Legal constraints conditioning the thermal use of groundwater for cooling purposes in urban areas—A comparative assessment of thermal impact areas

Abstract The cooling demand of cities is currently increasing due to urban heat island effects which could be covered with renewable energy to reach climate goals. An efficient solution is thermal cooling using groundwater. This approach involves introducing waste heat into the aquifer, which, at the same time, also raises some concerns among water authorities about potential impacts on groundwater quality. Hence, strict regulations were imposed on temperature spreading and maximum injection temperatures, limiting the thermal use of groundwater for cooling. However, regulations vary widely by country, because the influence of such temperature changes on groundwater conditions is still uncertain. In the present study, a summary of country-specific regulations is presented and evaluated with respect to possible impacts. Based on the review, a novel approach for the comparative assessment of the influence of different legal frameworks on city-wide relevant thermal impact areas is presented and implemented in three urban case studies in Germany, Switzerland and Spain, with various hydrogeological conditions. The results demonstrate that in the cities with higher hydraulic conductivities and strict regulations, the thermally impacted areas, defined by a 3K-isotherm, affect only a low percentage of the aquifer, comparable to those with less strict regulations. However, extensive thermal groundwater use by large systems, relatively high temperature spreading and aquifers with low hydraulic conductivities, lead to moderate percentages of impacted aquifer areas, especially by considering real operation data. The presented methodological approach provides a simplified tool to help city managers and local water authorities to define operational thresholds related to thermal-cooling use of groundwater.