Increasing Trends of Shallow Groundwater Warming in Vienna's Urban Aquifers

ABSTRACT Shallow groundwater in many cities around the world is subject to elevated temperatures that pose a risk to the groundwater quality and ecosystems. The objective of this study is to assess the suitability of different trend estimation methods for groundwater temperature and to specifically investigate the spatio‐temporal long‐term changes of water temperature in the urban groundwater of Vienna. Twenty‐year data records (2001–2020) from different sources were used to assess changes in air, soil, river and groundwater temperature regarding their annual mean and extreme percentile values. The effects of data quality, different trend methods, and various time periods of analysis were investigated. Block bootstrapping in combination with the Mann–Kendall trend test was found to be a suitable method for determining the significance of mean trends if the time‐series are short (10 years), as the underlying assumptions are lowest among all approaches. Between 2001 and 2020, the average annual temperature in Vienna increased by 0.9 K/decade for shallow groundwater and by 0.8 K/decade for air. However, the increase is not linear and has intensified in the later decade with an increase of 1.4 K/decade. The trends in extreme temperatures, represented by the lower (cold) / upper (warm) 10th percentile of air, soil and groundwater temperatures in the quantile regression, show the strongest increase in the lower 10th percentile of all air and soil temperatures. For groundwater, these extreme value trends are site‐specific and influenced by urban infrastructure and the interaction of groundwater with river water. These results underline the importance of spatially and temporally high‐resolution data and highlight the need for site‐specific aquifer characterisation for the sustainable use of shallow geothermal energy for heating and cooling. The trend of GWT rise in the urban aquifer needs to be considered in water management to avoid possible negative consequences for water quality and ecology.