The use of groundwater systems for heat storage increasingly gains interest among water managers, policy makers and researchers as a way to increase the efficiency of energy production and to allow the re-use of waste heat. Typically, mono-well storage systems are thought to require the use of separate aquifers. This study assessed the suitability of using heat and cold storage in a single deep geothermal aquifer for district heating and cooling. An integrated modelling approach was used for evaluating the controls on the energy efficiency of high temperature aquifer thermal energy storage (HT-ATES). The temperature difference (Delta T) of 40 degrees C between the injection temperatures for the cold and warm storages 20 degrees C and 60 degrees C was significant, which required accounting for transient variation of density and viscosity due to temperature and pressure within the modelling code SEAWAT. The developed model was applied for a geothermal reservoir from the Moesian platform, in the Bucharest area, Romania. The sensitivity of the system efficiency was analyzed with respect to the main physical (density, viscosity, longitudinal dispersivity) and operational design parameters (distance between warm and cold storage volumes, flow rates). Uncertainties in geological heterogeneity and the associated range in longitudinal dispersivity values (5-50 m) resulted in significant efficiency differences (80-55%). While reducing the lateral distance between multiple mono-well systems increased their overall efficiency due to positive thermal interference, a minimum vertical distance of (160 m) was required between the injection/extraction filters to prevent interaction between the cold and warm storage volumes. Overall, this study highlights the potential of using a cost-effective mono-well system for HT-ATES in single deep geothermal groundwater systems. (C) 2015 Elsevier Ltd. All rights reserved.