An inventory of 67 ATES systems showed that almost none of the investigated systems had a thermal balance (IF Technology 2007), meaning that cold or heat is discharged into the aquifer and long-term cooling or warming of groundwater is occurring. A study in Winnipeg, Canada, showed that using the aquifer solely for cooling purposes is not sustainable because of long-term rising groundwater temperatures (Ferguson and Woodbury 2006). Apart from reduced efficiency of UTES because of changing temperatures, downstream users of groundwater and aqueous ecosystems can be negatively affected (Ferguson 2009). To assess the long-term cumulative effects of heat discharge adequately, the autonomous trends caused by changing environmental stresses to the groundwater system should also be considered. For example, when analyzing the thermal impacts of UTES on the underground, the temperature effects of climate change and urbanization on the aquifer system should also be taken into account (Taniguchi and Uemura 2005, Ferguson and Woodbury 2007, Kooi 2008). The key question is: Can the effects of UTES be neglected compared to these stresses or should UTES be considered yet another thermal stress on a system that is already threatened?