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  • Hydrogeology
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  • 1
    In: Water Resources Research, July 2010, Vol.46(7), pp.n/a-n/a
    Description: Biodegradation of continuously emitted compounds that need a dissolved reaction partner, which is not jointly introduced with the contaminant into the subsurface, is mainly controlled by transverse dispersive mixing. Previous analytical approaches of evaluating mixing‐controlled bioreactive transport in steady state have been based on the assumption that the bulk aqueous‐phase concentration of the reactants is directly available to the specific biomass catalyzing the reaction. These models predict a very narrow stripe of active biomass with high specific biomass concentration. Experimental studies have indicated that such behavior may be unrealistic, particularly for anaerobic biodegradation. I extend the previous analysis to include kinetic solute uptake by the biomass, expressed as a first‐order mass‐transfer process coupled to dual Monod kinetics in the bio‐available domain. The approach is based on the evaluation of conservative components undergoing advective‐dispersive transport, the solution of a quadratic speciation problem within the immobile bio domain, and iterative simulation of linear transport of a single reactive constituent in steady state. Convergence is typically achieved within less than ten iterations. The comparison with simulations assuming instantaneous solute uptake by the biomass indicate that mass‐transfer kinetics may explain larger overlap of reactive constituents and a wider spatial distribution of specific biomass observed in experiments. Depending on the rate coefficient of mass transfer, the overall transformation of the contaminant may be significantly reduced or only slightly shifted to a region farther downstream.
    Keywords: Biodegradation ; Kinetic Mass Transfer ; Reactive Transport ; Monod Kinetics
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 2
    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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  • 3
    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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  • 4
    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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  • 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: Water Resources Management, Sept, 2013, Vol.27(12), p.4349(20)
    Description: Byline: Ashutosh Singh (1), Claudius M. Burger (1), Olaf A. Cirpka (1) Keywords: Urban hydrogeology; Groundwater management; MODFLOW; Numerical modeling; Response matrix; Optimization Abstract: In rapidly developing urban areas of emerging countries, increased water demand has led to enormous groundwater withdrawal, calling out for sustainable groundwater management. We suggest implementing a sustainable pumping rate concept based on numerical modeling of the managed aquifer. Sustainability is achieved by constraints regarding (1) a minimum groundwater discharge rate to gaining rivers (ecological constraint) and (2) a maximum drawdown along the city boundaries (social constraints) to prevent excessive groundwater depletion in the neighboring peri-urban and rural areas. The total groundwater extraction is maximized subject to these constraints, leading to specific extraction patterns throughout the city, depending upon the values set for the constraints. The optimization is performed by linear programming. For a given extraction rate, the two constraints can be traded off by the groundwater manager, causing different wells to be activated or deactivated. We demonstrate the applicability of the methodology by the example of the city of Lucknow, India, but it can be transferred to other cities facing conflicts of managing groundwater resources. Author Affiliation: (1) Center for Applied Geoscience (ZAG), University of Tubingen, Holderlinstr. 12, 72074, Tubingen, Germany Article History: Registration Date: 05/08/2013 Received Date: 15/03/2013 Accepted Date: 05/08/2013 Online Date: 18/08/2013
    Keywords: Hydrogeology -- Models ; Water Resource Management -- Models ; Aquifers -- Models ; Groundwater -- Models ; Developing Countries -- Models
    ISSN: 0920-4741
    E-ISSN: 15731650
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  • 7
    Language: English
    In: Journal of Contaminant Hydrology, Sept, 2012, Vol.138-139, p.22(18)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.jconhyd.2012.05.007 Byline: Olaf A. Cirpka, Massimo Rolle, Gabriele Chiogna, Felipe P.J. de Barros, Wolfgang Nowak Keywords: Transverse dispersion; Groundwater transport; Mixing-controlled reactions; Stochastic subsurface hydrology Abstract: We study plumes originating from continuous sources that require a dissolved reaction partner for their degradation. The length of such plumes is typically controlled by transverse mixing. While analytical expressions have been derived for homogeneous flow fields, incomplete characterization of the hydraulic conductivity field causes uncertainty in predicting plume lengths in heterogeneous domains. In this context, we analyze the effects of three sources of uncertainty: (i) The uncertainty of the effective mixing rate along the plume fringes due to spatially varying flow focusing, (ii) the uncertainty of the volumetric discharge through (and thus total mass flux leaving) the source area, and (iii) different parameterizations of the Darcy-scale transverse dispersion coefficient. The first two are directly related to heterogeneity of hydraulic conductivity. In this paper, we derive semi-analytical expressions for the probability distribution of plume lengths at different levels of complexity. The results are compared to numerical Monte Carlo simulations. Uncertainties in mixing and in the source strength result in a statistical distribution of possible plume lengths. For unconditional random hydraulic conductivity fields, plume lengths may vary by more than one order of magnitude even for moderate degrees of heterogeneity. Our results show that the uncertainty of volumetric flux through the source is the most relevant contribution to the variance of the plume length. The choice of different parameterizations for the local dispersion coefficient leads to differences in the mean estimated plume length. Article History: Received 14 January 2012; Revised 2 April 2012; Accepted 23 May 2012
    Keywords: Hydrogeology -- Analysis ; Monte Carlo Methods -- Analysis ; Groundwater -- Analysis
    ISSN: 0169-7722
    Source: Cengage Learning, Inc.
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  • 8
    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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  • 9
    In: Water Resources Research, January 2012, Vol.48(1), pp.n/a-n/a
    Description: We perform a salt tracer experiment, monitored by time‐lapse electrical resistivity tomography, in a quasi‐two‐dimensional sandbox with the aim of determining the hydraulic conductivity distribution in the domain. We use sodium chloride as a tracer, together with cochineal red for visual monitoring. The time series of observed resistance for each electrode configuration is characterized by its temporal moments. We invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by Pollock and Cirpka (2010). This is the first application of the approach to experimental data. The results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling. Using this estimation, a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox. The latter is compared to pictures taken during the experiment. These results show an even better agreement, indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present. We conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion. Use temporal moments to invert ERT monitoring data of salt‐tracer experiments Application to laboratory experiments has been successful Inverted results may be better than intended zonation of filling pattern
    Keywords: Electrical Resistivity Tomography ; Fully Coupled Inversion ; Salt Tracer Tests ; Temporal Moments
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 10
    Language: English
    In: Transport in Porous Media, 11/2012, 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 (sub v) , wavenumber, l, and frequency, omega , for a variety of values of the horizontal thermal Rayleigh number R (sub h) , and the vertical salinity Rayleigh number, S (sub 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 (sub r) = -1, -0.5, -0.1, 0, 0.1, 0.5, 1, for all the parameter cases in the treatment of normal modes. Copyright 2012 Springer Science+Business Media Dordrecht and Springer Science+Business Media B.V.
    Keywords: General Geochemistry ; Hydrogeology ; Convection ; Fluid Flow ; Hydrology ; Models ; Numerical Models ; Porous Materials ; Rayleigh Number ; Salinity ; Saturation ; Solutes ; Soret Effect ; Stability ; Temperature ; Thermal Gradient;
    ISSN: 0169-3913
    E-ISSN: 1573-1634
    Source: Springer (via CrossRef)
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