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  • 1
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.335-348
    Description: Stratigraphic heterogeneity is a key controlling factor for flow and transport in groundwater systems. In this case study, we have combined two- and three-dimensional images of electric resistivity with hydrogeological data to delineate such stratigraphic heterogeneity at a loop of River Steinlach close to Tübingen, Germany, where we estimated spatially varying aquifer parameters using established evaluation techniques. We developed a three-layer stratigraphic model consisting of heterogeneous sandy gravel overlain by a fine alluvium including top soil, and underlain by sandstone weathered at its top. The sandy gravel makes up the aquifer unit while we interpret the sandstone as aquitard. We classified the area into two stratigraphic segments based on resistivity data. The Southern segment consists of a thicker alluvium where the aquifer material contains more clay than in the Northern segment. Two different stratigraphic structures were also delineated at the Southern and Northern segments, respectively. These structures are visible in inversion results as low-resistivity features trending in the SE–NW in the Southern segment and NE–NW in the Northern segment. Both structures may be previous flow paths of River Steinlach, and the low permeability material overlying them may impair flow exchange in the area. We performed slug and pumping tests to estimate the hydraulic conductivity ( K ) of the aquifer. The K distributions show slight variation with higher values obtained at wells within the Northern segment and close to the river bank. A qualitative comparison of measured hydraulic conductivities with the resistivity distribution shows good agreement of the spatial patterns. The stratigraphic and hydraulic heterogeneities delineated in this work are important for experimental and modeling studies of flow, transport, and hyporheic exchange at the site.
    Keywords: Electric resistivity ; Hydraulic conductivity ; Stratigraphic heterogeneity
    ISSN: 1866-6280
    E-ISSN: 1866-6299
    Source: Springer Science & Business Media B.V.
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  • 2
    Language: English
    In: Transport in Porous Media, 2012, Vol.95(2), pp.425-446
    Description: In this article, we extend the analysis of Diaz and Brevdo (J. Fluid Mech. 681:567–596, 2011) of the absolute/convective instability dichotomy at the onset of convection in a saturated porous layer with either horizontal or vertical salinity and inclined temperature gradients to studying the influence of the Soret effect on the dichotomy in a similar model. Only longitudinal modes are considered. We treat first normal modes and analyze the influence of the Soret effect on the critical values of the vertical thermal Rayleigh number, R v , wavenumber, l , and frequency, ω , for a variety of values of the horizontal thermal Rayleigh number R h , and the vertical salinity Rayleigh number, S v . Our results for normal modes agree well with relevant results of Narayana et al. (J. Fluid Mech. 612:1–19, 2008) obtained for a similar model in a different context. In the computations, we use a high-precision pseudo-spectral Chebyshev-collocation method. Further, we apply the formalism of absolute and convective instabilities and compute the group velocity of the unstable wavepacket emerging in a marginally unstable state to determine the nature of the instability at the onset of convection. The influence of the Soret effect on the absolute/convective instability dichotomy present in the model is treated by considering the destabilization for seven values of the Soret number: S r = −1, −0.5, −0.1, 0, 0.1, 0.5, 1, for all the parameter cases in the treatment of normal modes.
    Keywords: Porous layer ; Inclined thermal and vertical solutal gradients ; Soret-driven thermosolutal convection ; Absolute/convective instability dichotomy
    ISSN: 0169-3913
    E-ISSN: 1573-1634
    Source: Springer Science & Business Media B.V.
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  • 3
    Language: English
    In: Transport in Porous Media, 2016, Vol.111(3), pp.591-603
    Description: Helical flow can occur in porous media if the hydraulic conductivity tensor is anisotropic. We study the structure of steady-state flow fields in three-dimensional anisotropic porous media formed by two homogeneous layers, one of which is anisotropic. We simulate transient transport of a conservative scalar in such flow fields by a hybrid streamline/smoothed particle hydrodynamics method and analyze dilution. We use stretching and folding metrics to characterize the flow field and the dilution index of a conservative scalar divided by the volume of the domain to quantify plume dilution. Based on the results of detailed numerical simulations, we conclude that nonlinear deformation triggers dilution and that plume dilution is controlled by two parameters: the contrast between the principal directions of the anisotropic layer, and the orientation of the hydraulic conductivity tensor with respect to the main flow direction. Furthermore, we show that in this kind of flow fields transverse dispersion is responsible for an increase in plume dilution, while the effect of longitudinal dispersion is negligible.
    Keywords: Anisotropic porous media ; Helical flow ; Flow topology ; Solute dilution
    ISSN: 0169-3913
    E-ISSN: 1573-1634
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  • 4
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.349-358
    Description: Performing tracer tests using artificial tracer compounds is a common practice to characterize natural streams regarding their (reactive) transport properties. Recently, the fluorescent compound resazurin was introduced as a reactive stream tracer to quantify hyporheic exchange and metabolic activity of streams. This tracer, together with its reaction product resorufin and a conservative tracer (in our case fluorescein), provides additional information about transport properties of the stream and its hyporheic zone and can therefore overcome restrictions that are commonly affiliated with the use of conservative tracers alone. However, all previously published studies using this tracer system were based on manual sampling of the water. This usually limits the number of measurements and thus the achievable temporal resolution, and potentially endangers data quality due to inadequate handling of samples. In this paper, a modified version of the GGUN-FL30 on-line fluorometer is presented in which the optics have been modified to allow measuring the concentrations of all three tracers simultaneously at intervals of 10 s. Experiments under controlled and natural conditions showed that the performance of the on-line fluorometer regarding tracer separation efficiency and practical detection limits is comparable to a high-performance laboratory spectrofluorometer. Furthermore, suggestions are given on how to correct tracer signal fluctuations caused by temporal changes in temperature and pH that might occur during a field tracer test.
    Keywords: Fluorometer ; Tracer test ; Groundwater/surface water interaction ; Resazurin
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 5
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.359-372
    Description: First results of a multi-disciplinary hyporheic monitoring study are presented from the newly established Steinlach Test Site in Southern Germany. The site is located in a bend of the River Steinlach (mean discharge of 1.8 m³/s) underlain by an alluvial sandy gravel aquifer connected to the stream. The overall objective is a better understanding of hyporheic exchange processes at the site and their interrelations with microbial community dynamics and biochemical reactions at the stream–groundwater interface. The present paper focuses on the distribution of lateral hyporheic exchange fluxes and their associated travel times at the Steinlach Test Site. Water level dynamics in various piezometers correspond to the different domains of hydraulic conductivity in the shallow aquifer and confirms hyporheic exchange of infiltrated stream water across the test site. Hydrochemical compositions as well as increased damping of continuous time series of electrical conductivity (EC) and temperature at the respective piezometers confirmed the inferred distribution of hyporheic flowpaths. Mean travel times ranging from 0.5 days close to the stream to more than 8 days in the upstream part of the test site could be estimated from deconvolution of EC and δ 18 O–H 2 O data. The travel times agree well with the presumed flowpaths. Mg/Ca ratios as well as model fits to the EC and δ 18 O data indicate the presence of an additional water component in the western part of the test site which most likely consists of hillslope water or groundwater. Based on the mean travel times, the total lateral hyporheic exchange flux at the site was estimated to be of the order of 1–2 L/s.
    Keywords: Hyporheic zone ; Stream–groundwater interaction ; Travel time distribution ; Deconvolution ; Monitoring
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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  • 6
    Language: English
    In: Mathematical Geology, 2003, Vol.35(1), pp.53-66
    Description: In many geostatistical applications, spatially discretized unknowns are conditioned on observations that depend on the unknowns in a form that can be linearized. Conditioning takes several matrix–matrix multiplications to compute the cross-covariance matrix of the unknowns and the observations and the auto-covariance matrix of the observations. For large numbers n of discrete values of the unknown, the storage and computational costs for evaluating these matrices, proportional to n 2 , become strictly inhibiting. In this paper, we summarize and extend a collection of highly efficient spectral methods to compute these matrices, based on circulant embedding and the fast Fourier transform (FFT). These methods are applicable whenever the unknowns are a stationary random variable discretized on a regular equispaced grid, imposing an exploitable structure onto the auto-covariance matrix of the unknowns. Computational costs are reduced from O( n 2 ) to O( n log 2 n ) and storage requirements are reduced from O( n 2 ) to O( n ).
    Keywords: Toeplitz ; circulant ; embedding ; spectral ; FFT ; convolution
    ISSN: 0882-8121
    E-ISSN: 1573-8868
    Source: Springer Science & Business Media B.V.
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  • 7
    Language: English
    In: Environmental Earth Sciences, 2013, Vol.69(2), pp.317-333
    Description: Sustainable water quality management requires a profound understanding of water fluxes (precipitation, run-off, recharge, etc.) and solute turnover such as retention, reaction, transformation, etc. at the catchment or landscape scale. The Water and Earth System Science competence cluster (WESS, http://www.wess.info/ ) aims at a holistic analysis of the water cycle coupled to reactive solute transport, including soil–plant–atmosphere and groundwater–surface water interactions. To facilitate exploring the impact of land-use and climate changes on water cycling and water quality, special emphasis is placed on feedbacks between the atmosphere, the land surface, and the subsurface. A major challenge lies in bridging the scales in monitoring and modeling of surface/subsurface versus atmospheric processes. The field work follows the approach of contrasting catchments, i.e. neighboring watersheds with different land use or similar watersheds with different climate. This paper introduces the featured catchments and explains methodologies of WESS by selected examples.
    Keywords: Water and solute fluxes ; Water quality ; Catchments ; Land-surface atmosphere exchange ; Processes and feedbacks ; Modeling ; Monitoring
    ISSN: 1866-6280
    E-ISSN: 1866-6299
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