Processes affecting groundwater chemistry in a zone of saline intrusion into an urban sandstone aquifer |
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| Institution: | 1. Faculty of Engineering and IT, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia;2. Institute of Filtration and Techniques of Separation (IFTS), Agen, France;1. Department of Chemical Engineering, Monash University, Melbourne, VIC 3800, Australia;2. Curtin Water Quality Research Centre (CWQRC), Department of Chemistry, Curtin University, Bentley, GPO Box U1987, Perth, WA 6845, Australia;3. Department of Civil Engineering, Curtin University, Bentley, GPO Box U1987, Perth, WA 6845, Australia;4. Department of Chemical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom |
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| Abstract: | Samples have been collected from inflows into railway tunnels in the Triassic sandstone aquifer beneath Liverpool and the Mersey Estuary, England, U.K. These provide a profile through a saline–freshwater mixing zone. Analyses were made of major anions and cations, δ34S and δ18O in SO4, δ13C in dissolved inorganic C and 87Sr/86Sr. The data demonstrate that the presence of a low permeability fault exerts a strong control on the local groundwater chemistry. On the estuary side of the fault, groundwater chemistry is dominated by mixing of intruding estuary water, which is modified by SO4 reduction and calcite dissolution, with fresh groundwater. The environment of SO4 reduction 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 isotopic composition, but an enrichment in residual SO4 δ18O of 1.5‰. Groundwater chemistry on the landward side of the fault is primarily the result of recharge in an urban environment. There is also evidence that saline water has been present in this region of the aquifer in the past and that this has now been flushed by fresh groundwaters. This saline water was either transported along the landward side of the fault from nearer the estuary or more probably transmitted across the fault. Both mechanisms would have been driven by large landward head gradients caused by heavy industrial abstraction earlier this century. This has produced a zone of groundwaters depleted in 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 variation since SO4 does not undergo significant ion exchange. A tracer test from a borehole to the tunnels showed multiple breakthroughs to some locations indicating a number of different flow paths through the aquifer. The maximum flow velocity recorded in this test was 140 m/d suggesting flow along fractures. |
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