INTERPRETING THE BOREHOLE WATER CHEMISTRY OF THE PERMO-TRIASSIC SANDSTONE AQUIFER OF THE LIVERPOOL AREA, UK

Using hydrogeological data, historical chemical data and the results o f studies in adjacent aquifers, an interpretation of the water chemist ry from a sparse network of boreholes is presented for the Liverpool a rea. The chemistry of the fresh groundwater samples is influenced by g eology, pollution and pumping history. The oldest waters, present wher e the sandstone is covered by Quaternary deposits, are calcite-saturat ed, contain little NO3- and have low SO42- and Cl- concentrations. How ever, water from the Collyhurst Sandstone are depleted in HCO3- whatev er the concentrations of the other anions. Samples from boreholes in a reas where the sandstones are not covered by Quaternary deposits are c haracterized by very low alkalinity and pH, and by high NO3-, SO42- an d Cl-. In the regions of the aquifer close to sandstone outcrop, or wh ere the Quaternary deposits are thin, the water samples have higher al kalinity and pH, and lower anion concentrations. Scattered throughout the region are boreholes yielding waters with very high SO42- concentr ations: where associated with industrial sites, these waters also have high NO3- concentrations and industrial pollution is suspected. In ru ral areas the high SO42- concentrations are derived from leakage throu gh the sulphur-bearing tills in response to pumping-induced lowering o f the piezometric surface. The distribution of borehole water types ca n be described with the help of a set of rules relating water type to hydrogeological features; these rules allow a map of hydrochemical dis tributions to be constructed. Saline groundwaters occur in the aquifer adjacent to the Mersey Estuary and have chemistry compositions equiva lent to slightly modified, diluted Estuary water. With the exception o f a single deep borehole sample, there is no indication of the widespr ead presence of ancient saline groundwaters in the base of the sandsto ne sequences as is found in the sandstones to the east of the study ar ea. However, slightly saline, reduced waters occur below the Mercia Mu dstone Group in the north of the area. Historical records give some in dication of the changes in water chemistry distributions through time.