### Quantifying the influence of subsurface constructions on groundwater temperature - underground and suburban railway, subterranean garage and cellar, district heating network, and ductwork

*Gabriella Zsófia Somogyi*

^{1}, Kai Zosseder

^{1}

^{1}Lehrstuhl für Hydrogeologie, Technische Universität München

P 9.10

*in*Urbane Hydrogeologie

Urban areas typically show local increases in groundwater temperature known as ʻheat-islandʼ effect (Oke 1973). It is revealed that hotspots of up to 4 K above the average rural groundwater temperature often exist, which stem from anthropogenic heat sources such as surface sealing, underground and suburban railway tunnel system, district heating networks, open geothermal systems, subterranean garages, and cellars as well as ductworks (Menberg et al. 2013). Local increases in groundwater temperature (GWT) was detected in Munich according to own measurements in more than 200 observations wells in 2012-2013. An increase of up to 3 K in groundwater temperature can produce a remarkable improvement both in the output and in the efficiency of a shallow geothermal system (Allen et al. 2003). However, this effect can also reduce the efficiency of cooling systems. The aim of this study is to clarify the thermal influence of every relevant subterranean construction on the urban groundwater body of Munich and to predict their effects. Our main interest is the total heat flow Q (x,y,z,t) from the construction to the aquifer which can be utilized to generate low enthalpy geothermal energy. The heat flow to the ground/groundwater from a subterranean construction depends on complex thermal processes in the ground and on large number of parameters involved in construction geometry. Early articles deal with the steady-state heat loss for the two-dimensional case and consider pure heat conduction in a homogeneous, semi-infinite ground with constant thermal properties (Mitalas 1982). Convective heat transfer under groundwater flow conditions is usually neglected or it is determined by the fact that the groundwater temperature is equal to the mean soil surface temperature (Claesson & Hagentoft 1991). Under consideration of heat transfer by flowing groundwater and the strong variability of the soil surface temperature, it is a coupled three-dimensional, time-dependent thermal conductivity and convective heat transfer problem, which can be approximated by numerical methods. The scientific approach of this study is based on the following steps: (1) determination of reference systems, analytical model, (2) laboratory-heat-flow-tank-experiments, (3) numerical heat flow model, and (4) field observations. Here, we present detailed calculations and modeling of heat flow from subsurface constructions to the ground, which can be carried out by means of the finite element method (FEM)-based computer software FEFLOW^{®}. The heat flow model in Fig. 2 was validated by comparison with data from the literature and analytical formulas. It shows the distribution of the temperature around a 2.5 m deep cellar, surrounded by groundwater with flow velocity of 0.7 m/d. This article also shows the first results of the current 2D-heat-flow-laboratory-tank experiments.

Fig. 2:Temperature distribution around a 2.5 m deep cellar with groundwater flow velocity of 0.7 m/d.

Allen, A., Milenic, D. & Sikora, P. (2003): Shallow gravel aquifers and the urban ʻheat islandʼ effect: a source of low enthalpy geothermal energy. ˗ Geothermics, **32**(4-6): 569-578.

Claesson, J. & Hagentoft, C.-E. (1991): Heat loss to the ground from a building - I. General Theory. - Building and Environment, **26**(2): 195-208.

Menberg, K., Bayer, P., Zosseder, K., Rumohr, S. & Blum, P. (2013): Subsurface urban heat islands in German cities. - Science of the Total Environment, **442**: 123-133.

Mitalas, GP. (1982): Basement heat loss studies at DBR/NRC. – DBR Paper No. 1045, Division of Building Research, National Research Council of Canada.

Oke, T.R. (1973): City size and the urban heat island. ˗ Atmospheric Environment, **7**: 769-779.