. Depending on the meteorological conditions in the respective year of the experiment, natural rainfall comes in addition.

A bromide tracer solution was injected into groundwater at a single location about 40 m upstream the weir over a period of approximately an hour, and was monitored using a set of groundwater tubes and the weir at the outlet over the following 3-4 days. Additionally, discharge and meteorological conditions were recorded. The experiments were repeated each summer from 2007 to 2019. In summer 2019, electrical resistivity tomography was done during the experiment. We measured a profile perpendicular to the flow direction covering the whole study site. This data shows how subsurface patterns could influence water flow on the soil-bedrock interface. We investigated tracer recovery rates against cumulated runoff since tracer application. Substantially different transit times and qualitatively different behaviour of the breakthrough curves were observed, even under steady state conditions. We present first results how these differences could be linked to the structure of the subsurface.

How to cite: Bogner, C., Steininger, F., and Hauhs, M.: Spatial heterogeneity and temporal variability in repeated hillslope tracer experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9563, https://doi.org/10.5194/egusphere-egu2020-9563, 2020.

Catchment descriptors are used to quantify and summarise catchment properties, representing the characteristics of a catchment.[ASMMR1]  It is meaningful to study how catchment descriptors relate to the integrated catchment dynamic, e.g. groundwater flow. Hydraulic conductivity (K) is a critical catchment descriptor and driving force for groundwater flow. It is well known that hydraulic conductivity is highly variable in space. Some studies have considered the impact of gradual decrease in hydraulic conductivity with depth on groundwater flow, but few considering the spatial variation in horizontal hydraulic conductivity. The purpose of this study was to investigate the effect of horizontal hydraulic conductivity on groundwater flow with an integrated hydrology model using virtual experiments. 

We study the variability of catchment-scale groundwater flow patterns for virtual catchments with identical average and/or dominant hydraulic conductivities, but different horizontal distributions. The results show that the variation in horizontal hydraulic conductivity influences the formation and development of regional flow patterns. We further study the implications of the variation in horizontal hydraulic conductivity for patterns and rates of recharge and discharge, as well as for the groundwater flux.

How to cite: Liu, H., Wagener, T., and Rahman, M.: Effects of horizontal hydraulic conductivity distributions on groundwater flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10525, https://doi.org/10.5194/egusphere-egu2020-10525, 2020.

Investigations at the long-term experimental catchment Vallcebre in the Pyrenees revealed that rainfall-runoff dynamics are highly variable due to the Mediterranean climatic conditions affecting the storage and release of water in the subsurface¹. In a changing climate, to the consequences of which could lead to more variations in catchment wetness due to an increase in both droughts and high intensity rainfalls, there is a strong need to better understand subsurface storage and runoff processes.

While our previous isotope studies (using ²H and ¹⁸O) demonstrated a pronounced heterogeneity of water flow in the unsaturated zone at the plot scale², we also observed that the contributions of young waters to catchment runoff are highly dependent on the catchments wetness³. These analyses provided a basis from which we present new insights into the relationship between subsurface runoff and storage dynamics applying StorAge Selection functions⁴ and end-member splitting analysis⁵. Thus, we combined modeling and data-driven approaches to disentangle the partitioning of subsurface waters into storage and runoff based on water age dynamics.

We gathered an extensive isotope data set with >550 rainfall samples and >980 stream samples taken at high temporal resolution (30 minutes to one week), with highest frequencies during high discharge to improve the coverage of rainfall-runoff events. Using this high-frequency isotope data set, we calibrated the StorAge Selection functions and put special emphasis on the representation of the isotopic response during high flow rainfall-runoff periods. We further tested if time-variant representations of StorAge Selection functions dependent on varying wetness improves the stream water isotope simulations and the ways in which isotope data from different compartments (groundwater and tree water) can assist in constraining the parameter space. Furthermore, end-member splitting analysis provided an independent view into the flow dynamics based on these long-term isotope data sets. As such, the analysis allowed us to derive estimates of the dynamics of rainfall partitioning into runoff and evapotranspiration. Therefore, the combination of the modeling and data-driven approaches enabled an assessment of the dynamics of subsurface runoff at the catchment scale underlining the relevance of heterogeneous flow pattern that were observed on the plot scale.

References

How to cite: Sprenger, M., Llorens, P., Gallart, F., and Latron, J.: Subsurface runoff and recharge dynamics in a Mediterranean catchment based on StorAge Selection functions and end-member splitting analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-829, https://doi.org/10.5194/egusphere-egu2020-829, 2020.

Subsurface water storage strongly influences runoff generation processes, regulates agricultural production and defines catchment buffering capacities to hydrometeorological extremes. Knowledge about the amount and spatio-temporal distribution of catchment storage can also be important for constraining and evaluating hydrological models. While it is still challenging to measure this directly, characterisation of catchment-scale storage is more likely to be achieved via a combination of estimation methods at appropriate scales. While stable water isotopes can provide insights into (timescales of) dominant stores and flow paths, novel cosmic ray sensors (CRS) offer insights into large scale water storage dynamics.

Here, we combined stable water isotope analyses with CRS data and rainfall runoff modelling to better understand subsurface storage dynamics and how these relate to catchment runoff generation. We focussed specifically on humid managed environments, such as in NE Scotland, where short-term changes in both storage and management activities occur predominantly at or near the surface. To understand spatial patterns in flow pathways and the evolution of water ages (as mean transit times), we conducted long-term (~5y) stable water isotope monitoring of a nested stream network in a 10km² mixed-agricultural catchment. Monitoring also involved artificial drains of agricultural fields and country roads. This was complemented with a short-term study (~14 months) of mobile soil water in key soil-land use units. Additionally, we characterised field scale near-surface storage dynamics in these same key soil-land use units using CRS technology. Finally, we explored the storage-discharge relationships based on these CRS storage estimates and the information content of these novel data for rainfall-runoff model calibration to better characterise catchment-scale storage dynamics.

The outcomes of both transit time and rainfall-runoff modelling highlighted the importance of near-surface storage dynamics for catchment functioning and streamflow generation. Predominantly young waters (<1 y) across the stream network were associated mainly with shallow soils and the extensive artificial field drainage, which short-circuits water delivery to the streams, especially during wet periods. Water ages in soil mobile water were also short (1 – 6 months) and subtle differences between the key soil-land use units were associated with land management practices, which either enhanced (artificial drainage, ploughing) or delayed (compaction) transit times in the soil. As CRS near-surface storage estimates related well to catchment scale storage dynamics (R²=0.91) and stream discharge (R²=0.71), we evaluated the effect of using CRS data in model calibration. Including it in the model calibration was especially useful during intermediate and wet periods. Overall, our results showed that a combined model calibration using discharge and CRS estimates provided a better representation of catchment internal dynamics, additionally reducing uncertainty during low flows.

In the context of a humid managed catchment, our results showed that the integration of water isotope analyses and CRS-derived storage estimates can provide unique insights into catchment scale sub-surface storage dynamics, runoff generation and the evolution of water ages in soils and streams. They also demonstrated the potential of these data for informing rainfall-runoff modelling frameworks, but further work is needed across a range of different environments to explore wider applications.

How to cite: Dimitrova Petrova, K., Geris, J., Wilkinson, M., Lilly, A., Verrot, L., Rosolem, R., and Soulsby, C.: Integrating water isotopes and cosmic ray sensor data with modelling to understand near-surface storage-discharge relationships in managed landscapes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19546, https://doi.org/10.5194/egusphere-egu2020-19546, 2020.

Catchment scale hydrological models all have some representation of the dynamics of subsurface flow and hence direct or indirect estimates of the celerities (velocities) involved. Parameters representing these celerities (for example recession coefficients of linear reservoirs) are often calibrated against runoff instead of being estimated directly from measured data. Such a procedure, when applied for hydrological models with (too) many parameters to be calibrated, may lead to unrealistic estimates of subsurface celerity due to equifinality issues. Our aim with this study is to obtain an estimate of the distributions of subsurface celerites corresponding to the distribution of saturation levels through recession analysis. Using the recession characteristic Λ=log(Q(t)/Q(t+∆t) and looking for sequences of recession in a moving average filtered time series of runoff, we find, for many catchments, no clear structure in the relationship between Q(t) and Λ.  In order to better understand the recession process we let the runoff be represented by four (parallel) unit hydrographs (UH) of different temporal scales. The UHs thus represent different subsurface celerities through their different temporal scales and different levels of saturation.  Only when there was a systematic build up of saturation from below, i.e. the slowest UH had to be filled to (a chosen max) capacity before the next UH received water, a clear structure between Q(t) and Λ emerged, where for each value of Q(t) the maximum Λ represented the true recession to be used for estimating the celerity.  At the tiny Muren catchment (7500 m2) in southern Norway we performed an infiltration test and estimated the saturated hydraulic conductivity to be 0.00045 m/s. The mean celerity estimated from recession analysis for the same catchment was found to be 0.00034 m/s, and when the distribution of celerities from the recession analysis was used in the Distance Distribution Dynamics (DDD) rainfall runoff model a Kling Gupta efficiency criterion of KGE = 0.86 was obtained for runoff simulations at 15 minutes temporal resolution.

How to cite: Skaugen, T., Møen, K., and Boje, S.: Comparing catchment scale subsurface celerities estimated from recession analysis and infiltration experiments. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8711, https://doi.org/10.5194/egusphere-egu2020-8711, 2020.

Stretching of fluid elements by a heterogeneous flow field, such as the flow through a porous media or geophysical flows such as atmospheric or oceanic vortices, is known to enhance mixing rates of scalar fields[1]. While the mechanisms leading to the elongation of material lines are well understood, predicting mixing rates still remains a challenge particularly when there is a reconnection (or aggregation) between several parts of the mixing interface, leading, at large mixing time, to a so-called coalescence regime[1][2]. In this presentation, we numerically study this coalescence dynamics through scalar transport in two different flow fields, the Rankine vortex and Stokes flow through a periodic bead pack[3]. The former is typical of large-scale turbulent flows [4] whereas the second is generic of small-scale laminar flows in porous media [5]. Both flows show a net elongation of the mixing interfaces, although at very different rates. To solve the transport problem in these flows, we use a Lagrangian method (the diffusive strip method[6]). This method allows us to reconstruct, at high resolution, the scalar concentration fields and to compute the evolution of the distribution of concentrations levels, scalar dissipation rate and scalar power spectrum in time. The signature of coalescence is clearly observed in both flows and we assess the influence of coalescence on the difference in mixing behaviour for the two flows. We finally discuss how coalescence may affect the reaction kinetics of mixing-limited reactive flows. The analysis proposed sheds light on fundamental aspects of transport and mixing in earth surface and subsurface flows.

[1] Emmanuel Villermaux. Mixing versus stirring. Annual Review of Fluid Mechanics, 51:245–273, 2019.
[2] Tanguy Le Borgne, Marco Dentz, and Emmanuel Villermaux. The lamellar description of mixing in porous media. Journal of Fluid Mechanics, 770:458–498, 2015.
[3] Régis Turuban, David R Lester, Tanguy Le Borgne, and Yves Méheust. Space-group symmetries generate chaotic fluid advection in crystalline granular media. Physical review letters, 120(2):024501, 2018.
[4] RT Pierrehumbert. Large-scale horizontal mixing in planetary atmospheres. Physics of Fluids A: Fluid Dynamics, 3(5):1250–1260, 1991.
[5] Brian Berkowitz, Andrea Cortis, Marco Dentz, and Harvey Scher. Modeling non-fickian transport in geological formations as a continuous time random walk. Reviews of Geophysics, 44(2), 2006.
[6] Patrice Meunier and Emmanuel Villermaux. The diffusive strip method for scalar mixing in two dimensions. Journal of fluid mechanics, 662:134–172, 2010.

How to cite: Sen, S., Singh, P., Heyman, J., Le Borgne, T., and Bandopadhyay, A.: Signature of coalescence during scalar mixing in heterogeneous flow fields, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11019, https://doi.org/10.5194/egusphere-egu2020-11019, 2020.

We investigate the dissolution of a single grain of soluble mineral by microfluidic experiments and numerical simulations. The experiments use gypsum cylinders (10 mm radius, 0.5 mm thick) cast from rehydrated CaSO4 hemihydrate. The numerical simulations used a finite-volume discretization of the reactive-transport equations with a mesh that conforms to the evolving shape of the mineral. Using the coefficients for dilute aqueous ions, we overpredict the dissolution rate by about 25%. However, including the Debye-Huckel correction for the ion activity gives a substantial reduction in diffusion across the boundary layer at the dissolving solid surface and brings the simulation time scale into quantitative agreement with experiment.

The asymmetry introduced by the flow causes the initially cylindrical sample to take on a shape resembling one half of a figure eight, with the tip pointing in the downstream direction. The simulations give a near perfect match to the experimental size and shape. We quantify the evolution of the volume of the grain and its surface area, as well as its overall shape as the function of the Peclet number. Next we discuss the differences between the geometric surface area and the reactive surface area of a dissolving grain and explore a potential use of these results to upscale the reactive transport problem and obtain the effective reaction rates in a multi-grain system.

How to cite: Szymczak, P., Dutka, F., Starchenko, V., Osselin, F., Magni, S., and Ladd, A. J. C.: Dissolution of a single mineral grain: comparison of microfluidic experiments with pore-scale simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12253, https://doi.org/10.5194/egusphere-egu2020-12253, 2020.

In order to obtain a deeper understanding of flow and transport processes in fractures, experimental investigations and numerical modelling have been carried out focusing on the effects of fracture surface morphology. To determine a possible relationship between the roughness of fracture surfaces and hydraulic and transport parameters, two different types of sandstones has been investigated. The sandstones were a coarse-grained, inhomogeneous and strongly anisotropic Flechtinger sandstone (Bebertal, Germany) and a fine-grained, rather homogeneous, isotropic Remlinger sandstone (Würzburg, Germany).

The sandstones were first cored with a diameter of 100 mm and a height of 150 mm and split into individual fissures. The resulting fracture surfaces were scanned using a 3D scan and surface images were generated. These surface images were used to determine the Joint Roughness Coefficient (JRC) and other measures of roughness. The roughness has been characterized along 1D profiles in each direction. Mean values and spread have been calculated for each surface. The fracture surfaces are self-affine so that little variation along both surfaces has been determined. Both sandstone halves were then joined together and the reassembled fractured rock core was examined experimentally. Darcy and tracer tests were carried out for the investigations and hydraulic (permeability, fracture opening width) and transport parameters (flow velocity, dispersivity, dispersion coefficient) were derived from the results and compared with each other and with the surface roughness. For the Darcy experiments, the cores were clamped in a specially designed Darcy cell and calculations were done based on equations for the cubic law. The transport parameters were determined using a salt tracer and by evaluating the breakthrough curves, recorded by measuring the electrical conductivity, with the moment analysis.

First results show a very clear separation between Remlinger and Flechtinger sandstone. Thus, the finer-grained Remlinger cores show lower JRC than the coarser-grained Flechtinger, as expected. Further, the Flechtinger cores have larger aperture opening widths than the Remlinger cores. First comparisons show a tendency to higher dispersivity with higher JRC, and thus with the Flechtinger than in the case of the Remlinger cores. Though, in-depth analysis reveals that the JRC alone might not be sufficient to characterize transport processes along fractures, as anisotropy, as well as roughness variability along the fracture surface can influence flow and transport. Numerical modeling of flow paths across the fracture surface are used to relate experimental results with the flow pattern across the rough surface.

How to cite: Frank, S., Heinze, T., Ribbers, M., and Wohnlich, S.: Investigating surface morphology and transport parameters of single fractures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5229, https://doi.org/10.5194/egusphere-egu2020-5229, 2020.

Reduced-order modeling is an emerging technique for cutting down the computational expenses incurred in terms of CPU time and usage associated with repetitive simulation of flow dynamics of natural aquifer systems. Identifying the patterns related to the evolution of aquifer response with time is the key to model order reduction methodology. However, the accuracy of reduced-order groundwater models is dependent on several factors. It has been observed that the accuracy decreases while accounting for random heterogeneity of natural aquifers. Besides, the imposition of different boundary conditions also tends to influence the accuracy of reduced-order models. In this work, we study the effects of Dirichlet and Neumann boundary conditions on reduced-order modeling of groundwater flow through randomly distributed heterogeneous porous media. For low dimensional modeling, we have performed Singular Value Decomposition of the ‘Snapshot Matrix’ to obtain a set of orthonormal basis functions. The ‘Snapshot Matrix’ is formed from the solution of a Finite Volume Method based full system groundwater flow model at some exponentially distributed time instants. The governing groundwater flow equation is then projected onto the reduced sub-space of orthonormal basis functions via Galerkin Projection to obtain the solution at each time-step. We have carried out the study on a two-dimensional square-shaped synthetic heterogeneous aquifer with multiple pumping wells operating simultaneously within the domain. Four illustrative case studies have been performed with the aquifer being subjected to: (1) Dirichlet condition on all boundaries; (2) Dirichlet condition on three boundaries while the remaining boundary is impermeable; (3) Dirichlet condition on two parallel boundaries while the other two boundaries are impermeable; (4) Three impermeable boundaries and Dirichlet condition on the remaining boundary. The study shows that the accuracy of the reduced-order model is maximum when all the four boundaries of the aquifer are subjected to a constant specified head (Dirichlet) boundary condition. The accuracy starts to go down as we start introducing impermeable boundaries withdrawing the Dirichlet boundaries. The error analysis is performed by comparing the error statistic parameters: Maximum Absolute Error, Mean Absolute Error (MAE), Root Mean Square Error (RMSE) and Normalized Root Mean Square Error (NRMSE) for the four case studies with respect to the results obtained from corresponding full system model runs. However, if we look into the computational expenses, the model takes lesser computation time per iteration as the complexity of boundary conditions increases. Although the reduction in the accuracy of the reduced-order model is observed with the introduction of impermeable boundaries, the error statistic parameters are within desirable limits. Hence, the proposed reduced-order modeling methodology can potentially be accepted as an accurate and efficient alternative for replication of high-dimensional full system groundwater flow models, and can also be applied for natural aquifers on a watershed scale.

How to cite: Dey, S. and Dhar, A.: Analyzing the Effects of Dirichlet and Neumann Boundary Conditions on Reduced-Order Modeling of Groundwater Flow through Heterogeneous Porous Media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6091, https://doi.org/10.5194/egusphere-egu2020-6091, 2020.

In certain reactive transport applications, strong coupling between geochemical reactions and hydrodynamics exists. Dissolution and precipitation of minerals, such as the conversion between gypsum and anhydrite [1] or the precipitation of nesquehonite during CO₂ sequestration [2], as well as gas bubble formation [3] are geochemical processes which modify the multiphase flow dynamics, with direct feedback on reactive transport processes. In addition, heat generation induced by sulphide mineral oxidation can lead to significant increases in temperature [4], impacting flow, transport and geochemical reactions. In these instances, commonly used reactive transport modelling approaches, which rely on decoupling flow and reactive transport processes, have limitations. For density dependent or two-phase flow problems in the presence of a gas phase, the coupling between flow and reactive transport can be accounted for through a Picard iterative approach [3,5,6]. However, this approach is computationally expensive, involving the solution of nonlinear problems multiple times during each timestep, and convergence properties are often poor. More recently, a weak explicit coupling approach was developed to capture the impact of chemistry on flow by integrating water as a component and perform a volume balance calculation [7]. In the current work, a compositional approach is implemented into MIN3P-THCm, in which the flow variables (pressure, density) are expressed based on mass variables. Hence, this global implicit approach does not require solving the flow problem, but instead integrates groundwater flow processes directly into the reactive transport equations. We show that this approach yields very similar results to the commonly used approaches for single and two-phase flow. Finally, we show that, in highly coupled systems, not considering these coupled effects may lead to significant errors in simulating system evolution, highlighting the benefits of the newly developed approach.

[1] Jowett, Cathles & Davis (1993). AAPG Bulletin, 77(3), 402-413.

[2] Harrison, Dipple, Power & Mayer (2015). Geochimica et cosmochimica Acta, 148, 477-495.

[3] Amos and Mayer (2006). Journal of contaminant hydrology, 87(1-2), 123-154.

[4] Lefebvre, Hockley, Smolensky & Gélinas (2001). Journal of contaminant hydrology, 52(1-4), 137-164.

[5] Henderson, Mayer, Parker, & Al (2009). Journal of contaminant hydrology, 106(3-4), 195-211.

[6] Sin, Lagneau and Corvisier (2017). Advances in Water Resources, 100, 62-77.

[7] Seigneur, Lagneau, Corvisier & Dauzères (2018). Advances in Water Resources 122, 355-366.

How to cite: Seigneur, N. and Mayer, K. U.: A compositional formulation for multiphase multicomponent reactive transport modelling of highly coupled systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5615, https://doi.org/10.5194/egusphere-egu2020-5615, 2020.

Transport behaviors of contaminants through a heterogeneous formation consisting multiple layers are complicated because of the different physical and chemical properties for each individual layer. Few analytical solutions for single-species contaminant transport in a multi-layer heterogeneous formation have been reported in the literature. Some contaminants of concern such as radionuclide, nitrogen and chlorinated solvents can decay or degrade to form new successor products during their transport processes, thus making migration of these contaminants much complicated. Clearly, analytical models for multispecies transport coupled by a series of decay reactions in a multi-layer formation are useful tools for synchronous determination of the fate and transport of the predecessor and successor species of decaying or degradable contaminants. This study attempts to develop an analytical model for the multispecies reactive transport of degradable or decaying contaminants through a multi-layer heterogeneous formation. The derived analytical model is shown to be correct and accurate as the consistent results of comparisons between the derived analytical model and the numerical model. The developed analytical model will provide a more reliable predicting tool for real world application.

How to cite: Chen, J.-S., Liang, C.-P., and Chang, C.-H.: An analytical model for multispecies reactive transport through a heterogeneous formation consisting multiple layers of differing physical and chemical properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2615, https://doi.org/10.5194/egusphere-egu2020-2615, 2020.

Analytical solutions to a set of simultaneous multispecies advective-dispersive transport equations sequentially coupled with first-order decay reactions have been widely used to describe the movements of decaying or degradable contaminants such as chlorinated solvents, nitrogens and pesticides in the subsurface. This study presents an exact analytical solutions for three-dimensional coupled multispecies transport in a semi-finite domain. The analytical model are derived for both the first-type and third-type inlet boundary conditions. A method of consecutive applications of three integral transformation techniques in combination with sequential substitutions is adopted to derive the analytical solutions to the governing equation system. The developed analytical model is robustly verified with a chlorinated solvent transport problem. It is applied to investigate the effect of inlet-boundary conditions on the multispecies plume migration and the model could be a very efficient tool that can be used to simulate the degradable contaminant sites.

請在此處插入您的抽象HTML。

How to cite: Liao, Z.-Y. and Chen, J.-S.: Exact analytical solutions for three-dimensional multispecies advective-dispersive transport equations sequentially coupled with first-order decay reactions in a semi-infinite domain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3590, https://doi.org/10.5194/egusphere-egu2020-3590, 2020.

The aim of the project is to develop and test the short-lived radionuclides in order to describe the contaminant transport processes radionuclides, tracer metals and nanoparticles in the environment. Furthermore, the aim is also to develop on-line detection methods to quantify the processes that influence their movement towards the biosphere. Use of short lived radionuclide in tracer tests brings an advantage of excellent detection and avoids contamination of rock samples/environment during experiments.

The research is focused predominantly on radio-tracers in various forms (solute/nanoparticles) and on development of advanced detection techniques for their monitoring and display. The following pre-selected radionuclides were considered for potential  irradiation (²⁴Na, ⁴²K, ⁶⁴Cu, ⁷²Ga, ⁷⁶As, ⁸²Br, ⁹⁹Mo, ¹⁴⁰La, ¹⁴²Pr, ¹⁹⁸Au, ¹⁶⁶Ho, ¹⁸⁸Re, ¹⁵³Sm). After thorough evaluation, holmium and rhenium compounds were selected for irradiation in the LVR- 15 reactor (CVŘ Řež), namely holmium oxide (Ho₂O₃) and ammonium perphenate (NH₄ReO₄). Those compounds were selected based on the computational analyses. Solutions of 50, 200, 300 MBq (¹⁸⁸Re) and 300 MBq (¹⁶⁶Ho) were finally prepared for detection tests. Paralelly, a method for the preparation of chromium oxide nanoparticles was introduced and tested.

A miniaturized spectral camera MiniPIX TPX3 has been developed for radionuclide detection. It is similar to the MiniPIX with a Timepix3 chip, a new generation of chips developed by the collaboration Medipix3. The camera has a resolution of 256 x 256 pixels with a pixel size of 55 x 55 µm (2 mm CdTe sensor).

The developed measurement system enables on-line monitoring and 3D visualization of the radioactivity distribution in the studied rock samples with respect to radionuclide distribution within the rock. Various measurement configurations were tested with respect to source activity, detector/collimator distance, and rock thickness to find optimal measurement parameters.

• The work described herein was funded by the project of the Ministry of Industry and Trade in the TRIO program (FV30430)

How to cite: Zuna, M., Dobrev, D., Havlová, V., Kůs, P., Doubravová, D., and Parma, P.: Determination of rock transport parameters using short-term radionuclides and nanoparticles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18967, https://doi.org/10.5194/egusphere-egu2020-18967, 2020.

Migration of contaminants (radionuclides, heavy metals, nanoparticles) in crystalline rock environment is driven mainly by advective processes in fractures. The main goal of our project is to develop tools for evaluation of migration and retention of potential contaminants in the rock environment. Since the naturally fractured environment is typically too complex to describe, it is common to mimic its behaviour by means of numerically simulated fracture network. The groundwork for applicable simulation of large-scale structures comes out from comprehension and verification of parameters for basic components such as a single fracture. For this reason, number of numerical simulations were performed to evaluate hydraulic and transport properties of an artificial and natural single fracture system by means of different modelling approaches. This will be presented in details in a separate conference contribution by Hokr et al.

Two granite blocks were split and reassembled to generate physical models with artificial fractures. Significant contribution to the exact model representation of the flow regime is the precise fracture topography description, derived from the method of the laser scanning. This allows the model resolution up to 100 µm for each of the two granite blocks used in the study and subsequently the identification of the preferential pathways of the contaminant spreading. Both blocks were customized for both on-line measurement of the selected parameters and sample collection for off-line measurement. This arrangement allowed us to perform series of migration experiments with different conservative (NaCl, KCl, KI, HTO) and sorbing (Pb(ClO₄)₂) tracers. The focus of the numerical modelling effort is to fully describe the hydraulic and transport properties of the fractured granite environment based on the data from experimental tracer tests. Pressure field distribution across the fracture and breakthrough curves at the sampled positions were used for the fracture parameters calibration and evaluation of the model overall reliability.

Several physical models with natural fractures were prepared from suitable sections of borehole cores coming from two locations in the Czech Republic (underground research center Bukov and Mrákotín quarry). Data from transmissivity measurements and conservative tracer breakthrough curves served as initial parameters for fracture description. Specially designed experimental set-up for conducting of migration experiments with very low flow rate was applied. Moreover in collaboration with HZDR (Leipzig, Germany) the unique combination of PET – µCT techniques was employed. Spatiotemporal images of the radioactive tracer (¹⁸F) concentration during conservative transport were recorded with positron emission tomography (GeoPET), and the underlying fracture structure was characterized by µCT-imaging. First results are proving the existence of preferential migration pathways within the studied natural fractures.

The activities were funded by Czech Technological Agency under Project No. TH02030543

How to cite: Jankovský, F., Havlová, V., Zuna, M., Polívka, P., Jankovec, J., Hokr, M., and Kulenkampff, J.: Development of tools for studying contaminant transport in fractured rock environment: laboratory migration experiments in physical models with artificial and natural fractures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19071, https://doi.org/10.5194/egusphere-egu2020-19071, 2020.

We present inverse modelling results of a laboratory hydraulic and tracer experiment in a single fracture in a granite block. The full set of experiments, artificial and natural fracture examples, and various tracers used, are described in details in the separate conference contribution by Jankovsky et al. Here we focus on two blocks 80 x 50 x 40 cm, split by an artificial fracture and conservative tracers. Several test realisations include a choice of different in/out holes and a use of the in-plane sensor grid (boreholes) either for pressure sensors or for tracer (conductivity) sensors. The measurement is consistent across the tests, although there are some anomalies.

The model of artificial fracture is based on fracture geometry obtained by laser scanning, providing (x,y,z) point cloud in 0.1 mm resolution. The two surfaces are scanned separately and then the coordinate systems connected from the scan of the completed block. The aperture is determined with uncertainty in the mutual movement of the surfaces, so various parameterization of its correction is included as part of the inverse modelling.

The solved problem is 2D with spatially variable parameters (element-wise). The Darcy flow is calculated with the transmissivity obtained by the cubic law. The transport model is a standard case of advection and hydrodynamic dispersion. The dispersivity parameters are meant as representing the dispersion in a smaller scale then the aperture field variations captured by the laser scanning data. This is also subject of the inverse modelling (optimization).

The flow and transport simulations within the inverse modelling are made by Flow123d, the in-house open-source code of the Technical University of Liberec. The inverse solver UCODE (freeware of the US Geological Survey) uses a gradient based method with parameter perturbation sensitivity evaluation. Other simulations are made with MODFLOW/MT3D and FEFLOW. Some of the differences are analysed and explained as numerical effects depending on discretisation.

Hydraulic and transport aperture could be independently determined either from the flow rate and pressure data or from the tracer breakthrough. Each block had different correction of the relative position of the surfaces and different hydraulic resistance. This can be caused by small surface irregularities or loose grains, not captured by the laser scan, hindering the perfect contact of the surfaces.

The activities were funded by Czech Technological Agency under Project No. TH02030543

How to cite: Hokr, M., Balvín, A., Jankovec, J., Grecká, M., and Jankovský, F.: Inverse model of single-fracture hydraulic and tracer experiments including a laser scanning data correction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21395, https://doi.org/10.5194/egusphere-egu2020-21395, 2020.

Agricultural fields are facing the problem of salinity, which is the key reason behind the decreased productivity of crop plants and reduced fertility of the soils. The ditch drainage system is extensively used for the elimination of salts present in agricultural fields. In this study, an analytical solution is developed for a ponded surface agricultural field with a fully penetrating ditch drainage system in presence of an additional sink in the study domain to improve the efficiency of saline water extraction. The considered study domain is taken of confined finite extent having homogenous and isotropic nature. The obtained analytical solution is compared well with a numerical model for a similar study domain. The analytical model is also validated for an existing analytical solution with no sink.  Results show that the travel time of water molecules containing salt concentration reduced drastically due to the presence of sink in the middle of the porous domain. The path line of saline water started deviating from the original position represents that the sink has a strong impact on discharge from the side drains. Therefore, the efficiency of the ditch drainage system is increased significantly with the influence of sink in the flow domain. The proposed study is expected to help in the understanding of solute transport flow dynamics in the ditch drainage system with the influence of source/sink in real field conditions. The analytical solution may also be useful in testing and comparing the numerical codes generated for such types of flow scenarios in the subsurface.

How to cite: Tiwari, S. and Yadav, B. K.: A Two-Dimensional Analytical Solution for Remediation of Salt Affected Site through Ditch Drainage System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1430, https://doi.org/10.5194/egusphere-egu2020-1430, 2020.

Velum^® is a novel contact nematicide with Fluopyram (FL) as active ingredient. Knowledge on its adsorption and transport characteristics is essential for both agricultural and environmental considerations. The main objective of this study was to quantify the transport characteristics of FL in a sandy soil from a non-cultivated area in Arava region, Southern Israel, with a special focus on the behavior in soil after drip application. In this regard, soil column transport experiments under saturated water flow conditions were conducted. In addition to FL, the transport experiments were performed with a bromide tracer. Four factors were examined: (i) pulse concentration, (ii) water flux (ii) pulse size and (iv) interrupted flow. Equilibrium adsorption isotherms were measured by batch experiments. The established breakthrough curves (BTCs) were analyzed with the convection–dispersion equation (CDE) in its chemical equilibrium and non-equilibrium forms. In addition, the validity of a two-site kinetics model was evaluated. All models were examined with and without a term, assuming irreversible sorption. The bromide BTCs were adequately fitted by analytical solutions of the equilibrium CDE using the CXTFIT code, suggesting that physical equilibrium is prevailing. The FL BTCs were fitted with two-site sorption and two kinetic sites models using HYDRUS-1D code. The experimental mass balance analysis demonstrated that the bromide mass was fully recovered, while only part of total FL applied was recovered, in particular, at low flow rate. The comparison between non-interrupted and interrupted water flow demonstrated that at a given flow rate, during the pulse input, the two BTCs are identical. However, following the flow interruption (60 hours), when the flow resumed, a sharp decrease could be observed in FL concentration. Thereafter, the two BTCs are re-converged, exhibiting similar desorption behavior. Possible explanations for FL transport characteristics seems to be low kinetics desorption and/or irreversible adsorption. Additional quantitative insights from the numerical analysis will be presented and discussed based on the goodness of fit and optimized parameters of each model.   

How to cite: Vasconcelos Barroca, M., Arye, G., and Ronen, Z.: Flow rate dependent transport of Fluopyram in saturated sandy soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22450, https://doi.org/10.5194/egusphere-egu2020-22450, 2020.

Many previous studies showed how the single well push-pull (SWPP) tracer test is a widely used in situ approach to define the aquifer characteristics. The SWPP test usually consists in two main phases: during the first one, the push phase, a tracer solution is injected into the aquifer through a single well; during the second one, the pull phase, the flow is inverted and the solution is extracted from the same well. The solute movement through the aquifer is driven by different phenomena advection dispersion mixing and dilution.By the analysis breakthrough curves (BTCs) obtained in the pull phase is possible to estimate significant transport parameters such as the dispersion and sorption coefficients. The  more common approaches for the BTC interpretation assume the aquifer as homogeneous.

We propose a semi-analytical physically based Lagrangian procedure for the SWPP analysis, mimicking the transport processes taking place in heterogeneous aquifer by a particle tracking procedure. We consider the well fully penetrate a stratified aquifer unbounded laterally. The isotropic log-hydraulic conductivity is normally distributed  log K=Y∈ N(0,σ_Y²) with given vertical integral scale I_Y,V and unbounded horizontal integral scale I_Y,H= ∞. The flow field is assumed to be steady state in both phases. The advective transport is driven by the local flow velocities different for each layer; the pore scale dynamics are modelled as Wiener process. Our procedure can be applied to a wide range of heterogeneity degrees Peclet numbers and test duration; the results emphasise how by different test set up it is possible to get different aquifer parameters: for instance a short test duration allows the estimate of the pore-scale dispersion while for longer test duration the solute experiences more formations emphasizing the  effects of the macrodispersion. Finally despite its simplicity our procedure is a useful tool for the SWPP interpretation in heterogeneous aquifers.

How to cite: Maggi, M. R., Di Gialleonardo, A., Perrotta, L., Petrella, G., Sperati, F., and Zarlenga, A.: A physically based Lagrangian procedure for the push-pull test analysis in heterogeneous aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4947, https://doi.org/10.5194/egusphere-egu2020-4947, 2020.

We derive an upscaled model for the prediction of the plume evolution in highly heterogeneous aquifers based a stochastic transport representation in terms of continuous time random walks. Transport is modeled through advective motion of idealized solute particles, which changes their speed at fixed distances. The series of particles speeds is modeled as a stationary Markov chain. The derived model is parameterized by the correlation length, mean and variance of the log-hydraulic conductivity, the mean hydraulic gradient and porosity. Furthermore, it can be conditioned on the conductivity and tracer data at the injection region. The model predicts the non-Fickian evolution of the longitudinal concentration profile observed during the MADE-1 experiment. The mass distribution is characterized by strong localization at the injection region and a strong forward tail. These features are explained by conductivity heterogeneity at the injection region, and the correlated motion of particles according to spatially persistent Eulerian flow speeds. 

How to cite: Comolli, A., Hakoun, V., and Dentz, M.: Dispersion upscaling in highly heterogeneous aquifers: The prediction of tracer dispersion at the Macrodispersion Experiment (MADE) site, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9640, https://doi.org/10.5194/egusphere-egu2020-9640, 2020.

Injection of grout material is widely used to create a temporary flow barrier at construction sites in the Netherlands. We investigate the long-term erosion behavior of a grout layer by means of semi-analytical expressions for groundwater flow and transport.

A typical grout injection contains sodium-meta silicate, water and solidifier forming a temporarily impermeable ‘waterglass’. The combination of a waterglass layer and vertical walls allow for dry excavations below the groundwater table. After construction is finished, the waterglass remains in the subsurface and erodes over time. A question concerning the potential risk to groundwater quality remains: How high is the concentration of dissolved waterglass in the groundwater leaving the site?

Numerical simulations allow to describe the flow and transport for site specific conditions. However, it’s missing an analytical expression to predict the transport behavior for arbitrary settings. We approximate the erosion behavior by a set of semi-analytical equations. The challenge here is the change in permeability of the waterglass layer from almost impermeable to fully permeable. We define a dilution ratio relating the flux into the construction site to the flux through the layer as a measure of dissolved waterglass concentration leaving the site. We also determine the impact of design parameters such as construction site aspect ratio, depth of the waterglass layer and its thickness. We checked our results against numerical simulations for a range of parameter settings. Preliminary results show that erosion is initially slow and accelerates until the temporary injection layer is completely gone.

How to cite: Zech, A., Dekker, J., and Sweijen, T.: What happens below construction pits? - The long-term erosion of temporary barriers to groundwater flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10417, https://doi.org/10.5194/egusphere-egu2020-10417, 2020.

The excessive use of fertilizers in conjunction with agricultural tile drainage can lead to high levels of nitrate (NO₃) in subsurface waters, which can pollute rivers, eventually causing eutrophication of larger water bodies. Among several water treatment alternatives, biological denitrification in edge-of-field woodchip bioreactors represents a popular option, given its relatively easy implementation, minimal maintenance and low cost. Since microbial denitrification is commonly catalyzed by facultative heterotrophic bacteria in anoxic environments, oxygen concentration is a key parameter to keep in consideration. To monitor dissolved oxygen concentration inside the test bioreactor in a non-invasive manner, oxygen optode sensors are used. Bioreactor performance also depends on several other factors, including hydraulic residence time, influent nitrate concentrations, and woodchips flow and transport parameters. One particular aspect that has been overlooked in the literature is how gas generation due to bacterial respiration can hinder the water flow through the bioreactor, hence reducing its effective porosity and denitrification performance. A continuous monitoring of inlet and outlet flow rates is applied to detect flow fluctuations during the experiments. Given the uncertainty and challenges associated with field observations, meter-scale experiments conducted under controlled boundary conditions and known porous media distribution represent a convenient method for studying the influence of water quality and hydraulic parameters on nitrate removal. The aim of this work is to characterize the bioreactor performance using woodchips with different particle sizes and to investigate the effect of retention time and inlet NO₃ concentration on NO₃ removal under variable flow conditions. For this purpose, a series of experiments are conducted in a 147.7 × 10 × 38.5 cm³ flume under well-controlled laboratory conditions.

How to cite: Trevisan, L.: Experimental analysis at the meter scale of denitrifying woodchips bioreactor performance under variable loading conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17486, https://doi.org/10.5194/egusphere-egu2020-17486, 2020.

The Permo-Triassic Sandstone aquifers of the Eden Valley, Cumbria UK, are a key water resource for public water supply in NW England as well as local agriculture and industries. Permo-Triassic Sandstone aquifers are characterised as having large storage capacities and moderate transmissivities, however, in the Eden Valley these characteristics vary greatly on a range of scales i.e. granulation seams (deformation bands) that are millimetres thick but have been shown to extend for hundreds of metres on analogous sandstones; silicified layers that are several metres thick and extending 10s to 100s of metres laterally; and lithological variation and faulting have been shown to juxtapose hydrogeological units with different hydraulic properties. Complex heterogeneous superficial deposits overlay 75% of the Permo-Triassic Sandstone aquifers and comprise glacial till, glacio-fluvial outwash deposits, river terrace deposits and alluvium. The lateral and vertical continuity of these superficial deposits is highly uncertain.

The complex geological and superficial deposits in the Eden Valley impose a control on flow processes and impact sub-surface runoff. Specifically, lenses of high conductivity sands and gravels within low conductivity clay till deposits coupled with the presence of low conductivity strata at ground level suggests that indirect recharge is an important sub-surface runoff component. Therefore, the magnitude and location of recharge to the Permo-Triassic Sandstone aquifers is highly uncertain. Published recharge estimates rely on baseflow separation techniques and thus do not distinguish between indirect and direct recharge. This highlights the uncertainty regarding the sub-surface flow processes active in the Eden Valley.

A methodology for characterizing the surface water – groundwater interaction spatially and temporally in an ungauged upland sub-catchment is presented.

A non-invasive approach has been implemented to investigate the relationship between the surface water and groundwater systems in the Eden Valley. This involved the design and installation of low-cost ultrasonic sensors that measure stream stage. The sensors have been installed at key locations within sub-catchments that incorporate limestone pavements, geological contacts and along fault trends in the headwaters of the Eden Valley. Flow gauging has been conducted along the reach of these streams to investigate the spatial variation in discharge. Data from the low-cost sensors and flow gauging have been used to estimate the magnitude of volumetric water exchange between the surface water and groundwater systems, as well as characterise this relationship spatially and temporally.

The thickness and composition of the superficial deposits along these stream reaches will be investigated via passive seismic survey. The superficial investigation and the volumetric water balance will be used to estimate indirect recharge in the upper Eden catchment. The results of which will be compared to localised recharge estimates calculated from groundwater level timeseries. This comparison will indicate the importance of indirect recharge within sub-surface runoff processes.

This ongoing research is a vital step in quantifying the relationship between the surface water and groundwater systems in a complex upland catchment. A knowledge of the active sub-surface runoff processes highlighted are key for reliably assessing the long-term security of groundwater resources in the Eden Valley.

How to cite: Colyer, A., Butler, A., Peach, D., and Hughes, A.: Characterising the role of heterogeneity on surface water-groundwater interaction in the Permo-Triassic Sandstone aquifers of the Eden Valley, NW England, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11755, https://doi.org/10.5194/egusphere-egu2020-11755, 2020.

In natural streams, vegetation considerably has an influence on the flow characteristics in a variety of ways. For example, vegetation distorts flow structure in both lateral and vertical directions and changes the magnitude of turbulence and shear flow. Due to these effects, diluted contaminants in river transport and disperse differently. Accordingly, many previous researchers have investigated the impact of vegetation on the mixing process. Most of them have estimated the dispersion coefficient since this is the crucial parameter to quantify the degree of dispersion of contaminants numerically. They mainly studied in diverse characteristics of vegetation, such as density or submergence, etc., and identified the change in hydraulic parameters involving the dispersion coefficient.

In this work, considering the vegetation distributed in various forms in the natural river, we studied the effect of vegetation patterns on the longitudinal mixing coefficient. Six types of spatial patterns considered in this study are represented numerically by introducing the standardized Morisita index. Laboratory experiments with artificial emergent vegetation were performed in multiple vegetation patterns, and the longitudinal dispersion coefficient was estimated from the measured concentration curves by applying the routing technique. And we analyzed the cause of change in dispersion coefficient by calculating not only the dispersion coefficient but also the magnitude of mean velocity, shear flow, turbulence, etc.

According to the experimental results, the mean velocity in the vegetated channel is almost the same regardless of the type of pattern but is always lower than that in the non-vegetated channel. The longitudinal dispersion coefficient gets larger as the arrangement changes from uniform to 2D clumped pattern. The cause of change in coefficient is closely related to the spatial velocity gradients in both lateral and vertical directions since the spatial heterogeneity of velocity increases the magnitude of shear flow.

How to cite: Park, H. and Hwang, J.: Experimental study on longitudinal mixing in open channel flow with various vegetation patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6660, https://doi.org/10.5194/egusphere-egu2020-6660, 2020.

Trenched hilsllope studies are logistically challenging but have provided valuable information on hillslope hydrological processes. For example, they have shown that subsurface stormflow can respond very quickly to rainfall and that subsurface storm flow often varies in a non-linear and threshold-like way with total rainfall or antecedent conditions. They have also highlighted the high spatial variability in subsurface stormflow due to surface or bedrock topography or spatial variability in soil and bedrock characteristics. However, still less is known about mixing and flow velocities along hillslopes.

Here we present the initial results of a tracer test at the Panola trenched hillslope in Georgia, USA. We applied chloride to the surface of the lower half of the hillslope and bromide as a line source. We measured the concentrations in subsurface flow at 2-m sections of the trench face and for two macropores during a five-month period that included two large rainfall events that caused subsurface flow, and several sprinkling experiments on parts of the hillslope. We used 20 lysimeter pairs and more than 50 wells and piezometers across the hillslope to track the transport of the tracer through the soil to the trench. The results highlight the variability in flow pathways, the considerable difference between celerity and velocity, as well as the fast tracer transport through the weathered bedrock

How to cite: van Meerveld, I. and Seibert, J.: A look inside the Panola trenched hillslope - initial results of a tracer test , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11675, https://doi.org/10.5194/egusphere-egu2020-11675, 2020.

Due to their prolonged water availability wetlands are of increasing importance for small scale agriculture in East Africa. In the inundating landscape of central Uganda, inland valley wetlands are a common landscape unit with high potential for crop cultivation year-round. Yet little is known about the hydrological processes which bring out these favourable conditions. This study focusses on the relevance of interflow processes from the slopes into the wetland regarding water and nutrient delivery from different land use types. Hereby special attention is given to water pathways at the transition from upland geology to valley sediments and to nutrient relocation along the slopes.

Electrical Resistivity Tomography (ERT) was used as a non-invasive method to characterise interflow pathways in the highly variable saprolite geology and for subsurface delineation of the valley sediments. The measurements were complimented by a drilling campaign and infiltration experiments in different depths. Interflow collection pits were installed at the slope toe in order to quantify water and nutrient fluxes towards the wetland during two consecutive years. Additionally, soil moisture and nitrate content in the soil water were quantified at various positions along the slope.

ERT-imaging supports the hypothesis of a separation between a confined shallow aquifer and the soil water in the wetland sediments. Drilling results and hydrogeochemical analysis of the interflow and this shallow groundwater indicate a connection of the two components via macropores in the upper saprolite at the slope toe. At the same time interflow is transferred to the soil water of the wetland via a sandy loam layer which is found on top of the confining clay-loam layer of the wetland sediments. Both processes are active even during the dry season and therefore water from the interflow is relevant for water storage (shallow aquifer) and agricultural production (soil water) in the wetland.

Interflow volume and nitrate content both show a fast reaction to rainfall events, while the amount of water and nutrients delivered to the wetland is related to the land-use on the slope. Nitrate content in the soil water on the slopes suggests a relocation of nutrients in the upper soil horizons towards the slope toe. As infiltration capacity of the soil’s A-horizon is higher compared to the B-horizon a second  lateral flow component appears to be present close to the soil surface.

The results of this study emphasize the relevance of subsurface flow for wetland hydrology and give first explanations of wetland-upland connectivity in a complex saprolite geology.

How to cite: Schepp, C., Diekkrüger, B., and Becker, M.: Linking slopes to the wetland: The relevance of interflow processes for water and nutrient input to an inland valley wetland in Uganda, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20214, https://doi.org/10.5194/egusphere-egu2020-20214, 2020.

The Mosel wine region (Rhineland-Palatinate, Germany) is the largest steep vineyard region in the world. Due to extreme slopes (>17°), tillage with heavy machinery, increase in extreme precipitation events and new planting of vines, these vineyards are among the agricultural systems most affected by soil erosion.

Due to viticulture since the Roman period and their special characteristics, almost all vineyard soils in the Mosel region are classified as Terric Anthrosols. Soils are characterized by a very high rock fragment content (schists and fluvial sediments) and a loose surface layer over a compact layer due to tillage or weathered parent material. This structure enables subsurface flows between these two layers, especially in periods of very high soil moisture.

There is a knowledge gap in the identification and quantification of transported soil particles in this subsurface flow. If these soil particles reach relevant amounts, superficial protective measures may be partially ineffective and the soil degrades despite the existing protection. In consequence, there is a need to develop a method to determine this subsurface particle transport in situ.

Here, we present a first experimental approach for assessing the occurrence of sub surface erosion of fine-grained soil particles within soils. With this, it is possible to prove this process and the development of a sediment trap prototype, based on a drainage pipe, for in situ measurements of subsurface soil erosion.

How to cite: Iserloh, T., Seeger, M., and Ries, J. B.: Experimental detection of subsurface particle transport in coarse steep vineyard soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5262, https://doi.org/10.5194/egusphere-egu2020-5262, 2020.

In the past years, it has been proposed that stream networks can accumulate genetic material over a given area. Accordingly, a sample of environmental DNA (eDNA) from streamflow at the outlet of a catchment can be used as an indicator of the upstream biodiversity. eDNA’s use in ecological studies is becoming more and more common and it seems reasonable to assume that eDNA might also offer a powerful tool as a hydrologic tracer. However, the original ecological proposition largely simplifies the complexity of any seasonal, diurnal, or spatial variation according to hydrologic flow paths and processes. From a hydrological perspective, this shortcoming is particularly problematic in Alpine headwater catchments, where the combination of snowmelt-dominated summer flow and particularly high climatic and geomorphologic heterogeneity results in hydrologic flow paths that are especially dynamic in space and time. 

We were interested to see if on one hand, eDNA could teach us something new about hydrologic (subsurface) flow paths, and on the other hand, if biodiversity assessment should consider hydrologic variation in detail. To do so, we sampled natural occurring eDNA at 11 points distributed over the 13.4 km², intensively monitored Vallon de Nant (1189-3051 m. a.s.l., Switzerland) between March and September 2017. We chose points corresponding to three different potential microhabitats and flow regimes (main channel, tributary, and spring) likely both inhabited by characteristic organismal communities and of interest for identifying hydrologic flow paths. We found that at moments when streamflow was increasing rapidly, biological richness in upstream points in the main channel and in tributaries was highest contrary to springs, where richness was higher when electrical conductivity was highest.  Thus, the main conclusion from our work is that elevated richness corresponds to moments in time when multiple mechanisms transport additional, probably terrestrial, DNA into water storage or flow compartments. These mechanisms could include overbank flow, stream network expansion, and hyporheic exchange. Our data demonstrates that biodiversity assessments using eDNA do need to consider hydrologic processes and shows that there is a potential future for eDNA among hydrologic tracers.  We will give recommendations in this talk about how to sample eDNA to answer hydrologic questions.

How to cite: Maechler, E., Ceperley, N., Salyani, A., Walser, J.-C., Larsen, A., Schaefli, B., and Altermatt, F.: The variation in genetic material of a high Alpine catchment reveals (sub)surface exchange, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9602, https://doi.org/10.5194/egusphere-egu2020-9602, 2020.

This research deals with the hydrological function of Peat Bog in a catchment where Peat Bog (formed by Histosol or other hydromorphic soils) covers a part of the area (40-60%). In this study, two soil types, creating two main hillslopes of the experimental catchment, form the dominant soil types (Podzol and Histosol) in the Šumava Mountains, Czechia. A modified HBV model was used for the estimation of the contribution of each soil type to common outflow and for the estimation of the water balance. According to previous research and field observations, dominant hydrological processes were described for each hillslope (soil). The model confirmed previous results concerning dominant preferential flows at Peat Bog hillslope and Podzol hillslope; moreover, it quantified a ratio between fast and slow flow in soils. At Peat Bog hillslope, the majority of outflow (67%) was formed from the upper soil layer (Acrotelm). In the mineral soil hillslope, a larger portion of runoff was generated from the lower soil layers or bedrock interface (61%). Peat Bog contributes to a stream mainly during rainfall events; however, the model showed also significant deep percolation at the Peat Bog hillslope and considerable contribution to baseflow during a year. Generally, more precipitation water was turned into runoff at the Peat Bog hillslope by the model, which was indicated by a lower rate of actual evapotranspiration (21% of precipitation), compared to 29% in the case of Podzol hillslope. If we consider land use changes in this locality in terms of expanding or reducing peat areas (draining, drains damming, droughts, etc.), this model could sufficiently estimate hydrological behaviour of local streams and thus, can be potentially used in hydrological planning by local authorities.

How to cite: Vlček, L., Šípek, V., Kofroňová, J., Kocum, J., Doležal, T., and Janský, B.: Understanding runoff formation in a basin with Peat Bog and Podzol hillslopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20264, https://doi.org/10.5194/egusphere-egu2020-20264, 2020.