Groundwater is a valuable natural resource, providing irrigation water and water for industrial processes and supplying about half of the world’s population with drinking water.
Groundwater temperature (GWT) is important for the physical and chemical state of groundwater. Insight in the GWT field has many practical applications. Groundwater temperature is being used as a tracer in research into the lateral and vertical flow of groundwater and hydraulic properties of the subsurface. Temperature-depth (TD) profiles have been used in paleo-climate reconstruction. Subsurface and groundwater temperature is also an important design parameter for underground infrastructure, such as power transmission networks.
GWT is determined by quasi-equilibrium between heat flow from the Earth’s interior (geothermal heat flow) and the long-term ground surface temperature (GST). GST is subject to various short- and long-term fluctuations and changes. These disturbances propagate downward as thermal waves, while being attenuated with depth. Diurnal fluctuations dominate GWT at very shallow depths (<0.5 m), while seasonal fluctuations at the surface propagate to a depth of approx. 15 m.
Human-induced climate change resulted in an observed surface air temperature (SAT) rise since approx. 1900. SAT in the Netherlands shows an accelerated increase since the late 1970’s. Climate change results in increasing GST and rising temperatures of infiltrating rain and surface water, resulting in higher GWT. Other drivers affecting GWT include changes in land use (agriculture, urbanization, deforestation) and the hydrological system by pumping or managed aquifer recharge. Furthermore, the subsurface temperature is affected by anthropogenic heat sources, such as underground constructions and infrastructure. Aquifers are increasingly being used worldwide for storage and recovery of (seasonal) thermal energy, such as underground thermal energy storage (UTES), or aquifer thermal energy storage (ATES). These may cause local, large temperature anomalies.
Insight into subsurface and groundwater temperatures, their spatial distribution and change in time is important for groundwater management and optimum use of the subsurface e.g. for UTES. The aim of this thesis is to improve the understanding of groundwater temperatures, their spatiotemporal distribution and drivers of groundwater temperature evolution in the Netherlands. The most important drivers of groundwater temperature change are climate change, land use change (in particular urbanization), and underground thermal energy storage. Based on this, the following main research questions have been formulated:
1.How are groundwater temperatures distributed in the Netherlands, and how are the mapped patterns explained?
2.What are the regional temporal trends in groundwater temperature. How are these trends related to changes in land use, climate and groundwater management or exploitation?
3.What is the impact of urbanization on subsurface and groundwater temperatures?
4.What is the impact of aquifer thermal energy storage (ATES) on local groundwater temperatures? Is this impact related to hydraulic and thermal heterogeneity of the subsurface? ....
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Subsurface temperatures are substantially higher in urban areas than in surrounding rural environments; the result is a subsurface urban heat island (SUHI). SUHIs and their drivers have received attention in studies world-wide. In this study, a well-constrained data set of subsurface temperatures from Amsterdam, The Netherlands, is presented. The study demonstrates that, through modeling of centuries-long (from fourteenth to twenty-first century) urban development and climate change, along with the history of both the surface urban heat-island temperatures and ground surface temperatures, it is possible to simulate the development and present state of the Amsterdam SUHI. The results provide insight into the drivers of long-term SUHI development, which makes it possible to distinguish subterranean heat sources of more recent times that are localized drivers (such as geothermal energy systems, sewers, boiler basements, subway stations or district heating) from larger-scale drivers (mainly heat loss from buildings and raised ground-surface temperatures due to pavements). Because these findings have consequences for the assessment of the shallow geothermal potential of the SUHIs, it is proposed to distinguish between (1) a regional, long-term SUHI that has developed over centuries due to the larger-scale drivers, and (2) local anomalies caused by anthropogenic heat sources less than one century old.

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