Interpreting the borehole water chemistry of the Permo-Triassic sandstone aquifer of the Liverpool area, UK

Abstract

Using hydrogeological data, historical chemical data and the results of studies in adjacent aquifers, an interpretation of the water chemistry from a sparse network of boreholes is presented for the Liverpool area. The chemistry of the fresh groundwater samples is influenced by geology, pollution and pumping history. The oldest waters, present where the sandstone is covered by Quaternary deposits, are calcite-saturated, contain little NO−3 and have low SO2−4 and Cl− concentrations. However, water from the Collyhurst Sandstone are depleted in HCO−3 whatever the concentrations of the other anions. Samples from boreholes in areas where the sandstones are not covered by Quaternary deposits are characterized by very low alkalinity and pH, and by high NO−3, SO2−4, and Cl−. In the regions of the aquifer close to sandstone outcrop, or where the Quaternary deposits are thin, the water samples have higher alkalinity and pH, and lower anion concentrations. Scattered throughout the region are boreholes yielding waters with very high SO2−4 concentrations: where associated with industrial sites, these waters also have high NO−3 concentrations and industrial pollution is suspected. In rural areas the high SO2−4 concentrations are derived from leakage through the sulphur-bearing tills in response to pumping-induced lowering of the piezometric surface. The distribution of borehole water types can be described with the help of a set of rules relating water type to hydrogeological features; these rules allow a map of hydrochemical distributions to be constructed. Saline groundwaters occur in the aquifer adjacent to the Mersey Estuary and have chemistry compositions equivalent to slightly modified, diluted Estuary water. With the exception of a single deep borehole sample, there is no indication of the widespread presence of ancient saline groundwaters in the base of the sandstone sequences as is found in the sandstones to the east of the study area. However, slightly saline, reduced waters occur below the Mercia Mudstone Group in the north of the area. Historical records give some indication of the changes in water chemistry distributions through time.