THE GROUNDWATER CHEMISTRY OF THE LOWER MERSEY BASIN PERMO-TRIASSIC SANDSTONE AQUIFER SYSTEM, UK - 1980 AND PRE-INDUSTRIALIZATION URBANIZATION

The Lower Mersey Basin Permo-Triassic Sandstone Aquifer lies in a heav ily urbanised and industrial area of NW England. Extensive pumping has lowered the piezometric surface considerably, causing the sandstone a quifer, which is overlain by a variable sequence of glacial deposits, to become extensively unconfined. The groundwater chemistry of the sys tem appears to be controlled by the geometry of the sandstones, faulti ng, the variable distribution and permeability of the Quaternary depos its, and by the pumping. The water in the present-day recharge areas i s affected by pollution, with high NO3, SO4, and Cl concentrations; it has pH values as low as 4.3. The oldest fresh waters in the area (age less than a few thousand years) are strongly ion exchanged as a resul t of the flushing of saline water from the system: these waters grade into similar but much less ion exchanged waters, with zero NO3, very l ow SO4, and high HCO3. A fourth group of waters appears to be the resu lt of mixing between the latter waters and pumping-induced leakage thr ough the Quaternary deposits. A final group of waters is characterised by very low alkalinity and is strongly associated with particular geo logical units poor in carbonates. Minor-ion distributions confirm the patterns suggested by the major ions. The distribution of the water ch emistry preserves some of the features of the pre-pumping system in th is low-transmissivity-high storage system, and reconstruction of the p re-pumping chemical distributions is attempted, to highlight the effec ts of industrial man on the system.