HYDROCHEMISTRY OF THE SALINE GROUNDWATERS OF THE LOWER MERSEY BASIN PERMO-TRIASSIC SANDSTONE AQUIFER, UK

Saline groundwater occurs at depth in the Triassic Sandstones of the northern end of the Cheshire Basin in northwest England. Major and minor ion data suggest that the origin of the salinity is dissolution of evaporite deposits; the salinity probably developed in an interval following the Tertiary inversion of the Basin which would have allowed faulted contact between the sandstones and the overlying evaporite sequence of the Mercia Mudstone Group. Isotope evidence suggests the saline waters to have been considerably diluted by presumably dispersive mixing with a meteoric water recharged under cool, and therefore Quaternary, climatic conditions. Comparison with other UK Permo-Triassic Basin brines suggests the Merseyside waters are not unusual, except in terms of the importance of exchange reactions and dilution by Quaternary recharge waters. These factors are due to the shallow maximum burial depth of the aquifer and the preservation of smectites, and also to the geometry of the acquifer and its low-permeability faults. In the light of the water chemistry data it is suggested, in a slight modification of the proposal of Naylor et al. (J. Geol. Sec. Lend., 146: 685-699, 1988), that the Cheshire Basin mineralization resulted from mixing of the relatively oxidized, SO4-rich evaporite brines with the reduced, SO4-poor, metal-rich, Ba-rich Coal Measures brines which were expelled through faults from the Coal Measures during Basin inversion: some degree of oxyhydroxide-stripping of the sandstones by the Coal Measures brines would have enhanced metal concentrations. The water resulting from the mixing of brines would have been far from equilibrium, and the barite and sulphide precipitation followed along the faults.