Peter Huggenberger

Using two- and three-dimensional georadar methods to characterize glaciofluvial architecture

Abstract The threat of pollution in the shallow subsurface has led to an increasing need to understand how complex heterogeneities in gravel aquifers influence groundwater transport. To characterize these heterogeneities, we have conducted extensive two-dimensional (2-D) and three-dimensional (3-D) ground-penetrating radar (GPR or georadar) surveys in two quarries within the Rhine valley of northeastern Switzerland. The quarries comprised gravel and sand deposited in a proglacial braided river system that followed the maximum Wurm-stage glaciation. After the surveys, the terrace walls beneath the two study sites were photographed as they were being excavated. By combining information extracted from the 2- and 3-D georadar images with the outcrop photographs, it was possible to correlate georadar facies with the various glaciofluvial architectural elements. The dominant elements were scour pools formed at or near the confluences of two stream channels and horizontally bedded and massive gravel sheets deposited during moderate to high water flow conditions and exposed during low flow conditions. Architectural elements were generally elongated parallel to the mean flow directions of the ancient river system. Variations in the strike of their long axes reflected lateral and vertical changes in the local flow direction. Time slices showed structural trends not evident on 2-D georadar images and photographs. Data interpretation was quicker, more complete and less ambiguous when georadar facies analyses were based on a combination of georadar vertical profiles and time slices than when based on georadar vertical profiles alone.

Mapping the architecture of glaciofluvial sediments with three-dimensional georadar

Three-dimensional (3-D) ground-penetrating radar (georadar) mapping offers new opportunities for determining the geometries and facies of surficial sedimentary units. To investigate the potential of this high-resolution technique and at the same time study the architecture of Quaternary glaciofluvial deposits, georadar data have been collected on a dense grid established across a sequence of braided-river gravels and sands in northeastern Switzerland. Results of this survey are striking 3-D images that provide many more details and much more reliable information on the heterogeneities of the shallow underground than are afforded by conventional georadar profile data. Continuous subhorizontal and oblique reflections can be traced throughout vertical sections and horizontal slices of the georadar data block to a depth of (Approx.)15 m. Clearly defined are the dominant flow direction of the ancient braided-river system, the boundaries between different sedimentary facies, and the level of the ground-water table. Trough-fill sediments and subhorizontal channel deposits observed on 7-m-high quarry walls can be followed confidently in the subsurface. The orientation, shape, and size of the troughs and the strike and dip of the cross-bedding are all well resolved.

Ground-probing radar as a tool for heterogeneity estimation in gravel deposits: advances in data-processing and facies analysis

Abstract Pleistocene gravely braided-river deposits in river valleys constitute a large fraction of the natural ground-water reservoirs in Switzerland. The knowledge of the distribution and variability of hydraulic conductivity within these deposits are key factors for the estimation of water residence times and of description of large-scale mixing procesess in aquifers such as macrodispersion. It has been shown elsewhere that the spatial variability of hydraulic conductivity is related to the composition and the characteristic dimensions of sedimentary structures, which are themselves related to the dynamics of ancient braided-river systems. In many contamination problems, sedimentological information is sparse and drill-core descriptions and pumping-tests only give a limited picture of the geometry of inhomogeneities. The ground-probing radar (GPR) method is a promising tool for resolving changes of physical properties in gravel deposits at the scale of natural inhomogeneities arising from changing sedimentary composition. However, the main limitation of GPR is the rapid attenuation of electromagnetic waves in subsurface sediments such as gravels, which leads to a limited penetration of the order of 10 to 15 m for a 250 MHz antenna. The objectives of our present work are: 1. (1) To show how digital processing methods similar to reflection seismics may be applied for velocity and profile processing. These methods can improve both the resolution of radar profiles, in particular at greater depths, and the determination of velocity distributions from CDP experiments. 2. (2) To examine whether and to what extent the characteristic lithofacies of Pleistocene gravel deposits can be recognized as mappable reflection patterns on ground-probing radar (GPR) reflection profiles in order to gain information about the geometry of inhomogeneities. Using modern digital data processing methods, such as band pass, high- or low-cut filtering, deconvolution and velocity analysis, much more significant information can be obtained from the recorded GRP field data-sets. Our results demonstrate that on GPR reflection images the basic fluvial forms such as (1) pool deposits generated at the junction of two channels, and (2) channel deposits may be distiguished. Their shape and characteristic spatial dimensions may be recognized from a series of profiles in different directions. Because the method can detect changes in the water content, the reflection image may be related even to small changes in the degree of saturation of the sediments. Thus reflectors can indicate the changing composition of sediments.

Interpretation of drill-core and georadar data of coarse gravel deposits

Abstract Pollution in the shallow subsurface has led to an increasing need of understanding how to quantitatively characterize both the heterogeneity of gravel aquifers and the influence of heterogeneity on groundwater flow and solute transport. Models play an important role in decision-making processes, especially in the context of better characterizing and in forecasting the behavior of a given geological system. The objective of the present paper is the derivation of a gradual lithofacies-based interpretation of outcrop, drill core, and ground penetrating radar (GPR or georadar) data of different quality. The presented method allows a probability estimation of drill core layer descriptions and radarfacies types representing defined sedimentary structure types. The method includes a determination of ‘initial structure type probabilities’ for grain-size categories and combinations thereof described in drill core layer descriptions as well as a subsequent differentiation of these structure type probabilities in an iterative process considering ‘additional information’ like main constituent, quantity, fraction, and sorting of single grain-size categories, color, chemical precipitation, layer thickness, and adjacent layer. The radarfacies types are calibrated with drill cores located in the vicinity of georadar sections. The calibration process consists of the assignment of the calculated structure type probabilities from the drill core layer descriptions to the corresponding radarfacies types considering the proportion in thickness between drill core layers and georadar structures. The structure type probabilities can be given for points along boreholes and a grid with arbitrary mesh size along georadar sections. The method is applied to field examples from the Rhine/Wiese aquifer near Basel in Switzerland. The resulting structure type probabilities can be used for conditioning stochastic simulations of geological models. However, the conditioned stochastic simulation of the Rhine/Wiese aquifer is the topic of another paper. The results show the importance of a detailed sedimentological analysis of outcrops and drill cores as well as its significance on the distinction of sedimentary structure types.

Analysis of aquifer heterogeneity within a well capture zone, comparison of model data with field experiments: a case study from the river Wiese, Switzerland.

Abstract. This paper describes two groundwater models simulating a well capture zone in a heterogeneous aquifer located near an infiltrating river. A deterministic, large-scaled groundwater model (1.8 × 1.2 km) is used to simulate the average behavior of groundwater flow and advective transport. It is also used to assign the boundary conditions for a small-scaled groundwater model (550 × 400 m) which relies on stochastically generated aquifer properties based on site-specific drill core and georadar data. The small-scaled groundwater model is used to include the large subsurface heterogeneity at the location of interest. The stochastic approach in the small-scaled groundwater model does not lead to a clearly defined well capture zone, but to a plane representation of the probability of a certain surface location belonging to the well capture zone. The models were applied to a study site, which is located in an area of artificial groundwater recharge and production, in Lange Erlen near Basel, Northwestern Switzerland. The groundwater at this site contributes to the city’s drinking water supply, and the site serves as a recreational area to the population of Basel. The river is channelized, but there are initiatives to restore the riverbank to more natural conditions. However, they conflict with the requirements of groundwater protection, especially during flood events. Therefore, a river section of 600 m in the vicinity of an unused and disconnected drinking water well was restored to study changes in the groundwater flow regime depending on hydrologic variations, water supply operation data, progress of river restoration, and subsurface heterogeneity. The results of the groundwater models are compared with data from two tracer experiments using Uranine and the natural Radon isotope Rn-222, and with physical, chemical, and microbiological data sampled in monitoring wells between the river and the drinking water well. The groundwater models document significant variations regarding the dimension of the well capture zone depending on changing boundary conditions and the variability of the hydraulic aquifer properties. The knowledge of the subsurface heterogeneity is important to evaluate transport times and distances of microorganisms from the infiltrating river or the riverbank to the drinking water well. The data from the monitoring wells show that chemical and microbiological processes predominantly occur in the hyporheic interstitial zone and the riverbank within a range of a few meters up to a few 10s of meters from the river. The methods presented here can be used to define and evaluate groundwater protection zones in heterogeneous aquifers associated with infiltration from rivers under changing boundary conditions, and under the uncertainty of subsurface heterogeneity. Furthermore, they allow one to study the site-specific operational alternatives associated with river restoration.

Exploring an aquifer system by integrating hydraulic, hydrogeologic and environmental tracer data in a three- dimensional hydrodynamic transport model

This article presents a numerical model of a part of an aquifer that is recharged by infiltration from the Swiss pre-Alpine river Toss in the Linsental (north-eastern Switzerland). The nearby city of Winterthur makes use of this aquifer as a resource of drinking water. The presented model is part of a larger interdisciplinary research program undertaken with the goal to evaluate the possible impacts of a planned revitalization of the severely canalized river Toss. Above all it should show the extent of decrease of the groundwater residence time if the river bed is allowed to move towards the drinking water wells. The flow model was constrained and calibrated by transport modelling of tritiogenic 3 He. This tracer reflects both the aging of the water (by accumulation of 3 He resulting from tritium-decay) as well as the two different components of the mixture (river water free of tritiogenic 3 He due to degassing, and groundwater enriched in 3 He due to accumulation). By simulating a Dirac- pulse-shaped input of a conservative tracer at different sources (river cells or upstream flux boundary cells) it is possible to determine the age distributions as well as the mixing ratios of the two types of water at the two pumping stations within the model area. The same calculations for a hypothetical river course passing directly beside the pumping stations indicate a decrease of the mean residence time of the pumped water together with an increase of the amount of the younger river water component. q 2001 Elsevier Science B.V. All rights reserved.

A numerical three-dimensional conditioned/unconditioned stochastic facies type model applied to a remediation well system

In this study a three-dimensional stochastic facies-based aquifer model was developed. The model can be used to numerically simulate flow and solute transport in heterogeneous groundwater aquifers. The stochastic generation process can be conditioned by using available facies information in one vertical plane or two orthogonal vertical ones. In this study the information was obtained from a facies interpretation of a vertical georadar profile in a natural gravel formation in Switzerland. In the domain outside the known profile, unconditioned lenses and layers were generated at random according to statistical information on coherent sedimentary structures based on observations in adjacent gravel pits [Jussel et al., 1994a]. The method was applied to a single extraction well designed to capture an initially block-shaped contaminant plume. A total of 80 conditioned and unconditioned synthetic aquifers was generated. The flow and transport simulations were performed using a finite element flow model and a random walk transport model. The results are presented as the ensemble of integral solute mass recovery curves of single realizations. One would expect conditioning to reduce the bandwidth of the recovery curves representing the uncertainty, but the results show that the bandwidth even increased. This effect was attributed to a discrepancy in the mean volumetric fraction of the different facies types in the conditioned and the unconditioned cases. Moreover, a simulation using a homogeneous model with constant equivalent flow and transport parameters overestimated the remediation efficiency. The influence of a linear, reversible equilibrium sorption on the remediation well efficiency was taken into account by an uncorrelated random field of the retardation factor based on values from the literature. However, the impact of the variability in hydraulic conductivity clearly exceeded the effect of the variability in the retardation factor.

Validation of a numerical indicator of microbial contamination for karst springs.

Rapid changes in spring water quality in karst areas due to rapid recharge of bacterially contaminated water are a major concern for drinking water suppliers and users. The main objective of this study was to use field experiments with fecal indicators to verify the vulnerability of a karst spring to pathogens, as determined by using a numerical modeling approach. The groundwater modeling was based on linear storage models that can be used to simulate karst water flow. The vulnerability of the karst groundwater is estimated using such models to calculate criteria that influence the likelihood of spring water being affected by microbial contamination. Specifically, the temporal variation in the vulnerability, depending on rainfall events and overall recharge conditions, can be assessed and quantified using the dynamic vulnerability index (DVI). DVI corresponds to the ratio of conduit to diffuse flow contributions to spring discharge. To evaluate model performance with respect to predicted vulnerability, samples from a spring were analyzed for Escherichia coli, enterococci, Clostridium perfringens, and heterotrophic plate count bacteria during and after several rainfall events. DVI was shown to be an indication of the risk of fecal contamination of spring water with sufficient accuracy to be used in drinking water management. We conclude that numerical models are a useful tool for evaluating the vulnerability of karst systems to pathogens under varying recharge conditions.

Enhanced vulnerability assessment in karst areas by combining mapping with modeling approaches

The objective of this work is to facilitate a sustainable regional planning of water resources in karst areas by providing a conceptual framework for an integrative vulnerability assessment. A combined mapping and modeling approach is proposed, taking into account both spatial and temporal aspects of karst groundwater vulnerability. The conceptual framework comprises the delineation of recharge areas, vulnerability mapping, numerical flow and transport modeling and the integration of information into a combined vulnerability map and time series. The approach is illustrated at a field site in northwest Switzerland (Gempen plateau). The results show that the combination of vulnerability mapping and numerical modeling allows the vulnerability distribution, both in the recharge and discharge areas, to be identified, and at the same time, the time dependence of karst groundwater vulnerability to be assessed. The combined vulnerability map and time series provide a quantitative basis for drinking water management and for regional planning.

Variable-density flow in heterogeneous porous media - laboratory experiments and numerical simulations.

Konz, M., Ackerer, P., Younes, A., Huggenberger, P., Zechner, E., 2009a. 2D Stable Layered Laboratory-scale Experiments for Testing Density-coupled Flow Models. Water Resources Research, 45. doi:10.1029/2008WR007118., a series of laboratory-scale 2D tank experiments were conducted and accurately simulated for density driven flow problems on homogeneous porous media. In the present work, we extended the numerical and experimental studies to heterogeneous problems. The heterogeneous porous medium was constructed with a low permeability zone in the centre of the tank and had well-defined parameters and boundary conditions. Concentration distributions were measured in high resolution using a photometric method and an image analysis technique. The numerical model used for the simulations was based on efficient advanced approximations for both spatial and temporal discretizations. The Method Of Lines (MOL) was used to allow higher-order temporal discretization. Three different boundary conditions, corresponding to different localizations of the inflow and the outflow openings at the opposite edges of the tank, were applied to investigate different flow scenarios in the heterogeneous porous medium flow tank. Simulation results of all three density coupled experiments revealed a density-dependent behavior of dispersion. Thus, a reduction of dispersivites was required to obtain a good matching of the experimental data. The high quality of the experiments enabled a detailed testing of numerical variable-density flow codes under heterogeneous conditions. Therefore, the experiments were considered to be reliable benchmark tests.