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 E. A. Mohamed, R. H. Worden To cite this version: E. A. Mohamed, R. H. Worden. Groundwater compartmentalisation: a water table height and geo- chemical analysis of the structural controls on the subdivision of a major aquifer, the Sherwood Sand- stone, Merseyside, UK. Hydrology and Earth System Sciences Discussions, European Geosciences Union, 2006, 10 (1), pp.49-64. hal-00304812 HAL Id: hal-00304812 https://hal.archives-ouvertes.fr/hal-00304812 Submitted on 8 Feb 2006 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Hydrology and Earth System Sciences, 10, 49–64, 2006 www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and SRef-ID: 1607-7938/hess/2006-10-49 Earth System European Geosciences Union Sciences 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 E. A. Mohamed and R. H. Worden Department of Earth and Ocean Sciences, University of Liverpool, 4, Brownlow Street, Liverpool, L69 3GP, UK Received: 19 April 2005 – Published in Hydrology and Earth System Sciences Discussions: 10 June 2005 Revised: 26 October 2005 – Accepted: 3 December 2005 – Published: 8 February 2006 Abstract. Compartmentalisation, the subdivision of an lates to the different, localised geochemical processes and aquifer into discrete and relatively isolated units, may be seawater intrusion but also relates to compartmentalisation of critical importance for the protection of groundwater al- due to faulting. Faults have limited the degree of mixing though it has been largely ignored in the groundwater litera- between the groundwater types thus retaining the specific ture. The Lower Triassic Sherwood Sandstone, in north west characteristics of each sub-basin. Highly localised seawa- of England, UK, may be a good example of an aquifer that ter intrusion is mainly controlled by low permeability fault has been compartmentalised by numerous high angle faults close to the Irish Sea and Mersey estuary. There is effec- with displacements of up to 300 m. The study was initiated tively no invasion of seawater beyond the faults that lie clos- to assess the local groundwater flow, the extent of seawater est to the coastline. Freshwater recharge to the aquifer seems invasion and the controls on recharge in the aquifer and to to be highly localised and mainly occurs by vertical percola- try to understand whether the aquifer is broken into discrete tion of rain and surface water rather than whole aquifer-scale compartments. groundwater flow. This study provides a detailed understand- Maps and schematic cross-sections of groundwater heads ing of the groundwater flow processes in Liverpool as an ex- for the years 1993, and 2002 were prepared to trace any ample of methods can be applied to groundwater manage- structural controls on the groundwater heads across the area. ment elsewhere. 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 1 Introduction 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 locally provides more than 75% of public wa- groundwater head seems to follow the topography in some ter supply in the UK and >35% in England and Wales as a places, although steep changes in groundwater head occur whole. Protecting groundwater supplies is important in in- across faults showing that they locally control the ground- dustrialised areas; understanding recharge and how, and in water head. The aquifer was thus provisionally subdivided what direction, water moves within an aquifer are important into several hydrogeological sub-basins based on groundwa- steps in developing long term practical strategies. There- ter head patterns and the occurrence of major structural fea- fore, much work has focused on the protection of water sup- tures (faults and a fold). plies by determining the areas contributing recharge to water- Using groundwater geochemistry data, contour maps of supply wells and by specifying regulations to minimize the chloride and sulphate concentration largely support the struc- opportunity for contamination of the recharge water by ac- tural sub-division of the area into hydrogeological sub- tivities at the land surface (Reilly and Pollock, 1993). A basins. Scrutiny of groundwater geochemical data, averaged key issue in this study is the degree of aquifer subdivision, for each sub-basin, confirmed the degree of compartmental- known as compartmentalisation, into discrete portions sepa- isation and the occurrence of sealed faults. The variation of rated by low permeability barriers. Compartmentalisation in the geochemical composition of the groundwater not only re- aquifers may be a result of faulting where fault zones typi- cally have low permeability due to a combination of catacla- Correspondence to: E. A. Mohamed sis and entrainment of clay minerals on the fault plane result- ([email protected]) ing in fault gouge. The thickness and degree of permeability © 2006 Author(s). This work is licensed under a Creative Commons License. 50 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer 1 3 5 C t r l o u x t a A f e t h h Permian & Triassic Formby t Sandstones f e H a t i u x ll l o ho t u Cr s e f a ul The study t area I n c e t l B u l a u f C n d r y ne o e r i l l 0 a sb l ic d t y f n a n A f u u a l y u t o le l s t B w no L K Mercia Mudstone i t h e L r l B i a v C n a e d y r f Ormiskirk Sandstone Fm p a o u l o t l Cro ss t l fault u Triassic Wilmslow Fm a f K i e r n k o d t a s l e e l Chester Pebble Fm Liverpool c c Sherwood Sandstone f a u E lt n Manchester Marl Fm a i Wirral m r e Collyhurst Fm P Carboniferous 0 5km River Mersey Fault line Fold Dip 4 8 Fig. 1. Maps of hydrogeologically important aspects of the area around Liverpool. of fault zonesFigure tends to1 increase(new version) with the extent of fault dis- of groundwater aquifers is the protection of resources and placement. Compartmentalisation is important since it fun- observance of ever-stricter environmental legislation. damentally controls how different parts of an aquifer are con- The objective of this study is to apply some of the con- nected and how, and in what direction, groundwater moves ceptual approaches adopted by the petroleum industry to as- within an aquifer. sess the degree of compartmentalisation of a locally impor- tant aquifer in the UK: the Triassic Sherwood Sandstone. We Compartmentalisation of fluid-bearing porous and per- will assess potential compartments within the aquifer using meable rocks has been a subject of great interest in the geological and groundwater head maps and then test these petroleum industry since understanding the degree of seg- initial ideas using geochemical data. The key scientific ques- mentation of oil fields has a major impact on production tions being addressed are: strategies (e.g. Smalley and England, 1992, 1994; Smalley et al., 1995). Compartmentalisation broadly occurs due to a 1. Is the Sherwood Sandstone (the UK’s most important combinationof impermeable faults, interbedded permeable aquifer) a “tank of sand” in the Merseyside area (an im- and impermeable rock units, lateral pinch-out of permeable plicit assumption in the hydrogeological map of Lewis sedimentary units. The degree of oilfield compartmentalisa- et al., 1989), or is it split into separate compartments or tion has been assessed using a combination of structural ge- sub-basins? ology, sedimentology, fluid pressure analysis and fluid geo- 2. Is it possible to use groundwater head data and water chemical differences (e.g. Smalley and England, 1992, 1994; geochemical data to define separate compartments or Smalley et al., 1995). There has been much less attention sub-basins within an aquifer? paid to the occurrence of compartmentalised groundwater- bearing aquifers. While the main driver for the study of com- The study area is located in the industrially important and partmentalised petroleum accumulations is the maximisation populous north west of England and encompasses the ur- of profit, the driver for the study of the compartmentalisation ban area of Liverpool (Fig. 1). The intensively investigated Hydrology and Earth System Sciences, 10, 49–64, 2006 www.copernicus.org/EGU/hess/hess/10/49/ E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer 51 Table 1. Regional stratigraphy of the bedrock underlying Liverpool and Greater Merseyside. Adapted from University of Birmingham, 1984 and Tellam, 1994. System Stage Lithostratigraphical Division Previous Division Thickness m Hydrogeological characters Soil Alluvium Flandrian 1>−35 Aquiclude Quaternary Terrace deposits (sands and gravels) Shirdley Hill Sand Formation (blown sand) Devensian Stockport Formation (till with glacial sands) 10–50 Aquiclude Mersey Mudstone Mudstone unit Keuper Marl 405 Aquiclude Anisian Group Tarporley Siltstone Fm Keuper Waterstone 30–60 Aquiclude Ormskirk Sandstone Fm Keuper Sandstone 181–295 Aquifer Triassic Wilmslow Sandstone Fm U.