Displays

Progress in experimental techniques and imaging methods have led to a leap in the understanding of 
microscopic transport and swimming mechanisms of motile particles in porous media. This is very different 
for the understanding and characterization of large scale transport behaviors, which result from the 
interaction of motility with flow and medium heterogeneity, and the upscaling of microscale behaviors. 
Only few works have investigated large scale dispersion of active particles in porous media, 
which mainly operate in the framework of Brownian dynamics and effective dispersion or 
are completely data driven. In this work, we use the particle tracking data of Creppy et al. [1] 
to derive the stochastic dynamics of small scale particle motion due to hydrodynamic flow variability 
and the swimming activity of bacteria. These stochastic rules are used to derive a 
continous time random walk (CTRW) based model for bacteria motion. The CTRW naturally accounts for 
persistent advective motion along streamlines [2]. In this framework, particle motility is modeled 
through a subordinated Ornstein-Uhlenbeck process that accounts for the impact of rotational diffusion on 
 particle motion in the fluid, and a compound Poisson process that accounts for the motion toward and around 
grains. The upscaled transport framework can be parameterized by the distribution of the Eulerian 
pore velocities, and the motility rules of the bacteria. The model predicts the propagators of the 
ensemble of bacteria as well as their center of mass position and dispersion for bacteria transport under different
flow rates. 

[1] A. Creppy, E. Clément, C. Douarche, M. V. D’Angelo, and H. Auradou. Effect of motility on the transport of bacteria populations through a porous medium. Phys. Rev. Fluids, 4(1), 2019.

[2] M. Dentz, P. K. Kang, A. Comolli, T. Le Borgne, and D. R. Lester. Continuous time random walks for the evolution of Lagrangian velocities. Physical Review Fluids, 1(7):074004, 2016.

How to cite: Dentz, M., Auradou, H., Creppy, A., Clément, E., and Carine, D.: Swimming-induced non-Fickian transport of bacteria in porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8928, https://doi.org/10.5194/egusphere-egu2020-8928, 2020.

In situ quantification of subsurface hydro-geomechanical properties is challenging and requires significant effort. Evolving research illustrates that subtle harmonic components in groundwater head measurements caused by Earth and atmospheric tides can be utilised to explore groundwater systems with little effort compared to traditional investigations. One long standing problem has been that, for dominant tidal components, Earth and atmospheric tides occur at the same frequency which prevents the use of the groundwater response to their individual forcing to infer subsurface properties. While Acworth et al. (2016) offered a way forward, their approach has assumptions that limit the applicability. Here, we illustrate an extended method that disentangles the borehole water level response and attributes magnitude and phase to their individual drivers. As a result, we obtain individual changes in harmonic properties of the drivers and their groundwater response (amplitude ratio and phase shift) using borehole water level records from different locations. In conjunction with groundwater flow and poroelastic theory, these properties can be used to infer the state of confinement, quantify specific storage and hydraulic conductivity as well as barometric efficiency of the formation. Further, because the stresses imposed by Earth and atmospheric tides are volumetric and uniaxial, respectively, their individual responses can be used to reveal strain anisotropy. Our new approach is passive, i.e. it only requires the measurements of atmospheric and groundwater pressure records, and can provide further insight into subsurface processes and properties using information hidden in standard pressure records.

Acworth, R. I., Halloran, L. J. S., Rau, G. C., Cuthbert, M. O., and Bernardi, T. L. ( 2016), An objective frequency domain method for quantifying confined aquifer compressible storage using Earth and atmospheric tides, Geophys. Res. Lett., 43, 11,671–11,678, doi:10.1002/2016GL071328.

How to cite: Rau, G., McMillan, T., Cuthbert, M., Andersen, M., Timms, W., and Blum, P.: Disentangling the groundwater response to Earth and atmospheric tides reveals subsurface processes and properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8665, https://doi.org/10.5194/egusphere-egu2020-8665, 2020.

How to cite: Lee, C., Bour, O., Ballard, J.-M., Simon, N., de la Bernardie, J., Paradis, D., Raymond, J., and Lefebvre, R.: Inferring high-resolution aquifer hydraulic conductivity and groundwater fluxes by active heat tracer using direct push fiber optics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9709, https://doi.org/10.5194/egusphere-egu2020-9709, 2020.

The reliable characterization of heterogeneous sedimentary aquifers, especially the identification of preferential flow paths and their connectivity remains a challenge in applied hydrogeology research and practice. However, aforementioned information is crucial for predicting subsurface flow and contaminant transport in complex deposits. Well established characterization methods such as outcrop analogue studies, hydraulic tomography, tracer testing, and direct push profiling suffer from uncertainty due to non-uniqueness of underlying inversions or insufficient temporal and/or spatial data resolution. Furthermore, the relation and effects of observed heterogeneity in hydraulic conductivity on transport is not always straight forward.

A promising novel approach to overcome the limitations of conventional hydraulic site characterization techniques is the joint application of tracer testing and direct push logging. We present a proof-of-concept field study, where conventional salt tracer testing was combined with vertical high resolution direct push electrical conductivity profiling. The method successfully captured tracer distribution in heterogeneous sedimentary deposits in-situ and visualized measured tracer distribution over time. Additional measurements, such as breakthrough-curves and surface geophysics can be easily integrated to set up ex-post simulations to further increase site-specific understanding of groundwater flow and transport processes.

How to cite: Vienken, T., Zech, A., Huber, E., Huggenberger, P., Kreck, M., Pohle, M., Dietrich, P., and Werban, U.: A novel approach towards the reliable characterization of complex sedimentary aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7219, https://doi.org/10.5194/egusphere-egu2020-7219, 2020.

Hydraulic tomography is known for imaging hydraulic conductivity of aquifers. In hydraulic tomography, the aquifer is stressed sequentially at several locations with pumping or slug tests while hydraulic heads are observed in different points. These hydraulic head data along with a numerical model are then used to reconstruct the hydraulic conductivity distribution of the aquifer through inversion process. The reconstructed distribution usually represents smooth-low resolution model of hydraulic conductivity which may be suitable for representation of groundwater flow with limited applicability to transport problems. Here, we investigate the added value of using groundwater fluxes measurement for the reconstruction of hydraulic conductivity in tomographic experiment. Vertical profile of groundwater flux may be estimated using active fiber optic distributed temperature sensor (FO-DTS) methods with FO cables installed by direct push so as it is in direct contact with formation. In active FO-DTS, FO cable is heated and heat is transported by conduction and convection. So different water fluxes result in different temperature behavior. This study is carried out in two parts. First, we conducted a synthetic analyze where we used a sequence of synthetic multivariate Gaussian aquifers with different tomographic configurations and datasets. This analysis showed that joint inversion of groundwater fluxes and hydraulic heads leads to better hydraulic conductivity resolution than using hydraulic heads solely. Inversion of groundwater fluxes alone is also superior than using only hydraulic heads. Then, insights gained from the synthetic study were used to guide the implementation of a field study at the Saint-Lambert experimental site located 40 km south of Quebec City, Canada. The tomography experiment was performed between 3 wells closely spaced (between 5 and 9 m) and two active FO-DTS cables. FO cables were installed vertically by a direct push drilling technique at mid-point between the central pumping well and two observation wells. Discrete intervals along the observation wells were also isolated with packers to monitor temperature and hydraulic heads at different depths in these two screened observational wells. First, the aquifer was constrained to pumping continuously for 24 hours at a constant rate of 10 LPM with simultaneously recording temperature (passive mode) and hydraulic heads in 8 discrete well intervals and in the pumping well itself as well as along the 2 FO-DTS with approximate resolution of 25 cm. Then, by analyzing the piezo-metric heads and making sure that steady-state conditions were achieved, the pumping was held at the same rate but heat was injected to fiber optic cables (active mode) for another 64-hour period. After this period, heating and pumping were stopped. Preliminary results show the feasibility of the active FO-DTS in capturing varying groundwater fluxes with depth, as reflected in the different temporal temperature trend. These temperature trends will be used to estimate the vertical groundwater flux profile from these temperature temporal trends at a vertical resolution of approximately 25 cm. Then estimated fluxes will be used for hydraulic tomography. Those experimental results along with the synthetic analyze are shown to be promising in improving characterization of hydraulic conductivity of aquifers.

How to cite: Pouladiborj, B., Bour, O., Linde, N., Paradis, D., Ballard, J.-M., de La Bernardie, J., Simon, N., Lee, C., longuevergne, L., and Lefebvre, R.: On the use of the ground water fluxes for hydraulic tomography: Theoretical and field-based assessments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9025, https://doi.org/10.5194/egusphere-egu2020-9025, 2020.

Understanding and computing fluid flow in porous media is relevant for many environmental and industrial applications. However, directly computing the flow on complex and large geometries is limited by computational capacities. To tackle this issue, many studies have aimed at relating flow distributions to the geometrical properties of the domain. However, understanding the relation between the the pore-scale structure and the experienced velocity distribution is still a challenge.
To improve this understanding we study well defined pore network models in 2D and 3D. We vary three main parameters: the coordination number that determine the amount of connections that a pore body has, the distribution of pore sizes, and geometric disorder.

We focus on the impact of these parameters on the flow organization in term of the distribution of flow speeds and local pressure gradients. We conclude that distribution of pore sizes and the coordination number are the main geometrical features that control the Eulerian speed distribution. 

How to cite: Puyguiraud, A., Uszes, P., and Dentz, M.: The role of coordination number and pore size distribution on flow organization in porous media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22124, https://doi.org/10.5194/egusphere-egu2020-22124, 2020.

Image analysis has become a standard method by which saltwater intrusion (SWI) is investigated in the laboratory. While the use of complex artificial neural networks is becoming a common analysis technique to obtain concentration fields, the standard methodology utilizes a classical algorithm which applies an augmented power-law function to each grayscale pixel. The classical method is methodologically rigorous, simple to implement numerically, and empirically robust. However, the power-law procedure involves substantial costs to the experimental process in producing calibration images for every aquifer and to computer processing times due to performing pixel-wise non-linear regression. We have developed three new classical image processing methods for SWI experiments in translucent glass-bead aquifers with the goal of optimizing the experimental and data analytic processes while maintaining accuracy and utility. First, a Laurent series provides similarly good fitting to optical grayscale data, while the function’s linearity reduces computation analysis time by a factor of a thousand—from over two hours to twenty seconds. For the second method, the Beer–Lambert Law is modified to include the optical effect of the glass beads. Applying this function form to images taken through a monochromatic light filter may decrease the number of calibration images, thereby saving the experimenter several hours of calibration time per experiment. Third, color image cameras provide different pixel intensity decreases between the three spectral channels which can be combined to produce a nearly linear correlation between source data and concentration, which gives an especially robust reduction in calibration images and rapid processing times. In our presentation, we will discuss the relative advantages and limitations of each method as they relate to the requirements and configuration of the laboratory under investigation and local analytic capabilities.

How to cite: Benner, E., Etsias, G., Hamill, G., Fernandez Aguila, J., Flynn, R., and McDonnell, M.: Optimizing Image Analysis Processing in Thin Transparent Aquifers: Application to Pixel Wise Regression of Salt-Water Intrusion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18127, https://doi.org/10.5194/egusphere-egu2020-18127, 2020.

Most single-point methods of measuring seepage fluxes across the surface water-groundwater interface in lakes, streams, and estuaries (e.g., volumetric, head-based, and thermal) have one trait in common: they produce seepage rate values, averaged over substantial periods of time, thereby limiting resolution of the intra-day dynamics. Recently, Solomon et al. (Water Resources Research, 2019, in review) presented a new instrument and modification to a previously tested concept (Solder et al., Groundwater, 2016). This instrument has an open-bottom permeameter (OBP) design, which is commonly used for investigating hydraulic conductivity of the interface with falling or rising head tests, but historically not used for flux estimates. The novel dynamic seepage meter (DSM) evaluates the transient water level in the OBP-based instrument with submillimeter accuracy, exceeding the performance of traditional pressure transducers. The initial dynamics of the water level response over fractions of an hour holds the necessary information to infer the natural seepage rate in both gaining and losing conditions. The tests can be repeated frequently in an automatic regime. If a single test lasts long enough, hydraulic conductivity, in addition to the seepage rate can also be accurately determined. Here, a detailed hydrodynamic theory of the flow systems inside and outside the OBP is presented and the accuracy of measured water fluxes is investigated with emphasis on interpretation of the data with ambient noise. The results of this study will facilitate rapid, accurate, and massive data collection in diverse field conditions. (Research was supported by the NSF grant EAR 1744719.)

How to cite: Zlotnik, V. A., Solomon, D. K., Gilmore, T. E., Genereux, D. P., and Humphrey, C. E.: Dynamic Seepage Meter: Theory with Application Examples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3718, https://doi.org/10.5194/egusphere-egu2020-3718, 2020.

Groundwater flow and contaminant transport are strongly influenced by the aquifer’s heterogeneity (Chao et al., 2000; Fernàndez-Garcia et al., 2004). Generally, the flow (and transport) variables, such as the effective conductivity K_eff, can be modelled as random space functions (RSFs) and determined by means of a self-consistent approximation (Severino, 2018). In particular, we aim at estimating the effective conductivity K_eff of a highly heterogeneous aquifer made of 12 different porous materials, whose K-values were experimentally measured.

A heterogeneous phreatic aquifer was built in the GMI Laboratory of the Department of Civil Engineering of the University of Calabria, inside a metal box (2 m x 2 m x 1 m). The thickness (0.35 m) of the aquifer was built by overlapping 7 different layers of 0.05 m, each consisting of 361 cells (19 x 19), with dimensions equal to 0.1 m x 0.1 m x 0.05 m. For each layer, each cell was filled with one of the 12 porous materials previously characterized in the lab, making the choice randomly. A central (pumping) well and 37 piezometers were located at different distances from the first according to a radial configuration.

A pumping test was carried out by a constant flow rate of 70 L/hour. The hydraulic head data, evaluated by using the Neuman method and verified in compliance with the boundary conditions, allowed an effective hydraulic conductivity value K_eff to be obtained.

Afterwards, this value was compared with K values measured in laboratory by permeameter for each of the 12 porous media used to build the heterogeneous aquifer considered here and with the main statistical parameters related to them. We found the K_eff value in a very good agreement with the expression obtained by the self-consistent approximation (Severino, 2018).

References

Chao C.-H., Rajaram H. and Illangasekare T. H. (2000). Intermediatescale experiments and numerical simulations of transport under radial flow in a two-dimensional heterogeneous porous medium, Water Resour. Res., 36(10), 2869– 2884.

Fernàndez-Garcia D., Illangasekare T. H. and Rajaram H. (2004). Conservative and sorptive forced-gradient and uniform flow tracer tests in a three-dimensional laboratory test aquifer. Water Resour. Res., Vol. 40, W10103, doi:10.1029/2004WR003112.

Severino G., 2018. Effective conductivity in steady well-type flows through porous formations. Stochastic Environmental Research and Risk Assessment, Vol. 5, https://doi.org/10.1007/s00477-018-1639-5.

How to cite: Brunetti, G. F. A., De Bartolo, S., Fallico, C., Severino, G., and Tripepi, G.: Laboratory device to investigate the heterogeneity’s influence on the effective hydraulic conductivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4782, https://doi.org/10.5194/egusphere-egu2020-4782, 2020.

Accurate groundwater level forecasting models is essential to ensure the sustainable utilization and efficient protection of groundwater resources. In this paper, a novel method for groundwater level forecasting is proposed on the basis of coupling discrete wavelet transforms (WT) and long and short term memory neural network (LSTM) . In this model, the wavelet transform is used to decompose the cumulative displacement into the term of trend and term of periodicity . The trend term reflects the long-term tendency of groundwater level variation, which is simulated by a linear regression method. The periodic term driven by external factors such as rainfall, the river stage and the distance from river, is modelled using a LSTM method. The distance from river and the distance from observation wells are used for spatiotemporal model interpretation. Finally, the trend term and periodic term are superposed to achieve the cumulative spatiotemporal prediction of groundwater level. A typical study area located in Haihe basin is taken as an example to validate the performance of the proposed model. The proposed mode (WT-LSTM) is compared with the regular artificial neural network (ANN) model and autoregressive integrated moving average (ARIMA) model. The results show that the prediction accuracy of WT-LSTM model is higher than ANN model and ARIMA model, especially during the flood period. Furthermore, the spatiotemporal groundwater level forecasting is not only included the observation of groundwater and precipitation, but should also take the influence factors of surface water into consideration. The proposed model gives a new sight in the prediction of groundwater level.

How to cite: qin, W., Lu, C., Sun, L., and Lu, J.: Spatiotemporal forecasting for groundwater level using a WT-LSTM model , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6343, https://doi.org/10.5194/egusphere-egu2020-6343, 2020.

Predicting groundwater level changing accurately is important to ensure the sustainable and efficient utilization of water resources. The study of groundwater model generally includes multi-source of data as input. The spatial and temporal patterns of groundwater is associated with surface water and rainfall, resulting in the difficulty of groundwater level predicting. Satellite data has been gradually valued and utilized more popularly. The effective fusion of remotely sensed data and ground observation data will greatly improve the spatial and temporal resolution of groundwater level mapping. At present, most of the models used solely the ground data or remote sensing data for prediction. The development and application of data analysis technology will effectively improve the level of prediction. How to merge multi-source data to enhance the accuracy is the goal of the study. In this study, the Beijing plain area will be selected as a typical research. Ground observed data and satellite remotely sensed data will be unified used for data fusion to predict groundwater dynamics. The groundwater level distribution after multi-source data fusion is simulated to analyze the evolution trend and spatial patterns of groundwater level in Beijing plain over the years. Analyzing the difference between single source data and multi-source data fusion is another goal in this study.

How to cite: Lu, J., Lu, C., Sun, L., and Qin, W.: Identification of the spatial and temporal variation of groundwater level in Beijing plain via the fusion of remotely sensed data and ground observation data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6741, https://doi.org/10.5194/egusphere-egu2020-6741, 2020.

The adsorption of hexavalent chromium Cr(VI) onto six different natural unconsolidated sediments (two loamy sand, two sandy loam, loam, silty loam clay) was studied using batch and column experiments. Equilibrium adsorption capacities and kinetic rate were calibrated using batch experimental data. Elovich, pseudo first- and second-order models were used to fit the kinetic adsorption data, respectively. Henry’s, Freundlich, and Langmuir isotherms were used to fit the equilibrium adsorption data. Four model selection criteria, Akaike information criterion (AIC), modified Akaike information criterion (AICc), Bayesian information criterion (BIC), and Hannan information criterion (HIC) were used to discriminate kinetic and equilibrium models. These criteria suggest that the selected optimal model depends on the sediment type. Specifically, we studied effects of different factors including pH, solid/solution ratio, particle size, and clay mineral content on adsorption capacities. Column experiments were performed and a deterministic equilibrium model as well as a chemical non-equilibrium model were applied to fit the breakthrough curves. Results revealed a high retention of Cr(VI) in sandy loam, loam and silty loam clay, and a high mobility in loamy sand. It was found that particle size and clay minerals played an important role in adsorption process. The results from this study provide important insight for us to understand the transport behaviors of Cr(VI) in porous media.

How to cite: Cao, Y., Dai, Z., Zhang, X., and Ma, Z.: Sorption model identification for chromium transport in unconsolidated sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12663, https://doi.org/10.5194/egusphere-egu2020-12663, 2020.

Groundwater resources are under increasing threat from human activity and climate change, making sustainable management critical. However, appropriate management generally requires extensive knowledge of the properties and characteristics of aquifers. In recent years, research into passive investigation methods utilising the impact of Earth and atmospheric tides (EAT) on the groundwater response have gained momentum. EAT occur at known frequencies of daily and sub-daily cycles per day (cpd) and present an inexpensive and viable opportunity for the characterization of groundwater systems at an unprecedented spatial and temporal resolution (McMillan et al., 2019). However, quantifying aquifer properties relies on accurate and reliable extraction of the harmonic properties (amplitude and phase) of tidal components embedded in groundwater level and atmospheric pressure records that are dominated by larger magnitude variations as well as other noise. Here, we use synthetic signals and real measurements to test and compare the performance of the Discrete Fourier Transform (DFT) with a generalised harmonic least squares amplitude and phase estimation (APES) approach for the harmonic tidal components. APES was implemented in Python in conjunction with a windowed de-trending function that serves as a high pass filter. The analysis focuses on three realistic aspects often encountered in groundwater monitoring: (1) the minimum record length required to reliably separate tidal components at nearby frequencies, (2) signal quantisation as a proxy for measurement resolution, and (3) the amount of sampling gaps or irregularly spaced sampling. Results indicate that APES outperforms DFT in quantifying the amplitude of the major tidal components M2 (1.93227 cpd) and S2 (2.0 cpd) on regularly sampled data, because it is not subject to spectral leakage. Furthermore, APES is superior in handling data gaps, missing values and outliers, yielding accurate amplitude estimates even for comparably small amounts of data and without requiring pre-processing such as data interpolation or resampling. This increases the data volume for the tidal analysis considerably and enables a much more extensive use of tidal analysis. Further investigation will focus on the methods’ performance in quantifying the phase of the M2 and S2 components.

McMillan, T. C., Rau, G. C., Timms, W. A., & Andersen, M. S. ( 2019). Utilizing the impact of Earth and atmospheric tides on groundwater systems: A review reveals the future potential. Reviews of Geophysics, 57, 281– 315. https://doi.org/10.1029/2018RG000630.

How to cite: Schweizer, D., Ried, V., Rau, G., Tuck, J., Stoica, P., and Blum, P.: Extracting the properties of Earth and atmospheric tidal harmonics from groundwater level records: a least-squares approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14639, https://doi.org/10.5194/egusphere-egu2020-14639, 2020.

In groundwater and other environmental compartments, compound-specific stable isotope analysis (CSIA) has been used for the determination of specific degradation pathways by analyzing the stable isotopes of two elements. This ‘dual-isotope’ or two-dimensional isotope’ analysis also allows for an estimation of the contribution of two different pathways contributing both to the overall degradation and stable isotope fractionation. Heterogeneous groundwater flow patterns lead to some yet acceptable uncertainities in the results of this method.  Recent CSIA approaches also allow for investigating the simultaneous stable isotope fractionation effects for three different elements. Such information on the stable isotope fractionation of three different elements of a degradable compound could be used for a quantitative analysis of the contribution of different degradation pathways in systems with three different pathways, but up to know there is no theoretical concepts providing such quantitative estimate.

The aim of the present study is to overcome this shortage and to present such theoretical concept for the quantification of single pathway contribution to the overall biodegradation in groundwater and other systems with three parallel degradation pathways. For this purpose the approach of Centler et al. (2013) for the analysis of dual-isotope analysis has been expanded to consider the fractionation of three different elements affected by three different pathways. The obtained analytical expression allows for the quantification of each pathway to total degradation based stable isotope enrichment factors and measured stable isotope signatures. The applicability of the concept is demonstrated using data from Wijker et al. (2013).

Centler, F., Hesse, F., and Thullner, M. (2013) Journal of Contaminant Hydrology, 152, 97-116.

Wijker, R. S., Bolotin, J., Nishino, S. F., Spain, J. C., and Hofstetter, T. B. (2013) Environmental Science & Technology, 47, 6872-6883.

How to cite: Thullner, M., Centler, F., and Hofstetter, T.: Quantifying the contribution of three competing pathways to total degradation in groundwater by a triple-isotope analysis approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14776, https://doi.org/10.5194/egusphere-egu2020-14776, 2020.

Seawater intrusion is a major issue worldwide, as coastal aquifers often act as the primary source of drinking water for more than one billion people. With climate change and projected population increases in coastal areas, this problem is anticipated to become more pressing over the next decades. Effective site characterisation strategies provide a crucial component in understanding subsurface saltwater migration. Density differences cause freshwater to float on seawater creating the classical saltwater intrusion saline wedge. However, tides often control coastal groundwater dynamics causing the emergence of an upper saline recirculation cell beneath the intertidal zone (Intertidal Recirculation Cell, IRC). Here we present the application of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques to characterize the coastal sand aquifer underlying Benone Strand (Magilligan, Northern Ireland) where tides induce an IRC. The aquifer is approximately 20 m thick and rests directly on Lr. Jurassic mudstones.

2D ERT profiles were generated at Benone beach using the SYSCAL Pro 72 ERI system (Iris Instruments). Two different array configurations (Wenner-Schlumberger and dipole-dipole) were used to provide both improved horizontal and vertical resolution. Because of the homogeneity of the sand, the ERT profiles made it possible to clearly define the configuration of the IRC and the fresh groundwater discharging “tube”. The presence of the tidally-driven recirculation cell causes fresh groundwater to flow below the IRC (“discharge tube”) and discharge in the vicinity of the low water mark. ERT data suggest that the IRC has a resistivity of approximately 1 Ωm and a thickness of 8 m. Resistivity increases below the IRC, but declines moving towards the low water mark. These findings suggest a possible mixing zone between saline water and the freshwater discharge. To verify the accuracy of the resistivity values measured in the ERT profiles, water samples were collected at various distances along a perpendicular transect from the high water mark to the low water mark. The electrical conductivities of the water samples were measured and compared with the resistivities obtained in the ERT profiles using Archie's law. Similar values were obtained in both cases.

A MALÅ ground penetrating radar system, operating at 50 MHz, 100 MHz and 500 MHz, was used to collect 2D GPR profiles at Benone beach from the low tide mark to beyond the high water mark. Findings suggested that the IRC attenuated the radar signal in all cases. However, GPR profiles were crucially important to demarcate the interfaces between freshwater and saltwater near the ground surface. GPR profiles obtained using higher frequencies (500 MHz) were the most informative.

The research work carried out at Magilligan allows us to conclude that the application of ERT and GPR techniques is effective in delineating seawater intrusion in aquifers where tides create an IRC. In addition, ERT profiles very clearly identified the IRC through field measurements (which in most cases is studied through numerical models and laboratory tests).

How to cite: Fernández Águila, J., McDonnell, M., Flynn, R., Ruffell, A., Benner, E., Etsias, G., Hamill, G., and Donohue, S.: Application of Electrical Resistivity Tomography and Ground Penetrating Radar to assess salinity in a coastal aquifer with tidally-driven saline recirculation cell, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18138, https://doi.org/10.5194/egusphere-egu2020-18138, 2020.

Abstract: In recent years, more and more attention has been paid to engineering projects, such as the remediation of contaminated groundwater, the restoration of water quality, and the seepage control of building foundations. For all these projects, detailed knowledge of the spatial distribution of aquifer hydraulic parameters is required. Inversion based tomography can be considered a promising subsurface investigation approach to obtain aquifer characterization with a high spatial resolution. However, single inversion cannot avoid parameter uncertainty and non-uniqueness problems. Combination of different independent inversions can help to reduce these problems. The purpose of this paper is to reconstruct cross-well hydraulic conductivity profiles by jointly using hydraulic tomography and thermal tracer tomography in a heterogeneous transient groundwater model.

In this study, based on a three-dimensional data set derived from an aquifer analogue outcrop study, a numerical ground water model is set up to simulate a number of short-term hot water injection tests in a tomographical array, and to perform 2D hydraulic tomography based on hydraulic travel time and attenuation inversions. Consequently, the hydraulic conductivity is calculated from the obtained diffusivity and specific storage values. Parallel to this, the temperature breakthrough curves of the active thermal tracers were utilized to reconstruct the cross-well hydraulic conductivity profiles by using travel-time-based thermal tracer tomography. Comparisons between the results and the “true values” of the analog have shown the satisfying accuracy of the subsurface investigation and advantages when using combined tomographical methods.

How to cite: Qi, J., Hu, R., Liu, Q., Hu, L., and Ptak, T.: Combination of Hydraulic and Thermal Tracer Tomography in a Heterogeneous Transient Groundwater Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21266, https://doi.org/10.5194/egusphere-egu2020-21266, 2020.

Stream flow is combination of two major portion of flows as direct runoff and baseflow. Study of baseflow and direct runoff is much needed to understand the hydrology of a watershed, including surface and sub-surface water interaction, and to assess the ecological functioning of streams. Tropical countries like India facing major challenges in water management; especially for irrigation and drinking water. In such regions identification of baseflow sources, knowledge of baseflow availability and analysis of their varied contribution to the stream is needful. Baseflow plays a critical role in maintaining streamflow, especially during pre and post monsoon periods.

Recursive digital filter technique is adopted for the daily stream flow data measured at river gauge stations on Kabini stream of Cauvery basin, to separate baseflow component from stream flow hydrograph. In terms of hydrogeology, since Cauvery basin occupied with hard-rock terrain, it is important to investigate the intra annual variation of groundwater discharge into the stream. In the present study an attempt has been made by considering daily stream flow data at two river gauge observation points, and annual baseflow and baseflow index is calculated through RDF method. The results obtained from RDF method are validated with the help of hydrogeochemical tracers by applying End Member Mixing Analysis (EMMA) to the hydrogeochemical data for the period of 2018-19 hydrological cycle.

How to cite: Nara, S. N. V., Muddu, S., and Ghosh, P.: Assessment of variation of baseflow contribution to stream flow in a hard-rock aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21691, https://doi.org/10.5194/egusphere-egu2020-21691, 2020.

How to cite: Ceriotti, G., Borisov, S., and de Anna, P.: Novel experimental methods for the identification of anoxic micro-niches in porous media., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22328, https://doi.org/10.5194/egusphere-egu2020-22328, 2020.

Subsurface processes in rocks are often sensitive to the presence of fractures and their ability to transport fluids, solutes and heat, which depends in turn on their geometry. Characterizing parameters like fracture length and orientation is therefore an important step towards making realistic flow and transport simulations. Despite advances in computational power, state-of-the-art approaches such as discrete fracture network (DFN) models still tend to be conditioned to conventional data sources, generally hydraulic tests or solute tracer tests, providing little constrains on 3-D fracture attributes (e.g. orientation). This highlights the need for novel experimental frameworks to better resolve complex flow patterns in fractured rocks with DFN models. Here we show how borehole thermal anomalies, caused by natural flow or by heat injection experiment, can be used to detect and characterize the orientation of permeable fractures. Heat has been increasingly used as a tracer in recent years owing to the commercialization of Raman scatter-based fiber optics distributed temperature sensing (DTS) systems tailored to environmental applications. We thus present a simple framework based on analytical and/or numerical methods to extract structural information on fractures intersecting or located nearby boreholes equipped with DTS systems. Our model assumes a single fracture, embedded in an impermeable rock mass, and is validated against a series of cross-hole thermal tracer tests performed in crystalline rock at the Grimsel Rock Laboratory, in Switzerland. Active heat injection was carried out by heating water using an electrical flow-through heater up to 45°C for a duration 40 days. Fluid injection took place across a discrete, 2-m long interval packing off a single flowing fracture. A continuous fiber optics loop was deployed along three fully-grouted boreholes, which managed to record thermal breakthroughs of 1-2°C up to 6-7 meters from the injection point.  We find that orientations constrained from thermal anomalies do not necessarily correspond to structural orientations of borehole fracture traces. This orientation is defined instead by the borehole axis and maximum thermal gradient along heat-carrying fractures. Such parameter provides information on the spatial organization of discrete flow paths and may offer an alternative calibration parameter to constrain flow and transport simulations on DFNs.

How to cite: Brixel, B., Klepikova, M., and Dentz, M.: Thermal imaging of sparse permeable fractures embedded in intact granite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22612, https://doi.org/10.5194/egusphere-egu2020-22612, 2020.

Airborne electromagnetics is a cost-effective and rapid method with which to map the regional distribution of saline groundwater in coastal areas, however the process of transforming observed data into salinity estimates comprises multiple sources of uncertainty. The resulting error primarily relates to two factors: (1) the transformation (or inversion) from airborne observations into physical properties, (2) the availability of lithological information to transform inversion results to salinity estimates. Recent research has shown that this uncertainty can significantly affect the accuracy of resulting groundwater salinity estimates, in particular the location of the fresh-saline interface. Reducing error relating to the two factors is not trivial. Firstly, as the inversion process is non-unique, an infinite number of models can fit the data. Secondly, the availability of lithological information on regional scales is generally low. To highlight potential sources of error and improve parameterization, we investigate the usefulness of combining airborne electromagnetic data with a 3D variable-density groundwater flow and coupled salt transport model. We quantitatively present findings using a synthetic model which was created using an existing large-scale (~100km²) 3D groundwater model based on real data from the Netherlands. The model is created in two steps: (1) the available groundwater model is run until a state of equilibrium is reached with the model boundaries and stress terms and (2), an airborne survey is simulated using standard geophysical forward modelling techniques, resulting in set of observations. The airborne observations are then inverted and used alongside a simulated lithological data acquisition programme, which are finally input as initial conditions to a groundwater model. As the groundwater model is assumed to be in a state of equilibrium, we show the effect of implementing an optimization framework that penalizes the rate of groundwater salinity fluctuations by iteratively changing the input parameters of both the inversion method and the lithological data. Results quantitatively highlight the effectiveness of implementing a simple, inter-disciplinary approach to airborne electromagnetic groundwater mapping.

How to cite: King, J. A., Essink, G. H. P. O., and Bierkens, M. F. P.: Improved parameterization of airborne electromagnetic surveys for groundwater salinity mapping using 3D variable-density groundwater flow models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22462, https://doi.org/10.5194/egusphere-egu2020-22462, 2020.

The importance of the benthic biolayer (the first few centimeters in the shallow part of the streambed) comes from the active biogeochemical reactions that happen within this thin layer. Currently, many studies use the simplified approach of using the constant profile to represent the diffusivity in the sedimented; however, other studies claim that the exponential profile is a better representation due to the turbulence penetration into the sediment bed. In this work, we are using an analytical model to simulate the temporal variation of solute concentration in water column in bedform morphology type by adopting two diffusivity profile; constant diffusivity profile, and exponential diffusivity profile. This rigorous analytical framework was built by Grant et al. 2019 (not published yet),  and is based on Duhamel’s Theorem. The model is used to fit a set of laboratory data that were performed on streams with dunes type bedforms, where temporal concentration variation is measured in the water column. Based on Root Mean Square Error (RMSE), coefficient of determination (R²), and modified Akaike Information Criterion (AICc), the exponential profile is superior over the whole range of Permeability Reynolds Number, and it can be considered as the best fit for the laboratory data compared to the constant diffusivity.  Additionally, the influence of sediment bed depth on the effective diffusivity, and therefore, on the benthic biolayer characteristics is investigated here by running the model with constant diffusivity profile in Infinite and finite sediment bed cases. An indicator () to determine whether the sediment bed depth influences the diffusivity within the sediment domain or not, is introduced here. when this indicator is larger than 1, the sediment bed depth will likely influence the diffusivity within the sediment. Based on our results, our analytical framework can be a predictive tool for the solute transfer into the benthic layer in bedform morphology type.

How to cite: Monofy, A. M. I. M. and Grant, S.: Turbulent Mixing in the benthic biolayer of streams with bedforms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-920, https://doi.org/10.5194/egusphere-egu2020-920, 2020.