Assessment of groundwater temperature measurements in an urbanized area

R. Cernak (SGIDS), 30 August 2021

Bratislava is in southwestern Slovakia at the foot of the Malé Karpaty Mts. sitting on both banks of the river Danube. It is a main source of the groundwater along with groundwater transfer from adjacent hills on the northwest part of the city. The share of geothermal energy use for heating and cooling is partly unknown due to a lack of information about the installations and utilization performance. There is also limited information about closed loop system installations.

Aim of the field measurements was to review and interpret the recorded values of groundwater temperatures in selected wells. The monitoring wells were chosen in the pilot area and in the city centre to evaluate the influence of factors which impact groundwater temperature in the study area (figure 1). Variables that affect the groundwater temperature are surface water temperature (river Danube) and anthropogenic factors that result in urban heat anomalies. Data were obtained by continuous measurements of groundwater level and groundwater temperature.

To a greater or lesser extent, three main factors affect all wells – air temperature, Danube river temperature and urban development. A close relationship with the temperature of the Danube was expected and confirmed from the boreholes on the right and left bank of the Danube and in vicinity to the river (VN138-1, VN138-3). Wells with high values of groundwater temperature in places of maximum urban development showed influence of anthropogenic factors on the subsurface temperature. The higher temperatures are more likely caused by direct utilization of groundwater in open loop systems used preferably for cooling of the buildings in newly developed area (City centre, wells VN4-2, VN5-7). Marginal areas are influenced by air temperature through paved surfaces or subsurface infrastructure installations. The latest mentioned can be assumed to be in higher density of installations in the centre than in the surroundings. The groundwater temperature profiles and time series of selected wells are shown in figure 2 and figure 3.

Field measurements show that to identify and accurately delimit urban heat islands, a more dense monitoring network is necessary. So far only limited data of open or closed loop installations are available. Relevant monitoring and management schemes, as they are currently developed in MUSE, will help to sustainably use shallow geothermal energy in the city.