Deterministic modeling of the impact of underground structures on urban groundwater temperature

Highlights

• Individual impact of underground structures on groundwater temperature were assessed.

• A Thermal Affected Zone is shown to occur approximately 100 m around the structure.

• The cumulative impact on ground water temperature was assessed in Lyon (France).

• The area impacted represents 10 times the total surface area of structures.

• The total annual heat flow from deep structures represents 4.5 GW · h.

Abstract

Underground structures have a major influence on groundwater temperature and have a major contribution on the anthropogenic heat fluxes into urban aquifers. Groundwater temperature is crucial for resource management as it can provide operational sustainability indicators for groundwater quality and geothermal energy. Here, a three dimensional heat transport modeling approach was conducted to quantify the thermally affected zone (TAZ, i.e. increase in temperature of more than + 0.5 °C) caused by two common underground structures: (1) an impervious structure and (2) a draining structure. These design techniques consist in (1) ballasting the underground structure in order to resist hydrostatic pressure, or (2) draining the groundwater under the structure in order to remove the hydrostatic pressure. The volume of the TAZ caused by these underground structures was shown to range from 14 to 20 times the volume of the underground structure. Additionally, the cumulative impact of underground structures was assessed under average thermal conditions at the scale of the greater Lyon area (France). The heat island effect caused by underground structures was highlighted in the business center of the city. Increase in temperature of more than + 4.5 °C were locally put in evidence. The annual heat flow from underground structures to the urban aquifer was computed deterministically and represents 4.5 GW · h. Considering these impacts, the TAZ of deep underground structures should be taken into account in the geothermal potential mapping. Finally, the amount of heat energy provided should be used as an indicator of heating potential in these areas.

Introduction

Half of the world's population now lives in cities. This phenomenon of urbanization is such that this proportion will reach 70% before 2050 (Un-Habitat, 2008). Land constraints lead to the vertical development of urban areas, with the construction of ever-deeper structures (Bobylev, 2009) (e.g., subways, building foundations, underground carparks). In parallel, the urban subsoil is now recognized as a space rich in resources: available water, geomaterials and geothermal energy (Li et al., 2013b, a), which play a vital role in ensuring sustainable territorial development (Goel et al., 2012), but for which regulations remain wanting (Foster and Garduño, 2013). This results in a lack of coordination and planning in the exploitation of this space, illustrated by conflicts over use (Bobylev, 2009).

In particular, the resilience of groundwater resources has become a major issue. On the one hand, 40% of the water distributed in the water supply networks of Europe comes from urban aquifers (Eiswirth et al., 2004), while on the other hand, geothermal energy is now considered as a strategic urban resource (Lund et al., 2011), (Herbert et al., 2013) since the European Council made a commitment to reduce greenhouse gas emissions by 20% by 2020 (European Commission, 2009). In parallel, underground structures involving impervious elements or pumping and re-injection devices, can impact groundwater flow (Attard et al., 2016c). Impervious structures can act as an obstacle to the flow (Epting et al., 2008), (Pujades et al., 2012). Draining structures can result in a fragmentation of urban flow systems (Attard et al., 2016b). In addition, underground structures can impact groundwater quality (Chae et al., 2008), (Attard et al., 2016a) and temperature (Hu et al., 2008), (Epting and Huggenberger, 2013), (Ferguson and Woodbury, 2004).

Regarding groundwater temperature, the heat island effect on groundwater due to urbanization has been clearly observed in many cities around the world (e.g. Zhu, K., et al., 2010, Taniguchi, M., et al., 2009, Menberg, K., et al., 2013a). In particular, Menberg et al. (2013b and Benz et al. (2015) used an analytical heat flux model and a GIS approach to demonstrate that underground structures makes up a significant share of the total anthropogenic heat flux into urban aquifers. In addition to that, Ampofo et al. (2004) used a mathematical model written with an engineering equation solver to investigate the heat load of an underground railway. The authors showed that heat absorbed by the earth surrounding a subway can reach 30% of the total heat generated by the structure. Finally, Epting and Huggenberger (2013) used a deterministic modeling approach to assess the thermal potential natural state and the present state of the groundwater body of Basel (Switzerland). The heat-transport model showed a major influence of deep underground structures on groundwater temperature. On the one hand, the total anthropogenic heat flux of a city into groundwater provides a potentially sustainable geothermal resource (Benz et al., 2015). However, according to (Hähnlein et al., 2013) the shallow geothermal system is only sustainable if the generated energy is mainly renewable energy. Consequently, the total anthropogenic heat flux should be quantified (Benz et al., 2015) and extracted in areas where groundwater temperatures are high (Allen, A., et al., 2003, Revesz, A., et al., 2016). On the other hand, groundwater cooling systems can be disturbed where temperatures are increased by anthropogenic heat flux Epting et al. (2013).

Understanding the thermally affected zone (TAZ) of both urban underground structures and groundwater heat pump systems (GWHPS) is important as it could facilitate the management of urban underground development and geothermal exploitation. Since the TAZ of GWHPS has been described thoroughly in previous works (Lo Russo et al., 2012), (Lo Russo et al., 2014), (Sciacovelli et al., 2014), the aim of this paper is to quantify the TAZ of two common underground structures on urban groundwater temperature: (1) impervious deep foundations, and (2) a structure with a drainage and re-injection system. In addition, the cumulative impact of these structures was assessed at the scale of the city of Lyon (France). The consequences on the geothermal potential of urban aquifers are discussed regarding the TAZ and the heat flow generated by underground structures.