Laboratory experiments have shown that compacted clays act as osmotic membranes when they separate aqueous solutions of unequal ionic concentration. Theoretically, osmotically induced differential hydraulic pressure in groundwater systems can be relatively high. The magnitude depends primarily upon concentration differences across the membrane, type of ions, type of clay, and pore size. In experiments, thin, compacted clay membranes commonly exhibit varying degrees of osmotic efficiency due to ion leak‐age through the clay. In natural systems the membrane and the solution containers are not as distinct and well defined as they are in the laboratory. Moreover, the membrane is commonly thick, inhomogeneous, and composite. In a buried Triassic basin at the Savannah River plant near Aiken, South Carolina, it is suspected that osmosis causes the saline water in the basin center to be slightly geopressurized in relation to freshwater in the overlying coastal plain aquifer. Two wells have heads of 7.88 and 12.98 bars (114.3 and 188.3 psi) above the head in the coastal plain aquifer. The head in each of these wells approximates the osmotic equilibrium head calculated from solution concentration of water produced by each well (12,000 and 18,500 mg/l, respectively). Other wells penetrating the top and edge of the Triassic basin probably penetrate a zone where ion leakage gives rise to less saline water. Thus these wells are not geopressurize