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Journal of Environmental Management 91 (2010) 2385e2395

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Journal of Environmental Management

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Hydrogeological study for improved nature restoration in duneecosystemseKleyne Vlakte case study, Belgium

A. Vandenbohede a,*, L. Lebbe a, R. Adams b, E. Cosyns c, P. Durinck d, A. Zwaenepoel c

aResearch Unit Groundwater Modelling, Department of Geology and Soil Science, Ghent University, Krijgslaan 281 (S8), 9000 Gent, Belgiumb International Marine and Dredging Consultants(IMDC), Coveliersstraat 15, 2600 Antwerpen, BelgiumcWest-Vlaamse Intercommunale(WVI), Baron Ruzettelaan 35, 8310 Brugge, BelgiumdGrontmij, Meersstraat 138A, 9000 Gent, Belgium

a r t i c l e i n f o

Article history:Received 17 July 2009Received in revised form31 May 2010Accepted 29 June 2010Available online 22 July 2010

Keywords:Water managementDunesHydrogeologyModelling

* Corresponding author. Tel.: þ32 9 2644652; fax:E-mail address: alexander.vandenbohede@ugent.b

0301-4797/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jenvman.2010.06.023

a b s t r a c t

In dune slacks a close coupling exists between changes in the hydrology and changes in speciescomposition and vegetation structure. Consequently, there is a need to underpin nature restorationprojects not only with ecologically relevant knowledge but also with scientifically sound hydrogeologicaldata. In this paper, this necessity is illustrated through a study of the Flemish Nature Reserve ‘TheZwindunes and Zwinpolders’ (Belgian coastal plain) as an example. The management plan for the naturereserve suggests rewetting part of it to enhance its ecological value. The groundwater aspect was studiedby means of field observations and mathematical modelling. First, fresh water head observation showeda mean groundwater flow from the nature reserve to the adjacent polder. Secondly, groundwater qualitywas studied with borehole measurements and water samples, resulting in a map of the fresh-salt waterdistribution and of water types. All available information was then put together in a density dependentgroundwater flow model. The aim of this model was the description of current flow and fresh-salt waterdistribution and to simulate the impact of three possible rewetting scenarios. Rewetting will beaccomplished by the infiltration of water in a depression, different lay-outs for which are considered.A zoomed in flow model based on a regional model was used to incorporate both local scale, which is ofimportance to ecology, and the larger scale, which determines general groundwater flow and fresh-saltwater distribution. This modelling indicated differences between scenarios and was used to decide onthe best rewetting strategy.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Groundwater levels, flow and groundwater quality are impor-tant factors determining coastal ecosystems (Grootjans et al., 1998;Stuyfzand, 1993). Understanding the link between groundwater(flow as well as quality), surface water, and ecosystems are neededfor their characterisation and management (Hancock et al., 2009).High groundwater levels benefit different plant communities thanthose benefited by low groundwater levels. These levels aretherefore an ecologically important aspect of groundwater (Klijnand Witte, 1999; Lammerts et al., 2001). A second aspect is waterquality (temperature, salinity, alkalinity, pH, cation and anionconcentrations, etc.) (Stuyfzand, 1993; Sival et al., 1997; Klijn andWitte, 1999). Of importance is that the scale of interest of ecolo-gists and hydro(geo)logists is different. Factors determining the

þ32 9 2644653.e (A. Vandenbohede).

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occurrence of different plant communities are in most cases verysite specific and consequently must be considered on a relativelysmall and local scale. Groundwater flow, on the other hand, isdetermined by larger-scale flowcycles (Tóth, 2009). The same holdsfor general groundwater quality, which is in many cases related tolarge scale flow cycles (Stuyfzand, 1999).

This paper illustrates the use of a groundwater study andgroundwater flowmodelling to optimise the restoration and futuremanagement of a coastal nature reserve in Flanders known as ‘TheZwindunes and Zwinpolders’. This reserve is part of the north-westEuropean coastal dunes, which are important ecosystems from thepoint of view of biodiversity conservation. They are characterizedby specific abiotic conditions resulting in pronounced gradients offor instance soil moisture (dry-wet), soil acidity (calcareouseacid)and salinity (salt-fresh), thus favouring a high biodiversity of plantsand animals. In most places however, the characteristic biodiversityis under threat as a consequence of urbanization, intensive agri-cultural exploitation, deforestation, irrigation, production ofdrinking water and various other activities that lower water tables

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in adjacent areas. Conservation and restoration of coastal duneecosystems is highly ranked on the European conservation agenda.Increasing the ecological values of different parts of the dune beltalong Europe’s north-west coast is steadily progressing (see e.g.Grootjans et al., 2002). This increase is being achieved throughvarious initiatives: mowing, grazing, rewetting, sod cutting,construction of dune slacks or a combination of these techniques.Decreasing extraction rates or reuse and/or the infiltration of water,for instance, restricts the effects of groundwater exploitation onphreatic aquifers (Geelen et al., 1995; van Breukelen et al., 1998;Vandenbohede et al., 2009; Van der Hagen et al., 2008). The crea-tion of tidal inlets brings brackish and salt water into the dunes(Vandenbohede et al., 2008) and polders (Degand et al., 2008)aiming at enhancing biodiversity. Grootjans et al. (2002) gives anoverview of projects where different techniques have been appliedand discusses the factors influencing their chances of success.

This paper focuses on rewetting as a restoration measure forpart of the nature reserve, the Kleyne Vlakte. The aim is the opti-misation of abiotic conditions in order to create new or betteropportunities for the endangered plant species, and their associ-ated fauna, mainly related to the grey dunes (e.g. vegetations of thePolygalo Koelerion), humid dune slacks (e.g. vegetations of theCaricion davallianae), dune-slack pools (Charetalia hispidae andParvopotamion), Atlantic salt meadows (Puccinellion maritimae) andwet hay meadows (Calthion palustris). The goal of the researchproject reported here was to distinguish between different resto-ration options and optimise hydrological interventions. Therefore,groundwater flow modelling is used as a tool.

2. Material and methods

2.1. Zwindunes and Zwinpolders

The Zwindunes and Zwinpolders are located in the most north-eastern section of the Flemish coast, between Knokke-Heist and theDutch-Belgian border (Fig. 1). The nature reserve has a surface areaof 222 ha and is situated within one of the two largest remainingnatural areas of the Flemish coast, hence its importance forconservation and management. The Zwindunes and Zwinpoldersconsists of low coastal dunes (maximal height of 17 mTAW, where0 mTAW is the Belgian reference level, 2.36 m below mean sea

Fig. 1. Location of study area and boundaries of the Flemis

level) with a large fossil beach plain on the landward side whichwas cut off from marine influence in the second half of the 19thcentury. In contrast with the coastal dune area, the topography ofthis fossil beach plain varies between 4 and 5 mTAW, hence thetoponym Kleyne Vlakte (small plain). The nature reserve is boun-ded by the North Sea (to the north), the town of Knokke (to thewest) and the Zwin estuary (to the east), fromwhich it is separatedby a dyke. Towards the south, there is a polder area (NieuweHazegraspolder), which is also separated from the nature reserveby a dyke. A brook, the Paardemarktbeek (Fig. 1), runs through thesouth-western part of the reserve. This brook originates in thewestern part of the nature reserve and flows towards the east,where it connects with the polder’s drainage system. Consequently,it drains water from the nature reserve towards the polder. At thelocation of “Golf” (Fig. 1), a groundwater extraction is present.

The Quaternary phreatic aquifer has a thickness of 25e30m andconsists of sandy deposits. At the bottom of the Quaternary aquifer,there are Pleistocene sand deposits overlain by sandbar andmudflat deposits. In the southwest corner of the nature reserve,a shallow clay layer is found. Finally, dune deposits witha maximum thickness of 8 m occur in the coastal dune area. TheQuaternary deposits are underlain by the Onderdijk Member(Formation of Maldegem), which is a clay layer of late-Eocene age.

During the 20th century large areas of the nature reserve wereinfluenced by human activities: cultivation, parcelling and theinstallation of a golf course and horse jumping area, an airport andwar infrastructure during World Wars I and II. Parts of the dune-system in the project area are now being subjected to invasion fromshrubs and grasses that are superseding both grey dunes and wetdune-slacks. Ecological values have also decreased as a result ofexcessive fertilization of the meadows on the fossil beach plainduring the last few decades. The Flemish Regional Governmentacquired the land in 2002. It was designated a Flemish NatureReserve by ministerial decree on 2nd December 2003.

In 2007 a management plan based on the recommendations ofan extensive study of the biotic and abiotic factors (Zwaenepoelet al., 2007) was approved. This plan formed the basis for theLIFE-nature project ZENO (Zwindunes Ecological Nature Optimi-sation), which runs from 2007 until the end of 2010. The mainobjective of ZENO is the restoration andmaintenance of the naturalhabitats that are typical of coastal dunes and their transition to salt

h Nature Reserve “The Zwindunes and Zwinpolders”.

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marshes (Atlantic salt meadows - Glauco-Puccinellietalia), fixeddunes with herbaceous vegetation or grey dunes, dunes with Salixarenaria, wooded dunes of the Atlantic coast, humid dune slacks orhard oligo-mesotrophic waters with benthic vegetation of Charaformations. To realise this objective, a number of ecological engi-neering interventions have been considered: the removal of vege-tation (shrub and exotic tree species), the remodelling or creation ofponds and the removal of old infrastructure (war remnants, oldhorse jumping etc.). Hydrogeological interventions, together witha restoration of the micro-topography in the Kleyne Vlakte, havebeen advised by Zwaenepoel et al. (2007). The aim of these inter-ventions is the rewetting of the Kleyne Vlakte with water storedwithin the nature reserve so that it no longer drains towards thepolder area.

2.2. Management interventions

The proposal of Zwaenepoel et al. (2007) is that the KleyneVlakte be rewetted through a number of interventions. First, thelandscape will be remodelled to create a number of depressions.The level of the floor of such depressions can be below or above themean (or lowest) water table. Consequently, the pools therebycreated will be fed by groundwater, with the level of the bottom ofeach determining in which period of the year and for how longwater will be present in it. An example of this intervention can beseen in Fig. 2 along the southern border of the Kleyne Vlakte. Anumber of others are planned in the eastern part of the KleyneVlakte (not shown in Fig. 2). Secondly, an intervention is intendedwith the aim of retaining the water presently drained by thePaardemarktbeek in the area. In the current situation, this water isevacuated very rapidly to the polder. Three scenarios for the real-isation of this intervention were considered. These were simulatedand evaluated using groundwater flow modelling.

In each of these three scenarios (Fig. 2), the course of thePaardemarktbeek will be altered. Instead of flowing south towardsthe polder area, its course will be changed to flow towards a newdepression. As a consequence, the downstream section will beabandoned and water from the Paardemarktbeek will flow into thedepression where it can infiltrate and recharge the aquifer. Thebottom level of the depression will be set at 3.0 mTAW, which isabout 1.5 m lower than the current surface level. To the east of thedepression, a new brook will be excavated which will act asa drainage channel for the water infiltrated in the depression. Thethree scenarios differ in the lay-out of this new brook. In the firstscenario, there are two north-south oriented sections, west andeast of an existing wooded area, to the north of which they are

Fig. 2. Different scenarios for management interventions, aiming to rewet the K

connected. The brook then extends towards the east, where itconnects with newly excavated depressions in the eastern part ofthe Kleyne Vlakte. In scenario 2 the connection between the twonorth-south oriented parts of the brook is located inside thewooded area. This difference from scenario 1 will have conse-quences for the drained water quality. The residence time of waterinfiltrated in the depression and drained by the brook will bedifferent for the eastern and western branches. This may possiblybe reflected in the drained water quality (for instance, with regardto alkalinity, pH and Eh) and consequently result in differences inplant communities. If the branches are connected in series(scenario 1), these water types will be better mixed than if they areconnected in parallel (scenario 2). The surface level of the upstreampart of the brook will be 3.5 mTAW, decreasing to 3.2 mTAW at theborder of themodel domain. These levels (and also the bottom levelof the infiltration depression) were decided on the basis of thegroundwater levels (mean, highest and lowest fresh water heads).In a third scenario the brook starts east of the forests and theexcavation depth of the brook is limited to reduce its drainagecapacity. Downstream, the brook fades away into the topography ofthe Kleyne Vlakte. Consequently, the upstream part of the brookdrains water, whereas the downstream part infiltrates water. Thesurface level of the upstream part of the brook is 3.7 mTAW,decreasing to 3.6 mTAW downstream.

2.3. Fresh water head observations

In the nature reserve, 22 shallow (<5 m) and six deep (�25 m)observation wells are present. Since 2003, the Agency for Natureand Forests has made regular (monthly) hydraulic head measure-ments. With the mean density of the water in the wells alreadyknown, head data were recalculated in fresh water heads. This wasnecessary to compare head measurements made in fresh, brackishand saline parts of the aquifer. This hydraulic head data were rep-resented in maps of mean fresh water head, variance of the freshwater head and monthly maps of the fresh water head.

2.4. Water quality data

Water analyses are available in 19 (mostly shallow) wells. Thewater samples were classified according to the classification ofStuyfzand (1993), which characterises different water types. Thedetermination of awater type implies the successive determinationof a main type, type, subtype and class of the water sample. Adescription of this classification is given as supplementarymaterial.

leyne Vlakte. The background is an aerial picture of the current situation.

Table 1Horizontal (Kh) and vertical (Kv) hydraulic conductivity used for the different unitsin the groundwater flow model.

Unit Kh (m/d) Kv (m/d)

beach deposits 13 0.5low permeable polder deposits 0.3 0.01channel ridge deposits 5 0.2permeable dune deposits 12 0.5low permeable dune deposits 0.2 0.01

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Secondly, water quality was studied using geophysical boreholemeasurements which were performed in a number of deep wells.For this purpose, an electromagnetic induction tool (EM39) whichmeasures the electrical conductivity of the sediments surroundingthe borehole (McNeill, 1986) was used. These conductivity valuescan be interpreted in terms of total dissolved solids of the porewater (Vandenbohede et al., 2008). Consequently, EM39 logsprovide information on the vertical fresh-salt water distribution.

2.5. Temperature logs

Temperature logs (temperature as a function of depth) wereperformed by lowering a temperature sensor very slowly (10 m/h)into a well. In doing so, the temperature of the water in the well,being in equilibrium with the temperature of sediment and porewater outside the well casing, was measured as a function of depth.Recharge velocity and general groundwater flow influence thistemperature profile (Vandenbohede and Lebbe, 2010). In general,two zones can be distinguished in such profiles (Parsons, 1970). Theupper zone, typically the first 10 m, is called the surficial zone.Temperature in this zone is subjected to yearly variations mainlydriven by seasonal heating and cooling of the land surface and thevarying temperature of the rechargewater. Temperature at the baseof the surficial zone equals the mean temperature of the rechargewater (ambient temperature). Below the surficial zone, ground-water temperature is determined by the geothermal gradient. Inrecharge areas, the thickness of the surficial zone is larger becauseof the downward flow. In seepage areas, where there is an upwardflow, the thickness of the surficial zone is reduced. Consequently,the temperature log, when combined with the fresh water headdata, was able to provide information about groundwater flow. Thethickness of the surficial zone was derived from the logs using thedepth at which temperature becomes constant or increasesbecause of the geothermal gradient.

2.6. Groundwater flow modelling

2.6.1. MOCDENS3D codeGroundwater modelling was used to integrate the available

hydrogeological information in one framework. It allows thedescription of the current system as well as that of the impact ofpossible restoration interventions. MOCDENS3D (Oude Essink,1998) was used as groundwater flow and solute transport model.This model simulates 3D density dependent groundwater flow andis based on the three-dimensional solute transport code MOC3D(Konikow et al., 1996), but adapted for density differences. VisualMOCDENS3D (Vandenbohede, 2007) is used as pre and post-processor. The groundwater flow equation is solved by the MOD-FLOW code (McDonald and Harbaugh, 1988), taking into accountdensity differences by using a buoyancy term in the basic flowequations. The advectionedispersion equation is solved by themethods of characteristics (Konikow and Bredehoeft, 1978):advection is simulated with particle tracking and dispersion witha finite-difference method.

2.6.2. Regional flow modelAs indicated in the introduction, while an ecological perspective

requires knowledge of changes in groundwater levels and qualityon a local scale, these aspects are determined by larger-scaleflow cycles. Therefore, an approach was followed whereby firsta regional groundwater flowmodel was made of the whole vicinityof the nature reserve. Thereafter, a detailed model of thesurroundings of the Paardemarktbeek was made in order tosimulate different scenarios of an intervention aimed at retainingas much water as possible from the brook in the nature reserve.

The boundary conditions of this latter model were derived from theregional model.

The regional model measures 3500 m by 4400 m and is dividedinto 70 rows and 88 columns. This model area is outlined in Fig. 1.The sides of each unit cell are 50 m. The aquifer is divided into12 layers, each with a thickness of 2 m. The boundary conditionsof the model are defined by the geological and hydrogeologicalcharacteristics of the study area. The clay of the Tertiary OnderdijkMember is considered impermeable for the purpose of this studyand forms the lower boundary. The North Sea, forming the northernboundary, is a constant head boundary. The southern boundary islocated in the polder area. The first layer has a constant head, whichis the polder drainage level. This drainage level varies along theboundary, depending on the level of the drainage ditches. Theeastern and the western boundaries are impermeable, wherebya flow perpendicular to the coastline is considered. The westernboundary of the model is situated in the centre of the groundwaterextraction in the Golf, which has been active since 1907 and atwhich the averagepumpflowratehas variedbetween120 000m3/yin the first part of the 20th century till 8 60 000 m3/y currently.

The geological model and the hydraulic parameters (horizontalhydraulic conductivities and the hydraulic resistances) of differentunits were derived from HCOV mapping. HCOV (HydrogeologischeCodering Ondergrond Vlaanderen e Hydrogeological Code FlemishSubsurface) maps the different units present in the FlemishSubsurface. An extension of HCOV, whereby the quaternarydeposits in the coastal plain were mapped in more detail (Lebbeet al., 2008b), was used in this study. Table 1 gives an overview ofthe different geological units which occur in the model domain andof the hydraulic parameters which are assigned to these units aftercalibration of the model. Recharge to the aquifer is varied based onthe vegetation and soil type and ranges between 250 mm/y in baredunes and 25 mm/y in polder meadows.

The initial fresh-salt water distribution of the model wasderived from a simulation of the historical fresh-salt water distri-bution in the region conducted by Lebbe et al. (2008a). Theseauthors simulated the changing groundwater flow and fresh-saltwater distribution from the 16th century onwards. They took intoaccount geomorphological evolution based on historical maps(Termote, 2004) and calibrated the fresh-salt water distributionand fresh water heads at the end of the 20th century with obser-vations. This calibrated fresh-salt water distribution was taken asinput for the currentmodel and the freshwater headswere used forthe definition of the boundary conditions. More information aboutthe model lay-out and the calibration of the model is given assupplementary material.

2.6.3. Scenario modellingFollowing the regional model a detailed model of the area

surrounding the Paardemarktbeek, the focus of the planned watermanagement interventions, was made in order to compare theimpact of the three different scenarios. This model has a surfacearea of 900 by 900 m. The aquifer is subdivided into 90 rows andcolumns (width of 10m) and 12 layers, eachwith a thickness of 2m.The tertiary clay layer also acts here as the impermeable lower

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boundary of the model. Constant head boundaries are present inevery layer. The value for this constant head was derived from theregional flow model, as was the initial fresh-salt water distributionand the recharge of the aquifer. Values for the hydraulic parameterswere also not altered. Four simulations were then performed:evolution over a period of 50 years retaining the current situation(i.e. no interventions) and the evolution over 50 years for each ofthe three considered management scenarios.

3. Results and discussion

3.1. Hydraulic head data

The monthly hydraulic head data were represented on mapsshowing the mean fresh water head and the variance of the freshwater head for the period 2003e2007 (Fig. 3). A map of the freshwater head in March 2005 and September 2005 is also shown. Themaps were based on the shallow observation wells, which meansthat they approximate the fresh water heads of the water table.

Fig. 3A shows the mean fresh water head during the period2003e2007. At the high water line a fresh water head of 4.2 m isencountered. This equals the high water level during a mean tidalcycle (Vandenbohede and Lebbe, 2006). Towards the polder, thereis a general decrease, indicating a mean groundwater flow from thenature reserve towards the polder area. The reason for this is theeffective drainage system in the polder area. The topography ofthe polder is between 3 and 5 mTAW and large parts are below thehigh water mark. The drainage system is needed for the manage-ment of the water levels in these low lying polders. In the naturereserve, no such drainage system, apart from the Paardemarktbeek,is present. Therefore, water which recharges the aquifer in thenature reserve flows towards the polder area where it is evacuatedby the latter’s drainage system. Fig. 3B shows the variance of the

Fig. 3. Mean fresh water head (mTAW) (A) for the period 2003e2007, variance on fresh watwater head (mTAW) for September 2005 (D).

fresh water heads for the period 2003e2007. The smallest vari-ances are seen near the high water line and the southern border ofthe nature reserve. Groundwater levels are in these cases deter-mined by the (constant) mean high water level and the polder’sdrainage system. Towards the east, variance increases in the KleyneVlakte which is due to the presence of a shallow (0.5 m) clay layer.The variance is largest in the centre of the coastal dune area.

Fig. 3C illustrates the fresh water heads when groundwater is atits highest level. This occurs in the period February to April of eachyear, when awater divide is present in the area, locatedmore or lessat the transition between the coastal dune area and the KleyneVlakte. This water divide of 4.5 mTAW is the result of the higherrecharge rate during the winter and spring. Water which rechargessouth of the divide flows towards the polder, as in the mean situ-ation. Water which recharges north of the divide flows towards thesea. When groundwater levels are at their lowest (Fig. 3D), thegroundwater flow is towards the polder as in the mean situation(Fig. 3A). Consequently, during most of the year there is a generalgroundwater flow of recharged water towards the polder drainagesystem. Only at the end of the winter and during (early) spring isthere a groundwater flow from the northern part of the naturereserve towards the sea.

3.2. Temperature logs

Temperature as a function of depth in four of the deep wells isgiven in Fig. 4. All logs were recorded in September 2007. The top ofthe profiles are equal to the level of the water table and reflects thehigher summer air temperature. Different temperatures wererecorded at the water table at different locations. This variance isdue to differences in the thickness of the unsaturated zone,different sediments and different surface coverings (e.g. vegetationversus concrete). Deeper in the surficial zone, the temperature

er head (m) for this period (B), fresh water head (mTAW) for March 2005 (C) and fresh

Fig. 4. Electrical conductivity and temperature as function of depth (expressed as mTAW) for a number of wells, as measured in September 2007, with map showing location of logs.

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decreases with depth until a mean ambient temperature is reached.For SB3, located in the centre of the coastal dune area, the thicknessof the surficial zone is about 7.6 m. It is notable that the surficialzone is thicker for SB1 and SB4 (8.9 and 8.8 m respectively). Thisincreased thickness is the result of an increased downward flowtowards the drainage system in the polder area at the southernborder of the dune area. In well B4, situated near the Paarde-marktbeek, the surficial zone has a thickness of about 7.3 m. This issmaller than in well SB3 because of the upward discharge of coolergroundwater towards the brook.

3.3. Groundwater quality

3.3.1. Electrical conductivity measurements (EM39)EM39measurements were carried out in the six deep wells. The

measurements in four of these are given in Fig. 4. Together withexisting data for the area, they allow the derivation of the currentfresh-salt water distribution of the area. This existing data is a mapof the depth of the 1500 mg/l total dissolved solid (TDS) isosurfacewhich was made by De Breuck et al. (1974) of the Belgian coastalplain and which is here updated with the EM39 measurements.This map shows the presence of a freshwater lens under the coastaldunes and under the largest part of the Kleyne Vlakte, whereasshallow salt water is present under the polder area. West of thenature reserve, a fresh water lens also occurs, hence the presence ofa groundwater extraction in the Golf.

The EM39 measurement in SB1, located at the southernboundary of the Kleyne Vlakte, shows low conductivity until�10 mTAW (depth below surface of 14 m). Deeper, conductivityincreases gradually until a large increase ismeasured at�14mTAW.These readings indicate fresh water until a depth of 14 m, a rela-tively large transition zone of about 4 m and salt water below 18 m.SB3 is located in the coastal dune area. The EM39 log here showssmall conductivities above �16 mTAW (20 m below surface level),indicating that the complete quaternary aquifer contains freshwater. SB4, located at the south-western boundary of the nature

reserve, shows the same profile. The depth of the fresh-salt waterinterface is here �11 mTAW (15 m below surface level). B4 is sit-uated along the Paardemarktbeek. Here, the EM39 readings showlow conductivity in the shallow part of the aquifer, with conduc-tivity increasing from a depth of 2 m below surface onwards. Thispoints to the fact that the fresh-salt water interfaces are relativelyshallow in the vicinity of the Paardemarktbeek.

Fig. 5 shows the resulting map of the fresh-salt water distribu-tion. A distinction is made between the fresh-salt water distribu-tion under the shore and sea (north of the dotted line in Fig. 5) andits distribution under the dunes and polders. Under the shore,fresh-salt water distribution is determined by the interaction of thesea and the shore, which is beyond the scope of this paper(Vandenbohede and Lebbe, 2006, 2007). As indicated before,a fresh water lens is present under the dunes. The southern borderof this fresh water lens is situated just south of the dyke betweenthe Kleyne Vlakte and the polder area. In the east, the Zwin estuaryforms the boundary of the fresh water lens. In the west, the dunearea becomes wider and here also fresh water is found. This is thearea where the water company is exploiting the fresh water lens inthe Golf. Under the Paardemarktbeek, salt water occurs at a muchshallower level.

3.3.2. Water analysesFig. 5 shows the water types according to the Stuyfzand classi-

fication in several of the shallow and two deep wells. For theshallow water samples, the main type is in most cases oligohaline-fresh (g, chloride concentration between 5 and 30 mg/l) or fresh(F, chloride concentration between 30 and 150mg/l). Alkalinity (thetype) is moderately high (3, bicarbonate concentration between244 and 488 mg/l) or high (4, bicarbonate concentration between488 and 976 mg/l). The subtype is in most cases CaHCO3 and thebase exchange index is positive.

These water types and the fresh-salt water distribution are theresult of the recent (hydro)geological evolution of the area(Vandenbohede and Lebbe, 2002). The coastal plain was originally

Fig. 5. Depth (m below surface level) of the 1500 mg/l interface (A) and water types according to Stuyfzand classification for several wells (B).

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a mudflat environment with incised tidal channels and gullies andthe aquifer was mainly filled with salt water. Subsequently, due tothe formation of dunes and land reclamation activities, the sea nolonger had access to the area. Consequently, the older connate saltwater became replaced by fresh recharge water, resulting in theformation of the freshwater lens under the dune area. In the polder,most of the rainwater is intercepted by the drainage system and isevacuated towards the sea. In this area, therefore, recharge withfreshwater is limited and freshwater is restricted to the uppermostpart of the phreatic aquifer. In the Zwin estuary, salt water can stillrecharge the aquifer and the occurrence of fresh water is limited.Under the Paardemarktbeek, displacement of salt by fresh waterwas severely limited by the drainage capacity of the stream. In theimmediate vicinity of the brook, there is an upward flow towards it.Therefore less fresh water recharges the aquifer, so that the saltwater remains present at relatively shallow depths.

Coinciding with the displacement of salt by fresh rechargewater is a geochemical evolution (Appelo and Postma, 2005;Vandenbohede and Lebbe, 2002). The fresh rainwater dissolvescalcite present in the sediments, resulting in an increase of thecalcium and bicarbonate concentration. Main cation and anionbecome calcium and bicarbonate and alkalinity increases tomoderately high to high. Cation exchange is a second importantreaction.When the sediments are filled with salt water, sodium andchloride are the main cation and anion in the pore water. Sodium isalso the dominating cation on the exchange complex. Flushing withfresh water results in an exchange of sodium from the exchangecomplex for calcium from the pore water, resulting in a NaHCO3

water type.When sodium has been flushed from the sediments, theCaHCO3 water type dominates again. Additionally, because marinecations (Naþ, Kþ and Mg2þ) are expelled from the exchanger, thebase exchange index becomes positive. This is what is observed inmost of the shallow wells where CaHCO3þ is found. In a deep wellin the centre of the fresh water lens, a NaMix water type is found.Here, the sediments are not flushed sufficiently for the completedisplacement of sodium by calcium and bicarbonate is still not thedominant anion (relative to, for instance, chloride). In the transitionzone between fresh and salt water, similar water types can befound. For instance, brackish NaCl water is found in a deep welllocated between the Kleyne Vlakte and the Zwin estuary. BrackishNaCl water is found at a very shallow level between the Kleyne

Vlakte and the polder area. In both cases, freshening has alreadyoccurred, hence the brackish main type, but the water is stilldominated by marine cations and anions.

3.4. Regional flow model

Fig. 6 shows a cross-section through the model domainaccording to layer 2, which corresponds with a level of 0 mTAW.Fig. 7 shows two vertical cross-sections whose locations are indi-cated on Fig. 6. In the nature reserve, a general flow towards thepolder is visible, as was derived from the observations. At thesouthern border, there is an intense flow towards the drainagesystem in the polder. This leads to higher vertical flow velocitiesand, among other things, to an increase of the thickness of thesurficial zone, as indicated by the temperature logs. In the polder,a fresh water head between 2.25 and 2.5 m is found, which is themean drainage level. In the dune area westward from the naturereserve, flow is mainly directed towards the water extraction. Thegeneral fresh-salt water distribution corresponds very well withthe map of Fig. 5. A fresh water lens is present under the naturereserve and the dune area westward and south-westward from it.Under the Paardemarkbeek brook, salt water occurs at a relativelyshallow level (Fig. 7A). As explained before, this is due to thedraining action of this brook, limiting the displacement of older saltby fresh recharge water. In the polder and under the Zwin estuary,salt water occurs at very shallow depth.

3.5. Scenario simulation

3.5.1. Groundwater flowThe depression in which water can recharge the aquifer and the

new brooks was modelled using the MODFLOW river package. Forthe depression, combination of the river stage, the level of thebottom of the river and the conductance of the riverbed was set sothat an annual volume of 20 000m3 could recharge the aquifer. Thisamount of water is currently available in the Paardemarktbeek, asconfirmed by flumemeasurements (Lebbe et al., 2009). For the newdrainage brooks, the contact factor was estimated based on thelithology of the subsoil.

Fig. 8 shows in horizontal cross-sections according to layer 2(0mTAW) the freshwater heads and projections of the flow vectors

Fig. 6. Horizontal cross-section through aquifer according to layer 2 (0 mTAW). Grey scale represents the TDS (mg/l), isolines the fresh water head (mTAW) and arrows are theprojection in the horizontal plain of the groundwater flow vectors. These vectors represent a displacement of 10 years. The nature reserve is outlined in black. A and B indicate thelocation of cross-sections of Fig. 7.

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for the current situation and the three scenarios. As explainedbefore, there is a general flow from the nature reserve towards thepolder when no intervention is planned. When the aquifer isrecharged through the depression, this general flow pattern ismaintained. Locally, in the vicinity of the depression and the newbrook eastward of it, there is a flow from the depression to the newbrook. In the vicinity of the depression, there is an increase of thefresh water head of about 0.5 m. At a distance of about 250 m fromthe depression, the rise in the water table is still in the order of0.1 m, providing better conditions for the conservation and resto-ration of C. palustris related vegetation (Zwaenepoel, 2009;Zuidhoff et al., 1996). Thus the infiltration of water in the depres-sion results in the creation of wet habitats in this part of the Kleyne

Fig. 7. Vertical north-south oriented cross-sections through aquifer. The location of the crosthe fresh water head (mTAW) and arrows are the projection in the vertical plane of groun

Vlakte. Differences between scenario 1 and 2 on the fresh waterhead are negligible. Indeed, the differences between these two andscenario 3 are also minimal. In all three cases the groundwater flowis mainly determined by the flow towards the polder and therecharge in the depression and less by the drainage of the brooks.

3.5.2. Water qualityFig. 9 shows the influence of these scenarios on the fresh-salt

water distribution in a vertical cross-section, oriented north-souththrough the centre of the model domain. The cross-section for thecurrent situation shows the general flow towards the polder andthe border of the freshwater lens under the Kleyne Vlakte. Brackishand salt water occurs at a relatively shallow level under the polder.

s-sections is indicated on Fig. 6 (A and B). Grey scale represents the TDS (mg/l), isolinesdwater flow vectors. These vectors represent a displacement of 3 years.

Fig. 8. Horizontal cross-sections through aquifer for layer 2 (0 mTAW) for the current situation and the three scenarios. Isolines represent the fresh water heads (mTAW) and arrowsare the projection in the horizontal plane of groundwater flow vectors. These vectors represent a displacement of 5 years.

A. Vandenbohede et al. / Journal of Environmental Management 91 (2010) 2385e2395 2393

In the first instance, the evolution of the fresh-salt water distribu-tion with no interventions was simulated. In this case, after 50years, the fresh water lens under the Kleyne Vlakte has stillincreased and brackish and salt water occur deeper in the aquiferunder the polder. This indicates that the fresh-salt water distribu-tion has, at the present time, not yet reached a dynamic equilib-rium. Fig. 9 shows also the fresh-salt water distribution 50 yearsinto the future for the three different scenarios. As was the case forthe fresh water heads, there are no differences between the firsttwo scenarios. Because of the recharge of the aquifer, there is anincreased flow of fresh water towards the polder, accelerating theenlargement of the freshwater lens under the Kleyne Vlakte and itsextension into the polder area. As a result, after 50 years salt watercan only be found in the lower part of the polder bordering thenature reserve. No major differences can be found betweenscenario 1/2 and scenario 3. Notice, however, that the interfaceoccurs fractionally deeper in the case of scenario 3.

The destination of the recharged water was calculated bymeansof particle tracking. A large number of particles were placed at thelocation of the depression and the flow lines of these particles werecalculated. In this way, it was determined if and how much of therecharged water (particles) would flow towards the polder andhowmuch to the new brook. In the case of scenarios 1 and 2, 62% ofthe water flows towards the polder whereas 33% flows towards the

Fig. 9. Vertical north-south oriented cross-section through aquifer in centre of model domarrows are the projection in the vertical plane of the groundwater flow vectors. These vectointo the future for the scenarios.

new brook east of the depression and 5% is intercepted by the newcourse of the Paardemarktbeek. In case of scenario 3, 87% of thewater flows towards the polder and only 7% towards the newbrook,with 6% being intercepted by the new course of the Paarde-marktbeek. This important difference in the scenarios is evidentlydue to the drainage efficiency of the brook. In scenarios 1 and 2, thesurface elevation of the bottom of the brook is significantly lower(0.1e0.4 m at the downstream part) than in scenario 3, hence themore effective drainage potential. Thus with scenarios 1 and 2a larger proportion of the recharged water is retained in the KleyneVlakte. This difference also explains the difference betweenscenario 1/2 and scenario 3 in fresh-salt water distribution visiblein Fig. 9. In scenario 3, a larger amount of recharged water flowingtowards the polder results in a larger increase of the fresh waterlens under the nature reserve than in scenarios 1 and 2.

With the particle tracking algorithm of Visual MOCDENS3D, it ispossible to calculate the residence times of the recharged waterbefore it is recovered by the brook to the east of it. This can berepresented by means of box plots as shown in Fig. 10. For eachfinite-difference cell comprising the brook, a box plot is made of theresidence times of particles flowing towards the particular cell.These box plots are then plotted as a function of distance along thebrook, from upstream to downstream. Consequently, Fig. 10 shows,as distance downstream from the brook, the statistic of the

ain. Grey scale represents the TDS (mg/l), isolines the fresh water head (mTAW) andrs represent a displacement of 5 years for the current situation and 1 year and 50 years

Fig. 10. Box plots of travel time of recharged water reaching the new brooks east of the depression for scenario 1 and 2 (A) and scenario 3. Box plots show lower quartile, medianand upper quartile values. The whiskers are lines extending from each end of the box to show the extent of the rest of the data. Outliers (þ) are data with values beyond the ends ofthe whiskers.

A. Vandenbohede et al. / Journal of Environmental Management 91 (2010) 2385e23952394

residence times of the water infiltrated in the depression drainedby the brook. For scenarios 1 and 2 (Fig. 10A) three different zonescan be distinguished along the course of the brook. For the sectionwest of the wooded area, the residence time of the recharged wateris about 1 year. For the section east of the wooded area, itis about5 years. Further downstream, residence times increase towards30 years at a distance of 340m from the beginning of the brook. Themore downstream part of the brook does not drain water which isrecharged in the depression. In scenario 3 (Fig. 10B), the residencetime of the recharged water before being drained by the brook isabout 5e6 years. These residence times show that, with scenarios1 and 2, the drained water is a mix of different residence times.Longer residence times will result in different water quality (alka-linity, pH, Eh, etc.), thus opening the potential for gradients in plantcommunities along the brook.

3.6. Ecological implications

Field observations show that the hydrological interventions areplanned in a part of the Kleyne Vlakte with groundwater flowtowards the polder. Therefore, scenario 3 is much less favourablethan scenarios 1 or 2. With scenario 3, less water is retained in thenature reserve after recharge (7%) than in the other two scenarios(33%) since it flows towards the polder’s drainage system. Addi-tionally, due to the limited drainage capabilities of the brook,scenario 3 results in a smaller increase of wet habitats. The mostimportant difference between scenarios 1 and 2 concerns theexpected difference in water quality in the two parallel parts of thebrook east of the depression. It is expected that the western branchwill drain water with a relatively short residence time. The easternbranch will drain water with a longer residence time. Additionally,the eastern branch will drain water which has gone througha longer and deeper flowcycle in the aquifer. Calcite dissolution andcation exchange are, as indicated by the naturally occurringgroundwater quality, the two most important reactions which willaffect the quality. The eastern branch (but not thewestern) will alsodrain an amount of pristine dune water. This will be a mix ofshallow oligahine-fresh CaHCO3þ water with a high alkalinity andmore mineralised water from deeper in the aquifer. Consequently,a higher mineral concentration compared to the western branch isexpected beside other differences (e.g. pH, Eh, etc.). This subtledifference in chemical composition of the water is expected toresult in differences in plant communities of the Ranunculion peltativersus Charetalia and Parvopotamion classes, the former growingunder slow flowing, slightly carbonate rich water, which is onlyslightly buffered, the latter growing in carbonate rich and clearwater (Van Katwijk and Roelofs, 1988). This is the case underscenario 2, and for this reason, it is scenario 2 that has been chosenfor implementation. Work on its realisation started in September of2009.

It is expected that environmental engineering followingscenario 2 will result in the establishment of certain target aquaticand riparian plant communities in the depression and in the brookswest of it (Lebbe et al., 2009). The target communities may showaffinity to different phytosociological groups e.g. Charetalia, Lem-netalia and Potametalia apart from Lolio-potentillion, Phragmitetaliaand Nasturtio-Glyceritalia. In the western branch one expects theestablishment of species with affinity to the Cardamino-Montionunder shadow conditions and to the R. peltati under conditionswith high light availability. In the eastern branch one expects theestablishment of Charetalia and Potamogeton densus. In the dun-eepolder transition area, the mean rise of groundwater level willcreate better conditions for dune slack communities and nutrientpoor grassland communities (Calthion and Eu-Molinion).

4. Conclusions

Because in dune slacks a close coupling exists between changesin the hydrology and changes in species composition and vegeta-tion structure (Grootjans et al., 1998), there was a need to underpinthe nature restoration project at the Flemish Nature Reserve ‘TheZwindunes and Zwinpolders’ with detailed hydrogeological data,together with ecologically relevant knowledge. The interest ofecologists in groundwater is twofold: groundwater flow and levels,and groundwater quality. The former was studied here using timeseries of groundwater levels from a large number of observationwells which were represented in maps showing mean, highest andlowest groundwater levels. Groundwater quality was studied bymeans of water samples from a number of observation wells usinggeophysical borehole measurements. Water types, using theStuyfzand classification (1993), were used to classify the samplesand yield information about mineralisation, alkalinity, dominatingcation and anion, and base exchange index. Geophysical boreholemeasurements revealed the fresh-salt water distribution. In thecase of ‘The Zwindunes and Zwinpolders’, this information wasvitally important for the design of possible scenarios for rewettingpart of the nature reserve (the Kleyne Vlakte). Groundwater levelsgave information about the general groundwater flow and wereused to optimise the excavation depth of depressions and brooks.Groundwater quality data was needed to evaluate the impact of theinterventions on it. The impact of different scenarios was furtherstudied using groundwater flow modelling.

Groundwater flow modelling combines all available hydro-geological information (geological model, water quality data,drainage levels, water extraction data, etc.) and thus offers a valu-able tool for groundwater management in nature reserves andthe planning of restoration interventions. The output of ground-water flow models are fresh water head and fresh-salt waterdistribution in the model domain. These can be used to describecurrent and future hydrogeological situation and plan hydrological

A. Vandenbohede et al. / Journal of Environmental Management 91 (2010) 2385e2395 2395

interventions. Obviously, a groundwater flow model is only asgood as the data one uses to construct the model. The geology ofthe model area must be known relatively well, data on hydrology(e.g. drainage levels and recharge) and sediment properties (e.g.hydraulic conductivities) must be present and head and/or waterquality data to calibrate the model must be available. This was thecase for the example of the ‘The Zwindunes and Zwinpolders’ andthe simulation made it possible to make well-founded distinctionsbetween the different rewetting scenarios.

An ecological point of view is often a distinctly local one,whereas groundwater flow and water quality are in many casesdetermined on amore regional scale. This mismatch was addressedhere by first constructing and calibrating a regional model.Secondly, a local model was constructed with a focus on thehydrological intervention in the Kleyne Vlakte. The boundaryconditions of this local model were supplied by the regional model.Working with more detailed (zoomed in) models derived fromlarger scale models is a way to meet the ecologist’s need for localinformation, while at the same time incorporating larger scalegroundwater flow and quality pattern.

Acknowledgements

AVDB is supported by the Fund for Scientific Research d Flan-ders (Belgium), where he is currently a postdoctoral fellow. Thisstudy was commissioned by the Agency for Nature and Forests,Flemish Government and is a part of the European LIFE-natureproject ZENO. We acknowledge the numerous remarks andsuggestions from three anonymous reviewers.

Appendix. Supplementary Material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jenvman.2010.06.023.

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