Abstract: The water chemical conditions of groundwater in coastal areas are complex. In order to find out the harm of groundwater quality to groundwater source heat pump system (GWHP), the harm mechanism was studied. Through the hydro-chemical analysis of the phreatic and First confined water in the Yancheng City, the main ion sources and causes were determined, and the harm of Mg2+, Ca2+ and Fe2+ was analyzed. Finally, the saturation index Is was calculated by PHREEQC, and the potential harm of groundwater overflow mixing and groundwater recharge superposition to the ground water source heat pump was analyzed. The results show that the TDS and hardness of groundwater in Yancheng area are high. Na+,Mg2+, Ca2+, Fe2+ HCO3- and  Cl- are the main abnormal ions. The diving ρ(TDS) is 180.42-4 497.00 mg/L, ρ (Na+)，ρ (Mg2+)，ρ (Ca2+) are 278.16，72.91，72.74 mg/L, respectively. The anions ρ(HCO3-) and ρ (Cl-) are 487.33 mg/L and 298.76 mg/L, respectively. The ρ (TDS) of  First confined water was 508.50-17 182.00 mg/L, ρ(Na+), ρ (Ca2+) and  ρ (Fe2+) were 1 595.06, 564.57 and 4.50 mg/L, respectively, and the ρ (Cl-) was 4 560.06 mg/L. The process of groundwater migration is influenced by various water-rock reactions such as dissolution and intrusion of ancient seawater. Major problems such as Fe(OH)3 scaling, CaCO3 and CaMg(CO3)2 precipitation and scaling will occur in the long-term operation of GWHP.

Key words: complex hydro-chemical conditions, ground water source heat pump system, groundwater scaling, groundwater