Groundwater Compartmentalisation

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

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E. A. Mohamed, R. H. Worden. Groundwater compartmentalisation: a water table height and geochemical 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

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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 typically 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

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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 important 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. Mottled Sandstone 205–480 Aquifer Scythian Sherwood Sandstone Chester Pebble Beds Fm. 316–375 Aquifer Group Bunter Pebble Beds Pebble Beds Unit L. Mottled Sandstone Unit L. Mottled Sandstone 0–>80 Aquifer Upper Bold/Manchester Marl Fm Manchester Marl 10–225 Mixed aquifer-aquiclude Permian Lower Collyhurst Sandstone Fm Collyhurst Sandstone 283–720 Aquifer Carboniferous Westphalian Stages A–D >1500 Aquiclude

area extends from Widnes to Liverpool and as far north as ports for the Geological Survey by Taylor (1957) and Land Formby. (1964). Both authors presented maps with groundwater con- Despite the local and national importance of the Sherwood tours. The high degree of faulting in the aquifer and its pos- Sandstone aquifer, few previous studies substantially men- sible importance for internal boundaries occurring within the tioned the roles of lithology and geological structure as con- Permo-Triassic aquifer was noted by University of Birming- trols on groundwater geochemistry. ham (1981). In an assessment of the saline intrusion into The occurrence and movement of groundwater in the dis- the Sherwood Sandstone aquifer it was suggested that wher- trict were discussed by Stephenson (1850) including the im- ever fault movement was small the effect on the transmis- portance of fissures and the fact that there was a finite amount sivity might be small depending on the nature of the fault groundwater. He also recognized the occurrence of seawa- zone (University of Birmingham, 1984). Large displace- ter intrusion into the aquifer. The role of regional topogra- ment, however, may juxtapose impermeable strata against phy as a control on groundwater contours for the Sherwood permeable strata leading to a significant local reduction of aquifer (in the east of England) was discussed by Strahan in the effective hydraulic conductivity. Just outside of the stud- Hull (1882). Significantly, Morton (1866) discussed the loca- ied area, the Roaring Meg fault has low permeability and re- tion of wells as a possible control on yield and water quality stricts saline groundwater movement (Tellam et al., 1986). and concluded that the highest yielding wells were located However the published synthetic hydrogeological map of the on faults. In contrast, Moore (1902) examined the porosity area has groundwater head contours that ignore all faults across a fault plane in the Sherwood Sandstone aquifer on the and has seemingly been drawn on the assumption that the adjacent Wirral peninsula, just southwest of the area of inter- aquifer is not sub-divided into compartments (Lewis et al., est, and concluded that porosity-reduction occurred across 1989). Barker (1996) investigated the geochemistry and iso- fault planes since they were cemented by calcite and pyrite. tope of the saline-freshwater mixing zone beneath Liverpool Also a porosity-occluding clay-rich fault gouge was found in and stated that the change in groundwater chemistry was due two zones up to 0.5 m thick. to the presence of the fault and implies that it acts as a hydro- The Mersey Railway Tunnel, excavated through the logical barrier to groundwater flow. aquifer, connects the Liverpool area to the Wirral (essentially the industrial town of Birkenhead) and it was recorded that The main objectives of this study will be focused on the abstraction from the tunnel led to deterioration of groundwa- role of the structural geology of the area explicitly trying to ter quality (Wedd et al., 1923). establish the impact of faults on groundwater conditions us- The hydrogeology of the Sherwood Sandstone aquifer in ing records of groundwater head and geochemistry variations the Merseyside and Manchester areas was described in re- over the period 1993 to 2002. www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 52 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

10 10 Croxteth fault 3 3 N 20 40 50 Formby 60 Formby 60 70 10 60 30 50 40

Crosby fault onayfault Boundary

0 0 50 40 40 Litherland fault

Liverpool Bay 60 30 Kingsway fault Knowsley Anticline 40 80 Cross fault 70

70 West fault Kirkdale fault 50 70 60 50 9 40 9

Liverpool 30 Ecclestone 60 70 40 20 Wirral Wirrel 10 Sampled wells locations 5km 0 0 5km River River Mersey ()a Mersey 3 (b) 5 4 5 3 4 8 8

Fig. 2. (a) Topographic map with contours in meters, (b) Distribution of boreholes across the aquifer for which data have been used. Notice the location of faults and the boreholes distribution.

2 Geology formation is made up of two units; the Lower Mottled Sand- stone Unit, composed of red and brown, fine to medium 2.1 General geology grained weakly cemented sandstone, followed by the Peb- ble Beds Unit, composed of red or yellow, cross laminated, Permo-Triassic sedimentary rocks represent the principle coarse to medium grained pebbly sandstones. The overly- aquifer units in the area. The regional stratigraphy is rep- ing Wilmslow Sandstone Formation is characterised by soft, resented in Table 1 with the regional distribution of rocks fine-grained sandstones. The Ormskirk Sandstone Formation at the surface given in Fig. 1. Permo-Triassic sedimentary has two lithological units; a lower one directly overlying the rocks rest unconformably over Upper Carboniferous well- Wilmslow Formation consists of greyish-yellow to red sand- indurated sandstones and mudstones that are typically con- stone and upper one composed of medium and fine grained sidered to denote the low permeability base of the aquifer. sandstone. These older rock units crop out in the east and north east of the area (Fig. 1). The Upper Triassic Mercia Mudstone Formation, a major The Permo-Triassic sedimentary succession starts with the argillaceous sequence overlying the Sherwood Sandstone, is Collyhurst Sandstone Formation, which is red to grey in considered to be the upper low-permeability boundary of the colour and fine to medium grained. The stratigraphically- entire aquifer. It is composed of two units: the lower one be- equivalent Manchester Marl and Bold Formations con- ing the Tarporly Siltstone and the upper one being the Mercia formably overlie the Collyhurst Sandstones. The Manchester Mudstone. Marl, in the north, is composed of calcareous siltstone or The Quaternary system represents a very short length of shale with thin bands of fossiliferous limestone, while the geological time and is characterised in the area by a restricted Bold Formation, in the south, is made up of fine to very fine- depositional thickness, which nevertheless is of hydrogeo- grained sandstones. logical significance. Thicknesses of 5 to 10 m are com- The major sandstone-dominated unit that represents the mon, but significant parts of the area are without Quaternary bulk of the aquifer in the area, the Sherwood Sandstone cover (e.g. Liverpool, Kirkby, and Ormskirk; University of Group, overlies the Manchester Marl and Bold Formations. Birmingham, 1984). Lithologically, these deposits are domi- The base of this group is the Chester Pebble Formation. This nated by glacial tills and sands followed by well-sorted sands

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 53

10 10 3 -5 3 10 -5 Formby A` A A` A Formby 15 30 0 20 25 0 5 5 5 5 40

0 0 10 35 15 30 15 25 Liverpool 20 Liverpool 20 B` bay B` bay 25 10 0 30 25 -10 15 5 B -15 B 10 9 0 9 Liverpool Liverpool -5 5

Wirral C` -5 C C` C Wirral 0 0 -5 -10 0 5km 0 5km River -5 River a. ) Water table map Mersey b. ) Water table map Mersey 3 5 3 1993 4 2002 4 8 8

Fig. 3. Water table height in meters relative to the ordinance datum (OD, mean annual sea level); (a) 1993, and (b) 2002. Contouring was based on the assumption employed during the drafting of the published hydrogeological map (Lewis et al., 1989) that the aquifer was a uniform entity with no sub-compartments. locally intercalated with peat beds. River alluvium of silt, with some smaller east-west cross faults. The faults are nor- sand and clay has mainly developed in the valley of the River mal with an average 60◦ dip (Jones et al., 1938). The throws Alt, where the recent estuarine alluvium of sand, silt and mud of these faults vary from a few meters to >300 m. Examples occurs (University of Birmingham, 1984; Tellam, 1994). of faults with large displacements are the Boundary fault, the Croxteth fault and the Eccleston West fault (Tellam, 1983; 2.2 Structural geology and Fig. 1). In the Sherwood Sandstone in the north of England, there is a correlation between degree of displace- The structural geology of the area is dominated by a variety ment and fault zone thickness. Fault zones may either be of faults as represented on published geological maps and ﬁlled with gouge or cataclasite depending on the lithologies cross sections from the British Geological Survey (and its on both side of the fault; some of the greatest faults in the predecessors) and other published work (e.g. Morton, 1866; area are likely to have potentially impermeable fault zones Wedd et al., 1923; Shackleton, 1953; University of Birming- of >1 m thickness (e.g. Beach et al., 1997; Chadwick, 1997; ham, 1984). Cowan, 1996; Rowe and Burley, 1997). These major faults Although faults dominate the geological structure from (e.g. the Croxteth fault) have been implicated in controlling the hydrogeological perspective (Fig. 1), there is also a sub- the occurrence of minor shallow petroleum accumulation to tle low amplitude fold with its axis running approximately the north of the study area (Kent, 1948; Lees and Taitt, 1945) northeast to southwest. This fold is known as the Knowsley with the suggestion that the fault zones are effectively imper- anticline (Fig. 1) and it results in a general, low angle north- meable. The faults that seem to cause the greatest geolog- west dip over the northern part of the study area and south- ical complexity occur at the boundary between the Permo- east dip in the vicinity of Halewood, Speke and Widnes. Traissic and Carboniferous rocks (the Boundary fault, Fig. 1; There are other small syn- and anti-forms to the north of the University of Birmingham, 1984). Minor faults in the Qua- Knowsley anticline but they have a very limited effect on the ternary deposits have been noted by Aitken (1871), Reade general dip of the strata. (1884) and Taylor (1958). Such faults are syndepositional The area is intensively disrupted by numerous large faults. and have no signiﬁcant effect on groundwater movement. These generally trend north-northwest – south-southeast www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 54 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

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Fig. 4. Schematic cross sections illustrated the variation of the water table height in meters across the faults in years 1993 and 2002. The locations of the cross sections are shown on the map.

Figure 4 (new version) 2.3 Topography though the Environmental Agency has only recently been re- sponsible for collecting and monitoring these data. Head data Although the area has generally low relief, the bedrock to- have been collected at least monthly from most of the moni- pography is locally characterised by narrow, linear lows (up toring boreholes. The EA staff and the Nottingham EA lab- to about 30 m above sea level) separated by hills that reach up oratory followed recommended BSI sampling and analytical to about 70 m above sea level (Fig. 2a). These topographic methods. Acidity, temperature, conductivity and total salin- features broadly run northwest to southeast. Travelling fur- ity were measured immediately after sampling. Previously ther north, the narrow depression opens out to become a wide cleaned sample bottles were rinsed with the groundwater and plane in the northwest area around Formby, the flood plain then filled to the top. Airtight caps were fitted to each bottle of the River Alt, with 10 m maximum elevations above sea prior to transportation to the laboratory. Alkalinity titrations level. were performed within a few hours of sampling. Ion chro- matography was used for the major anions, and atomic ab- sorption and ICP-techniques were used for the major cations. 3 Methods In order to check the quality of the data, all the groundwater geochemical data that were used in this study were assessed In this study 75 groundwater head records and 78 ground- for charge balance using Geochemist Workbench. Ground- water geochemical analyses collected from 70 groundwater water analyses with a charge imbalance >5% were rejected wells (Fig. 2b) have been utilised for two discrete year pe- from the data set. riods (1993, 2002). These groundwater head data and geochemical analyses were supplied from the UK Environmen- We concluded that automated geostatistical methods, ideal tal Agency (EA). Head data have been collected from a large for gridded data but not suited to the irregular location of number of boreholes in the area for more than 70 years al- sampling points in this study (Fig. 2b), would not be used

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E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer 55

10 10 3 5 3 5 100 75 50 25 Formby Formby 25 25 50

0 0

200 B B 100 100 75 75 100 400 200 200

100 300 200 500 A 400 A 500 1000 50 1000 50 50 9 10000 9 10000

25 25

100 D D C 4000 C 50 4000 50 75 0 5km 0 5km 1000 100 75 100 1000 a) Cl- concentration 1993 500 b) Cl- concentration 2002 3 4 3 4 500 8 8

Fig. 5. Aqueous chloride concentrations in mg/L contoured for; (a) 1993 and (b) 2002. Contouring of chloride concentration was based on the assumption that the aquifer is not compartmentalised by the structure elements. for contouring groundwater head variation and the geochem- distinct, though imperfect, NNW-SSE lineation that accords ical data. Instead we have use manual contouring methods to the main fault pattern (Figs. 1 and 2b). to draw maps of water table elevation and groundwater geochemistry. A number of schematic cross-sections representing topographic and groundwater elevation across some of the main faults in the study area were prepared to trace any effect of the faults on groundwater head height across the fault lines 4 Results (Fig. 4). These show that there are significant fluctuations of groundwater head height across some of the faults (Fig. 4). 4.1 Groundwater head variation in space and time Schematic cross-sections illustrate the changes of groundwater head in passing from one side of a fault to another for The groundwater head distribution maps from the years 1993 the years 1993, and 2002. Cross-section A-A0, which tracks and 2002 have been drawn using the available data from across the Croxteth fault in the north, shows a very large monitoring boreholes. The groundwater head varied from change of the groundwater head across the main fault, where 30 m above mean annual sea level (ordinance datum; OD) to the small faults have no significant effect on the groundwater as much as 15 m below OD for the years listed above. In head. Cross-section B-B0 also traverses the Croxteth fault, the first instance we followed the assumptions implicit in the Litherland faults and other small faults to the west (Fig. 4). published regional hydrogeological map (that the aquifer was This clearly illustrates the dramatic change of groundwa- neither subdivided by faults nor constrained by folds into dis- ter head across the Croxteth and Kirkdale faults. Further- crete compartments; Lewis et al., 1989) and freely contoured more, cross-section A-A0, and B-B0 show that there is no the groundwater head across the entire area (Fig. 3). It is simple relationship between local topographic elevation and noteworthy that the water contours drawn in this way have a groundwater head. These observations of groundwater head www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 56 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

10 10 3 5 3 75

Formby 100 50 Formby 100 75 50 50 50 75 100

0 0

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300 75 50 200 50 50 300 200 400 A 50 A 50 400 500 500 1000 1000 9 9 75

25 25 50 50 300

200 D D C C 0 5km 75 400 0 5km 100 50 300 2- 2- 75 a)SO4 concentration b)SO concentration 4 5 3 1993 4 3 2002 4 100 200 8 200 8

Fig. 6. Aqueous sulphate concentrations in mg/L contoured; (a) 1993 and (b) 2002. Contouring of chloride concentration was based on the assumption that the aquifer is not compartmentalised by the structure elements. displacement across faults, especially those contrary to the Distribution maps of the concentrations of aqueous chlo- topographic slope, are best interpreted in terms of the faults ride and sulphate (mg/l) from the chosen two years (1993 and representing low permeability zones in the aquifer that have 2002) have been prepared (Figs. 5 and 6). These maps were limited the movement of groundwater. Cross-section C-C0 prepared following the initial assumption that the aquifer was traverses the Eccleston West, Boundary and Croxteth faults neither subdivided by faults nor constrained by folds into and other small faults in between (Fig. 4). Although the discrete compartments and freely contoured the chloride and groundwater head follows the topographic elevations, there sulphate concentrations across the entire area. Two cross sec- is a pronounced steep change of groundwater head across the tions traversing the faults were constructed to illustrate the fault lines implying the signiﬁcant effect of faults on ground- degree of change in the chloride and sulphate concentrations water head in these southern parts of the study area. along the faults (Figs. 7a and b).

4.2 Groundwater geochemistry As expected for groundwater in temperate climatic regions, chloride concentrations are relatively low (<100 mg/l) Chloride, as the geochemically most conservative element for much of the area for most of the time (Fig. 5). However, in the natural waters, can give direct indication of the de- there are notable exceptions. The coastal strip near where gree of mixing between different types of groundwaters and the Mersey channel is at its narrowest has chloride concen- also the existence of seawater intrusion, while sulphate may trations in excess of 1000 mg/l for some of the time and the give a useful insight into other non-conservative processes area is relatively elevated all of the time. Also the southeast- causing geochemical variations. This paper is not an in- ern part of the aquifer has locally elevated chloride reaching depth study of the controls of geochemical variations in this up to 4000 mg/l. Noteworthy is an area slightly to the west of aquifer. Rather, the geochemical data are here used to assess the centre of the area that has less dramatic chloride concen- the degree of compartmentalisation. trations but is also locally elevated above background. The

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 57

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A 0 5km B C 0 5km D (b)

Fig. 7. SchematicFigure cross 7 sections(new version) illustrated the variation of the (a) chloride concentration across the faults in years 1993 and 2002 and (b) sulphate concentration across the faults in years 1993 and 2002. The locations of cross-sections showed on the maps (Figs. 6 and 7, respectively).

elevated groundwater salinity in some of the near-coastal re- dle of the block bounded to the east by the Croxteth fault, gions is the result of seawater invasion. water table height does not follow the general reduction to- Maps of the dissolved sulphate concentrations have some wards the coastline and instead rises to more than 25 m and similarities to the chloride maps although they are not exactly 30 m above sea level (Figs. 5a and b, respectively). This ele- the same (Fig. 6). Sulphate is elevated in parts of the coastal vated groundwater table has a sharp western margin marked strip and just to the west of the centre of the study area but by the Kirkdale fault where the groundwater table falls to it is also relatively concentrated at the northern edge of the 10 m below sea level in the direction of the Mersey estuary area. (Fig. 4). To the east of Croxteth fault, the water table elevation follows the Knowsley anticline structure (Fig. 1). The fold axis can be recognized as a groundwater divide where 5 Discussion the water table falls away from the fold axis. In conclusion, the water table in this part of the aquifer (east of Croxteth 5.1 Structural geology and groundwater head fault) is to a fairly large extent unrelated to the regime in the west of this fault. The block to the east of the Croxteth For the two discrete years for which water table height con- fault can be separated by the anticline axis into northern and tour maps have been prepared (Fig. 3) it is noteworthy that southern parts. In the southern part, to the east of the Eccle- water table heights do not match topographic variations re- ston West fault a very sharp and dramatic drop of water table sulting in anomalously low and high water tables relative to height occurs ultimately falling to below sea level (Fig. 4). the overlying land surface. The water table reaches maxi- In general, the variations in the water table contours seem to mum elevations in the north eastern part of the study area be controlled by the regional fault and fold lines. (more than 30 m above sea level) and broadly tends to de- crease toward the coastline and the Mersey estuary (Fig. 4). Following the analysis represented in Fig. 4, an alternative This reduction of the water table height towards the coast is contouring scheme was used for groundwater head for the not steady across the whole area. There are sudden changes two discrete year periods based on the assumptions that the in the water table height over very short distances. In the Croxteth, Ecclestone West and linked Litherland-Kirkdale middle of northern area just west of the Croxteth fault where faults are all low permeability zones strongly limited the the water table drops from >5 m above sea level to as much groundwater flow. The Knowsley anticline will also act as as 5m below sea level (Fig. 4). Therefore, the Croxteth fault a local modifier of the groundwater movement and effec- is a fairly well pronounced groundwater barrier. To the mid- tively act as a groundwater divide. On this modified basis www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 58 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

10 10 3 5 3 5 -5 0 -5 10 5 Formby 5 Formby 0 10 15 5 0 15 0 5 30 35 20 40 20 35 25 5 25 10 30

10 0 0 15 15 25 20

15 20 Liverpool 20 Liverpool bay 10 bay 20 5 30 5 -10 15 Wirral 25 20 0 25 10 9 9 Liverpool 0 Liverpool 15 0 0 -15 20 -5 10 20 5 Wirral -5 Fault 5 -5 Fold 15 15 Contour 10 0 10 lines 5 -10-10 5 0 5km -5 0 5km -5 River 0 River a. ) Water table map 0 0 Mersey b. ) Water table map Mersey 5 3 1993 4 3 2002 4 8 8

Fig. 8. Water table height in meters relative to the ordinance datum (OD, mean annual sea level); (a) 1993 and (b) 2002. The maps were prepared using the same data as Fig. 3 but, in contrast to Lewis et al. (1989), contouring was based on the assumption that the aquifer is compartmentalised by the Boundary, Croxteth, Litherland-Kirkdale and Ecclestone West faults and the Knowsley anticline (see Fig. 4). the groundwater head map was tentatively re-contoured for wood Sandstone will be dominated by cataclasis, grain size the two years assuming the aquifer was compartmentalised diminution and possibly cementation (Beach et al., 1997; by these geological structures (Fig. 8). Figure 8 can be com- Chadwick, 1997; Cowan, 1996; Rowe and Burley, 1997). pared to Fig. 3 to reveal the consequence of the recognition However, in the extreme east of the area, the Boundary Fault, of the low hydraulic conductivity fault zones. The head con- with its great displacement (>300 m), juxtaposes the low per- tours are distinctly interrupted in the vicinities of the Crox- meability, mudstone-rich Upper Carboniferous against the teth, Ecclestone West and linked Litherland-Kirkdale faults. Sherwood Sandstone, therefore it has probably got the lowest permeability values (Moor, 1902; Tellam, 1983) and so will The previous discussion shows that at least some of the have had greatest effect on groundwater movement. faults that dissect the regional Permo-Triassic Sherwood Sandstone aquifer and represented on the geological map On the basis of fault distribution patterns and water table (Fig. 1) could be low permeability zones that have limited elevations, the aquifer could be subdivided into several hy- the movement of groundwater. The close similarity in the drogeological sub-basins with boundaries defined largely by hydrogeological properties of the Permo-Triassic sandstone faults. However, the subtle regional fold (Fig. 1) may also formations suggest that low permeability characteristics of influence groundwater movement patterns with groundwater fault zones are mainly due to the low permeability of the flow being away from the antiform axis. These sub-basins filling materials. Note that the Sherwood Sandstone aquifer (Fig. 8) are thus defined by the fold axis and the most sig- is hugely dominated by fine to coarse grained sandstones. nificant faults (those that have affected water table elevation Fault movement within the Sherwood Sandstone aquifer is most extensively and those with the most significant impacts unlikely to result in the dominant juxtaposition of intrafor- on geological outcrop patterns. Hydrogeological sub-basin 1 mational mudstones and sandstones. Fault zones in the Sher- (B1) occupies the area to the north of the Knowsley anticline

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 59

10 10 3 53 5

Formby 75 Formby 25 50 50 50 25

0 0

75 100 75 100 300 200 200 Liverp Liverpool300 bay bay 200 400 100 ool 400 25 100 75 25 75 500 50 50 1000 50 1000 10000 10000 9 9 Liverpool 50 25 Liverpool 25 25 100 25 50 75 Wirral 100 100 50 Wirral 100 50 75 200 75 1000 75 100 100 1000 4000 4000 75 200 5000 5km River 0 5km 0 200 River Mersey a) Cl- concentration 1993 b) Cl- concentration 2002 3 Mersey4 300 3 4 8 8

Fig. 9. Aqueous chloride concentration in mg/L contoured for (a) 1993 and (b) 2002. The maps were prepared using the same data as Fig. 5 but, contouring was based on the assumption that the aquifer is compartmentalised by the Boundary, Croxteth, Litherland-Kirkdale and Ecclestone West faults and the Knowsley anticline. and is bounded by the Croxteth fault in the west and Bound- The initial contour maps of chloride and sulphate concen- ary fault in the east. Sub-basin B2 lies to the south of the tration (Figs. 5 and 6) have been redrawn and re-contoured Knowsley anticline and is limited in the west by the southern (Figs. 9 and 10) using the new concept that the Croxteth, extension of the Croxteth fault and the Carboniferous sedi- Ecclestone West and linked Litherland-Kirkdale faults limit ments and Ecclestone West fault in the north and east. Sub- water movement and that the Knowsley anticline also acted basin B3 lies to the east of the Eccleston West fault. Sub- as a barrier to movement (thus compare Figs. 9 and 10 to basin B4 lies between the Croxteth fault to the east and the Figs. 5 and 6). Note that the Boundary fault is the effective Litherland-Kirkdale faults to the west. Sub-basin B5 has its eastern limit of the aquifer as it brings aquiclude Carbonifer- eastern margin defined by the linked Litherland and Kirkdale ous rocks against Permo-Triassic aquifer (Fig. 1). faults. At this stage, the interpretation is somewhat tentative being based solely on water table elevations in relation to Chloride distribution maps for the two years (Figs. 5 and topography and fault patterns, structural geology (faults and 9) reveal that the areas of very high chloride concentra- fold) and outcrop patterns. Independent geochemical data tion (that >1000 mg/l) predominantly lies very close to the will now be employed to help ascertain the validity of the Mersey Estuary or the open sea. In the western part, the con- interpretation of compartmentalisation. tours of chloride concentration run approximately NNW-SSE and are parallel to, and bounded to the east by, the linked 5.2 Sub-basin definition and groundwater geochemistry Litherland and Kirkdale faults. This seems to broadly con- cur with the definition of sub-basin B5. However, the very The previous analysis included no water geochemical evi- highest chloride concentrations in this area are constrained dence for the compartmentalisation of the aquifer into dis- to an area west of a small fault near the coast. This is also crete sub-basins. Geochemical data should be capable of as- the area that has some of the lowest water table elevations sessing the plausibility of the tentatively assigned sub-basins. (up to 15 m below sea level). This area is conceivably a www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 60 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

10 10 3 5 75 5 3 100 50 50 25 75 75 50 75 Formby Formby 100 100

50 100 50 75 0 0 75 75 75 100 200 Liverpool 200 Liverpool 25 bay 25 bay 100 50 200 75 100 50 300 400 400 25 25 500 500 50 50 1000 1000 50 9 9 75 25 Liverpool 75 75 Liverpool 50 25 50 25 50 Wirral 75 Wirral 100 75 100 100 100 100 200 100 300 50 200 400 0 5km 50 0 5km 300 River River A) SO 2- concentration 2- Mersey 4 Mersey D) SO4 concentration 3 1993 4 3 2002 4 8 8

Fig. 10. Aqueous sulphate concentration in mg/L contoured for (a) 1993 and (b) 2002. The maps were prepared using the same data as Fig. 6, but contouring was based on the assumption that the aquifer is compartmentalised by the Boundary, Croxteth, Litherland-Kirkdale and Ecclestone West faults and the Knowsley anticline. separate sub-basin (B6) and is likely to be defined by the fault The sulphate concentration contour maps have some simi- that runs west of, and sub-parallel to, the linked Litherland larities with chloride concentration contour maps (Figs. 6 and and Kirkdale faults, which is known as the Kingsway fault 10). As for chloride, one of the areas with the highest sul- (Fig. 2b). This fault was considered previously by (Barker, phate concentrations lie close to the Mersey River bounded 1996) to be a sealed fault that limits eastwards seawater inva- by the linked Litherland and Kirkdale faults although sul- sion. In the extreme southeast of the area (near Widnes), the phate concentrations =∼1000 mg/l only occur very close to the elevated chloride concentrations in the groundwater (locally coast supporting the existence of a separate sub-basin in B5, >4000 mg/l) seem to be sharply defined. This represents the here defined as B6 using the water geochemistry data (Figs. 6 B3 sub-basin. It is possible that the locally elevated chlo- and 10). Based on the sulphate as well as the chloride con- ride concentrations are limited to the west by the Ecclestone centration data, in the extreme southeast of the area, sub- West fault and Boundary fault (Figs. 5 and 9). Away from the basin B3 is defined to the west by the Eccleston West fault coast, there are some locally elevated chloride concentration and Boundary fault. To the middle of B4 is an area of anoma- values not easily correlated with the values in neighbouring lously high sulphate concentration (>200 mg/l; Figs. 6 and areas. In sub-basin B4 there is an area with chloride concen- 10). In the north of the study area, to the east of the Croxteth trations >200 mg/l that is anomalous with respect to the re- fault the sulphate concentration locally rises to >100 mg/l mainder of the sub-basin. The Croxteth fault seems to define while to the west of the Croxteth fault the sulphate concentra- differences in chloride concentration between the tentatively tion fall to <50 mg/l (Figs. 6 and 10). This seems to confirm assigned sub-basins B1, B2 and B4, where the Litherland- that the Croxteth fault is an effective seal between discrete Kirkdale faults define the differences in chloride concentra- compartments (B1, B2 and B4). The chloride and sulphate tion between the tentatively assigned sub-basins B5 and B4. concentration maps seem to support the Knowsley anticline

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 61 as a feature that effectively separates sub-basins B1 and B2 (Figs. 6 and 10). The maps of water geochemistry broadly support the no- Formby tion of sub-basins although the initial sub-division of the area based on water table and the occurrence of structural geo- Croxteth fault logical features is not wholly confirmed. Initially assigned sub-basins B1to B5 seem to be broadly robust. An extra sub- B1 basin to the western edge of B5 may be operative and has been labelled B6. The newly identified compartments to the aquifer are de- lineated in Fig. 11. In this figure the aquifer is divided into 0 Litherland fault areas (B1 to B6) that the water table head and geochemical data have identified. The compartments (B1 to B6) are seem- fault Boundary ingly isolated from each other. This has significant implica- B5 tions for recharge patterns, pollution movement and public Anticline water supply issues. Liverpool bay The compartmentalisation of the aquifer based on con- B4 Knowsley toured maps of water table height (Fig. 8) and geochemistry Cross fault (Figs. 9 and 10) can be tested further by examining individ- ual and averaged geochemical characteristics of each of these Kirkdale fault newly defined aquifer compartments. Although the faults are 9 not the main reason for the geochemical variation between Ecclestone fault Liverpool B2 sub-basins, they can plausibly highlight any differences that B6 exist. The use of averaged geochemical parameters can give an overall view of any geochemical differences. Illustrations Wirral employing all the geochemical data are not particularly help- ful since the figure is very crowded and is not easy to inter- pret. These averaged data have been added to binary cross- B3 plots (Fig. 12). The average geochemical parameters and standard deviation cross-plots of each sub-basin (Fig. 12) in- 0 5km River Mersey dicate that the geochemical composition of groundwater in Chemistry basins 4 sub-basins B1, B2 and B4 are fairly similar but sub-basins B3, B5 and B6 are all quite different and seemingly unrelated either to one another or to sub-basins B1, B2 and B4. Fig. 11. Suggested division of hydrogeological sub-basins based on water table height and water geochemical data. The broadly similar geochemical compositions of waters in sub-basins B1, B2 and B4 could be a result of similar geochemical processes in these parts of the aquifer leading to similar evolutionary pathways. The wide inland extension of gree of geochemical overlap between groundwater samples B3 sub-basin (Fig. 11) seems to result in a high degree of from sub-basins B1 and B2 although there is a significant mixing between saline and fresh groundwaters leading to the difference between the cross-plotted data (Fig. 12) suggest- wide range of geochemical parameters (as reflected by the ing that these sub-basins are indeed discrete aquifers. B1 magnitude of the standard deviation values) in this sub-basin. has very low chloride and moderately low sulphate whereas In contrast, B6 sub-basin where the tendency for mixing be- B2 has moderately low chloride and very low sulphate. In tween saline- and fresh-groundwater is limited by the sealed order to differentiate between the geochemical evolution of fault (Kingsway fault, Fig. 11) the variation range of geo- the groundwaters in B1, B2 and B4 average sulphate and chemical parameters is very limited (Fig. 12). The geochem- chloride cross-plots have been developed (Fig. 13). This fig- ical evolution of groundwater in B5 is seemingly unique and ure indicates that, the main recharge of the groundwater in is not particularly related to any other sub-basin. these three sub-basins seems to be the same, however, the Sub-basin B4 has a small degree of overlap with B1 and groundwater evolution in each sub-basin follows a differ- B2 (Fig. 12a) possibly suggesting that there is limited wa- ent direction. In B1, both parameters are inversely corre- ter movement across the Croxteth fault. The notion that B4 lated where the sulphate concentration increases in the same is discrete from B1 and B2 is largely supported by the geo- way as chloride concentration. In B2 there is no correlation chemical data. B4 has moderately elevated sulphate concen- between the two parameters. High chloride anomalies are trations relative to B2 and moderately high magnesium con- recorded only in the samples close to the Mersey River in centration relative to B1 and B2 (Fig. 12). There is some de- B2 (Fig. 11). In B4 sulphate and chloride concentrations are www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 62 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

360 300 B3 B5 340 250 320

200 300

l l

/ / g

g 150 280

m m

- - 2 2 4 4 260 O 100 B1 B4 O B6 S S B2 240 50 220 0 200 0 50 100 150 200 250 1000 2000 3000 4000 ( ) a Cl- mg/l Cl- mg/l 60 450

B5 430

40 B6 l

l 410

/ / g

B4 g

m m

+ +

2 B1 2 B3 g

g B2 390

M M

20 370

0 350 50 100 150 200 250 100 120 140 160 180 200 220 (b) - - HCO3 mg/l HCO3 mg/l 700 B6 1500 600

500 1300

l l

/ / g

400 g

m m

B3 +

+ 1100 a

a 300

N N

200 900 B5 100 B2 B1 B4 700 0 0 20 40 60 80 100 650 750 850 950 (c) ++ ++ Ca mg/l Ca mg/l

Fig. 12. GeochemicalFigure cross-plots; 12 (new(a) version)average chloride concentrations vs. average sulphate concentrations (b) average magnesium concentrations vs. average bicarbonate concentrations, (c) average sodium concentrations vs. average calcium concentrations. Note, for simpliﬁcation the very high chloride concentration samples (>4000 mg/l) in B6 were excluded in this graph as this sub-basin is separated from the others by the Kingsway fault.

directly correlated which may be a result of them having a Therefore the degree of compartmentalisation of the major related set of controls in this sub-basin. aquifer in the area tested by the groundwater heads is broadly supported well by the variation in the geochemical composi-

The similarity in the overall groundwater geochemistry in tion of groundwater. compartments B1, B2 and B4 (Figs. 12 and 13), with their own localised head variations, seems to support the conclusion that faults are zones of low hydraulic conductivity. If 6 Significance there had been no barriers, then the regional movement of water in all three zones would likely have resulted in re- It has been known that petroleum accumulations are com- gional geochemical evolution patterns. Instead, the ground- monly divided into discrete fluid compartments for a number water has broadly similar composition in all three discrete of years. The data typically used to assess the degree of com- compartments and there has certainly not been regional scale partmentalisation include fluid pressure data, fluid contact change resulting in differences along regional flow paths. (e.g. oil-water) depths, oil geochemistry, gas geochemistry,

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 63 associated water geochemistry, and the physical properties 250 of petroleum (Smalley and England, 1992, 1994; Smalley High anomaly et al., 1995). By analogy we have here used water table 200 samples to the middle of the

l B4 height variations (equivalent to ﬂuid pressure variations and / sub-basin g B1 B4

ﬂuid contact depths in oil ﬁelds) and water geochemistry data m

- 150 2 (equivalent to petroleum geochemistry data). This approach 4 SO has proved to be very effective in determining the degree of 100 B2 compartmentalisation in a near-surface aquifer. These data B2 Samples near are typically available from aquifers in developed regions 50 to the Mersey and the approach could be widely applied with only minor River extra expense. 0 The compartmentalisation in the Sherwood sandstone 0 25 50 75 100 125 150 175 200 225 aquifer is primarily a result of the occurrence of a series of - Cl mg/l major faults that traverse the sandstone-dominated bedrock. The faults have vertical displacements of up to several hun- Fig.Figure 13. 13The (new cross version) plots of concentrations of sulphate versus chlo- dred meters and probably have meter-thick zones composed ride for the B1, B2 and B4 sub-basins samples. of low permeability fault gouge and cataclasite resulting from the ancient movements of the faults (Beach et al., 1997; Chadwick, 1997). Faults are also commonly implicated in spatially restricted to the local compartment. Point-source the lateral compartmentalisation of oil fields although verti- pollution is unlikely to spread to the entire aquifer. cal compartmentalisation in oil fields can also be the result of stratification of permeable and impermeable sedimentary units, growth of diagenetically-cemented layers etc. 7 Conclusions Why does the compartmentalisation of this coastal aquifer matter? The water table close to the coastline is, in some lo- 1. Analysis of water table height variations and groundwa- cations, 10 m below sea level (as a result of anthropogenic ter geochemistry reveals different sub-basins, or com- activities). However, the landward movement of seawater partments, within the Triassic Sherwood Sandstone in the aquifer has been locally limited by faults. Where the aquifer in the Liverpool area in the UK. faults are sub-parallel to the coastline the landward invasion is spatially restricted to the near-shore region of the aquifer. 2. NNW-SSE oriented geological faults, with vertical dis- In stark contrast, where the faults are approximately perpen- placements of up to 300 m, traverse the Triassic and dicular to the coastline, the landward invasion of seawater represent the vertical margins of discrete compartments is more extensive since there is nothing preventing the ad- within the aquifer. vance into the coastal aquifer. Managing groundwater resources is done most effectively with knowledge of compart- 3. The combination of fluid pressure gradients (as repre- mentalisation and its controls. These would enable predic- sented by water table height), water geochemistry data tion of groundwater movements. Adopting the notion that and an appreciation of the dominant geological struc- the aquifer is merely a “tank of sand” (as is implicit in the re- tures in an aquifer (e.g. major faults) proved effective gional hydrogeological map; Lewis et al., 1989) would lead in defining the degree of compartmentalisation in the to a whole host of errors in predicting occurrence and extent aquifer. of saline intrusion. The region in question has relatively high ground to the 4. Lateral influx of seawater is limited by the occurrence of east where the Sherwood sandstone sub-crop could poten- low hydraulic conductivity faults running sub-parallel to tially be a site of active recharge to the entire aquifer. The the coastline in the west of the aquifer. These faults are existence of groundwater compartments due to NNW-SSE perpendicular to the coastline in the south of the aquifer oriented sealing faults suggests that this scenario is perhaps and have permitted more extensive landward invasion of unlikely. The NNW-SSE sealing faults will restrict E-W seawater. recharge. Recharge of the aquifer is thus most likely via vertical percolation of rainfall. This is supported by the occur- 5. Geochemical data show that mixing of groundwater be- rence of a water table high in the SW of the area (under a tween different compartments is limited by NNW-SSE local topographic high) and a long way from the relatively trending faults. This reveals that lateral recharge of the high ground in the east. aquifer from higher ground in the east will be limited. One of the practical implications for the occurrence Recharge must be by vertical percolation thus spatially of compartmentalisation is that any lateral spread of limiting the extent of any groundwater contamination contaminants due to poor environmental protection will be by anthropogenic activities. www.copernicus.org/EGU/hess/hess/10/49/ Hydrology and Earth System Sciences, 10, 49–64, 2006 64 E. A. Mohamed and R. H. Worden: Groundwater compartmentalisation of major aquifer

Acknowledgements. The work was funded through a PhD award Reilly, T. E. and Pollock, D. W.: Factors affecting areas controlling from the Egyptian Education and Cultural Bureau, London. The recharge to wells in shallow aquifers, U.S. Geological Survey Department of Earth & Ocean Sciences at the University of Water Supply Paper 2412, 1993. Liverpool is duly acknowledged. The authors would like to thank Rowe, J. and Burley, S. D.: Faulting and porosity modifica- the Environmental Agency staff J. Ingram and D. Billington for tion in the Sherwood Sandstone at Alderley Edge, northeastern supplying us with the data set used in this study. Cheshire: an exhumed example of fault-related diagenesis, in: Petroleum geology of the Irish Sea and adjacent areas, edited by: Edited by: J. Carrera Meadows, N. S., Trueblood, S. P., Hardman, M., and Cowan, G., Special Publication of the Geological Society, 124, 315–324, 1997. References Shackleton, R. M.: Geology in a scientific study of Merseyside, edited by: W. Smith, Published for Brit. Assoc. by Uni. Press of Aitken, J.: On faults in drift at Stockport, Cheshire. Transactions of Liverpool, 1953. the Manchester, Geological Society, 10, 46–49, 1871. Smalley, P. C. and England, W. A.: Assessing reservoir compart- Barker, A. P.: Isotopic studies of groundwater diagenesis. PhD The- mentalization during field appraisal: how geochemistry can help, sis, Leeds University, 139 p., 1996. SPE 25005, 1992. Beach, A., Brown, J. J., Welbon, A. L., McCallum, J. E., Brock- Smalley, P. C. and England, W. A.: Reservoir compartmentalization bank, P., and Knott, S.: Characteristics of fault zones in sand- assessed with fluid compositional data, SPE Reservoir Engineer- stones from NW England: application to transmissibility, in: ing, 175–180, 1994. Petroleum geology of the Irish Sea and adjacent areas, edited Smalley, P. C., Dodd, T. A., Stockden, I. L., Raheim, A., and by: Meadows, N. 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