The determination of the origin of saline groundwaters is sometimes crucial for the successful exploitation of an aquifer system. Although hydrochemical methods provide a direct approach to the problem, modification of intruding water can often give rise to interpretational difficulties. By means of a case history from the Permo-Triassic sandstone aquifer of the Widnes-Manchester area, the problems encountered are illustrated. The major ion chemistries of (a) saline waters originating from upconing of deep old saline water and (b) saline groundwaters resulting from recent intrusion are compared using computer modelling techniques with a third group of waters of unknown provenance. It is concluded that in some cases an apparent chemical equivalence, in which the same chemical product can arise through more than one set of processes, can occur; in such cases major ion analyses alone are not of use in determining groundwater origins. Explicit discussion of equivalence problems has rarely been undertaken in the hydrochemical literature, and the results presented have implications beyond saline intrusion studies.