The objective of the study is to optimize the performance and thereby to mitigate the environmental impact of ground source heat pump (GSHP) systems using multiple borehole heat exchangers (BHEs) by including variable energy loads. Hence, an optimization procedure is developed that is able to predict temperature distributions in the subsurface. Optimized BHE fields are able to keep the maximum temperature change in the subsurface about 18% lower than BHE fields which feature equal flow rates for all BHEs. The long-term temperature anomaly can be mitigated and the possibility of extracting a higher amount of energy, while keeping temperature thresholds or environmental constraints, arises. (C) 2012 Elsevier Ltd. All rights reserved.