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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

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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 Chalk in a highly condensed Upper Chalk of Jukes-Browne (1880), Penning & succession. This is condensed to less than 1 m thick Jukes-Browne (1881) and Jukes-Browne & Hill in some parts of the Wessex Basin area (Mortimore (1903, 1904). During the 1980s and early 1990s 1983). The dominant horizon within the Chalk Rock fieldwork and analyses of satellite imagery (Marsh also varies across the region. The lowest hard- 1993) progressively showed that the Chalk had far grounds (i.e. at the base of the Lewes Chalk) pre- more structure and geomorphology than is pre- dominate along the southern margins of the region. sented on the 1:50 000 BGS maps using only the Along the northern margin of the region, higher three traditional divisions. In the landscape, these hardgrounds predominate. In other areas the Chalk features are represented by a series of escarpments Rock, while still retaining hard nodular layers, is a and dip slopes that appeared to relate closely to coarse, gritty, shelly chalk (Mortimore 1983). the Chalk lithostratigraphy defined in Sussex Hence the Chalk Rock, for simplicity, is shown as

Fig. 1. The outcrop of the Chalk and associated aquifers across Southern England: (a) 1:50 000 scale rivers, surface water abstractions and discharges, and protected conservation sites designated under the EU Habitats Directive; (b) solid geology formations from British Geological Survey 1:250 000 scale mapping showing groundwater abstraction pressures illustrated as ‘equivalent recharge circles’; and, (c) areas covered by the regional recharge and runoff models (dotted lines), and associated groundwater models (solid lines) across Southern and South West Environment Agency Regions. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

132 R. W. N. SOLEY ET AL.

Fig. 2. Chalk lithological and fracture stratigraphy showing some of the key horizons associated with enhanced groundwater fissure flow and spring lines from the Hampshire and Dorset modelling areas.

a broad band on the stratigraphical diagram (Fig. 2). affects index properties such as matrix porosity. Where well developed, the Chalk Rock frequently The porosity measurements for the various beds acts as a flow horizon, enhanced locally by cavities shown in Figure 2 are based on mercury intrusion seen on borehole camera logs. porosimetry (Bloomfield et al. 1995; Mortimore & Within the formations are a number of other key Pomerol 1998) on mini-cores taken from the cored marker beds including marl seams, hardgrounds and BGS Faircross Borehole Berkshire (Mortimore & flint bands (distinctive engineering properties Pomerol 1998). Plotted stratigraphically, these described in Table II of Mortimore & Pomerol measurements show a typical range of matrix poros- 1998). Many are groundwater flow horizons in ity for the different chalk materials and formations both the saturated and unsaturated zones (local (Fig. 2), although this is much larger than the effec- perched water). Each of the key marker horizons tive porosity associated with gravity drainage and is identified in Mortimore et al. (2001). Chalk specific yield. material also varies in composition (i.e. type of car- As well as the chalk material, measurements on bonate grains related to the original rock-forming fracture orientations, persistence, style and fre- calcareous algae, and post-depositional process of quency show that these are also stratigraphically compaction and mineralization), and this variation distributed (figure 2 in Mortimore 2001). Many of Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 133 the fractures are of synsedimentary origin and relate towards the SE beneath the middle reaches of the to changing stress fields during the formation of the Rivers Avon and Stour, tributaries of the Stour Chalk. Fractures are consistently well developed including the River Allen and the Tarrant, and where contrasting lithologies are present (e.g. beneath the upper half of the Piddle and Frome Plenus Marls–Melbourn Rock; Glynde Marls– catchments. The lower reaches of all these rivers base Lewes Chalk/Chalk Rock), and these tend to flow over the clay, silt and sand formations of the be enhanced groundwater flow horizons. Combining Palaeogene that support minor shallow groundwater the information on chalk properties and fracturing flow systems isolated from the regionally important related to the lithostratigraphy makes the new underlying aquifers by the London Clay. The new maps produced by BGS even more valuable BGS 1:50 000 maps show that, as well as these to hydrogeology. folds and broad basinal structures, the Chalk is also extensively faulted. Outcrops of the lower Chalk formations com- Conceptual and numerical modelling of the monly form topographic scarp features, hills or Wessex Basin aquifer system ridges above the more flat lying Upper Greensand valleys (Fig. 3a, b). The drainage network over the The development of the Wessex Basin recharge, Chalk is very sparse in comparison with the Jurassic runoff and groundwater model was completed in and Palaeogene catchments, but valleys typically February 2010 (Entec UK 2010). The model was extend well upstream of mapped water courses. extended westward to cover the whole Wessex These are dry for most of the time, flowing only Basin from a study area that initially focused on during winter when groundwater levels rise to over- the catchment of the Hampshire Avon where EU flow into more extensive ‘winterbourne’ reaches. Habitats Directive investigation drivers were more A BGS structural model providing a three- urgent. Much of the conceptual and numerical model dimensional interpretation of the elevations of the features discussed here are summarized from Ham- new stratigraphic formations (as in Fig. 3c) was an pshire Avon reports for the Environment Agency essential building block of the Wessex Basin con- (Entec UK 2005a) which, in turn, built on the under- ceptualization. Figure 4 is a north–south cross- standing established in previous modelling studies section through these formations and includes the of three sub-catchments – the Bourne and Nine inferred elevations of three horizons that, in differ- Mile River (Environment Agency 2004), the Wylye ent parts of the Wessex Basin, are associated with (Komex 2004) and the Lower Avon and Dorset enhanced fissure flow into abstraction wells, as Stour (Water Management Consultants 2004). well as with locations of river flow loss and spring discharge. These are the Melbourn Rock and Topography, stratigraphy, structure and Plenus Marls at the base of the Holywell Chalk, saturated aquifer formations the Chalk Rock, and the Whitway Rock (which is also called the Stockbridge Rock further east) – as The Wessex Basin model area in the Environment shown in Figure 2. Other notable zones of preferen- Agency’s South West Region includes the surface tial permeability development also occur around and groundwater catchments drained by the Rivers the top of the Lewes Chalk. It should be emphasized Frome, Piddle, Dorset Stour, and Hampshire Avon that secondary bed-parallel or fracture-related (Figs 1c & 3a). The topographic form of the domi- fissure transmissivity may be developed in most nantly rural landscape (Fig. 3a) reflects the parts of the Chalk sequence, depending on the his- outcrop (Fig. 3b) and structure (Fig. 3c) of the geo- tory of active groundwater flow, but the horizons logical formations. The principal lithostratigraphic noted are often considered to be particularly units of the aquifer system are, from bottom to important. top, the Upper Greensand, Chalk and the West Observed groundwater levels were contoured for Park Farm Member, and these are bounded by the periods representing minimum (drought) and underlying Gault Clay (mapped in blue on Fig. 3b) maximum (wet winter) conditions, and these sur- and the confining London Clay (mapped in pink). faces are included in Figure 4. Beneath interfluve These strata are gently folded into an open syn- areas unsaturated depths can be over 50 m and the cline beneath Salisbury Plain which has a shallow aquifer formations at the groundwater table are eastward plunge (Fig. 3c). A series of anticlines often not the same as those mapped at surface bring the older Upper Greensand and lower Chalk outcrop. By intersecting contoured groundwater formations to crop out beneath the Upper Avon, levels with the stratigraphic model surfaces, a the River Wylye and the River Nadder. Much of ‘water table geology’ map can be developed, as the upper reaches of the Rivers Nadder and Stour shown in Figure 5a for the minimum groundwater drain the less permeable underlying Jurassic clay level period. Such maps are helpful for the develop- and limestone formations. The dip steepens ment of numerical model parameterization because, Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

134 R. W. N. SOLEY ET AL.

Fig. 3. Eastward-looking mapped projections, surfaces and drainage of the Wessex Basin model area illustrating the relationships between (a) scarp and dip slope topography and the river network; (b) the outcrop of the main aquifer formations, and the aquicludes that underlie and confine the aquifer system; and (c) the folded and faulted geological structure apparent in the base of the Chalk surface from the British Geological Survey geological model. while there are many local exceptions, the for- groundwater flow across bedding is more restricted mations do have distinctive general characteristics. in comparison with bed-parallel flow. Both the Upper Greensand and West Park Farm Figure 5b shows the Upper Greensand surface Member have a higher specific yield (ranging from and elevation from the BGS geology model and 5 to 20%) than the Chalk (typically between 0.5 includes the projected lines of the rivers. This is a and 2%). They are also typically less transmissive. westward looking view of part of the Wessex The lower Chalk formations tend to be marlier Basin, which is presented in an eastward view in than those further up the sequence such that second- Figure 3. A shallow syncline is again seen to be ary developed permeabilities are often lower, and plunging gently eastward beneath the Rivers Avon Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 135

Fig. 4. A north–south cross-section through the Wessex Basin along a line shown in Figure 3a. The folded and faulted new Chalk lithostratigraphic formation elevations and inferred hardground and marly horizons (at assumed metres above base (m a b) of the formations) are based on the BGS structural model. High and low groundwater levels are drawn from hand-contoured observations and river discharge elevations. Unsaturated depths may be over 50 m beneath interfluves so that formations saturated at the water table are different from those mapped at the surface.

and Bourne, which drain Salisbury Plain towards the bed of the upper reaches of the Bourne – where flow River Test. is lost, and also returns it close to the surface further down gradient – where perennial flow is estab- lished. The intersection of structure, topography Development of hardground flow horizons and and drainage thereby provides a route for flow winterbournes into, through, and out of this horizon that has enhanced bed-parallel transmissivity because it is The importance of enhanced transmissivity devel- harder for water to flow across it. Geophysical opment and groundwater flow associated with hard- logging of the abstraction boreholes located next ground horizons is illustrated by the spatial and to the Bourne confirms that the Whitway Rock temporal variation of flows gauged down the River is an important flow horizon, but the loss of flow Bourne (Fig. 6a). As its name suggests, only the in the middle reaches is mostly a natural fea- lower reaches of the river have perennial, year- ture of the hydrogeology (Environment Agency round flow. For much of the time, flow accreting 2001, 2004). to its headwaters is lost so that there is no water in During low groundwater level periods, much of its middle reaches. The whole river only flows the water leaking into the Whitway Rock ‘under- during wet winter months when recharge and drain’ discharges at a lower elevation into the groundwater levels are highest. River Avon to the west, or into tributaries of the This behaviour is explained by the relatively River Test in the east (Fig. 6c). Such ‘underdrai- high elevation of the Bourne in between the more nage’ is less likely to be associated with the Chalk deeply incised Rivers Avon and Test, and by the Rock beneath the River Bourne because Figure 6b transmissivity associated with fissure flow around shows that, being deeper than the Whitway Rock, the Whitway Rock horizon (Fig. 6b). The Salisbury the syncline does not bring it closer to the surface Plain syncline brings this horizon to crop out in the again in this location – there is ‘nowhere for the Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

136 R. W. N. SOLEY ET AL.

Fig. 5. (a) A westward looking ‘water table geology map’ of saturated Upper Greensand, Chalk and West Park Farm Member formations within part of the Wessex Basin, projected onto the mimimum groundwater level surface. The main rivers discussed in the text are labelled. (b) The underlying geological structure from the British Geological Survey model indicated by the base of the Upper Greensand. A shallow dipping syncline beneath Salisbury Plain plunges gently beneath the Rivers Avon and Bourne towards the River Test. The structure dips more uniformly towards the SE beneath the lower reaches of the Rivers Avon, Stour and tributaries.

water to flow to’ in the south (Environment Agency on Fig. 3a). West of the Avon, therefore, the 2001, 2004). However, on the west side of the River most important flow horizon is the Chalk Rock – Avon, the Chalk Rock is shallower and Figure 4 responsible for regional flows across surface catch- shows that it crops out on both the up-gradient ment divides, a well-defined inflow horizon in northern margins of Salisbury Plain, and on the abstraction boreholes, and associated with spring down-gradient side, in the Wylye Valley (and its discharges into the lower reaches of the Rivers tributaries, the Rivers Chitterne and Till – located Chitterne and Till. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 137

The Whitway Rock occurs above the water table the interfluves and do not act to significantly attenu- further west but, like other similar horizons, it may ate recharge. If there are storm rainfall events result- still be associated with the enhanced fissuring and ing in intensive recharge when groundwater levels the lateral movement of recharge down-dip within are already high, extremely rapid rises and falls in the unsaturated zone. level are observed that are also apparent with lower amplitudes next to the winterbourne. Figure 7 includes schematic annotations devel- Recharge, unsaturated zone flow, and oped as part of the conceptual model to help explain interfluve–valley contrasts in hydraulic the contrasts between Chalk valley and interfluve. conductivity profiles Fissure transmissivity in the Chalk is mostly asso- ciated with bedding plains (typically sub-horizontal, Further to the south and west, the Chalk dips more particularly above lower permeability tabular flint uniformly southeastwards, beneath the confining or marly horizons), or with perpendicular jointing. Palaeogene. Therefore, around the middle reaches The blue dashed lines on Figure 7 represent these fis- of the River Avon, the River Stour and its tributaries sures, the thickness of which indicates their degree of the River Allen and Tarrant, hardground flow hor- development. Immediately beneath the shallow soils izons, although locally important, have not devel- across the interfluves, these schematically suggest a oped over such large areas to cross surface water large number of pressure-relieved small cracks and catchments. Chalk transmissivity is extremely vari- weathered fractures with the dominant direction of able. Some boreholes drilled may be totally dry, unsaturated zone flow being downward – through whereas others nearby intersect high-yielding the Chalk matrix, and along fissure surfaces over a fissures. However, some general contrasts between wide area. Processes of unsaturated flow and interfluve and valley locations are apparent that recharge to the water table have been described pre- relate to the development of secondary fissure viously by Mathias et al. (2005), with most volumes permeability through dissolution and the history of draining relatively slowly through the bulk of the groundwater flow. rock matrix and small fractures. Figure 7 is a cross-section drawn from an obser- However, Figure 7 suggests that larger fissures vation borehole next to the Tarrant – a winterbourne are also found in the unsaturated zone, which may valley labelled on Figure 3a – to an interfluve obser- carry some flow relatively rapidly in response to vation borehole approximately 1 km to the east. intensive recharge events. Bedding plain fissures Both these groundwater level records include the (particularly associated with hardgrounds) could five year period plotted from 1998 to 2003 during collect water from a wider area and focus it downdip which data loggers were fitted to capture short-term into a relatively fewer number of larger vertical fluctuations. Water levels in both boreholes fall to joints. As dissolution continues and the recharge roughly the same minimum elevation – just below flows down through the unsaturated zone, it becomes the bed of the Tarrant (so that it is losing flow, or more saturated with respect to calcium carbonate is dry). During such summer and autumn periods, and therefore less chemically aggressive. Once at the dominant gradient is southward – towards the the water table, the saturated aquifer hydraulic gra- main River Stour. The bed elevation of the Stour dient becomes important in determining the direc- over its middle reaches has eroded below its Chalk tion and rate of flow (whereas geological structure tributaries because of the relatively high-energy may be more important under gravity in the unsatu- flood flows associated with its flashy Jurassic rated zone). upper catchment. This helps to explain why tribu- As a dual porosity fracture/matrix aquifer, the taries like the Tarrant lose water as they approach flow response of the Chalk where soils are thin the lower drainage elevation of the main river appears to depend on the intensity of the stresses channel. put on it. Hence, a sharp storm may result in the acti- The observed groundwater level response to vation of an otherwise abandoned, low-storage winter recharge is very rapid in both boreholes, fissure system above and around the water table, even though the unsaturated depth at the end of which rapidly conveys water through the unsatu- the summer recession may be over 25 m beneath rated zone and away to river and winterbourne dis- the interfluve. In the valley, groundwater discharges charges. Ireson et al. (2009) describe ‘same-day’ into the winterbourne so the amplitude of ground- Chalk groundwater level responses to high-intensity water fluctuations is limited in comparison with rainfall at an experimental site where the unsatu- the interfluve. Away from the valley, levels rise by rated depth is c. 70 m. Rapid baseflow responses 10–15 m to increase the gradients driving flow are commonly gauged throughout the Wessex Basin towards the Stour, and also towards the Tarrant. river network if rainfall exceeds 10 mm/day, even The chalky loam soils across the Wessex Basin are during summer months when regional soil moisture typically thin and free-draining, particularly across deficits have been established. Short-term nitrate Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

138 R. W. N. SOLEY ET AL. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 139

Fig. 7. Observed groundwater level hydrographs on a schematic section across the Tarrant winterbourne illustrating contrasts in fissure development and hydraulic conductivity variation with depth (VKD) between valleys and interfluves.

spikes observed in the raw water abstracted from ongoing Chalk dissolution could be expected to public supply wells may provide further evidence have resulted in a gradual lowering of minimum of rapid transport from the soil in such events. interfluve groundwater levels until, as around the During prolonged, slower-recession periods, low Tarrant, they are the same as in the adjacent valleys. flows are supported by gravity drainage from the These winterbourne valleys and river corridors matrix and smaller fissures. The Chalk water table are exposed to much more concentrated fluxes of will always ‘drain away’ from this unsaturated throughflow than the interfluves because they are fissure system until the rate of drainage is exceeded the groundwater ‘collector drains’ (Fig. 7). Unsatu- by the rate of recharge. Over long periods of geo- rated depths are also much less so recharge water is logical time and exposure to rainfall recharge, more chemically aggressive. As a result, bedding

Fig. 6. (a) Spot flows gauging survey results for the River Bourne demonstrating its ‘winterbourne’ character. For much of the time flow is lost from its upper section such that the middle reaches are dry. (b) The geological cross-section drawn along the line of the river indicates that enhanced fissure transmissivity and ‘underdrainage’ associated with the Whitway Rock horizon helps to explain this behaviour, and the syncline structure evident in Figure 5b returns of some of the water to the lower perennial reaches of the river. (c) Map of the River Bourne with arrows to show that, during the summer, some of the Whitway Rock water flows along the axis of the syncline towards the lower level River Avon to the west and River Test to the east. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

140 R. W. N. SOLEY ET AL. plain and jointed fissuring are developed to greater lower permeability features help to explain the depth beneath minimum groundwater levels and mapped distribution of springs and rivers. The transmissivities can become very high. Virtually all source of the River Bride, which flows away from large Chalk groundwater abstractions in the Wessex the Wessex Basin to the west, is a scarp slope Basin area are therefore located in valleys. spring from the Upper Greensand. Spring flow Figure 7 also includes two schematic profiles of from the lowest formations of the Chalk and from horizontal hydraulic conductivity with depth (VKD) the Upper Greensand provides perennial baseflow drawn to contrast typical valley and interfluve loca- into the River Hooke, which is reliable enough to tions. Along valleys, permeabilities are developed support a headwater surface abstraction. A very down to 50 m or more beneath the water table. Some large karstic spring emerges from Jurassic Lime- horizons have particularly enhanced flow capacities, stones that are faulted against the main Chalk but these will only carry flows from under interfluve aquifer at Upwey on the southern boundary of the areas if water can easily get into them from these Wessex Basin. Upper Greensand inliers are also areas (rather than from further up the valley). mapped at the headwaters of the Sydling, South Hydraulic conductivities may be extremely high in Winterbourne and River Win, although some the near surface, but the seasonal fluctuations in spring flow may also be associated with the Plenus groundwater levels are small so that there is little Marls outcrop or with faulting, as at the source of variation in transmissivity between the winter and the North Winterbourne, the Bere Stream and the the summer – it remains high all the time. River Piddle. In interfluve areas, however, Figure 7 suggests The cross-section in Figure 8b (after Colley that fissure development is much shallower beneath 2005) is drawn along the Devil’s Brook (labelled the minimum water table. The hydraulic conduc- on Fig. 8a). Groundwater levels measured around tivity profile has been extended up through the unsa- the Plenus Marls suggest that this formation, which turated zone to the ground surface as if it was dips southwards, acts as an important barrier here saturated. This is no longer the case, but the profile as well, promoting discharge to the Brook and limit- drawn assumes that fissuring is present above the ing the upstream propagation of drawdown asso- present day water table because of the preceding ciated with abstraction from a public supply long history of recharge and flow. When ground- borehole. Downstream of the Plenus Marls the water levels are low, transmissivities are also very Brook is perched above the water table where it low, unless ‘underdrained’ by an active flowing hard- flows past the borehole. Some of the abstracted ground. However, transmissivities increase rapidly water is returned to the stream to augment flows as recharge reactivates flow in fissures above the through the nearby village for amenity purposes. minimum water table. Some of the rivers on Figure 8a follow fault lines and these may also promote the development of pre- ferential groundwater flow paths along them. Fault- Springs and flow accretion supported by the ing along the North Winterbourne, for example, Upper Greensand or associated with marls, may be associated with the observed loss of flow faults and clay formations where the path of the bourne turns eastward to the Stour. These fault lines extend south towards the Around the northern and western margins of the Bere Stream where discharges from springs and Wessex Basin, extensive areas of the relatively artesian watercress farms can only be accounted high-specific-yield and lower-transmissivity Upper for as cross-catchment transfers from the north. Greensand sustain springs and support baseflow Groundwater from the Chalk and West Park into river headwaters. Faults and the valley outcrop Farm Member also discharges at a number of promi- of hardgrounds and lower permeability marl hor- nent watercress and fish farms at the edge of the con- izons can also act as barriers to groundwater flow fining London Clay (e.g. near Empool, Fig. 8a). that bring the water table to the surface and promote Recharge through the West Park Farm Member is spring discharge. Both the underlying Gault Clay undersaturated with respect to calcite and may fur- and confining London Clay are also effective aqui- ther enhance local secondary permeability deve- cludes with springs emerging where they crop out. lopment. There are few areas now where runoff Scarp springs flow from the base of the Upper from the Palaeogene flows onto the Chalk, so the Greensand over the Gault Clay, and dip slope large active swallow holes and sinks that occur groundwater outflows from locally karstic springs around the karstic margins of the London Basin along the West Park Farm Member margins of the are not a feature of the Wessex Basin. However, Palaeogene because flow into the confined zone is several large solution collapse features close to the very limited. Chalk–Palaeogene (London Clay) contact are evi- Figure 8a shows the western catchments of the dence of large-scale secondary fissure development Rivers Frome and Piddle and illustrates how these in the past. 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MODELLING THE CHALK OF SOUTHERN ENGLAND 141

Fig. 8. (a) Mapped relationships of scarp and dip slope springs and river lines with the outcrop of the Upper Greensand, the intersection of the low-permeability Plenus Marls with the dry period water table, the confining London Clay, and with faulting across the Frome and Piddle catchments. (b) Section down the Devil’s Brook with observed groundwater levels, losing and gaining reaches, and inferred groundwater table: Plenus Marls promote discharge to the Brook and act as a barrier to drawdown associated with public supply abstraction.

Numerical model codes and structure code (Heathcote et al. 2004) with a groundwater model built in a version of the MODFLOW code The numerical Wessex Basin model is constructed adapted by the Environment Agency to include the on a regular 250 m grid and combines a daily rep- representation of variable hydraulic conductivity resentation of rainfall, routed runoff, evapotran- with depth (i.e. MODFLOW-VKD, Environment spiration and recharge simulated using the 4R Agency 1999; Taylor et al. 2001). Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

142 R. W. N. SOLEY ET AL.

The combination of 4R with the MODFLOW groundwater–surface interaction and accumulate Stream Package (Prudic 1989) was chosen because routed river flows combined with runoff from the of the importance of simulating the spatial and tem- 4R simulation. Chalk winterbourne behaviour poral variations in the impacts of groundwater suggests there is little resistance to baseflow dis- abstraction across the network of intermittent and charge from the aquifer, so modelled stream con- perennial rivers. Finite element codes would have ductances are generally high. However, leakage advantages in representing groundwater flow features rates back to the aquifer are typically limited, par- associated with hardground-enhanced fissuring, but ticularly during low-flow periods when the stream did not deal as well with groundwater–surface may flow perched on lower permeability bed sedi- water interaction and total river flow accumulation. ments above the water table. In the MODFLOW MODFLOW is also the industry standard for the Stream Package, this can be represented by raising Environment Agency’s National Groundwater Mod- the elevation of the stream bed bottom elevation to elling System (Whiteman et al. 2012). close below the stream stage (water level) until The runoff and recharge model extends over the rates of modelled leakage match observed losses. total catchments of all the Wessex Basin rivers, Establishing accurate and smoothly interpolated including the Jurassic upper-catchments of the stream water level profiles, controlled by ground Nadder and Stour, and the Palaeogene lower surveys wherever possible, is a very important catchments (Fig. 9a). Time-series of surface water task in the model build because winterbourne abstractions and discharges are included in 4R behaviour is often related in part to these drainage runoff and shallow groundwater interflow routing. boundary elevations. This provides a credible simulation of total flows Most of the groundwater leaves the model in from both the Jurassic and Palaeogene parts of the interaction with these stream cells (Fig. 9a), or model area, and routes accumulated flows into from abstraction cells. There can be large overlap- MODFLOW stream cells that simulate interaction ping areas between neighbouring models (e.g. with groundwater in the main aquifer system (see the Wessex Basin, Test and Itchen and East inset detail left of Fig. 9a). Hampshire models, as in Fig. 1c). This is because On the Upper Greensand, Chalk and Reading Chalk groundwater divides between catchments Beds outcrop processes simulated in 4R are domi- can move markedly between seasons or in response nated by recharge, including a by-pass of soil moist- to abstraction pressure. It is therefore preferable to ure deficits related to rainfall intensity. The 4R and extend the model beyond its ‘core area’ of interest MODFLOW models are not coupled. The arrival of to a neighbouring river boundary. recharge at the water table is subject to lags that are General Head Boundaries have been locally spatially distributed according to a fixed grid of introduced along the Chalk at the coast around unsaturated depth. The lags do not change according Lulworth (shown in red on Fig. 9a) to allow ground- to the modelled heads, but a rapid ‘same-day water outflow to the sea. fracture flow’ route can be optionally activated if recharge exceeds a daily intensity threshold. Investigating alternative numerical The MODFLOW model has three layers to rep- resent the contrasting storage and flow properties implementations of the conceptual model of of the Reading Beds (layer 1), the Chalk (layer 2) Chalk transmissivity development and the Upper Greensand (layer 3). It runs with three stress periods per month of variable length Use of a single MODFLOW layer representing the (10 days, 10 days, plus the remainder of the month) Chalk has advantages in avoiding cell drying and and incorporates the influence of groundwater re-wetting instabilities, but requires marked sim- abstractions – the largest being for public supply plification of the three-dimensional complexities and watercress-farm and fish-farm support. evident in the ‘real’ multiple fissure, hardground, The combined simulation starts with a five year marl and matrix flow structures described in ‘warm-up’ period from 1965 – relatively short Figures 6–8. because of the low specific yield of the Chalk – During the early stages of modelling with the before the calibration period running from 1970 MODFLOW-VKD code, alternative ways of sim- to 2009. plifying the conceptual understanding of transmis- sivity development were investigated (Fig. 10). Model boundary conditions Initial model runs assumed that hydraulic con- ductivity was constant with depth. Manually defined The extent of active 250 m MODFLOW cells is zones were drawn to distinguish the formations shown by the black boundary drawn on Figure 9a. within which the water table resides (Fig. 10a), to The MODFLOW Stream Package (Prudic 1989) is differentiate between valleys and interfluves, and used as the top boundary condition to model to introduce higher-transmissivity zones associated Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 143 with the plunging axes of synclines (Fig. 10c, also Chalk Rock and Whitway Rock – were inferred Fig. 5b). from the BGS structural model and compared with However, manual zone definition, while essen- the minimum water table. Where the hardground tial during subsequent more localized model refine- is above the minimum water level it is assumed to ments, can be overly influenced by the modeller’s have no influence on the modelled transmissivity. preconceptions. It also does not make best use of Where it occurs at greater than a fixed depth readily available information on the topography beneath the minimum water table (estimated with and distribution of unsaturated depth, and the con- reference to examples like the Bourne in Fig. 6b to ceptual understanding of how these can influence be 35 m), it is also assumed to be inactive. Over the transmissivity distribution. Figure 10d is the the areas where the hardgrounds lie closer beneath ‘steppy’ transmissivity profile down a north–south the water table, they are assumed to add a fixed section line within the model based on such manu- amount of additional transmissivity to the profile ally drawn hydraulic conductivity zones and assu- by lowering the inflection point to a fixed depth of ming the minimum water level and model base 25 m below the water table. shown above in Figure 10b. The transmissivity This approach uses the VKD profile available in changes from valley to interfluve are sharp rather the code, but is still a marked simplification of the than transitional, and there are only small seasonal ‘real system’. In reality the extra transmissivity variations in interfluve transmissivity as water lies within the fissuring associated with the hard- levels change. Overall this numerical implemen- ground, as illustrated schematically in Figure 7, tation did not capture many of the key character- and this may become an effective drain over larger istics of the conceptual model, and the resulting areas as groundwater levels rise. Nevertheless the flow simulation was poor. Therefore, alternatives result is that the hardgrounds generate broad areas were sought using the MODFLOW-VKD code. of enhanced transmissivity where they occur within The inset at the bottom right of Figure 7 shows the the shallow Salisbury Plain syncline, as shown simple VKD hydraulic conductivity profile and around the Bourne in Figure 10e. In these areas some of the parameters used to define it which are the effect of the VKD profile in amplifying seasonal discussed below. variations in transmissivity is reduced – the ‘under- The Figure 10a minimum water level formation drainage’ mechanism makes VKD less important. map was retained to define variations in the ‘unde- A more continuously varying transmissivity pro- veloped’ base hydraulic conductivity of the Chalk file results (Fig. 10f ), which is probably a better (Kbase). However, the majority of the transmissiv- approximation of the variation in the real system ity calculated in the Chalk layer is developed than the use of simpler manually defined zones of above the VKD ‘inflection point’ elevation, as fixed properties. This numerical implementation shown in Figure 10b. This is distributed continu- captured many of the important features of the con- ously across the model using the topographic and ceptual model and also produced a better simulation minimum groundwater level surfaces mapped of flows. during conceptualization. The inflection point is The numerical modelling continued to refine the set at the lowest elevation of (topography, 45 m) controls influencing this parameterization. A number and (minimum water level, 1 m). Above the inflec- of local overrides were built in where necessary, but tion point the hydraulic conductivity increases up to the regional approach summarized appears to be a maximum developed Kmax. reasonably sound, within the constraints of the The depth of more developed saturated Chalk is MODFLOW-VKD code used. greatest in river and dry valleys where the water table is close to the surface (e.g. areas shown in Model transmissivity distribution and faults brown on Fig. 9a). Here water level fluctuations are also minimal so there is little variation in the Figure 11a shows the modelled transmissivity dis- high resulting transmissivities throughout the year. tribution based on low groundwater levels simulated On the interfluves, the seasonal water level ampli- by the Wessex Basin run finally agreed as ‘fit for tude is much greater and is associated with purpose’ following refinement against observed marked increases in hydraulic conductivity such groundwater levels and river flows. It has been cal- that winter transmissivities are much greater than culated to combine the dry period transmissivity during dry periods. associated with all three model layers according to A further refinement to the regional approach their saturated depth. was important to represent the additional transmis- The distribution reflects the influences of litho- sivity that may be associated with developed hard- stratigraphy, structure, hardground ‘underdrainage’ ground ‘underdrainage’ at depth beneath some and barrier zones representing the Plenus Marls interfluve areas. The elevations of the most promi- and the down-gradient outcrop of the Chalk Rock, nent potential flow horizons – the Melbourn Rock, as previously introduced. Some of the faults Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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MODELLING THE CHALK OF SOUTHERN ENGLAND 145 identified on recent BGS maps have been rep- groundwater divide northward to the Chalk Rock resented in the model (Fig. 11a) using the Horizon- outcrop, promoting preferential southward drainage tal Flow Barrier package (Hsieh & Freckleton of the Chalk in the Great Ridge area, down geologi- 1993), where they appear to be essential to simulate cal dip towards the tributaries of the Nadder. observed spring heads, or to improve the simula- Within the confined Chalk and West Park Farm tion of confined aquifer heads. The need for refine- Member aquifers to the south and east of the ment of regional concepts and parameters is also Wessex Basin, modelled transmissivities are low locally apparent. (140 m2/day). To the north of Poole Harbour, inves- Around the western and northern margins of the tigations into the viability of an aquifer storage and Wessex Basin, the modelled transmissivity of the recovery scheme provided additional information Upper Greensand is relatively low compared with on confined aquifer flows and storage properties. the Chalk. Its saturated depth above the Gault Following test abstraction, rates of recovery were Clay is typically shallow (5–30 m), and is multi- slow. An associated groundwater modelling study plied by hydraulic conductivity in the range 2.5– (CH2M HILL 2000) suggested that faults in this 5m/day to give transmissivities of c. 20–150 m2/ area act as barriers to flow. These were incorporated day. Combined with specific yield in the range 5– into the Wessex Basin model as well because, 10%, these headwaters are a source of relatively without them, the simulated groundwater levels in reliable baseflow to many of the streams and rivers. the confined zone were too responsive to unconfined The flow barrier associated with the Plenus recharge, and rates of simulated recovery were Marls is represented as a one-cell-wide zone of too rapid. lower transmissivity at its interpreted intersection In a number of areas manually zoned local over- with minimum water levels (i.e. as highlighted in rides have been essential to improve the simulation Fig. 8a). As a line of cells with vertical faces, this of flows and heads. These include the introduction is clearly a simplification of the real dipping combi- of particularly high transmissivities down the nation of lower and higher permeability horizons Gussage Valley. This is the only tributary of the shown in Figure 8b. This zone is important in River Allen where artificial lining of the river bed matching the observed distribution of springheads, has been carried out in association with stream often in intersection with faults implemented using support discharges to maintain some flow during the Horizontal Flow Barrier Package, and particu- the summer. This suggests much higher transmissiv- larly in the tributary catchments of the Rivers ities (up to 7100 m2/day) than are developed Piddle and Frome. according to the regional parameterization rules. Contrasts between valley and interfluve transmis- Similarly high transmissivity zones are required sivities are most apparent up the main river corridors, beneath the Till catchment to represent cross- and in the mid-catchments of Rivers Allen, Piddle catchment flows in the Chalk Rock and around the and Frome. Beneath Salisbury Plain (the Chitterne, top of the Lewes Chalk Formation into the River Till and Bourne catchments), the development of Avon. Data from detailed pumping test investi- enhanced flow horizons within the shallow open gations and monitoring of a public supply source synclinal structure is apparent in the larger areas of in the Chitterne valley have also been used to more regionally uniform high transmissivity. improve parameterization in this area, and to On both the north and south side of the River justify the introduction of a low transmissivity line Wylye anticline structure, the outcrop of the Chalk of cells between the Rivers Chitterne and Till in Rock is represented as a line of low-permeability order to match observed drawdown patterns. cells in a similar manner to the Plenus Marls, even Abstraction at Chitterne is associated with a rapid though flow within this horizon is responsible for drawdown response up to 6 km away towards the wide area of high transmissivity to the north. the SE. However, this stops where there is a step Within the constraints of a single MODFLOW layer, in rest water levels, indicating the existence of this representation was the only effective way of a barrier – perhaps fault-related. In formations modelling enhanced flow within the horizon to where much of the transmissivity may be associated spring heads in the Rivers Chitterne and Till, with the development of karstic fissuring, such which discharge from it at outcrop. To the south, local discontinuity features are inevitable and the barrier zone is necessary to shift the modelled should serve as a warning to treat impact predictions

Fig. 9. (a) Wessex Basin model boundaries and comparisons of average river flows and groundwater levels against measured records for the flow gauging stations and observation borehole sites used during model refinement. Black boundary shows the extent of active MODFLOW cells with red General Head Boundaries along coastal outflows and blue Stream Package cells inland. Inset shows detail of 4R runoff routing across 250 m grid and through MODFLOW stream cells. (b) Comparison of modelled and measured groundwater levels at three contrasting locations. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

146 R. W. N. SOLEY ET AL. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 147 at local scales of a few model cells with great accompanies investigation of a particular question caution. or scenario test. With continued pumping at Chitterne, drawdown develops more gradually, suggesting a slower verti- Model use, update and ongoing refinement cal drainage from the water table. This response – rapid ‘semi-confined’ fissure drawdown, followed The Wessex Basin model has already been used for by slower ‘unconfined’ dewatering – is not easily over 100 predictive scenario runs in support of captured by the single Chalk MODFLOW layer. Habitats Directive investigations for the River Avon, Along the southern margin of the Wessex Basin and water resource management decisions associ- all the main aquifers are folded to vertical within the ated with abstractions elsewhere. Examples of flow narrow (c. 500 m wide) outcrop of the Purbeck impact predictions for Leckford and Clarendon – Hills. The numerical model grid spacing of 250 m two groundwater sources next to the intermittent is too coarse to retain the layering in this area; the and perennial reaches of the Bourne – are described combined Upper Greensand–Chalk–West Park in Soley et al. (2012). This work has been carried out Farm Member aquifer system is all represented on behalf of both the Environment Agency and the within layer 2. Investigations and groundwater level water company (Wessex Water). The model con- monitoring around Lulworth suggest a locally tinues to be updated annually as an important inves- complex and faulted flow system with spring out- tigation platform for options appraisal that is used flows to the sea only occurring in bays and coves alongside evidence from field tests. where coastal erosion has broken through the verti- Wherever new questions are asked of the model, cal barrier formed by the Plenus Marls and lower further local refinement will usually be beneficial if Chalk formations. time and resources allow, where possible incorpor- ating understanding from field investigations. Modelled flows and groundwater levels Both the conceptual understanding and numeri- cal datasets are also feeding into more integrated Figure 11b includes flows simulated at six of the studies of diffuse pollution problems, particularly 42 gauging stations that were routinely reviewed concerning predictions of future nitrate trends. during the refinement process alongside the ground- water records from some 284 observation boreholes. These six flow hydrographs illustrate that the fit is Other regional models across the southern acceptable at most gauging stations, although pre- Chalk: similarities and distinctive features dictions become less reliable for locations further upstream (not shown) as the catchment area reduces, Figure 12 includes maps of each of the other mod- which is typical for many groundwater models. elled areas across the southern Chalk. These all A more comprehensive summary of the model include the solid geology, rivers, locations of calibration is provided by comparing modelled and surface water abstractions and discharges, and measured average flows and heads at all the locations ‘equivalent recharge circles’, centred on ground- used during the refinement process (Fig. 9a). The water abstractions and drawn to give a simple and groundwater level calibration is much more vari- consistent indication of abstraction rates relative to able – many hydrographs are reasonably matched estimated recharge. Key features of each model (e.g. those shown in Fig. 9b), but others are not. are summarized below. In some cases this is because no attempt has been made to improve the fit – either because there is Test and Itchen uncertainty as to how representative the observed borehole record is of the regional groundwater The Test and Itchen are large chalk spring-fed rivers flow system modelled, or because the area has not in Hampshire flowing through Southampton and yet been the focus of local refinement that typically Winchester, respectively, before discharging into

Fig. 10. Alternative approaches to the use of manual zoning and variable hydraulic conductivity profiles with depth (VKD) in the numerical distribution of transmissivity, as trialled early in the development of the Wessex Basin model. (a) Minimum groundwater level geology map of part of the Wessex Basin model, also showing areas where the contoured water level is within 4 m of the ground surface. (b) Schematic short north–south cross-section through the modelled Chalk showing the elevation of the ground level, groundwater level and base of the model. (c) An illustration of manual transmissivity zoning, and (d) a north–south profile of the resulting transmissivity. (e) A map of transmissivity developed for an early model run from the known elevations of the ground surface, minimum groundwater level and hardground horizons using VKD MODFLOW functionality, and (f) the more continuously varying transmissivity profile which results. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

148 R. W. N. SOLEY ET AL. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 149

Southampton Water. Preferentially fissured zones of gravel deposits around Chichester, which has led enhanced Chalk permeability are important here, to flooding problems. some associated with stratigraphic flint bands or At the far western boundary of the model the hardgrounds such as the Stockbridge Rock (Fig. 2, syncline is deeper and its intersection with the Giles & Lowings 1990, Entec UK 2005b). A series estuarine clay-filled valley forces groundwater to of plunging synclines have combined with stratigra- discharge in a series of large springs around Swan- phy and periglacial flow processes to produce bourne Lake (Fig. 12). Because of its coastal loca- karstic groundwater flow pathways within the catch- tion, an appreciation of the history of sea-level ments of the upper Itchen and its Alre tributary, west changes is locally important in helping to under- towards the River Test and two of its tributaries – stand the development of Chalk flow horizons. the Anton and Dun. Some karstified pathways may have developed Underdrainage associated with the Stockbridge when sea-levels were lower but no longer carry Rock in a syncline plunging westward to crop out much natural flow because the discharge elevation in the Alre and Upper Itchen is associated with has risen. high transmissivities and a concentration of water- The interaction between groundwater levels and cress beds and fish farm operations. This feature the different elevation springs and winterbournes captures water from a large area towards the results in groundwater abstraction impact predic- eastern scarp, which is topographically located tions that can be spatially and temporally very com- within Thames Region, and enhances the reliability plex. Both the Havant and Bedhampton Springs and of low summer flows in the Itchen beyond that Worlds End sources are included as examples in expected from its surface catchment. Soley et al. (2012). The Upper Greensand is generally less important than in the Wessex Basin area and a one-layer Brighton and Worthing MODFLOW-VKD model was built. The catchment includes stream support schemes for the River Alre The Brighton and Worthing Chalk Blocks (Figs 1 & and River Candover (as described in Southern 12) comprise the only large area of the Chalk aqui- Water Authority 1979). These schemes were fer within the Environment Agency’s Southern initially implemented for sewage dilution, but are Region for which no groundwater model has been now utilized for ecological benefit. Their operation built within the last 10 years, although the area can be automatically triggered within MODFLOW was modelled during the early 1970s (Nutbrown according to simulated river flows through the use et al. 1975). A recharge model for the area was con- of a Stream Support Module (Lewis & Power 2006). structed in the late 1990s (Entec UK 1999) and updated in 2008. Rainfall gradients are relatively East Hampshire and Chichester steep from the coast northwards up the hills of the South Downs. The higher ground is under- The main driver for the development of the East represented in the rain gauge network which, Hampshire and Chichester Chalk model was to together with the absence of gauged outflows, assess licensed abstraction impacts on rivers and makes groundwater resource estimation difficult. freshwater inflows to a series of harbour conserva- A new groundwater modelling study is proposed tion sites (Fig. 12; Entec UK 2007, 2008). A by Southern Water within the next few years. London Clay and overlying Palaeogene filled syn- The South Downs Chalk aquifer has a long his- cline here separates the main Chalk outcrop in the tory of careful management, due to the intensive north from a smaller anticline outcrop of the demand and the risk of saline intrusion (Jones & aquifer next to three of the harbours. Along the Robins 1999). Groundwater sources situated nearer axis of the syncline, there are points where it is rela- to the coast are pumped to intercept winter outflows, tively shallow and where karstic flow paths have with sources further inland used to support sum- been developed that continue draining groundwater mer demands and maximize the development of from the northern block to discharge at springs aquifer storage. such as at Havant and Bedhampton. Recharge To optimize this operation, the groundwater raises groundwater levels in the north until they model will need to consider density variations and overflow into winterbourne reaches of the Rivers must also simulate observed heads and fissure flow Wallington, Ems and Lavant. Groundwater can processes more accurately than has been the case also flow over the Palaeogene syncline within for other models described in this paper.

Fig. 11. (a) Wessex Basin transmissivity distribution at the end of model refinement, simulated for November 2003 (low groundwater level). Faults modelled using the Horizontal Flow Barrier Package are shown as black lines. (b) Comparison of modelled and measured flows for the largest rivers. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

150 R. W. N. SOLEY ET AL.

Fig. 12. Maps of other southern Chalk model areas showing the geology, rivers, abstractions and discharges, and coastline. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

MODELLING THE CHALK OF SOUTHERN ENGLAND 151

East Kent marshes. However, the water balance of these protected wetlands is dominated by rainfall, runoff The East Kent Groundwater Model (Fig. 12) was and local controls on surface water levels, rather developed for regional aquifer management and to than by groundwater abstraction impacts. quantify the impacts of abstraction on Chalk streams and their ecology (Mott MacDonald 2006). The model was constructed with three Future conceptual and numerical modelling layers: the Palaeogene in the north and river gravels in the main valleys, with the Chalk divided challenges into the upper more fissured Chalk of the Margate, Now that regional numerical models have been built Seaford, Lewes and New Pit formations and the to cover most of the Chalk aquifer system, the first less fissured Chalk of the Holywell, Zig Zag and priority should be to ensure that they are made West Melbury formations. Combined with the use easy to update, improve and use. They provide a of VKD, this approach allowed simulation of the platform to support water resource decision-making contrasting hydraulic properties of the Chalk for- in the context of climate change predictions, and to mations, but the conceptual model also recognized help investigate other issues such as diffuse pol- structural and topographic controls on permeability. lution and (possibly) groundwater flooding. A distinctive feature of this area is the secondary However, in addition to maximizing the benefits Chalk scarp, which is set well back from the main from the models as they have been developed to scarp, coinciding with the outcrop of the Lewes date, there are a number of areas where ongoing Chalk, and is linked with a zone of high transmis- research and development would be helpful. sivity. The streams and dry valleys follow the Chalk Understanding of Chalk stratigraphy will con- structure and both valleys and some fault zones are tinue to develop into the future. This should associated with enhanced transmissivity. include an improved appreciation of lateral vari- The thick unsaturated zone results in consider- ations in the thickness, lithology and developed able delays in recharge reaching the water table, rep- hydrogeological characteristics of the various resented by applying a smoothing function where formations. These are probably less uniform than the water table is more than 45 m deep. In the Folk- is currently assumed. estone and Dover area, the topographic features and Recharge and unsaturated zone flow processes associated transmissivity zones are oblique to the through fissures and the matrix remain an important coast, which protects the aquifer from potential area of investigation, including the possibilities for saline intrusion. a more coupled simulation of the daily shallow surface processes with the longer stress period satu- North Kent rated groundwater model, with unsaturated zone delays dynamically linked to modelled groundwater The groundwater resources of North Kent (Fig. 12) levels. The dual porosity characteristics of Chalk were simulated in a two-layer model, representing groundwater level and flow responses to ‘short the Chalk as a single layer with VKD, and the over- sharp’ storms compared with the slower water lying Palaeogene deposits (Water Management table drainage during summer recessions is not yet Consultants 2006). This was the first model to use completely represented in the models. This has a a water table geology map as a means of understand- bearing on both diffuse pollution studies (e.g. ing the controls on flow and allocating hydrau- nitrate ‘spikes’), the resilience of low flows and lic properties. Overall, the valleys and dry valleys the observed rapid recovery of flow after drought dominate the transmissivity distribution together periods. Rapid groundwater flooding is also occur- with complex interactions at the transmissive ring more frequently, as intense rainfall events Palaeogene margin. become more common during both winter and Groundwater hydrographs measured at the top of summer. This points to the importance of soil the North Downs anticline are unusually flat, poss- moisture by-pass mechanisms and the short-term ibly because of lateral diversion of recharge from activation of unsaturated zone pathways above the the top of the scarp downdip within the thick unsa- zone of normal water table fluctuation. turated zone. Further down the dip slope, the water Improved numerical modelling techniques table shows seasonal fluctuations. should also be sought to better capture the concep- The area is very heavily abstracted, locally tual understanding of preferential bed-parallel resulting in drawdown of groundwater levels Chalk hydraulic conductivity development, further below the main flow horizon in the Chalk. The enhanced at some hardground horizons. The general groundwater flow direction is to the NE, MODFLOW-VKD code is helpful in providing a approximately downdip, with discharge by diffuse simple and stable simulation of the broad seasonal leakage and spring flow to the North Kent variations in interfluve and valley transmissivity Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

152 R. W. N. SOLEY ET AL. development. However, an alternative approach to also become active so that winter transmissivities defining the vertical profile of hydraulic conduc- on the interfluves increase rapidly as heads rise. tivity more flexibly could improve the represen- Winter–summer flow response contrasts are often tation of the flow structures described in Figures highly non-linear and this is a ‘dual porosity’, 6–8. This might allow a more three-dimensionally fissure–matrix aquifer. There is little saturated realistic interaction between groundwater levels storage of water carried from one year to the next, and discrete flow horizons or marly barriers, or but river flows may be more resilient to drought with more general bed-parallel anisotropy, without because of steady drainage from the unsaturated the need for multiple layers with all the typical zone matrix (Mathias et al. 2005), or because the wet/dry MODFLOW instability problems that groundwater catchment is much larger than topo- could result. Extension of similar ideas into the graphic divides would suggest. However, ground- recharge models might also improve the represen- water level and flow recovery is surprisingly rapid tation of lateral recharge displacement downdip when recharge returns. within the unsaturated zone. The outcrop of marl and hardground horizons is A Brighton and Worthing Chalk groundwater often associated with springs, flow accretion or flow model is planned that is intended as a platform to loss. Faulting may also be locally important as a optimize the management of public water supply barrier to flow, or possibly to promote flow along sources in the face of saline intrusion risks. Credible the fault. Around the Wessex Basin, in particular, simulation of groundwater levels will be critical, as the Upper Greensand is a very important headwater will an appropriate representation of flows to and aquifer, providing a relatively reliable source of from the sea and salinity risks to the abstraction summer flow. Moving from west to east, transmis- sources, taking into account the existence of sivity development along the boundary between fissure flow horizons that may have been developed the Chalk and Palaeogene outcrops becomes more when sea-levels were lower. marked, as does the importance of understanding the history of groundwater flow in relation to pre- vious periods of lower sea-levels. Conclusions The numerical models demonstrate that it is possible to achieve a reasonable simulation of the The construction and refinement of regional models regional flow system using MODFLOW, typically of the Chalk aquifer system across southern England with a single layer representing the Chalk. It is has been described. These have built on the improved always important to ‘get the simple things right’, definition of lithostratigraphy and structure provided such as the elevations of the river and spring dis- by recently revised BGS mapping. There are many charge boundaries, and the use of the MODFLOW similarities in the conceptual models of runoff, Stream Package (Prudic 1989) is essential because recharge, groundwater flow and discharge developed of the winterbourne characteristics of many water- from Dorset in the west to Kent in the east. courses. The MODFLOW-VKD code can be used The Chalk Downs are hilly and lack the low- in association with ground- and groundwater-level permeability glacial Till cover and thicker soils data, and saturated formation maps to provide an that blanket some of its outcrop in East Anglia and appropriate regional transmissivity distribution as further north. Long exposure to recharge and a starting point for model refinement. Further local groundwater flow has developed secondary fissure adjustments and manual overrides will typically be transmissivity that, because of the existence of necessary to improve calibration. More flexibility lower permeability marls or hardgrounds that in representing profiles of hydraulic conductivity promote flow on top of them, often leads to bed- with depth within a layer might capture the concep- parallel anisotropy. In the unsaturated zone, this tual characterization of the Chalk more completely may result in lateral, downdip displacement of in future. recharge. In the saturated system, preferential flow These models provide an excellent platform to paths will be developed toward the lowest discharge support water resources decisions alongside infor- boundary of least resistance. Shallow synclines may mation from field investigations. The associated often encourage the development of ‘underdrainage’ understanding and datasets are also being used in within hardground horizons, which can drain large diffuse pollution studies. areas, sometimes pulling groundwater across surface water divides. Many thanks go to my fellow authors for their contri- Intermittent winterbourne streams are a common butions (Tim, Rory, Paul, Jane, Giles and Ian); to the feature everywhere. In the winter, these wet up, other Environment Agency hydrogeologists and hydrolo- reducing drainage path lengths and increasing the gists who have managed and provided important inputs speed of the aquifer’s flow response to recharge. to these projects (J. Grundy, A. Matthews, S. Ritchie, Abandoned fissures within the unsaturated zone K. Croker and B. Howlett, amongst others); to J. van Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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