No abstract

This paper describes two groundwater models simulating a well capture zone in a heterogeneous aquifer located near an infiltrating river. A deterministic, largescaled 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 Aquatic Sciences 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 sitespecific operational alternatives associated with river restoration.

Abstract. 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.

Aunque los resultados de este estudio son específicos del caso, pueden transferirse directamente a otras áreas urbanas, tanto el acercamiento conceptual global, como las metodologías aplicadas.

Statistical information on sedimentary structures determined from natural gravel deposits in north-eastern Swit zerland is used to characterize transport phenomena of aquifers. These aquifers essentially consist of a back ground gravel matrix with embedded lenses of different hydraulic properties and internal structure. One of the embedded materials is highly conductive open framework gravel which leads to a bimodal probability density function of the hydraulic conductivity of the overall gravel material. Based on an analysis of this information three approaches to modeling macrodispersion are suggested. In the first one distinct unconditioned realizations of synthetic block shaped facies type aquifers are generated numerically such that they exhibit the same statistical properties with respect to facies geometry and hydraulic properties as the deposits themselves. Numerical experi ments simulating saturated flow and transport were subsequently performed with the help of a three-dimensional finite element flow model and a corresponding random walk transport model. A total of 100 experiments allowed estimates of apparent, time dependent macrodispersivity values. In the second approach the statistical parameters characterizing the overall gravel deposit were directly applied to an analytical unimodal stochastic model of apparent macrodispersivity according to Dagan (1988). For the modeling of the bimodal nature of the hydraulic conductivity an analytical anisotropic stochastic model was developed based on the isotropic model of Rubin (1995). This third approach was again applied using the appropriate statistical parameters. Differences in the results of the three models are discussed. They can be attributed mainly to uncertainty in the input parameters, and to the complex sedimentary structure predominant in the natural gravel deposits which were investigated. RÉSUMÉDes informations statistiques sur les structures sédimentaires déterminées a partir de depots de graviers dans le nord-est de la Suisse sont utilises pour caractériser les phénomènes de transport dans l'aquifère. Les matrices de ces aquifères sont principalement constituées de graviers avec des lentilles scellées aux propriétés hydrauliques et structures internes variées. Un de ces matériaux constitue un réseau tres permeable au sein de l'aquifère, d'oü l'idée d'utiliser une fonction de densité de probabilité bimodale pour la conductivité hydraulique de l'ensemble du matériau compose de graviers. A partir de l'analyse de cette information, trois approches sont suggérées pour modéliser la macrodispersion. Dans la première, différentes realisations non conditionnelles d'aquifères synthé-tiques caractérisés par des facies en forme de bloc, possédant les mêmes caractéristiques statistiques en terme de facies, de geometrie et de propriétés hydrauliques que les depots étudiés, sont générées numériquement. Des tests numériques simulant le transport et l'écoulement saturé ont été realises a l'aide d'un code 3D aux elements finis et un modèle lagrangien de type ...