Saline intrusion into an urban sandstone aquifer

A saline-freshwater mixing zone has been sampled by taking advantage: of rail tunnels beneath the Mersey Estuary and Liverpool city centre. SO4 in the saline tunnel waters below the estuary is shown to be enriched in O-18 by 1.5 parts per thousand but without any significant change in delta(34)S, despite showing a decrease in SO4 concentration and a decline in SO42-/Cl-. This is the result of a tidally-driven estuarine cycle of bacterial SO4 reduction at high tide, followed by draining of some pore water with S-34 enriched SO4 and reoxidation of part of the newly formed monosulphides at low tide. A major low permeability fault separates these Cl rich waters from freshwaters on the landward side of the fault. The saline waters on the estuary side of the fault are affected by mixing with meteoric waters and calcite dissolution. On the landward side of the fault the freshwaters have been modified by ion exchange induced by flushing of an earlier saline intrusion.