Abstract The authors have been conducting demonstration and performance assessment of the ground-coupled heat pump (GCHP) system optimized for the air conditioning system of an experimental house in the artificial island in Fukuoka City, Japan, since 2005 to evaluate the subsurface thermal environmental changes caused by heat exchange with the ground. The authors monitored the underground temperature and groundwater level in some observation wells around the heat exchanging well, and performed numerical simulation of the underground temperature change with a groundwater simulator “FEFLOW”. The simulation result for a 20-year operation by using the numerical model, which had replicated the measured heat exchange rate during the 40-day heating operation from 15 December 2006, predicted that the cumulative increase or decrease of subsurface temperature will not be seen. Therefore, it is thought that there is little impact to the subsurface thermal environment around this GCHP system.