Aquifer Thermal Energy Storage (ATES) is the process of storing heat in an aquifer by injecting water of a different temperature than the ambient groundwater, storing it, then recovering the heated or cooled water for use in a heat exchange energy system. This paper describes numerical modeling of ATES system in an effort to alleviate the problems encountered by the system. The computer code Heat and Solute Transport in 3D (HST3D) and ATES SYM were used to construct two dimensional models of aquifer. Temperature values calculated using the model for each simulation was compared with measured temperatures in the production and observation wells. For the case of one hot injection well and one cold injection well, the agreement between the HST3D model with the given data is approximately 90%. The final calibrated two-dimensional model agreed to the measured data to within 88.0%. Comparison between the HST3D and ATES SYM models for Sussex Health Centre ATES system, agrees to approximately 96%. It can be concluded that the HST3D model has been successfully verified and may be used as a valid analysis tool.