[Show abstract][Hide abstract] ABSTRACT: Aquifer storage and recovery (ASR) methods are increasingly used to overcome the temporal imbalance between water demand and availability. Common ASR recharge methods utilize large-diameter injection wells or surface infiltration basins and trenches, and can be costly to implement. A new low-cost ASR recharge method is currently being developed. This approach is based on recharge via gravity in small-diameter wells installed with direct-push (DP) technology. Numerical modeling is used here to assess the potential of this new approach under conditions commonly faced in field settings. The primary objective is to investigate if a battery of small-diameter DP wells can serve as a viable alternative to a surface basin under typical field conditions, while the secondary objective is to assess which subsurface parameters have the greatest control on DP well performance. Simulation results indicate that gravity recharge via small-diameter wells appears to have a distinct advantage over recharge via surface infiltration basins. For example, two 0.05-m shallow vadose-zone wells with 9-m screens can recharge water at a greater rate than a 60 m2 basin. Also, results reveal that, contrary to an infiltration basin, the recharge rate in a DP well has a much stronger dependence on the horizontal component of hydraulic conductivity than on the vertical component. Moreover, near-surface layers of low hydraulic conductivity, which can significantly reduce the recharge capacity of a surface basin, have a relatively small impact on the recharge capacity of a well as long as a significant portion of the well screen is installed below those layers. Given that installation and operation costs can be low in comparison to common ASR recharge methods, this new approach appears to have great potential for recharging good quality water in shallow unconsolidated aquifers. A field investigation has recently been initiated to follow up the findings of this simulation assessment.
Journal of Hydrology 09/2014; 517:54–63. · 2.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the last ten years (approximately) a lot of natural attenuation scenarios have been modeled under the assumption that the degradation of pollutants occurs mainly at the reaction front. This was particularly facilitated by using numerical methods which readily covers a wide range of different hydrogeological scenarios to predict associated plume length. In contrast, due to experimental difficulties, only part of the scenarios has so far been simulated in laboratory experiments in order to assess the suitability of the models under controlled conditions. This paper deals with the design and evaluation of natural attenuation scenario at laboratory scale in a tank experiment which is based on a vertical 2D experiment (tank dimension 200 cm * 2 cm * 15 cm). The study aims at the maximum spreading of electron donors (contaminant) reacting with the vertically entering electron acceptors (e.g. oxygen). In the experiment, this scenario was simulated by a base-acid pair with pH = 11.3 and pH = 2, respectively. An indicator mixing with both acid and base was used to visualize the reaction front. Glass beads (diameter 1.55 mm to 1.88 mm) was used as porous media. Experiments were evaluated after reaching steady conditions.
[Show abstract][Hide abstract] ABSTRACT: Large thermal extractions and extensive implementation of groundwater heat pumps (GWHP) necessitate a validation of the sustainability of their use and possible detrimental effects on groundwater. The goal of this work is to develop a regional heat transport model (of ~13 km × 5 km) for real site conditions. This model should consider all relevant transport processes, despite the large area under investigation. The model is based on a two-dimensional, transient-calibrated groundwater flow model for the “Leibnitzer Feld” (Styria, Austria). The two-dimensional horizontal model is linked via the FEFLOW interface manager with a newly developed “Multi-Layer-Model”-tool, which reproduces thermal aquifer–atmosphere interaction. Based on the regional heat transport model, scenarios are delineated for heating and cooling purposes for large GWHPs, which are appropriate for a small manufacturing business, an administrative building and 10 family homes. First of all, these have large spacing and thereafter, effects of area-covering usage of geothermal systems are evaluated for five administrative buildings located in close proximity to one another (200–350 m) and also for a large number of smaller heat extractions (each representing a one family house system). Modeled spatial and temporal temperature effects on the shallow aquifer are discussed. It was possible to present a simulation of realistic heating and cooling scenarios. This simulation may be introduced into practice once some further simplifications to the system are made. Locally limited heat plumes (max. length: 625 m) were observed for the manufacturing business. Any thermal effects coming from the geothermal systems were shown to be temporally stable. As such, no distinct trend of reduced annual temperatures could be observed.
[Show abstract][Hide abstract] ABSTRACT: We present a novel approach for the numerical simulation of the gelation of silicate solutions under density-dependent flow conditions. The method utilizes an auxiliary, not density-dependent solute that is subject to a linear decay function to provide temporal information that is used to describe the viscosity change of the fluid. By comparing the modeling results to experimental data, we are able to simulate the behavior and the gelation process of the injected solute for three different compositions, including long-term stability of the gelated area, and non-gelation of low concentrations due to hydro-dynamic dispersion. This approach can also be used for other types of solutes with this gelling property and is useful in a variety of applications in geological, civil and environmental engineering.
Journal of contaminant hydrology 11/2013; 157C:1-10. · 2.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A groundwater model characterized by a lack of field data about hydraulic model parameters and boundary conditions combined with many observation data sets for calibration purpose was investigated concerning model uncertainty. Seven different conceptual models with a stepwise increase from 0 to 30 adjustable parameters were calibrated using PEST. Residuals, sensitivities, the Akaike information criterion (AIC and AICc), Bayesian information criterion (BIC), and Kashyap's information criterion (KIC) were calculated for a set of seven inverse calibrated models with increasing complexity. Finally, the likelihood of each model was computed. Comparing only residuals of the different conceptual models leads to an overparameterization and certainty loss in the conceptual model approach. The model employing only uncalibrated hydraulic parameters, estimated from sedimentological information, obtained the worst AIC, BIC, and KIC values. Using only sedimentological data to derive hydraulic parameters introduces a systematic error into the simulation results and cannot be recommended for generating a valuable model. For numerical investigations with high numbers of calibration data the BIC and KIC select as optimal a simpler model than the AIC. The model with 15 adjusted parameters was evaluated by AIC as the best option and obtained a likelihood of 98%. The AIC disregards the potential model structure error and the selection of the KIC is, therefore, more appropriate. Sensitivities to piezometric heads were highest for the model with only five adjustable parameters and sensitivity coefficients were directly influenced by the changes in extracted groundwater volumes.
[Show abstract][Hide abstract] ABSTRACT: Conduit Flow Process (CFP) couples a pipe flow model to MODFLOW-2005. In this hybrid model the karst aquifer is conceptualized as a dual flow system consisting of highly conductive conduits and a less conductive porous/fissured rock matrix. The code is partly based on the hybrid model Carbonate Aquifer Void Evolution (CAVE). The existing CFP functionality is enhanced by processes and boundary conditions to facilitate the practical applicability for karst characterization and management. Modifications comprise (A) enhancements to flow routines and (B) addition of heat and solute transport.
Enhancements to flow routines are intended to improve CFP capabilities for water abstraction from karst aquifers, e.g., large scale hydraulic tests. These applications require one to consider fast-responding storage in connection with the conduit network. Hence, conduit-associated drainable storage was implemented in CFP. Beyond this, specific boundary conditions were added, namely a constraint for the fixed head boundary that limits in- or outflow from karst conduits by a user-defined threshold. It is demonstrated that the enhanced CFP flow routines can qualitatively reproduce water abstraction scenarios.
Implementation of mass and heat transport routines allows one to apply CFP to tracer test analysis. Existing heat and solute transport routines from CAVE were updated, enhanced, and converted to CFP. With this, CFP computes convective heat transport with matrix conduction for laminar and turbulent flow. A conduit interacts with the matrix via a thermal boundary layer and matrix conduction is considered by a localized 1D radial approach perpendicular to the conduits. Solute transport considers advection and dispersion for laminar and turbulent flow. Additionally, solutes in conduits can interact with the surrounding rock, resulting in matrix diffusion. The correct implementation of heat and solute transport is demonstrated by schematic model studies. Likewise, these model studies illustrate the sensitivity of different transport processes useful for future field application.
NGWA Groundwater Summit, San Antonio, TX, USA; 04/2013
[Show abstract][Hide abstract] ABSTRACT: Thermal management of aquifers requires knowledge on interactions and
heat transport processes not only on a local but also on a more regional
scale. Therefore, prediction of temperature developments due to thermal
use and other anthropogenic impacts necessitate the use of large scale
numerical models based on field temperature measurements. This
contribution presents different modelling strategies for the thermal
management of shallow rural and urban groundwater bodies. Depending on
the settings and the relevant management topics different boundary
conditions have to be considered. Whereas, thermal regimes within rural
groundwater bodies primarily are governed by natural boundaries and the
interaction with the atmosphere, in urban areas also the influences of
urbanization and heated subsurface constructions have to be considered.
Therefore, the setup of modelling tools as basis for the thermal
management of groundwater bodies in different settings requires
different interaction processes to be focused on. The study is
illustrated by selected examples of a rural groundwater body located in
the "Leibnitzer Feld" (Austria) and an urban groundwater body located in
the city of Basel (Switzerland). The two case studies differ in their
respective hydro-geological setting, above all in the vertical extents
of the saturated and unsaturated zone. Therefore, specific modelling
approaches are used to focus on a reliable description of the main
governing impacts. The regional models evaluate current and future
thermal use of the groundwater bodies and highlight the advantages
arising from a regional view of heat transport processes.
[Show abstract][Hide abstract] ABSTRACT: Karst aquifers are characterized by highly conductive conduit flow paths
embedded in a less conductive fissured and fractured matrix resulting in
strong permeability contrasts with structured heterogeneity and
anisotropy. Groundwater storage occurs predominantly in the fissured
matrix. Hence, most karst models assume quasi steady-state flow in
conduits neglecting conduit associated drainable storage (CADS). The
concept of CADS considers storage volumes, where karst water is not part
of the active flow system but rather hydraulically connected to conduits
(for example karstic voids and large fractures). The disregard of
conduit storage can be inappropriate when direct water abstraction from
karst conduits occurs, e.g. large scale pumping. In such cases, CADS may
be relevant. Furthermore, the typical fixed head boundary condition at
the karst outlet can be inadequate for water abstraction scenarios
because unhampered water inflow is possible. The objective of this paper
is to analyze the significance of CADS and flow-limited boundary
conditions on the hydraulic behavior of karst aquifers in water
abstraction scenarios. To this end, the numerical hybrid model
MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is enhanced to account
for CADS. Additionally, a fixed-head limited-flow (FHLQ) boundary
condition is added that limits inflow from constant head boundaries to a
user-defined threshold. The affect and proper functioning of these
modifications is demonstrated by simplified model studies. Both
enhancements, CAD storage and the FHLQ boundary, are shown to be useful
for water abstraction scenarios within karst aquifers. An idealized
representation of a large-scale pumping test in a karst conduit is used
to demonstrate that the enhanced CFPM1 is potentially able to adequately
represent water abstraction processes in both the conduits and the
matrix of real karst systems.
Hydrology and Earth System Sciences Discussions 04/2013; 10(4):4463-4487. · 3.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The impact of source thickness on steady-state plume length is studied using modifications of the analytical expressions provided in Liedl et al. (2005, 2011) for 2D and 3D scenarios. For comparison, 2D and 3D numerical experiments were performed, and the following three important conclusions were obtained: first, the modified expressions overestimate the plume length only up to a factor of 2 when the source thickness (M
) is at least 50 % of the aquifer depth (M). Second, overestimates do not exceed plume length by a factor of 10 (2D scenario) or 5 (3D scenario) for 25 % < M
/M < 50 %. Third, numerical techniques are recommended for M
< 25 %. In addition, it was observed that the degradation from the top dominates for M
/M > 50 %. As far as the numerical experiments are concerned, it is important to note that the employed finite element approach was applied to the transformed transport equation provided in both Liedl et al. works. This transformation, which can also be applied to more complex scenarios than those studied here, eliminates reaction terms from the model equations and therefore largely facilitates numerical computations.
[Show abstract][Hide abstract] ABSTRACT: This paper deals with numerical modeling of groundwater systems. We present the implementation of an approach to solve a moving boundary problem for a dynamic water table within an invariant finite element mesh. The modeling software is successfully validated against laboratory experiment data for an unconfined, density-dependent benchmark. The validated software is applied to a regional-scale study area and sufficiently calibrated for a steady state of pre-development conditions. Transient mass transport
scenario simulations show good concordance with salinity measurements satisfyingly supporting the model setup.
Journal of Computational and Applied Mathematics 12/2012; 236(18):4798-4809. · 1.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, we present a framework for numerical modeling of CO2 injection into porous media for enhanced gas recovery (EGR) from depleted reservoirs. Physically, we have to deal with non-isothermal, compressible gas flows resulting in a system of coupled non-linear PDEs. We describe the mathematical framework for the underlying balance equations as well as the equations of state for mixing gases. We use an object-oriented finite element method implemented in C++. The numerical model has been tested against an analytical solution for a simplified problem and then applied to CO2 injection into a real reservoir. Numerical modeling allows to investigate physical phenomena and to predict reservoir pressures as well as temperatures depending on injection scenarios and is therefore a useful tool for applied numerical analysis.
Journal of Computational and Applied Mathematics 12/2012; 236(18):4933–4943. · 1.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three commonly used thermal equations of state for carbon dioxide, as well as the ideal gas law, have been compared against a large number of measurement data taken from the literature. Complex equations of state reach a higher accuracy than simple ones. The inaccuracy of the density function can cause large errors in fluid property correlations, such as heat capacity or viscosity. The influence of this inaccuracy on the results of numerical simulations have been evaluated by two examples: The first one assumes isothermal gas expansion from a volume, while the second one considers heat transport along a fracture. For both examples, different equations of state have been utilized. The simulations have been performed on the scientific software platform OpenGeoSys. The difference among the particular simulation results is significant. Apparently small errors in the density function can cause considerably different results of otherwise identical simulation setups.
[Show abstract][Hide abstract] ABSTRACT: A groundwater model characterized by a lack of field data to estimate
hydraulic model parameters and boundary conditions combined with many
piezometric head observations was investigated concerning model
uncertainty. Different conceptual models with a stepwise increase from 0
to 30 adjustable parameters were calibrated using PEST. Residuals,
sensitivities, the Akaike Information Criterion (AIC), and the
likelihood of each model were computed. As expected, residuals and
standard errors decreased with an increasing amount of adjustable model
parameters. However, the model with only 15 adjusted parameters was
evaluated by AIC as the best option with a likelihood of 98%, while the
uncalibrated model obtained the worst AIC value. Computing of the AIC
yielded the most important information to assess the model likelihood.
Comparing only residuals of different conceptual models was less
valuable and would result in an overparameterization of the conceptual
model approach. Sensitivities of piezometric heads were highest for the
model with five adjustable parameters reflecting also changes of
extracted groundwater volumes. With increasing amount of adjustable
parameters piezometric heads became less sensitive for the model
calibration and changes of pumping rates were no longer displayed by the
sensitivity coefficients. Therefore, when too many model parameters were
adjusted, these parameters lost their impact on the model results.
Additionally, using only sedimentological data to derive hydraulic
parameters resulted in a large bias between measured and simulated
Hydrology and Earth System Sciences Discussions 08/2012; 9(8):9687-9714. · 3.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Coming from the zero inertia (ZI) equations, an analytical model to describe sheet flow phenomena with a special focus on rainfall runoff processes is developed. A slight modification of the ZI equations, which draws upon the concept of a momentum-representative cross-section of the moving water body, leads—after comprehensive mathematical calculus—to an analytical solution describing essentially one-dimensional, shallow overland flow. In a test series, the analytical ZI model is applied together with three numerical models, one based on the Saint-Venant equations, one on the kinematic wave equations, and another one on diffusion wave equations. The test application refers to a typical rainfall runoff situation, i.e., rather shallow overland flow on a hillslope as a consequence of excess rainfall. Contrary to the analytical model, the comparative analysis clearly shows the difficulties of the numerical solutions in terms of exactness and robustness when approaching typical shallow water depths. This problem of numerical models is tackled by applying small time and space discretization, which, however, comes along with higher CPU execution times. Besides the good computational efficiency and freedom of any numerical inconvenience, the new analytical model outperforms the numerical models for typical overland flow simulations. This particularly refers to a highly satisfactory fulfillment of the mass balance and a nearly perfect match of peak flow rates.
Journal of Hydraulic Engineering 05/2012; 138(5):391-399. · 1.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Numerical modeling of interacting flow and transport processes between different hydrological compartments, such as the atmosphere/land surface/vegetation/soil/groundwater systems, is essential for understanding the comprehensive processes, especially if quantity and quality of water resources are in acute danger, like e.g. in semi-arid areas and regions with environmental contaminations. The computational models used for system and scenario analysis in the framework of an integrated water resources management are rapidly developing instruments. In particular, advances in computational mathematics have revolutionized the variety and the nature of the problems that can be addressed by environmental scientists and engineers. It is certainly true that for each hydro-compartment, there exists many excellent simulation codes, but traditionally their development has been isolated within the different disciplines. A new generation of coupled tools based on the profound scientific background is needed for integrated modeling of hydrosystems. The objective of the IWAS-ToolBox is to develop innovative methods to combine and extend existing modeling software to address coupled processes in the hydrosphere, especially for the analysis of hydrological systems in sensitive regions. This involves, e.g. the provision of models for the prediction of water availability, water quality and/or the ecological situation under changing natural and socio-economic boundary conditions such as climate change, land use or population growth in the future.
KeywordsCoupling–Modeling–Surface–subsurface–Soil–root water flow–Reactive transport–Density-dependent flow