PROCESSES AFFECTING GROUNDWATER CHEMISTRY IN A ZONE OF SALINE INTRUSION INTO AN URBAN SANDSTONE AQUIFER

Samples have been collected from inflows into railway tunnels in the T riassic sandstone aquifer beneath Liverpool and the Mersey Estuary, En gland, U.K. These provide a profile through a saline-freshwater mixing zone. Analyses were made of major anions and cations, delta(34)S and delta(18)O in SO4, delta(13)C in dissolved inorganic C and Sr-87/Sr-86 . The data demonstrate that the presence of a low permeability fault e xerts a strong control on the local groundwater chemistry. On the estu ary side of the fault, groundwater-chemistry is dominated by mixing of intruding estuary water, which is modified by SO4 reduction and calci te dissolution, with fresh groundwater. The environment of SO4 reducti on in the tidal estuary is one of repeated reduction and re-oxidation of S in an open system and has resulted in virtually no change in S is otopic composition, but an enrichment in residual SO4 delta(18)O of 1. 5 parts per thousand. Groundwater chemistry on the landward side of th e fault is primarily the result of recharge in an urban environment. T here is also evidence that saline water has been present in this regio n of the aquifer in the past and that this has now been flushed by fre sh groundwaters. This saline water was either transported along the la ndward side of the fault from nearer the estuary or more probably tran smitted across the fault. Both mechanisms would have been driven by la rge landward head gradients caused by heavy industrial abstraction ear lier this century. This has produced a zone of groundwaters depleted i n Ca and radiogenic;Sr and enriched in Na as a result of ion exchange between the fresh groundwaters and the aquifer previously occupied by more saline water. Sulphur isotopic composition, however, shows no var iation since SO4 does not undergo significant ion exchange. A tracer t est from a borehole to the tunnels showed multiple breakthroughs to so me locations indicating a;number of different how paths through the aq uifer. The maximum flow velocity recorded in this test was 140 m/d sug gesting flow along fractures. (C) 1998 Elsevier Science Ltd. All right s reserved.