In this work, a parametric study is done with computational methods for groundwater flow and heat transport in order to describe the dominant physical phenomena in an Aquifer Thermal Energy Storage (ATES). Optimum operating conditions and geological parameters are deduced from it. The intended use of the ATES is the domestic water or space heating. The stock is seasonal and functions without heat pump. Parametric study is organized as a series of reference simulations relating to the thickness of the aquifer and the injection flow rate. The other parameters and operating conditions of are kept constant. Other simulations are done and compared to the reference simulations in order to describe the influence of injection temperature, hydraulic conductivity, anisotropicity, injection frequencies, phases of rest, outflow rate and doublet wells. The cells of natural convection are strongly affecting the performances of an ATES. The thermal losses by conduction trough bedrock and caprock is an unfavourable factor as well, but to a lesser extent however. The increase of inflow rate always improves the efficiency, whereas an optimum aquifer thickness exists for each inflow rate. This optimum is characterised by weak natural convection phenomena. A greater thickness means strong density-viscosity effects, while a thinner aquifer forms a horizontally stretched stock, with increased conductive losses through the bedrock and cap rock. A maximum thermal recovery rate exists for a given flow rate. This maximum increases with inflow rate. The injection temperature behaves on the stock in the same way as the aquifer thickness for a given inflow: there is an optimal temperature. However it is in general preferable to increase the injection temperature instead of the inflow rate. In terms of optimization of the thermal power at disposal, the most favorable scenario is an increasing flow during the discharge. The precise scenario is different for each problem. The performances of a ATES are the complex result of the many parameters which enter in plays. One will recommend an inflow rate of at least 100m33/d during 6 months, and even rather 500 - 1000m3/d with an injection temperature of 80°C at least. An injection temperature of 60°C will have to be associated with an inflow of 1000m3/d at least. The geological targets proposed in Switzerland are the quaternary alluvial deposits filling the former glaciated valleys, fluvioglacial deposits and alluvial fans.