Geothermal energy can be extracted from an aquifer, where the groundwater is used as heat exchange medium while heat and cold are stored in the surrounding material in the aquifer and to some extent in the groundwater. Application of aquifer storage for the use of geothermal energy is mainly used in large scale facilities and is limited to sites with suitable aquifers in the form of ridges, sandstone and limestone aquifers. Löwenströmska hospital in the municipality of Upplands Väsby, north of Stockholm, is located nearby the northern part of the Stockholm esker. This means that it can be profitable and environmentally beneficial for the hospital to examine the possibilities of aquifer storage in the esker material next to its property. The purpose of this master thesis has been to investigate if geothermal energy storage with a seasonal storage of heat and cold can be applied within Löwenströmska hospital’s property area using groundwater modeling. A hydraulic groundwater model was constructed in MODFLOW based on a simplified conceptual model of the groundwater system. The hydraulic groundwater model was calibrated and validated against observed groundwater levels before and after a pumping test. The hydraulic groundwater model was then used to implement a fictitious geothermal energy storage with MT3DMS. MT3DMS is a modular function used with MODFLOW, which can be modified to simulate heat transport. The result shows that the geothermal energy storage can store seasonal heating and cooling of about 4 GWh, which covers 85 % of the hospital’s heating demand with an assumed SP-factor of 4, and the entire cooling demand. To cover 50 % of the peak heating power it was calculated that a flow of 63 l/s was needed, and according to the model this is possible. The geothermal energy storage does not need to be completely in energy balance, since the aquifer is recharged with its natural groundwater. The location of the wells influences which flows that are needed to create energy balance. A too close placement of the wells leads to a thermal breakthrough. The hydraulic conductivity of the esker material affects the amount of energy that can be stored. A higher hydraulic conductivity provides greater energy losses and a lower hydraulic conductivity favors the energy storage but gives a greater influence area. A number of assumptions have been made in the model construction of the hydrogeological model and further investigation of the geological and hydrogeological conditions are desirable to improve the model.