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

International audienceThe 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. The study area is located in the northwest of England and encompasses the urban area of Liverpool and surrounding countryside and extends east-west from Liverpool to Widnes and as far north as Formby. The Irish Sea marks the western margin of the area while the Mersey estuary defines the southern margin. The Triassic sandstone in this area has been, and remains, an important aquifer although industrialisation and groundwater exploitation have led to significant water quality problems. Maps of water table for the years 1993, 1997, 2000 and 2002 and schematic cross-sections of the water table height along the faults were prepared to trace any effect of these faults on water table height across. Studying the water table maps and cross sections revealed that: 1) there are substantial differences in water table height across some of the NNW-SSE trending faults implying that groundwater flow is strongly limited by fault, 2) an anticline in the east of the area acts as a groundwater divide and 3) the water table seems to follow the topography in some places, although steep changes in water table occur across faults showings that they locally control the water table elevation. The aquifer was thus provisionally subdivided into several hydrogeological sub-basins based on water table height 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 relate 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 no effectively no invasion of seawater beyond the faults that lie closest to the coastline. Freshwater recharge to the aquifer must be highly localised and will mainly occur by vertical percolation of rain and surface water rather than whole aquifer-scale groundwater flow