| Compartmentalisation, the subdivision of an aquifer into discrete and
relatively isolated units, may be of critical importance for the protection
of groundwater although it has been largely ignored in the groundwater
literature. The Lower Triassic Sherwood Sandstone, in north west of England,
UK, may be a good example of an aquifer that has been compartmentalised by
numerous high angle faults with displacements of up to 300 m. The study was
initiated to assess the local groundwater flow, the extent of seawater
invasion and the controls on recharge in the aquifer and to try to
understand 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 groundwater
heads across the area. Studying the contour maps and cross sections revealed
that: 1) there are substantial differences in groundwater head across some
of the NNW-SSE trending faults implying that groundwater flow is strongly
limited by faults, 2) an anticline in the east of the area acts as a
groundwater divide and 3) the groundwater head seems to follow the
topography in some places, although steep changes in groundwater head occur
across faults showing that they locally control the groundwater head. The
aquifer was thus provisionally subdivided into several hydrogeological
sub-basins based on groundwater head patterns and the occurrence of major
structural features (faults and a fold).
Using groundwater geochemistry data, contour maps of chloride and sulphate
concentration largely support the structural sub-division of the area into
hydrogeological sub-basins. Scrutiny of groundwater geochemical data,
averaged for each sub-basin, confirmed the degree of compartmentalisation
and the occurrence of sealed faults. The variation of the geochemical
composition of the groundwater not only relates to the different, localised
geochemical processes and seawater intrusion but also relates to
compartmentalisation due to faulting. Faults have limited the degree of
mixing between the groundwater types thus retaining the specific
characteristics of each sub-basin. Highly localised seawater intrusion is
mainly controlled by low permeability fault close to the Irish Sea and
Mersey estuary. There is effectively no invasion of seawater beyond the
faults that lie closest to the coastline. Freshwater recharge to the aquifer
seems to be highly localised and mainly occurs by vertical percolation of
rain and surface water rather than whole aquifer-scale groundwater flow.
This study provides a detailed understanding of the groundwater flow
processes in Liverpool as an example of methods can be applied to
groundwater management elsewhere. |