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System of the Chalk Across Southern England Modelling the Hydrogeology and Managed Aquifer

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System of the Chalk Across Southern England Modelling the Hydrogeology and Managed Aquifer Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012 Geological Society, London, Special Publications Modelling the hydrogeology and managed aquifer system of the Chalk across southern England R. W. N. Soley, T. Power, R. N. Mortimore, P. Shaw, J. Dottridge, G. Bryan and I. Colley Geological Society, London, Special Publications 2012, v.364; p129-154. doi: 10.1144/SP364.10 Email alerting click here to receive free e-mail alerts when service new articles cite this article Permission click here to seek permission to re-use all or request part of this article Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes © The Geological Society of London 2012 Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012 Modelling the hydrogeology and managed aquifer system of the Chalk across southern England R. W. N. SOLEY1*, T. POWER2, R. N. MORTIMORE3, P. SHAW4, J. DOTTRIDGE5, G. BRYAN6 & I. COLLEY7 1AMEC Environment & Infrastructure, Copper Beeches, St Kew, Bodmin, Cornwall PL30 3HB, UK 2AMEC Environment & Infrastructure, 17 Angel Gate, City Road, London EC1 V 28H, UK 3ChalkRock Ltd, 32 Prince Edwards Road, Lewes, Sussex BN7 1BE, UK 4Environment Agency, Guildbourne House, Chatsworth Road, Worthing, West Sussex BN11 1LD, UK 5Mott MacDonald, Demeter House, Station Road, Cambridge CB1 2RS, UK 6Environment Agency, Manley House, Exeter, Devon, EX2 7LQ, UK 7Hyder Consulting UK Ltd, 3 Kew Court, Pynes Hill, Rydon Lane, Exeter, EX2 5AZ, UK *Corresponding author (e-mail: [email protected]) Abstract: Six regional recharge and groundwater models have been recently developed of the Chalk and Upper Greensand from Dorset to Kent. Updated Chalk stratigraphy and mapping have improved understanding of geological structure and the development of preferential ground- water flow pathways along hardground horizons. Where shallow dipping folds bring these into the zone of active groundwater flow, extensive ‘underdrainage’ may result in marked differences between surface and groundwater catchments. Hardgrounds and marls are also associated with spring discharges, as are some faults and the clay formations that underlie or confine the aquifer system. Higher specific yield within the Upper Greensand helps support summer baseflow, as do local groundwater discharges from augmentation schemes, watercress and fish farm operations. The aquifer system has been successfully modelled using the ‘variable hydraulic conductivity with depth’ version of MODFLOW. Depths of secondary permeability development have been dis- tributed according to ground and groundwater level data. Interfluve–valley contrasts overlie a base hydraulic conductivity set according to the formation saturated at the water table and enhanced by active hardgrounds. Local parameter overrides may also be needed. The Wessex Basin conceptual and numerical model is described before summarizing similarities and contrasts from the other five regional model areas. The Cretaceous Chalk is a white fine-grained typically thin and the dominance of groundwater limestone that crops out over extensive parts of flow pathways is immediately apparent from the southern and eastern England and supports more sparse distribution of the mapped river network in groundwater abstraction than any other aquifer in comparison with less permeable catchments the UK. It has a relatively low specific yield but (Fig. 1). Several of the clear water Chalk stream can develop very high transmissivities through sec- habitats are designated as UK conservation sites ondary development of fissure permeability associ- and two rivers – the Avon and the Itchen – are pro- ated with dissolution of calcium carbonate by tected under the European Union (EU) Habitats groundwater flow – a self-reinforcing geomorpho- Directive (Council of European Communities logical process. It is underlain by the Upper Green- 1992), as are several harbour and transitional sand Formation and overlain by the West Park Farm water body habitats where chalky freshwaters Member (formerly termed the Reading Beds); both enter the sea (Fig. 1a). Whiteman et al. (2012) are silty sand lithologies which locally enhance provide a wider explanation of the role of ground- storage. These aquifers are typically in hydraulic water modelling in these regulatory processes. continuity and are underlain by Gault Clay and con- Figure 1a shows that baseflow also supports fined by the London Clay. many large surface water abstractions and is impor- There is little superficial cover over much of the tant in diluting treated effluent discharged back into Chalk landscapes across southern England; soils are the rivers. There are numerous direct groundwater From:Shepley, M. G., Whiteman, M. I., Hulme,P.J.&Grout, M. W. (eds) 2012. Groundwater Resources Modelling: A Case Study from the UK. Geological Society, London, Special Publications, 364, 129–154. http://dx.doi.org/10.1144/ SP364.10 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012 130 R. W. N. SOLEY ET AL. a Swale & Thanet Coast Avon Itchen Conservation sites Portsmouth,Langstone, Surface Water Poole Chichester & Pagham Abstractions Harbour Harbours Discharges b 'Equivalent recharge circles' i.e. circular areas of estimated long term . British Geological Survey recharge equal to the licensed long term . 1:250,000 Solid Geology Map groundwater abstraction rate c Thames London North Region Kent South West Region Southern Region East Kent 0 50km Brighton & London Clay Thanet Sands/ Worthing Reading Beds Upper Chalk Wessex Test & E Hampshire Middle Chalk Basin Itchen & Chichester Lower Chalk Upper Greensand Recharge & Runoff Models Gault Clay Groundwater system Lower Groundwater Models modelled Greensand Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012 MODELLING THE CHALK OF SOUTHERN ENGLAND 131 abstractions, particularly around London (Fig. 1b). (Mortimore 1983, 1986; Mortimore & Pomerol Groundwater also provides a vital local source of 1996) and in Dorset (Bristow 1994; Bristow et al. reliable supply to smaller towns and villages, avoid- 1995). As a result, these two lithostratigraphies ing the need for long pipelines and pumping. Many were combined and field mapping demonstrated fish-farming and watercress-farming operations that the units could be mapped using traditional depend on artesian and pumped chalk groundwater. and modern methods (Bristow et al. 1997). In This paper summarizes the recharge and ground- 1999 this new stratigraphy was ratified by a joint water models that have been recently developed for committee of the Geological Society and BGS this aquifer system (Fig. 1c) – within the Southern (Rawson et al. 2001) and the main mapping units and South West Regions of the Environment given formation status (Fig. 2). The names given Agency of England Wales (the environmental regu- to the formations are taken from the geographic lator responsible for water management). Revisions locations where the formations and their boundaries to the lithostratigraphic definition of Chalk for- are defined and, for the White Chalk Subgroup, mations are summarized first. These provide more where type sections have been described in detail detail beyond the traditional Lower, Middle and and where the formations are most complete strati- Upper units that are mapped on the regional figures graphically (Mortimore 1983, 1986). in this paper, and have improved the understanding In Wessex, the new stratigraphy is broadly appli- of structure and the variation in hydrogeological cable throughout the region (see for example the characteristics through the sequence. Thereafter new BGS 1:50 000 sheets for Dorchester (Sheet 328, the paper describes the conceptual and numerical 2000; Westhead 1992, 1993), Winchester (Sheet development of the Wessex Basin model (Fig. 1c) 299, 2002, Booth 2002) and Alresford (Sheet 300, that has been completed most recently, focusing 1999; Farrant 2002)). There are several local vari- on features and mechanisms that are widely appli- ations including, for example, the thinning west- cable across many areas of the aquifer system. wards of the Plenus Marls (figure 9 in Bristow Brief accounts of the other five regional model et al. 1997), the presence of a hard porcellenous areas follow (Fig. 1c), emphasizing similarities as chalk layer at the boundary of the Lewes and well as recognizing the distinctive features encoun- Seaford Chalk formations (e.g. the Bar End Hard- tered. Remaining modelling challenges are set out grounds around Winchester, Mortimore & Pomerol before conclusions are finally drawn together. 1987), and a further hard porcellenous chalk layer near the boundary of the Seaford and Newhaven Chalk formations (the Stockbridge Rock/Whitway Chalk lithostratigraphy for hydrogeology Rock Member, Winchester Sheet, Booth 2002; Newbury Sheet 267, 2006). This latter hard layer Southern England Chalk has been completely re- is probably the lateral equivalent of Barrois’ Sponge mapped by the British Geological Survey (BGS) Bed, a regionally important marker bed. using nine Chalk formations (Bristow et al. 1997; The Chalk Rock in its type area of Berkshire Mortimore 2001; Rawson et al. 2001; Hopson (Bromley & Gale 1982) is a complex of hard, indu- 2005). These nine formations (Fig. 2) replace the rated, mineralized layers representing most, if not traditional three divisions into Lower, Middle and all, of the Lower Lewes
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