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  • 1
    Language: English
    In: Journal of Hydrology, February 2017, Vol.545, pp.42-54
    Description: Groundwater resources management requires operational, regional-scale groundwater models accounting for dominant spatial variability of aquifer properties and spatiotemporal variability of groundwater recharge. We test the Ensemble Kalman filter (EnKF) to estimate transient hydraulic heads and groundwater recharge, as well as the hydraulic conductivity and specific-yield distributions of a virtual phreatic aquifer. To speed up computation time, we use a coarsened spatial grid in the filter simulations, and reconstruct head measurements at observation points by a local model in the vicinity of the piezometer as part of the observation operator. We show that the EnKF can adequately estimate both the mean and spatial patterns of hydraulic conductivity when assimilating daily values of hydraulic heads from a highly variable initial sample. The filter can also estimate temporally variable recharge to a satisfactory level, as long as the ensemble size is large enough. Constraining the parameters on concentrations of groundwater-age tracers (here: tritium) and transient hydraulic-head observations cannot reasonably be done by the EnKF because the concentrations depend on the recharge history over longer times while the head observations have much shorter temporal support. We thus use a different method, the Kalman Ensemble Generator (KEG), to precondition the initial ensemble of the EnKF on the groundwater-age tracer data and time-averaged hydraulic-head values. The preconditioned initial ensemble exhibits a smaller spread as well as improved means and spatial patterns. The preconditioning improves the EnKF particularly for smaller ensemble sizes, allowing operational data assimilation with reduced computational effort. In a validation scenario of delineating groundwater protection zones, the preconditioned filter performs clearly better than the filter using the original initial ensemble.
    Keywords: Data Assimilation of Hydraulic Heads ; Ensemble Kalman Filter ; Kalman Ensemble Generator ; Groundwater-Age Tracers ; Phreatic Aquifer ; Groundwater Recharge ; Hydraulic Conductivity ; Specific Yield ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 2
    Language: English
    In: Journal of Hydrology, 27 November 2014, Vol.519, pp.3386-3399
    Description: The travel-time distribution between rivers and groundwater observation points and the mixing of freshly infiltrated river water with groundwater of other origin is of high relevance in riverbank filtration. These characteristics usually are inferred from the analysis of natural-tracer time series, typically relying on a stationary input–output relationship. However, non-stationarity is a significant feature of the riparian zone causing time-varying river-to-groundwater transfer functions. We present a non-stationary extension of nonparametric deconvolution by performing stationary deconvolution with windowed time series, enforcing smoothness of the determined transfer function in time and travel time. The nonparametric approach facilitates the identification of unconventional features in travel-time distributions, such as broad peaks, and the sliding-window approach is an easy way to accommodate the method to dynamic changes of the system under consideration. By this, we obtain time-varying signal-recovery rates and travel-time distributions, from which we derive the mean travel time and the spread of the distribution as function of time. We apply our method to electric-conductivity data collected at River Thur, Switzerland, and adjacent piezometers. The non-stationary approach reproduces the groundwater observations significantly better than the stationary one, both in terms of overall metrics and in matching individual peaks. We compare characteristics of the transient transfer function to base flow which indicates shorter travel times at higher river stages.
    Keywords: Travel-Time Distribution ; Bank Filtration ; Non-Stationarity ; Nonparametric Inference ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Journal of Hydrology, 15 November 2013, Vol.505, pp.352-363
    Description: Dissolved oxygen (DO) is an important groundwater-quality parameter, especially within the context of drinking-water production by riverbank filtration. In riverbank sediments, a strong decrease of DO over the distance of a few meters has frequently been observed. The consumption rates may vary in time, which puts the representativeness of common, sporadic DO measurements in groundwater, based on monthly or even yearly sampling, into question. We present a new modeling approach that allows efficiently estimating DO concentrations in alluvial groundwater from measured DO concentrations in the river under various temperature and discharge conditions. The model is based on the stochastic–convective reactive approach and assumes a time-invariant lognormal travel-time distribution of the stream tube ensemble connecting the river and a groundwater observation well. DO consumption, resulting from aerobic respiration, is modeled by zero-order kinetics. According to high-resolution DO time series measured in the Thur River (NE-Switzerland) and an adjacent observation well, the DO consumption rate appears to depend on river temperature and discharge. While the temperature dependence of aerobic respiration is well known, the discharge dependence is probably related to an increased trapping of particulate organic matter (POM) within the riverbed during high-discharge events, thus enhancing the POM availability and DO consumption rate. We propose an empirical equation that quantifies the dependence between discharge and the DO consumption rate. The estimated parameterization at our field site suggests that an increasing discharge within the narrow window of 20–50 m /s enhances the DO consumption rate by a factor of 4. By considering the measured DO in the river and including the dependence of the DO consumption rate on both discharge and temperature, the model was able to capture the diurnal, short-term (days to weeks), and seasonal dynamics of the observed DO within the alluvial aquifer. The temperature dependence of the DO consumption rate was found to be more important on a seasonal time scale, while the effect of discharge dominated the DO behavior during hydrological events extending over a few days to weeks. The presented modeling approach can be transferred to other riverbank-filtration systems to efficiently estimate DO concentrations in alluvial aquifers under various climatic and hydrologic conditions and, hence, assess the risk of approaching anoxic conditions in a changing climate.
    Keywords: Riverbank Filtration ; Climate Change ; Oxygen Consumption ; Stochastic–Convective Reactive Transport ; POM ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
    Source: ScienceDirect Journals (Elsevier)
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  • 4
    Language: English
    In: Journal of Hydrology, 13 February 2014, Vol.509, pp.631-631
    Keywords: Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
    Source: ScienceDirect Journals (Elsevier)
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  • 5
    Language: English
    In: Journal of Hydrology, 2010, Vol.380(1), pp.154-164
    Description: Vertical temperature profiling in the river beds of losing streams has been shown to be useful in obtaining seepage rates. We present a method for high-resolution vertical temperature profiling in surface-water sediments for detailed quantification of seepage flux over depth and time. The method is based on fiber-optic distributed temperature sensing, in which temperature profiles along an optical fiber are obtained by making use of Raman scattering. An optical fiber was wrapped around a 2 in. PVC tube and installed vertically within the streambed sediment. The wrapping transfers the spatial resolution along the fiber of 1 m to a vertical resolution of about 5 mm. The high-resolution temperature profiler was tested at a losing reach of the Swiss prealpine River Thur resulting in a 20-day long temperature time series with a temporal resolution of 10 min. The time series are analyzed by means of dynamic harmonic regression to obtain the diurnal contributions of the measured time series at all depths and time points. The time for the diurnal temperature signal to reach the observation depth and the associated attenuation of the signal are calculated from the phase angles and amplitudes of the diurnal contributions. The time shift results in an apparent celerity of diurnal temperature propagation, which is converted into an apparent seepage rate by fitting the data to the analytical solution for convective–conductive heat transfer in a semi-infinite, uniform, one-dimensional domain with a sinusoidal surface temperature. The high spatial resolution allows the location of discontinuities in the river bed which would have remained undetected if temperature had been measured only at a few individual depths to be identified. This is a particular strength of the fiber-optic high-resolution temperature profiler. The time series also give evidence of sporadic high infiltration rates at times of high water tables.
    Keywords: River–Groundwater Interaction ; Fiber-Optic Distributed Temperature Sensing ; Time Series Analysis ; Seepage Flux ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 6
    Language: English
    In: Journal of Hydrology, May 2018, Vol.560, pp.97-108
    Description: Mobile-immobile transport models can be effective in reproducing heavily tailed breakthrough curves of concentration. However, such models may not adequately describe transport along multiple flow paths with intermediate velocity contrasts in connected fields. We propose using the mobile-mobile model for simulating subsurface flow and associated mixing-controlled reactive transport in connected fields. This model includes two local concentrations, one in the fast- and the other in the slow-flow domain, which predict both the concentration mean and variance. The normalized total concentration variance within the flux is found to be a non-monotonic function of the discharge ratio with a maximum concentration variance at intermediate values of the discharge ratio. We test the mobile-mobile model for mixing-controlled reactive transport with an instantaneous, irreversible bimolecular reaction in structured and connected random heterogeneous domains, and compare the performance of the mobile-mobile to the mobile-immobile model. The results indicate that the mobile-mobile model generally predicts the concentration breakthrough curves (BTCs) of the reactive compound better. Particularly, for cases of an elliptical inclusion with intermediate hydraulic-conductivity contrasts, where the travel-time distribution shows bimodal behavior, the prediction of both the BTCs and maximum product concentration is significantly improved. Our results exemplify that the conceptual model of two mobile domains with diffusive mass transfer in between is in general good for predicting mixing-controlled reactive transport, and particularly so in cases where the transfer in the low-conductivity zones is by slow advection rather than diffusion.
    Keywords: Mobile-Mobile ; Mobile-Immobile ; Mixing ; Reactive Transport ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 7
    In: Ground Water, July 2010, Vol.48(4), pp.569-579
    Description: In most groundwater applications, measurements of concentration are limited in number and sparsely distributed within the domain of interest. Therefore, interpolation techniques are needed to obtain most likely values of concentration at locations where no measurements are available. For further processing, for example, in environmental risk analysis, interpolated values should be given with uncertainty bounds, so that a geostatistical framework is preferable. Linear interpolation of steady‐state concentration measurements is problematic because the dependence of concentration on the primary uncertain material property, the hydraulic conductivity field, is highly nonlinear, suggesting that the statistical interrelationship between concentration values at different points is also nonlinear. We suggest interpolating steady‐state concentration measurements by conditioning an ensemble of the underlying log‐conductivity field on the available hydrological data in a conditional Monte Carlo approach. Flow and transport simulations for each conditional conductivity field must meet the measurements within their given uncertainty. The ensemble of transport simulations based on the conditional log‐conductivity fields yields conditional statistical distributions of concentration at points between observation points. This method implicitly meets physical bounds of concentration values and non‐Gaussianity of their statistical distributions and obeys the nonlinearity of the underlying processes. We validate our method by artificial test cases and compare the results to kriging estimates assuming different conditional statistical distributions of concentration. Assuming a beta distribution in kriging leads to estimates of concentration with zero probability of concentrations below zero or above the maximal possible value; however, the concentrations are not forced to meet the advection‐dispersion equation.
    Keywords: Hydrogeology -- Analysis ; Hydrogeology -- Models ; Groundwater -- Analysis ; Groundwater -- Models ; Advection (Earth sciences) -- Analysis ; Advection (Earth sciences) -- Models;
    ISSN: 0017-467X
    E-ISSN: 1745-6584
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  • 8
    Language: English
    In: Journal of Contaminant Hydrology, 2010, Vol.111(1), pp.36-47
    Description: The contaminant mass discharge crossing a control plane is an important metric in the assessment of natural attenuation at contaminated sites. For risk-assessment purposes, the mass discharge must be estimated together with a level of uncertainty. We present a conditional Monte Carlo approach that allows estimating the statistical distribution of mass discharge. The approach is based on conditioning multiple realizations of the hydraulic conductivity field on all data available. We jointly determine a first-order decay coefficient in each realization, leading to conditional statistical distributions of all estimated parameters and the total mass discharge. The resulting statistical distribution of contaminant mass discharges can be used in the assessment of risks at the contaminated site. The method is applied to data of hypothetical test cases, which gives the opportunity to compare estimation results to the true field. As concentration data, we account for pointlike measurements obtained in multi-level sampling wells. The obtained empirical distribution of mass discharge crossing the multi-level sampling fence could be well fitted by a log-normal distribution.
    Keywords: Groundwater ; Steady-State Concentration ; Mass Discharge Distribution ; Bayesian Inference ; Engineering ; Environmental Sciences ; Geography
    ISSN: 0169-7722
    E-ISSN: 1873-6009
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  • 9
    Language: English
    In: Journal of Contaminant Hydrology, June 2013, Vol.149, pp.13-26
    Description: Colloidal particles can act as carriers for adsorbing pollutants, such as hydrophobic organic pollutants, and enhance their mobility in the subsurface. In this study, we investigate the influence of colloidal particles on the transport of pesticides through saturated porous media by column experiments. We also investigate the effect of particle size on this transport. The model pesticide is lindane (gamma-hexachlorocyclohexane), a representative hydrophobic insecticide which has been banned in 2009 but is still used in many developing countries. The breakthrough curves are analyzed with the help of numerical modeling, in which we examine the minimum model complexity needed to simulate such transport. The transport of lindane without particles can be described by advective–dispersive transport coupled to linear three-site sorption, one site being in local equilibrium and the others undergoing first-order kinetic sorption. In the presence of mobile particles, the total concentration of mobile lindane is increased, that is, lindane is transported not only in aqueous solution but also sorbed onto the smallest, mobile particles. The models developed to simulate separate and associated transport of lindane and the particles reproduced the measurements very well and showed that the adsorption/desorption of lindane to the particles could be expressed by a common first-order rate law, regardless whether the particles are mobile, attached, or strained.
    Keywords: Groundwater Transport ; Lindane ; Colloidal Particles ; Sorption ; Particle-Facilitated Transport ; Modeling ; Engineering ; Environmental Sciences ; Geography
    ISSN: 0169-7722
    E-ISSN: 1873-6009
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  • 10
    In: Water Resources Research, January 2015, Vol.51(1), pp.261-280
    Description: Characterizing the topology of three‐dimensional steady‐state flow fields is useful to describe the physical processes controlling the deformation of solute plumes and, consequently, obtain helpful information on mixing processes without solving the transport equation. In this work, we study the topology of flow in three‐dimensional nonstationary anisotropic heterogeneous porous media. In particular, we apply a topological metric, i.e., the helicity density, and two complementary kinematic descriptors of mixing, i.e., stretching and folding, to investigate: (i) the flow field resulting from applying a uniform‐in‐the‐average hydraulic gradient within a fully resolved heterogeneous three‐dimensional porous medium with a nonstationary anisotropic covariance function of the locally isotropic hydraulic log conductivity; (ii) the flow field obtained by averaging a set of Monte Carlo realizations of the former field; (iii) the flow field obtained considering the blockwise uniform anisotropic effective conductivity tensor computed for the fully resolved case. While in the fully resolved case, the local helicity density is zero as a consequence of the local isotropy of hydraulic conductivity, it differs from zero in the other two cases. We show, therefore, that this topological metric is scale dependent and should be computed at the appropriate scale to be informative about the leading patterns of plume deformation. Indeed, streamlines are helical in all three cases at scales larger than the characteristic scale of spatial variability. We apply stretching and folding metrics to investigate the scales at which plume deformation is more influenced by helical motion than by the effect of small‐scale spatial heterogeneity in the hydraulic‐conductivity field. Under steady‐state flow conditions, stretching, which quantifies the increasing length of an interface, dominates at short distances from a given starting plane, while folding, which describes how this interface is bent to fill a finite volume of space, dominates further downstream and can be correlated with the appearance of large‐scale secondary motion. We conclude that three‐dimensional flows in porous media may show a complex topology whose analysis is relevant for the description of plume deformation. These results have important implications for the understanding of mixing processes, as shown in detail in the companion paper focusing on solute transport. Macroscopic helical flow occurs in 3‐D nonstationary isotropic media Helicity density is scale dependent and is used to describe flow topology Stretching and folding metrics are used to describe plume deformation
    Keywords: Topology ; Helicity ; Stretching ; Folding ; Nonstationarity ; Anisotropic Correlation Structure
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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