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The U-type ground heat exchanger is a significant component of the ground source heat pump (GSHP) system, transferring heat from the soil for the air conditioning requirements. Heat accumulation becomes a common problem for the practical application of such systems in south China. Moreover, the buried pipes containing heat exchangers are constructed in a group form, making the heat accumulation situation more severe and complex. A 3-dimensional simulation model of a 9-well buried pipe group was established and validated by a thermal response test conducted in Nanjing, China. On the basis of this model, the influence of underground seepage water temperature and flow rate on thermal distribution was investigated. Subsequently, the heat transfer capacity of single-wel depth in a heat exchanger under varying conditions was calculated. It is concluded that a serious heat accumulation occurs without groundwater seepage. With seepage flow, the heat accumulation can be dissipated. Lower seepage temperature has a modestly beneficial effect on the heat dissipation. The heat accumulation can be better dissipated and the heat transfer capacity per well depth can be improved with greater seepage flow rate.