Aquifers are being used more and more as heat reservoirs. Thermopumps provide energy for heating and, in the case of solar installations, the energy is stored in groundwater during summer to be used in winter. But groundwater has to be protected against the excessively high temperatures which modify water quality, and against chemical pollutants. Theoretical and experimental studies have been conducted in Lausanne since 1970 (Laboratoire de mecanique des sols de l’Ecole Polytechnique Federale de Lausanne LMS — EPFL). This chapter describes a physical model constructed for the study of natural convection.