Groundwater compartmentalisation: a water table height and geochemical analysis of the structural controls on the subdivision of a major aquifer, the Sherwood Sandstone, Merseyside, UK

摘要

Compartmentalisation, the subdivision of an aquifer into discrete andrelatively isolated units, may be of critical importance for the protectionof groundwater although it has been largely ignored in the groundwaterliterature. The Lower Triassic Sherwood Sandstone, in north west of England,UK, may be a good example of an aquifer that has been compartmentalised bynumerous high angle faults with displacements of up to 300 m. The study wasinitiated to assess the local groundwater flow, the extent of seawaterinvasion and the controls on recharge in the aquifer and to try tounderstand whether the aquifer is broken into discrete compartments.

Maps and schematic cross-sections of groundwater heads for the years 1993,and 2002 were prepared to trace any structural controls on the groundwaterheads across the area. Studying the contour maps and cross sections revealedthat: 1) there are substantial differences in groundwater head across someof the NNW-SSE trending faults implying that groundwater flow is stronglylimited by faults, 2) an anticline in the east of the area acts as agroundwater divide and 3) the groundwater head seems to follow thetopography in some places, although steep changes in groundwater head occuracross faults showing that they locally control the groundwater head. Theaquifer was thus provisionally subdivided into several hydrogeologicalsub-basins based on groundwater head patterns and the occurrence of majorstructural features (faults and a fold).

Using groundwater geochemistry data, contour maps of chloride and sulphateconcentration largely support the structural sub-division of the area intohydrogeological sub-basins. Scrutiny of groundwater geochemical data,averaged for each sub-basin, confirmed the degree of compartmentalisationand the occurrence of sealed faults. The variation of the geochemicalcomposition of the groundwater not only relates to the different, localisedgeochemical processes and seawater intrusion but also relates tocompartmentalisation due to faulting. Faults have limited the degree ofmixing between the groundwater types thus retaining the specificcharacteristics of each sub-basin. Highly localised seawater intrusion ismainly controlled by low permeability fault close to the Irish Sea andMersey estuary. There is effectively no invasion of seawater beyond thefaults that lie closest to the coastline. Freshwater recharge to the aquiferseems to be highly localised and mainly occurs by vertical percolation ofrain and surface water rather than whole aquifer-scale groundwater flow.This study provides a detailed understanding of the groundwater flowprocesses in Liverpool as an example of methods can be applied togroundwater management elsewhere.