HS8.1.4

Recent works have shown that laminar flows through porous media generate Lagrangian chaos at pore scale, with strong implications for a range of transport, reactive, and biological processes in the subsurface. The characterization and understanding of mixing dynamics in these opaque environments remains an outstanding challenge. We present a novel experimental technique based upon high-resolution imaging of the scalar signature produced by push-pull flows through various porous materials (beads, gravels, sandstones) at high Péclet number. We show that this method provides a direct image (see below) of the invariant unstable manifold of the chaotic flow, while allowing a precise quantification of the incompleteness of mixing at pore scale. In the limit of large Péclet numbers, we demonstrate that the decay rate of the scalar variance is directly related to the Lyapunov exponent of the chaotic flow. Thus, this new push-pull method has the potential to provide a complete characterization of chaotic mixing dynamics in a large class of opaque porous materials.

How to cite: Heyman, J., Lester, D. R., and Le Borgne, T.: Push-pull flows reveal the scalar signature of chaos in porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-665, https://doi.org/10.5194/egusphere-egu21-665, 2021.

Solute mixing mediated by flow in porous media plays a significant role in controlling reaction rates in subsurface environments. In many practical cases, incomplete mixing—inhomogeneous solute concentrations—occurs at the pore-scale, limiting local and thus upscaled reaction rates, and renders their prediction based on effective dispersion coefficients derived from dispersion models (or by assuming Taylor-Aris dispersion) inaccurate. We perform solute transport experiments in transparent, quasi-two-dimensional, soil analog models to investigate the relationships between pore-scale solute dispersion and mixing under different flow conditions. We use Fluorescein as a conservative tracer and record its fluorescence intensity in monochrome images at fixed time intervals. We convert the fluorescence intensity to solute concentration fields based on a calibration curve obtained with various homogeneous solute concentrations and subsequently compute concentration gradients. Our images provide evidence for incomplete mixing at the pore-scale and show strong gradients transverse to the overall flow direction. We fit the mean longitudinal concentration profile to an analytical solution of the advection-dispersion equation and compute the effective longitudinal dispersion coefficient. Based on the lamellar mixing theory, we also infer an effective diffusion coefficient relevant to the mean concentration gradient’s dynamics. By comparing these two diffusion/dispersion coefficients in saturated flow conditions, we show that while their values are similar at low Péclet, their scaling behaviors as a function of Péclet are different. Hence, as pointed out by several previous studies, modeling reactive transport processes requires accounting for a mixing behavior driven by a diffusive process that cannot entirely be described by the solute dispersion coefficient. We extend this work by varying the saturation degree in the experiments and our samples' structural heterogeneity to investigate how flow desaturation and porous medium structure impact solute mixing.

How to cite: Borgman, O., Régis, T., Géraud, B., Tanguy, L. B., and Yves, M.: Impact of flow conditions on pore-scale solute mixing: experiments in heterogeneous 2D porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8636, https://doi.org/10.5194/egusphere-egu21-8636, 2021.

Due to the increasing challenges to preserve water quality and supply at global scale, groundwater flow modeling has become a tool of pivotal relevance for remediation, implementation of policies, and design of applications for recharge management. The strain towards faster and more reliable hydrogeological simulations triggered the development of upscaled and multi-scale approaches employing different diffusion and dispersion models that are still the object of much debate in the community. Our ongoing study focuses on the up scaling of solute transport through heterogeneous geological domains by means of an extensive three-dimensional simulation study, based on a new open-source C++ library, built on top of the well-know finite-volume library OpenFOAM®. We integrate the whole workflow, from geostatistical random field generators to flow and transport solvers with integrated post-processing capabilities. The robustness, scalability and flexibility of the library makes it suitable framework for the development, testing, and application of upscaling techniques.  

Being the subsurface inaccessible by nature, the appeal to geostatistical techniques is a well-established approach to construct a realistic domain for flow and transport simulations. However,  additional challenges are posed by the numerical simulation of highly heterogeneous materials. Indeed, the problem is twofold: on one side it is not always possible to characterize the heterogeneity in a deterministic way, while on the other side numerical methods which are effective for elliptic and parabolic equations solved over homogeneous domains might suffer in heterogeneous media. Both challenges were effectively tackled using the open-soruce library OpenFOAM whose implementation and capabilities will be illustrated. Preliminary results on flow and transport simulations performed on truncated pluri-Gaussian permeability fields will be shown and the influence of geostatistical metrics (e.g. correlation lengths, variance, geological entropy) on the flow and transport results (e.g. average velocity and breakthrough curves) analysed.

Extensions to variable-density, mobile-immobile, and multi-rate mass transfer formulations are also presented in the context of the EU project “SECURe”.

How to cite: Pescimoro, E., Icardi, M., Municchi, F., and Bianchi, M.: An open-source integrated solution for flow and transport in random heterogeneous porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2290, https://doi.org/10.5194/egusphere-egu21-2290, 2021.

We present a new method to estimate the displacement covariance and macrodispersivity for solute transport in bounded formations. Here we use circulant embedding, which is based on the fast Fourier transform and is much more efficient than eigen-decomposition for the factorization of random spatial fields. We compute the displacement covariances using the analysis of variance approach and introduce an interpolation process to significantly reduce the number of forward simulations. Once the effect of each eigenvector on the displacement covariance is obtained, it is unnecessary to rerun the simulator for different spatial covariance functions or anisotropy ratios, which saves a large amount of computational effort. The proposed method is validated in various tests in two-dimensional and three-dimensional examples and accurately matches the results from the Monte Carlo simulation. It is found that the longitudinal dispersivity is not sensitive to the boundaries, while the transverse and vertical dispersivities are greatly affected. The method is applied to the Borden site and provides a better explanation of the observed data after considering the effect of vertical boundaries. These results show that our method could serve as a promising tool for studying and predicting the characteristics of solute transport in heterogeneous media.

How to cite: Liao, Q., Lei, G., Zhang, D., and Patil, S.: Scale-dependent dispersion for solute transport in bounded formations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1746, https://doi.org/10.5194/egusphere-egu21-1746, 2021.

Characterization of spatially variable aquifer properties is a necessary first step towards modeling flow and transport. An emerging technique in hydraulic tomography, known as diffusivity tests, consist of injecting (or pumping) a volume of water through short segments of a well for a short time and measuring the travel time of the peak of the head signal at different points in the surrounding aquifer volume. In our stochastic model, the specific storage is assumed to be constant, while the hydraulic conductivity of the heterogeneous aquifer is modeled as a random lognormal field. The axi-symmetric anisotropic structure is characterized by a few parameters (logconductivity mean and variance and horizontal and vertical integral scales). The mean and variance of the peak travel time are then determined as a function of distance from an instantaneous source by solving the flow equation using a first-order approximation in the logconductivity variance. The mean travel time is recast in terms of the equivalent conductivity, which decreases from the harmonic mean near the source to the effective conductivity in uniform flow for a sufficiently large distance. Similarly, the variance drops from its maximum near the source to a small value.

A different type of tomographic test is the constant-rate pumping one. We propose to apply the first order stochastic approach to the data from the Boise Hydrogeophysical Research site (BHRS) to characterize the aquifer properties by estimating heterogeneity statistical parameters. Equivalent properties are first calculated by matching a homogeneous aquifer solution to the pointwise data to obtain a spatially varying hydraulic conductivity (K_eq) and storativity (S_s,eq). Then the statistical properties of K and S_s are to be computed by a best fit between the theoretically derived statistical moments of the equivalent random properties (K_eq, S_s,eq) and those from field measurements. Our preliminary results indicate that the proposed stochastic methodology is robust and reliable as well as computationally more efficient than the conventional hydraulic tomography techniques.

How to cite: Cheng, K. B., Dagan, G., and Rabinovich, A.: Stochastic Modeling of Diffusivity and Constant-Rate Pumping Tests in Heterogeneous Aquifers in a Tomographic Setup and Its Application to Field Measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2680, https://doi.org/10.5194/egusphere-egu21-2680, 2021.

Six conceptually different transport models are applied to the MADE-1 field tracer experiement as a first major attempt for model comparison. The objective was to show that complex mass distributions in heterogeneous aquifers can be predicted without calibration of transport parameters - solely making use of structural and flow data.

The models differ in their conceptualization of the heterogeneous aquifer structure, computational complexity, and use of conductivity data obtained from various observation methods (Direct Push Injection Logging - DPIL, Grain Size Analysis, Pumping Tests and Flowmeter). They agree in the underlying physical transport processes, none of them considering mass transfer. Predictive capability is assessed by comparing results to observed longitudinal mass distributions of the MADE-1 experiment. We deal with data uncertainty indicated by decreasing rates of recovered mass by focusing the comparison on measures, such as peak location, position and shape of bulk mass and leading tail, and we do not normalize observation data.

Comparison of models reveals that the predictions of the solute plume agree reasonably well with observations if the models are underlined by a few parameters of close values: mean velocity, a parameter reflecting log-conductivity variability and a horizontal length scale related to conductivity spatial correlation. The robustness of the results implies that conservative transport models with appropriate conductivity upscaling strategies of various observation data provide reasonable predictions of plumes longitudinal mass distribution as long as key features are taken into account.

How to cite: Zech, A., Attinger, S., Bellin, A., Cvetkovic, V., Dagan, G., Dentz, M., Dietrich, P., Fiori, A., and Teutsch, G.: A Comparison of Six Transport Models of the MADE-1 Experiment Implemented with Different Types of Hydraulic Data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9469, https://doi.org/10.5194/egusphere-egu21-9469, 2021.

Open-loop shallow geothermal systems, which exploit shallow aquifers as a heat source or sink, have a great potential to reduce greenhouse gas emissions related to the heating and cooling of buildings. In order to limit the depletion of groundwater resources water is generally reinjected into the same aquifer after the heat exchange, as a consequence a thermal plume develops within the aquifer. Furthermore a share of the reinjected water may come back to the abstraction wells, inducing a progressive thermal alteration of the abstracted water temperature that may even result in the plant failure. This phenomenon, known as thermal recycling, strongly depends on the hydraulic conductivity of the aquifer. The design models commonly adopted in the practice assume a homogeneous domain with constant hydraulic conductivity, this assumption, however, is not realistic: neglecting the natural heterogeneity of hydraulic properties of the porous medium may result in large prediction errors.

In this study, we aim to quantify the impact of the different heat transport dynamics in aquifers on the thermal plume development. A stochastic model, which explicitly considers the spatial variability of the hydrological properties, such as the hydraulic conductivity, is developed for low enthalpy geothermal systems. The thermal breakthrough curve at the extraction well is obtained by applying a Lagrangian model and assuming a steady state velocity field. Relevant quantities of thermal recycling, such as the thermal breakthrough time, are adopted for the evaluation of the effects of the hydrogeological and geometrical parameters of the systems.

The results of our study emphasize how the correct representation of the aquifer heterogeneity is fundamental in the design of shallow geothermal systems and in the correct heat plume assessment.

How to cite: Zarlenga, A., Di Dato, M. I., D'Angelo, C., and Casasso, A.: Effects of the aquifer heterogeneity in heat production from low enthalpy geothermal systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4417, https://doi.org/10.5194/egusphere-egu21-4417, 2021.

Hydro-geochemistry of groundwater plays an important role in understanding the characteristics of a geothermal system. Mixing zones of geothermal deep waters and shallow groundwater can be identified through chemical distribution maps and help identify geothermal flow paths. The flow paths can be used to calculate the chemical values of the geothermal water leading to a characterization of the heat source. In combination with knowledge about regional structures, the geochemical distribution can further reveal unknown geothermal zones.

In the present study, the geochemical distribution of the groundwater is studied from samples collected from shallow and deep wells, with special reference to the regional structures present in the terrain. The study area was selected as a 20 • 20 km area centered around the Padiyathalawa hot spring field in Sri Lanka. From the results, two main geochemical anomalous zones are identified, especially with the increased values of electrical conductivity (EC), total dissolved solids (TDS), and Sulphate distribution maps. Those two zones include the hot spring itself as well as an area in ~10 km distance in the NE direction from the hot spring. Both zones are characterized by crosscutting structures of dolerite dykes and shear zones. Due to the shear zones, there are deep-seated fractures facilitating water flow from deeper layers towards the surface. This uprising water mixes with the shallow groundwater, affecting the general geochemical values of the shallow groundwater system.

Common minerals in Dolerite in Sri Lanka are Pyroxenes, Feldspar, Ilmenite, Magnetite, and Pyrite with minor amounts of other minerals. The increased EC values in both before mentioned zones relate with higher amounts of iron due to dissolution and mixing processes in regions with fractured Dolerite. Similarly, the increased concentration of Sulphates in the groundwater can be related to Pyrite from the fractured Dolerite, as microbial oxidization of Pyrite leads to origin of Sulphates. The increase of TDS can be interpreted as shallow water mixing with deep geothermal water, which contains a higher amount of minerals from the fractured dolerites.

The similar geochemical anomalies in those two zones can be associated with cross-cutting Dolerite dikes and existing faults in the shear zones at greater depth, subsequently mixing uprising deep geothermal water with shallow groundwater. A similar geochemistry and tectonic setting suggest similar flow paths from the underground and therefore also similar geothermal conditions at both spots. However, due to the rural and remote region, only one of the two before mentioned areas is known as a hot spring field. Thermal signatures dissipate much more quickly in the shallow groundwater than the mineral composition and might not be significant for measurement. Geochemical signatures of groundwater can therefore be a substantial help to locate geothermal springs, identify source mechanisms and characterize fluid flow paths.

How to cite: Bandara, D., Heinze, T., Premathilake, M., and Wohnlich, S.: Regional structures influencing the groundwater geochemistry around geothermal springs: A case study from Padiyathalawa, Sri Lanka, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2378, https://doi.org/10.5194/egusphere-egu21-2378, 2021.

Per- and polyfluoroalkyl substances (PFASs) are fluorinated anthropogenic compounds of which numerous have a high thermal stability, high surface tension, water and oil repellence. PFASs are used e.g. as surfactants, in agrochemical products and in fire extinguishing foams. Due to negative effects to the environment and human health some compounds are already declared as Persistent Organic Pollutants (POPs) or Substances of Very High Concern (SVHC). The behaviour of PFASs in environmental matrices can differ dependent on the molecular structure and environmental conditions. Sorption, for example, depends on soil characteristics, compound properties such as the carbon chain length and the functional group. When PFASs are found in soils, the knowledge of factors affecting the leaching behaviour in soil can be helpful to assess plant availability and risk of groundwater contamination.

In some contamination cases, e.g. Rastatt/Baden-Baden, Germany, PFASs precursors such as polyfluoroalkyl phosphate diesters (diPAPs) are present in soil and are transformed into PFAAs (perfluoroalkyl acids), a sub-group of PFASs, which are stable in the environment. To study the behaviour of diPAPs in soil, 8:2 diPAP and 6:2 diPAP were applied to 50 cm soil-filled columns with a concentration of 1 mg per kg soil. For two years, the columns are watered on 3-5 days per week with an average weekly natural rainfall of Baden-Württemberg. The leaching water is collected and analysed every two weeks. After the first year, there was no detection of precursors in the leaching water. The main products were, in accordance with literature, PFPeA (Perfluoropentanoic acid) and PFHxA (Perfluorohexanoic acid) for the precursor 6:2 diPAP and PFHpA (Perfluoroheptanoic acid) and PFOA (Perfluorooctanoic acid) for the precursor 8:2 diPAP. The breakthrough peak of PFPeA (864 µg/l) and PFHxA (487 µg/l) was found in week nine after the start of the experiment, the major peak of PFHpA (124 µg/l) in week 15 and the peak of PFOA (303 µg/l) in week 25. Even though the major breakthrough of PFAAs was over after the first year, all of them are still found in leaching water in relatively constant concentrations: ~40 µg PFPeA/l, ~20 µg PFHxA/l (6:2 diPAP) and ~5 µg PFHpA/l, ~50 µg PFOA/l (8:2 diPAP). Thus, as a preliminary result after the first year, there is a higher mass of PFPeA and PFHxA in the leachate compared to PFHpA and PFOA, which can be led back to higher transformation rates of 6:2 diPAP than 8:2 diPAP.

After two years, the soil columns will be cut horizontally in sections, and the soil will be analysed for precursors and PFAAs in order to calculate a mass balance. This will provide more insight into the transformation and mobilisation behaviour of investigated diPAPs and PFAAs.

How to cite: Weidemann, E., Lämmer, R., Stahl, T., Göckener, B., Bücking, M., Breuer, J., Kowalczyk, J., Just, H., and Gassmann, M.: Soil Column Experiments to Study Leaching and Transformation Behaviour of 8:2 diPAP and 6:2 diPAP, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2700, https://doi.org/10.5194/egusphere-egu21-2700, 2021.

Per- and poly-fluoroalkyl substances represent a new group of anthropogenic environmental pollutants.
For instance, one of the most commonly used firefighting methods today especially within airports is the use of aqueous film forming foams (AFFFs). AFFFs contain PFAS, which give the foam the ability to easily spread over a large area. Some PFAS species have demonstrated adverse health effects already at low concentrations such as liver damage and reproductive harm. The strong chemical bonding between carbon and fluorine also results in PFAS to being resistant to thermal- and chemical degradation. Thus, they degrade very slowly in nature. In Sweden, the limit for PFAS in drinking water is 90 ng L^-1. This means that water needs to be treated for PFAS in the water treatment plants in cases of contamination. where water with a concentration over the limit would be used.

Bålsta is a growing city, with is proximity to Sweden’s largest city and capital Stockholm and Sweden’s fourth largest city Uppsala. Both located about 50 kilometres away, makes Bålsta an attractive city for people to live in and commute to their job in in the larger cities. With this growth new drinking water sources are required in the future Vreta-Bålsta area is an esker with good aquifer properties. However, it is an industrial area, and in the area lies a landfill which has been used as a training ground for firefighting exercises. During these exercises AFFFs have been used and the area is polluted with an unknown amount of PFAS. As the area is planned to be used for artificial infiltration and extraction of groundwater, it needs to be tested for potential PFAS contamination. 

This study aims to investigate by means of numerical modelling different scenarios of PFAS transport from the contaminated. Using data about the topography, soil depth of the area together, hydraulic conductivity, porosity, precipitation and runoff a model of the area was constructed within GMS. The package MODFLOW was used to simulate the groundwater flow. Using the groundwater flow solution, with the PFAS transport is simulated with the package MT3DMS to produce a result of a possible spread of PFAS within the studied area to see which parts that could be contaminated or become contaminated in the future.

The results given from the model was that PFAS from the landfill would transport northeast bound. With the southern part of the area would stay clear from all contamination from the landfill.

How to cite: Pettersson, K., Mossadeghi Björklund, M., and Tatomir, A.: Numerical Modelling of PFAS Transport and Groundwater Flow in the Vreta-Bålsta Esker, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6542, https://doi.org/10.5194/egusphere-egu21-6542, 2021.

Emerging contaminants could exert combined toxic effects, including synergetic and antagonistic ones, that cannot be identified by chemical analysis tools. The biological early warning systems (BEWS) perform a real-time and continuous (24 h) monitoring of physiological and/or behavioural parameters of organism alterations, potentially correlated to water pollution. They are based on the response of living sentinels (i.e. molluscs, algae, crustaceans, fish) to a contaminant or mixture of them. Early warnings can be sent by SMS, e-mail, etc. to operators, in order to activate response actions. Belonging to different trophic levels, the crustacean D. magna, the alga C. vulgaris and the mollusc P. casertanum have been used to control Gran Sasso-Sirente aquifer in three different locations. Drinkable water of Teramo province and irrigation water of L’Aquila have been continuously monitored by the commercial tools “bbe® Daphnia Toximeter (DTOX)” and “bbe® Algae Toximeter (ATOX)”, respectively. In Tirino river spring, a novel sensor “SmartShell” has registered for the first time the valve movements of the autochthonous bivalve. After the first testing period, DTOX and ATOX did not register any typology of alarms in the potable and irrigation water. The valve movements of P. casertanum have been examined through spectral analysis in order to evaluate the behavioural rhythms useful for further investigation on their alterations as early warnings. The objective has been to reinforce the aquifer protection by installing instruments internationally recognised as efficient tools and exploring new proposals for guaranteeing human and ecosystem health.

How to cite: Di Giacinto, F., Berti, M., Carbone, L., Caprioli, R., Colaiuda, V., Lombardi, A., Tomassetti, B., Tuccella, P., De Iuliis, G., Pietroleonardo, A., Mascilongo, G., Di Renzo, L., D’Alterio, N., and Ferri, N.: Aquatic organism behaviours at multiple trophic levels for water monitoring: the case of Gran Sasso - Sirente aquifer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15955, https://doi.org/10.5194/egusphere-egu21-15955, 2021.

Ivermectin (IVM) is one of the few pharmaceutics that are still used in a preventive, systematic manner in extensive cattle breeding in our study region in the Ardèche region (France), amongst others. It is an efficient antiparasitic agent with an extreme acute toxicity for most invertebrates, especially aquatic organisms like daphnia (ng/l), and is also highly toxic to different fish species (µg/l). Due to its strong sorption to soil and sediment and quick photodegradation, early environmental risk assessments (ERA) conclude a low risk for aquatic organisms. More recent studies conclude an inacceptable risk for daphnia and dung organisms. One of the critical parameters between these contradictory conclusions is IVM export from cow dung and transfer towards the streams.

The study region is characterized by a Mediterranean climate with a dry summer and intense convective storm events leading to regular flash flood events that coincide with the cattle treatment seasons in spring and autumn. The study region encompasses the Claduègne catchment which is part of the OHMCV observatory and the OZCAR and eLTER research infrastructures.

The key question concerning the risk for aquatic organisms is to what extent and in which conditions IVM is mobilized and transferred from cow dung to soil and river via surface runoff and percolation in this environment prone to rapid flow processes. We approach this question on the scale of 60*30*22 (L*W*D) cm3 intact soil mesocosms, for which we developed an adapted field sampling and laboratory experimentation case. Soil mescosms are collected in the Claduègne catchment. IVM is applied in form of spiked cow dung at realistic environmental concentrations before simulating several rainfall events, representative of this Mediterranean region. Runoff and drainage water are sampled for major anions (including Br- tracer), non-particulate organic carbon and IVM concentrations on a high temporal frequency in order to gain an insight on the intra- and inter-event dynamics of water and IVM transfer. Tested parameters include dung ageing, soils types, initial soil humidity and consecutive rainfall events.

The first results highlight the importance of runoff for the overall export of IVM on the event scale. Concerning the water flux, initial humidity is found to determine the runoff / drainage partitioning as well as the rapidity of percolation through the occurrence of preferential flow. In this context, hydrophobicity seems to play an important role.

How to cite: Hachgenei, N., Spadini, L., Nord, G., Morel, M.-C., Lespine, A., Mora, H., Courvoisier, F., Sutra, J.-F., and Duwig, C.: Ivermectin transfer through percolation and surface runoff from intact soil mesocosms under rainfall simulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12343, https://doi.org/10.5194/egusphere-egu21-12343, 2021.