Heat accumulation in shallow aquifers: managing a growing resource

Heat loss from buildings, infrastructure and enhanced heat flow from sealed surfaces increase the temperatures of shallow groundwater often more than global warming. A worldwide analysis of thousands of wells reveals that the temperature at every second location is higher than expected, and local anthropogenic heat sources that exist for decades contribute to subsurface waste heat accumulation down to a depth of around 100 m. At some places, such as in the city centre of Cologne, heating of groundwater by several degrees of Celsius appears to have even reached a maximum. Here, long-term temperature records reveal stabilizing thermal conditions in the shallow aquifer. This also means that the geothermal potential has increased significantly, possibly to a critical level for maximum stored heat in place. Still, the natural geothermal resources together with the artificially stored resources are often overlooked. In many regions, recycling only the energy lost to the subsurface could (1) fulfil a substantial part of the heat demand of buildings, and (2) increase the efficiency of heat pumps with a more favourable thermal regime during the heating period. This resource is growing.  On the global scale, by the end of this century nearly 75% of the heat demand could be covered by recycling the heat that accumulates in the subsurface from anthropogenic heat loss and in response to climate change. Especially in densely populated areas, continued heat accumulation mitigates the risk of overexploiting the geothermal potential of shallow aquifers. Sustainable thermal management of aquifers must integrate concepts of heat recycling to avoid long-term warming of groundwater. For this, integrated spatial planning is needed. Shallow geothermal systems such as groundwater heat-pump installations have to be spatially organized in urban districts to achieve optimal use of the geothermal resource. They can maintain controlled cooling of the groundwater while benefitting from enhanced waste heat flux. As an example, we discuss the thermal interference of urban infrastructure and geothermal wells for the city of Lyon, which are spatially arranged based on hydraulic and thermal criteria to benefit from urban groundwater heating.