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  • Aquifers
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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: 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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  • 3
    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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  • 4
    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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  • 5
    In: Water Resources Research, July 2010, Vol.46(7), pp.n/a-n/a
    Description: We present a method for the determination of hydraulic conductivity from monitoring of salt tracer tests by electrical resistivity tomography (ERT). To ensure that the underlying principles of flow, transport, and geoelectrics are obeyed in the inversion, we perform a fully coupled hydrogeophysical analysis using temporal moments of electrical potential perturbations. In the predictive mode, we use moment‐generating equations with corresponding adjoint equations for the evaluation of sensitivities. For inversion, we apply the quasi‐linear geostatistical inversion approach. The method is tested in a synthetic case study mimicking a laboratory‐scale quasi two‐dimensional sandbox, in which 48 electrodes and 8 piezometers are used. The hydraulic conductivity field is estimated from the mean arrival times of electrical potential perturbations and hydraulic heads. The estimated hydraulic conductivity field reproduces most features with, however, a loss of variability. Even though only the temporal moments of the electrical signals are used for inversion, the transient behavior is satisfactorily recovered. Also, the spatial patterns of concentration arrival times in the true and estimated cases are in good agreement, so that the propagation of the tracer plume can be followed fairly accurately. We test the effects of large measurement errors and erroneous prior information on the performance of the inversion. While prior statistical parameters are of minor importance in detecting the major pattern of hydraulic conductivity, a large measurement error could have an important impact on the solution. Also, the choice of electrode configurations appears to be important. In particular, strictly surface‐based geoelectrical surveys do not seem to be very suitable for identifying spatial patterns of hydraulic conductivity by ERT monitoring of salt tracer tests within aquifers.
    Keywords: Electrical Resistivity Tomography Ert ; Tracer Test ; Geostatistical Inversion ; Hydrogeophysics
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 6
    Language: English
    In: Ground water, 2011, Vol.49(4), pp.491-502
    Description: Infiltrating river water carries the temperature signal of the river into the adjacent aquifer. While the diurnal temperature fluctuations are strongly dampened, the seasonal fluctuations are much less attenuated and can be followed into the aquifer over longer distances. In one-dimensional model with uniform properties, this signal is propagated with a retarded velocity, and its amplitude decreases exponentially with distance. Therefore, time shifts in seasonal temperature signals between rivers and groundwater observation points may be used to estimate infiltration rates and near-river groundwater velocities. As demonstrated in this study, however, the interpretation is nonunique under realistic conditions. We analyze a synthetic test case of a two-dimensional cross section perpendicular to a losing stream, accounting for multi-dimensional flow due to a partially penetrating channel, convective-conductive heat transport within the aquifer, and heat exchange with the underlying aquitard and the land surface. We compare different conceptual simplifications of the domain in order to elaborate on the importance of different system elements. We find that temperature propagation within the shallow aquifer can be highly influenced by conduction through the unsaturated zone and into the underlying aquitard. In contrast, regional groundwater recharge has no major effect on the simulated results. In our setup, multi-dimensionality of the flow field is important only close to the river. We conclude that over-simplistic analytical models can introduce substantial errors if vertical heat exchange at the aquifer boundaries is not accounted for. This has to be considered when using seasonal temperature fluctuations as a natural tracer for bank infiltration.
    Keywords: Models, Theoretical ; Rivers ; Temperature ; Water Cycle
    ISSN: 0017467X
    E-ISSN: 1745-6584
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  • 7
    In: Groundwater, April 2015, Vol.53(S1), pp.139-148
    Description: Numerical and laboratory studies have provided evidence that combining hydraulic tomography with tomographic tracer tests could improve the estimation of hydraulic conductivity compared with using hydraulic data alone. Field demonstrations, however, have been lacking so far, which we attribute to experimental difficulties. In this study, we present a conceptual design and experimental applications of tracer tomography at the field scale using heat as a tracer. In our experimental design, we improve active heat tracer testing by minimizing possible effects of heat losses, buoyancy, viscosity, and changing boundary conditions. We also utilize a cost‐effective approach of measuring temperature changes in situ at high resolution. We apply the presented method to the 8 m thick heterogeneous, sandy gravel, alluvial aquifer at the Lauswiesen Hydrogeological Research Site in Tübingen, Germany. Results of our tomographic heat‐tracer experiments are in line with earlier work on characterizing the aquifer at the test site. We demonstrate from the experimental perspective that tracer tomography is applicable and suitable at the field scale using heat as a tracer. The experimental results also demonstrate the potential of heat‐tracer tomography as a cost‐effective means for characterizing aquifer heterogeneity.
    Keywords: Hydrogeology ; Aquifers ; Tracers (Biology) ; Tomography;
    ISSN: 0017-467X
    E-ISSN: 1745-6584
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  • 8
    In: Water Resources Research, May 2014, Vol.50(5), pp.4149-4162
    Description: Models of microbial dynamics coupled to solute transport in aquifers typically require the introduction of a bacterial capacity term to prevent excessive microbial growth close to substrate‐injection boundaries. The factors controlling this carrying capacity, however, are not fully understood. In this study, we propose that grazers or bacteriophages may control the density of bacterial biomass in continuously fed porous media. We conceptualize the flow‐through porous medium as a series of retentostats, in which the dissolved substrate is advected with water flow whereas the biomasses of bacteria and grazers are considered essentially immobile. We first model a single retentostat with Monod kinetics of bacterial growth and a second‐order grazing law, which shows that the system oscillates but approaches a stable steady state with nonzero concentrations of substrate, bacteria, and grazers. The steady state concentration of the bacteria biomass is independent of the substrate concentration in the inflow. When coupling several retentostats in a series to mimic a groundwater column, the steady state bacteria concentrations thus remain at a constant level over a significant travel distance. The one‐dimensional reactive transport model also accounts for substrate dispersion and a random walk of grazers influenced by the bacteria concentration. These dispersive‐diffusive terms affect the oscillations until steady state is reached, but hardly the steady state value itself. We conclude that grazing, or infection by bacteriophages, is a possible explanation of the maximum biomass concentration frequently needed in bioreactive transport models. Its value depends on parameters related to the grazers or bacteriophages and is independent of bacterial growth parameters or substrate concentration, provided that there is enough substrate to sustain bacteria and grazers. One‐dimensional transport model with substrate‐bacteria‐grazer interactions Steady state bacteria concentration is constant over a certain length Grazing may explain the carrying capacity of bacteria in groundwater ecosystems
    Keywords: Groundwater Ecology ; Grazer ; Retentostat ; Reactive Transport ; Microbial Dynamics ; Top‐Down Control ; Linearized Stability Analysis ; Carrying Capacity
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 9
    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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  • 10
    In: Water Resources Research, December 2016, Vol.52(12), pp.9390-9411
    Description: We present a semianalytical model for the transport of solutes being subject to sorption in porous aquifers. We couple a travel time‐based model of advective transport with a spherical diffusion model of kinetic sorption in nonuniform material mixtures. The model is formulated in the Laplace domain and transformed to the time domain by numerical inversion. By this, three‐dimensional transport of solutes undergoing mass transfer between aqueous and solid phases can be simulated very efficiently. The model addresses both hydraulic and reactive heterogeneity of porous aquifers by means of hydrofacies, which function as homogeneous but nonuniform subunits. The total exposure time to each of these subunits controls the magnitude of sorption effects, whereas the particular sequence of facies through which the solute passes is irrelevant. We apply the model to simulate the transport of phenanthrene in a fluvio‐glacial aquifer, for which the hydrofacies distribution is known at high resolution, the lithological composition of each facies has been analyzed, and sorption properties of the lithological components are available. Taking the fully resolved hydrofacies model as reference, we evaluate different approximations referring to lower information levels, reflecting shortcomings in typical modeling projects. The most important feature for a good description of both the main breakthrough and tailing of phenanthrene is the nonuniformity of the porous medium. While spatial heterogeneity of chemical properties might be neglected without introducing a large error, an approximation of the facies' composition in terms of a uniform substitute material considerably compromises the quality of the modeling result. Efficient semianalytical transport model for sorbing solutes in heterogeneous porous media Hydraulic and reactive heterogeneity are considered by spatially distributed nonuniform hydrofacies Results show major significance of facies nonuniformity to accuracy of simulated solute breakthrough
    Keywords: Mathematical Model ; Solute Transport ; Kinetic Sorption ; Aquifer Heterogeneity
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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