Annette Affolter

The thermal impact of subsurface building structures on urban groundwater resources – A paradigmatic example

Shallow subsurface thermal regimes in urban areas are increasingly impacted by anthropogenic activities, which include infrastructure development like underground traffic lines as well as industrial and residential subsurface buildings. In combination with the progressive use of shallow geothermal energy systems, this results in the so-called subsurface urban heat island effect. This article emphasizes the importance of considering the thermal impact of subsurface structures, which commonly is underestimated due to missing information and of reliable subsurface temperature data. Based on synthetic heat-transport models different settings of the urban environment were investigated, including: (1) hydraulic gradients and conductivities, which result in different groundwater flow velocities; (2) aquifer properties like groundwater thickness to aquitard and depth to water table; and (3) constructional features, such as building depths and thermal properties of building structures. Our results demonstrate that with rising groundwater flow velocities, the heat-load from building structures increase, whereas down-gradient groundwater temperatures decrease. Thermal impacts on subsurface resources therefore have to be related to the permeability of aquifers and hydraulic boundary conditions. In regard to the urban settings of Basel, Switzerland, flow velocities of around 1 md-1 delineate a marker where either down-gradient temperature deviations or heat-loads into the subsurface are more relevant. Furthermore, no direct thermal influence on groundwater resources should be expected for aquifers with groundwater thicknesses larger 10m and when the distance of the building structure to the groundwater table is higher than around 10m. We demonstrate that measuring temperature changes down-gradient of subsurface structures is insufficient overall to assess thermal impacts, particularly in urban areas. Moreover, in areas which are densely urbanized, and where groundwater flow velocities are low, appropriate measures for assessing thermal impacts should specifically include a quantification of heat-loads into the subsurface which result in a more diffuse thermal contamination of urban groundwater resources.

Improved water resource management for a highly complex environment using three-dimensional groundwater modelling

A three-dimensional groundwater model was used to improve water resource management for a study area in north-west Switzerland, where drinking-water production is close to former landfills and industrial areas. To avoid drinking-water contamination, artificial groundwater recharge with surface water is used to create a hydraulic barrier between the contaminated sites and drinking-water extraction wells. The model was used for simulating existing and proposed water management strategies as a tool to ensure the utmost security for drinking water. A systematic evaluation of the flow direction between existing observation points using a developed three-point estimation method for a large number of scenarios was carried out. It is demonstrated that systematically applying the developed methodology helps to identify vulnerable locations which are sensitive to changing boundary conditions such as those arising from changes to artificial groundwater recharge rates. At these locations, additional investigations and protection are required. The presented integrated approach, using the groundwater flow direction between observation points, can be easily transferred to a variety of hydrological settings to systematically evaluate groundwater modelling scenarios.RésuméUn modèle tridimensionnel d’eau souterraine a été utilisé pour améliorer la gestion des ressources en eau pour une zone d’étude dans le Nord-Ouest de la Suisse, où la production d’eau potable est à proximité d’anciens sites d’enfouissement et des zones industrielles. Pour éviter la contamination de l’eau potable, la recharge artificielle des eaux souterraines avec de l’eau de surface est utilisée pour créer une barrière hydraulique entre les sites contaminés et les puits d’exploitation d’eau potable. Le modèle a été utilisé pour simuler les stratégies existantes et proposées de gestion de la ressource en eau comme outil pour assurer la plus grande sécurité pour l’eau potable. Une évaluation systémique de la direction des écoulements entre les piézomètres existants en utilisant une méthode d’estimation à trois points développée pour un grand nombre de scénarios a été réalisée. Il est démontré que l’application systématique de la méthodologie développée aide à identifier les endroits vulnérables qui sont sensibles aux changements des conditions aux limites, telles que celles résultant des changements de taux de recharge artificielle des eaux souterraines. Dans ces endroits, des études et protection complémentaires sont requises. L’approche intégrée présentée, en utilisant la direction de l’écoulement des eaux souterraines entre les piézomètres, peut être facilement transférée à une variété de contextes hydrogéologiques pour évaluer de manière systématique des scénarios de modélisation des eaux souterraines.ResumenSe utilizó un modelo tridimensional de agua subterránea para mejorar la gestión de los recursos hídricos de un área de estudio en el noroeste de Suiza, donde la producción de agua potable está cerca de los antiguos vertederos de residuos y zonas industriales. Para evitar la contaminación del agua potable, la recarga artificial de agua subterránea con agua superficial se utiliza para crear una barrera hidráulica entre los sitios contaminados y los pozos de extracción de agua potable. El modelo se utilizó para simular las estrategias existentes y propuestas de gestión del agua como una herramienta para garantizar la máxima seguridad para el agua potable. Se llevó a cabo una evaluación sistemática de la dirección del flujo entre puntos de observación existentes utilizando un método de estimación de tres puntos desarrollado para un gran número de escenarios. Se demuestra que la aplicación sistemática de la metodología desarrollada ayuda a identificar lugares vulnerables que son sensibles a las condiciones cambiantes de los límites, como los que surgen de los cambios en las tasas de recarga artificial de agua subterránea. En estos lugares, se requieren investigaciones adicionales y de protección. El enfoque integrado que se presenta, que utiliza la dirección del flujo de agua subterránea entre los puntos de observación, puede transferirse fácilmente a una variedad de ambientes hidrológicos para evaluar sistemáticamente los escenarios a partir del modelado del agua subterránea.摘要在瑞士西北部一个研究区,饮用水生产靠近过去的填埋场及工业区,在这里利用三维地下水模型提高水资源的管理水平。为了避免饮用水遭到污染,采用地表水人工补给地下水,在污染的场地和饮用水抽水井之间构建水力屏障。模型用作进行模拟现有的和建议的水管理策略的工具,以确保饮用水的绝对安全。利用开发出的针对各种不同方案的三点估算法对现有观测点之间的水流方向进行了系统评估。证明系统应用开发的方法有助于确定易受物探的位置,这些位置对变化的边界条件诸如人工地下水补给量导致的边界变化非常敏感。在这些位置,需要进行额外的调查和保护。所展示的利用观测点之间水流方向的综合方法可容易地应用到各种水文地质背景下,系统地评估地下水模拟方案。ResumoUm modelo tridimensional de águas subterrâneas foi utilizado para aprimorar o gerenciamento de recursos hídricos para uma área de estudo no noroeste da Suíça, onde a produção de água potável está próxima de antigos aterros e áreas industriais. Para evitar a contaminação da água potável, a recarga artificial de águas subterrâneas com água superficial é utilizada para criar uma barreira hidráulica entre os locais contaminados e os poços de extração de água potável. O modelo foi utilizado para simular estratégias existentes e propostas de gerenciamento da água, como uma ferramenta para garantir a máxima segurança para a água potável. Foi realizada uma avaliação sistemática da direção do fluxo entre os pontos de observação existentes utilizando um método de estimativa de três pontos desenvolvido para um grande número de cenários. Demonstra-se que a aplicação sistemática da metodologia desenvolvida ajuda a identificar locais vulneráveis que são sensíveis a modificações nas condições de contorno, tais como os decorrentes de mudanças nas taxas de recarga artificial de água subterrânea. Nesses locais, são necessárias investigações adicionais e proteção. A abordagem integrada apresentada, utilizando a direção do fluxo das águas subterrâneas entre os pontos de observação, pode ser facilmente transferida a uma variedade de configurações hidrológicas para avaliar sistematicamente cenários de modelagem de águas subterrâneas.

Hypotheses and Concepts

Within this chapter, we present and discuss several hypotheses and some concepts which we consider as important for urban geology. The first section deals with adaptive subsurface and groundwater resource management in urban areas with a focus on the definition of “system and risk profiles.” The second section discusses the importance and role of “flow across boundaries.” The third section describes an approach for the assessment of “vulnerability” of urban groundwater resources and includes a discussion on how to define “quality control systems.” In the last section, we discuss impacts of anthropogenic and climate change to quantitative and qualitative aspects of groundwater resources in the city Basel.

Examples and Case Studies

The presented examples and case studies illustrate specific applications of adaptive management of water resources in the region of Basel, Northwestern Switzerland. Such concepts together with the setup of tools and process-oriented experiments allow testing hypotheses. The applied methods facilitated us to fill several gaps of knowledge of subsurface processes. The examples focus on questions with practical as well as research. Most topics are relevant for urban areas and the sustainable use of subsurface resources in general.